Engineers have not yet found any obvious manufacturing problems or defects that might explain the potentially catastrophic foam loss that marred shuttle Discovery's July 26 launching. While a variety of possible fixes are under study, it's not yet known how long it might take to identify root causes and as a result, there is virtually no chance of launching the next shuttle flight in September as originally planned, a top NASA manager said today.

"There is nothing that became very obvious to us from an overall standpoint that said there was anything truly unique about this tank that's different than the other tanks that would easily clear us to go ahead an fly the next tank," said Bill Gerstenmaier, manager of NASA's space station program.

"So if I take the preliminary results I've gotten to date from the team, I think more than likely we're going to have to make some kind of minor engineering modification to the tank and to do that probably impacts the September launch window. We probably won't make the September launch window."

NASA had hoped to follow up Discovery's mission by launching the shuttle Atlantis around Sept. 9. But Discovery's launch was delayed and because Discovery must be ready for takeoff as a rescue vehicle in case of major problems with Atlantis, only four days - Sept. 22 through Sept. 25 - are available for the second post-Columbia flight. And that was before Discovery was diverted to a California landing, which will add a week to its ground processing.

Gerstenmaier discussed initial findings from so-called "tiger teams" of engineers trying to find out what caused relatively large pieces of foam insulation to fall off Discovery's external fuel tank during the climb to space July 26.

The largest and most alarming piece was a 0.9-pound chunk of foam that separated from a so-called protubereance air load - PAL - ramp used to smooth the flow of supersonic air over a cable tray and two pressurization lines. The foam peeled away two minutes and seven seconds after blastoff, just two seconds or so after Discovery's solid-fuel boosters were jettisoned.

Last week, NASA confirmed that during the tank's assembly, engineers had to repair minor damage in the same area of the PAL ramp. But Gerstenmaier said today the damage in question measured just two-tenths of an inch across and "that alone probably wouldn't be enough to cause the foam loss we saw.

"We still need to confirm this (but) there's probably another underlying cause."

The external tank is equipped with two PAL ramps, one that runs along the outside of the upper liquid oxygen section and a 38-foot six-inch-long ramp that runs along the upper part of the hydrogen section. The hydrogen ramp, where Discovery's debris originated, is made up of foam that is sprayed on and shaped by hand. It weighs a total of 22 pounds and measures less than six inches thick.

The PAL ramps were not modified in the wake of the Columbia disaster and engineers were hopeful the foam loss seen in Discovery's launch was caused by a manufacturing defect or some other isolated, non-generic problem that would not require a fleet-wide redesign.

But Gerstenmaier appeared to play down that possibility today, saying "we didn't find any obvious root cause," generic or otherwise.

"Probably the root cause will be some combination of events," he said. "But we didn't see anything that showed us the specific root cause. We just need to keep looking. There's no immediate answer or problem that jumps out at us."

While the PAL ramp foam has received the most attention, foam also separated from four other areas of the tank:

• LH2 tank acreage: Two divots, one measuring about four inches across and the other eight to 10 inches in diameter, were seen in an area where foam is sprayed on by machines.

• Intertank/LH2 flange closeout: An area that was heavily modified in the wake of Columbia, the intertank flange is where a cylindrical section of the tank joins the lower part of the oxygen section and the upper part of the hydrogen section. In past flights, foam shedding from this area was commonplace. In Discovery's flight, two divots were seen, both measuring about three inches across.

• Bipod fitting closeout: Columbia was brought down by a large 1.67-pound piece of foam that broke away from an aerodynamically shaped ramp insulating one of the two struts holding the hose of the shuttle to the tank. The bipod ramps were eliminated and replaced by heaters. But a piece of foam measuring 8.4 by 7.3 inches broke away near one of the exposed bipods 148 seconds after launch.

• LH2 ice/frost ramps: Foam ramps are used to smooth the flow of air over fitting that attach two pressurization lines to the tank. The pressurization lines run adjacent to the PAL ramps. During Discovery's launch three of these ice/frost ramps suffered minor foam loss.

Despite the obvious problems, Gerstenmaier said overall, Discovery's tank performed well.

"When you look at the amount of foam on the tank and the performance overall, the tank really did a pretty phenomenal job," he said. "When you look at it emotionally, you kind of feel one way. But then when you start looking at the hard engineering data, you sure get a different feeling."

The tank was launched with 4,192 pounds of sprayed-on foam insulation. Ascent and post-separation photography showed just 1.2 pounds of foam was liberated.

"So in terms of the aggregate, that's a pretty small number, one out of 4,192," Gerstenmaier said. "It's still not the performance we'd really like, but it gives you a feel for how the tank performed overall."

While the intertank flange area was the subject of a major post-Columbia redesign, "we had two small divots three inches in diameter. But again, that's pretty phenomenal performance compared to what we used to see on previous flights. We had lots of foam loss in that area and this is a dramatic reduction."

Gerstenmaier said a wide range of possible fixes were under discussion, including the removal of current PAL ramps so they can be rebuilt with a "clean spray" in a controlled fashion. Other ideas include using fiber or mesh-like material in specific areas to add strength.

The loss of foam near the bipod struts may have been caused by a post-Columbia heater wire that allowed air to get underneath the foam. Nitrogen in the air then could have liquified due to the ultra low temperature of iiquid hydrogen. During launch, that liquid would have changed back into a gas, possibly blowing out overlying foam.

"We may have some potential for cryo ingestion through a heater wire that runs through there," Gerstenmaier said. "We may make a minor change to the heater wire to try to alleviate that if that appears to be a problem."

Whether such fixes might work remains to be seen.

"We're going to have to really understand why this foam came off," he said. "We're going to have to spend the time to do the root cause analysis and understand with a high probability what the underlying mechanism, or mechanisms, were that contributed to this foam loss. Then the techiques we use to either put the foam back on or to change it have to accommodate and rule out and remove all those leading causes that could have led to the foam loss.

In the end, he said, "I think you're going to find differing mechanisms for different areas."

"There may be a different mechanism for foam loss associated with each of these different areas," he said. "Then the question is can we control that?"

The shuttle Discovery's crew, braving the hellish fire of re-entry for the first time since Columbia's ill-fated descent two-and-a-half years ago, flew safely back to Earth today, gliding to a predawn California touchdown to close out an action-packed mission.

With veteran commander Eileen Collins at the controls, Discovery swooped to a ghostly, tire-smoking touchdown on runway 22 at Edwards Air Force Base in California's Mojave Desert at 8:11:22 a.m. EDT, one day late because of concern about cloudy weather in Florida. The crew had two shots at a Kennedy Space Center landing today, but off-shore storms forced entry flight director LeRoy Cain to divert the shuttle to California.

It was the 50th shuttle landing at the famed Air Force test center and only the sixth carried out in darkness. But Collins, a 1990 graduate of Air Force Test Pilot School at Edwards, had no problems, guiding Discovery to a picture-perfect touchdown.

Barreling down the runway at more than 200 mph, pilot James "Vegas" Kelly deployed a large braking parachute, the shuttle's nose dropped and the spaceplane slowly rolled to a halt.

"Houston, Discovery, wheels stopped," Collins radioed.

"Roger, wheels stopped, Discovery," called astronaut Ken Ham from mission control. "And congratulations on a truly spectacular test flight. Stevie Ray, Souichi, Andy, Vegas, Charley, Wendy and Eileen, welcome home, friends."

"Thank you, those are great words to hear," Collins replied. "We're happy to be back and we congratulate the whole team on a job well done."

Discovery's high-speed touchdown was the final chapter in the 114th shuttle mission, a voyage spanning 5.8 million miles and 219 complete orbits since blastoff July 26 from pad 39B at the Kennedy Space Center.

Collins, Kelly and their crewmates - flight engineer Stephen "Stevie Ray" Robinson, Andrew Thomas, Wendy Lawrence, Charles Camarda and Japanese astronaut Soichi Noguchi - underwent brief medical checks before a traditional walk-around inspection on the runway.

"We had a fantastic mission, we are so glad to be able to come back and say it was successful, we resupplied the international space station, we met the test objectives of the space shuttle program, we brought Discovery back in great shape as you can see," Collins said on the landing strip.

"The crew was really anxious to walk around and see what the outside looked like and it looks fantastic. ... I just want to say thanks to all of those who worked this mission. For many of us, it's been four years since we've (started training) for STS-114 and this is a wonderful moment for all of us to experience."

NASA Administrator Michael Griffin agreed, praising Collins for a successful mission and joking that "I'm thinking about resigning my position. ... She's smarter, more personable, better looking, a better pilot."

"I think the crew performed beyond fantastically well," Griffin said. "The flight directors who haven't controlled a shuttle for two-and-a-half years performed fantastically well, the space station program, their flight controllers, their engineering team, were so good that nobody ever noticed them and they have not yet gotten the credit they really deserve for being able to deal with two and a half years of downtime on shuttle resupply. ... Everywhere you looked there was outstanding success."

Shuttle program manager Bill Parsons called Discovery's flight a "wildly successful mission" and Bill Readdy, associate administrator for spaceflight, said "Eileen made it look like a cakewalk."

Said Michael Leinbach, launch director at the Kennedy Space Center: "On July 26, we all sat here and said the only thing better than that launch would be on landing day. I'm here to tell you it really is truly better. Discovery's home, the crew is safe and we've come full circle now."

But with Discovery's safe return, NASA's full attention now shifts to figuring out what caused multiple pieces of foam insulation to fly off Discovery's external tank during launch. Columbia was brought down by wing damage caused by a foam debris strike during launch and the No. 1 priority of NASA's return to flight was fixing the insulation to minimize foam shedding.

But during Discovery's launch, a 0.9-pound chunk of foam peeled away from the tank just after solid booster separation. Two other relatively large pieces separated from the tank near the point where the shuttle's nose attaches to bipod struts and a fourth piece broke away from another area.

Media attention has focused on the largest piece, which came from a so-called protuberance air load - PAL - ramp designed to smooth the flow of supersonic air across pressurization lines and electrical cables. Going into Discovery's mission, the hydrogen PAL ramp, which was not modified in the wake of Columbia, had not suffered any known foam loss since 1983.

Last week, NASA confirmed that the area that broke away during launch had been slightly damaged and repaired, using standard procedures, during the tank's assembly at Lockheed Martin's Michoud, La., plant. NASA managers are hopeful whatever caused the foam to separate will be traced to an isolated problem with tank No. 121 and not a generic issue requiring a fleet-wide redesign.

But the PAL ramp foam is only part of NASA's problem and it's not yet known what might been needed to address the other areas of foam separation. One of those, from an area called the intertank flange, was modified in the wake of Columbia and engineers did not expect to see any significant debris.

"Now that we have Discovery on terra firma, we'll go work those other issues," Parsons said. "We've got the teams in place to get that data, that information, and I think in the next week or so we'll get some preliminary findings about where we're headed."

The shuttle Atlantis had been scheduled for launch on the second post-Columbia mission in September, but resolution of the foam problem made that target window problematic. Now, with Discovery back on the ground in California, a September launch is no longer thought to be possible. Discovery must be ready for launch as a rescue vehicle in case Atlantis suffers any significant damage on its flight and today's West Coast landing will add a week or so to Discovery's turn-around time.

Atlantis' launch already had slipped to no earlier than Sept. 22, giving it just four days to get off the ground before the September launch window closes. It now appears Discovery will be unable to support that window, even if the foam problem can be resolved in time.

The only other available launch window before the end of the year is a three-day period in November. Another short window opens in January but the first lengthy opportunity to send a shuttle back to the international space station is in March.

Griffin would not speculate on how long it might take to return the shuttle fleet to space.

"I don't want to guess," he said. "We're going to try as hard as we can to get back in space this year because we have a big construction project we're working on and we need the shuttle to do it. So we're going to try as hard as we can."

But he quickly added: "We're not going to go until we're ready to go."

Despite the disappointment over the launch-day foam events, Discovery's crew chalked up a near flawless mission, delivering tons of supplies to the space station and staging three spacewalks to install a new stabilizing gyroscope, an external tool and spare parts depot and to demonstrate potential heat-shield tile and wing leading edge repair techniques.

The astronauts also carried out an unprecedented inspection of the shuttle's heat shield, using a large sensor-equipped boom to examine the wing leading edges and nose cap and utilizing cameras on the station to photograph the orbiter's underside in great detail.

During that inspection, engineers spotted two protruding "gap fillers" sticking up from between tiles on Discovery's belly. Because of concern the gap fillers could trigger re-entry turbulence - and thus expose the underside tiles to extreme temperatures for longer periods - Robinson carried out emergency repairs during the crew's third spacewalk, plucking the gap fillers out with his gloved fingers.

The only other problem of any significance was a torn insulation blanket just below Collins' left-side cockpit window. After an extensive analysis, mission managers concluded Discovery could safely return to Earth as is and the damaged blanket was left in place.

Post-landing close-up television views showed most of the blanket survived Discovery's fiery re-entry with only minor signs of heat-related discoloration. It appeared that a small section at the extreme front end of the thin, 20-inch-long blanket might have ripped away during Discovery's passage through the dense lower atmosphere, but the on-orbit analysis appeared to be vindicated.

Collins and company had hoped to land Monday in Florida, but off-shore clouds and rain forced a one-day delay. More of the same was on tap today, and after ordering the crew to pass up the first of two Florida landing opportunities, Cain diverted Discovery to California.

"How would you feel about a beautiful clear night with a breeze down the runway in the high desert of California?" Ham radioed around 5 a.m.

"We are ready for whatever we need to do," Collins replied.

"OK, that's going to be our plan," Ham said. "The official forecast is holding electrified clouds off the coast of the Cape, which we're not going to send you through."

Flying upside down and backward on a northwest to southeast trajectory over the Indian Ocean, Discovery's flight computers fired up the ship's twin orbital maneuvering system rockets at 7:06:18 a.m. for a two-minute 42-second burn that slowed the ship by 187 mph, just enough to drop the far side of the orbit deep into the atmosphere.

Collins then flipped the shuttle around, putting it in a wings-level, nose-up orientation at a 40-degree angle of attack. After a half-hour freefall, Discovery fell into the discernible atmosphere at an altitude of 400,000 feet - 75 miles - above the South Pacific Ocean.

Within minutes, Discovery was enveloped in a fireball as atmospheric friction converted the shuttle's energy of motion into heat. During Columbia's decent on Feb. 1, 2003, instruments on the shuttle recorded the first signs of abnormal heating just four minutes and 50 seconds after entry interface as plasma began.

But today, Discovery's fall to Earth appeared problem free as the shuttle streaked toward California.

Flying upside down and backwards just off the tip of Madagascar over the Indian Ocean, shuttle commander Eileen Collins and pilot James Kelly fired Discovery's twin braking rockets at 7:06:18 a.m. to begin the hourlong descent to a California landing.

The deorbit rocket firing lasted two minutes and 42 seconds, changing the shuttle's velocity by about 187 mph and dropping the far side of its orbit deep into the atmosphere. Touchdown on runway 22 at Edwards Air Force Base, Calif., is expected at 8:12 a.m. EDT, 54 minutes before sunrise local time.

NASA Administrator Mike Griffin, spaceflight chief Bill Readdy, shuttle program manager Bill Parsons and launch director Mike Leinbach will participate in a post-landing news conference at the Kennedy Space Center at 9:30 a.m. Entry flight director LeRoy Cain will review Discovery's descent from orbit during a 10 a.m. briefing. NASA hopes to hold a post-landing news conference with Discovery's crew sometime between 2 p.m. and 3 p.m. EDT. All three briefings will be carried on NASA television.

This status report will be updated after Discovery lands or as conditions warrant.

The Discovery astronauts were waved off from a Florida landing today and told to set their sights on a landing at Edwards Air Force Base in California's Mojave Desert because of concerns about off-shore storms near the Kennedy Space Center.

Already waved off from the first of two Florida landing opportunities, Discovery's crew was told to divert to Edwards during a 5 a.m. call from mission control.

"How would you feel about a beautiful clear night with a breeze down the runway in the high desert of California?" astronaut Ken Ham radioed from Houston.

"We are ready for whatever we need to do," shuttle skipper Eileen Collins replied.

"OK, that's going to be our plan. What we had is continued instability at the Cape. Just within the last 15 minutes or so, we had another couple of small cells popping up around 15 to 20 miles that came out of nowhere, just off the coastline and growing. Predicted to have some lightning activity here shortly. So the official forecast is holding electrified clouds off the coat of the Cape, which we're not going to send you through. So for now, you can just hold where you are in the checklist. we're going to think about the best way to get in a good config for Edwards."

"We copy all," Collins said. "We'll hold right here. We've been thinking about Edwards, so we'll be ready for that."

"Great."

This will be the 50th shuttle landing at Edwards - the first since June 19, 2002 - and only the sixth night landing at the Mojave Desert flight test facility. But Collins is a 1990 graduate of Air Force Test Pilot School at Edwards and and has flown scores of approaches and landings in shuttle training jets.

Here is a timeline of major events with an updated time for the deorbit rocket firing (in EDT; includes STS-107/Columbia entry milestones for reference):

Rev 219 Deorbit to EDW

06:46 AM......Mission Control 'go' for deorbit burn
06:52 AM......Mission specialists seat ingress
07:01 AM......Single APU hydraulic power unit start

07:06:18 AM...Deorbit ignition (dT: 2:42; dV: 187 mph)
07:09:00 AM...Deorbit burn complete

07:40:11 AM...Entry interface: altitude 400,000 feet (75.7 miles)
..............Range to landing: 5,024 statute miles

07:44:41 AM...STS-107: EI+04:30 - 1st unusual data
07:45:01 AM...STS-107: EI+04:50 - 1st unusual heating

07:45:11 AM...1st roll command to left

07:46:55 AM...STS-107: EI+06:44 - Start peak heating
07:47:02 AM...STS-107: EI+06:51 - WLE temps 2,650
07:47:16 AM...STS-107: EI+07:05 - Plasma into wing
07:49:02 AM...STS-107: EI+08:51 - WLE temps 2,800

07:52:27 AM...1st left-to-right roll reversal

07:53:02 AM...STS-107: EI+12:51 - WLE temps 2,900
07:54:02 AM...STS-107: EI+13:51 - WLE temps 2,880
07:54:58 AM...STS-107: EI+14:47 - Tire alarm
07:55:02 AM...STS-107: EI+14:51 - WLE temps 2,850
07:55:34 AM...STS-107: EI+15:23 - Initial loss of signal
07:56:07 AM...STS-107: EI+15:56 - Last valid data
07:56:55 AM...STS-107: EI+16:44 - End peak heating

08:05:30 AM...Velocity less than mach 2.5
08:07:40 AM...Velocity less than mach 1
08:08:45 AM...Shuttle on the heading alignment cylinder
08:11:48 AM...Landing

Offshore thunderstorms within NASA's 30-nautical-mile safety zone forced NASA flight director LeRoy Cain to delay the shuttle Discovery's re-entry today by one more orbit, pushing landing back to 6:43 a.m. at the earliest. If the Florida weather doesn't improve, the astronauts will be forced to head for California instead at 8:12 a.m. EDT.

"At this time, we're going to ask you guys to watch the Earth go by for one more rev and we're going to officially wave off of this attempt," astronaut Ken Ham radioed the crew just after 3 a.m. "Our current obs (observed weather) at the Cape is we have rain showers and thunderstorms withiin 30 (nautical miles) and an electrified anvil (cloud) and we don't see any way that we're going to be able to clear that within the time for touchdown."

The Spaceflight Meteorology Group at the Johnson Space Center is "holding some hope for the second opportunity at the Cape," Ham continued. "The official forecast for that is a slight chance of rain showers within 30. I know we've told you that a few times in the last couple of days. All we can do is watch it and hope it gets more stble."

He told commander Eileen Collins to hold off on the crew's re-entry checklist "with no activities and pick up again at the IMU (inertial measurement unit) align where you are right now. How copy?"

"We copy all that, Houston, and we understand how unstable the weather is in Florida is so we're not surprised at all," Collins replied. "I've been in your shoes many times, so I understand how it goes."

Alignment of the shuttle inertial measurement unit occurs roughly one hour before the rocket firing that drops the ship out of orbit. Here's an updated timeline for Discovery's second Florida landing opportunity (times in EDT; includes STS-107/Columbia entry milestones for reference):

Rev 218 Deorbit to KSC

05:17 AM......Mission control 'go' for deorbit burn
05:23 AM......MS seat ingress
05:32 AM......Single APU start
 
05:37:27 AM...Deorbit ignition (dT: 2:43; dV: 188 mph)
05:40:10 AM...Deorbit burn complete

06:10:45 AM...Entry interface: altitude 400,000 feet (75.7 miles)
..............Range to landing: 5,004 statute miles

06:15:15 AM...STS-107: EI+4:30 - 1st unusual data
06:15:35 AM...STS-107: EI+4:50 - 1st unusual heating

06:15:43 AM...1st roll command to left

06:16:36 AM...STS-107: EI+05:51 - WLE temps 2,520
06:17:29 AM...STS-107: EI+06:44 - Start peak heat
06:17:36 AM...STS-107: EI+06:51 - WLE temps 2,650
06:17:50 AM...STS-107: EI+07:05 - Plasma into wing
06:19:36 AM...STS-107: EI+08:51 - WLE temps 2,800
06:23:36 AM...STS-107: EI+12:51 - WLE temps 2,900
06:24:36 AM...STS-107: EI+13:51 - WLE temps 2,880
06:25:32 AM...STS-107: EI+14:47 - Tire alarm
06:25:36 AM...STS-107: EI+14:51 - WLE temps 2,850
06:26:08 AM...STS-107: EI+15:23 - Initial LOS
06:26:41 AM...STS-107: EI+15:56 - Last valid data
06:27:29 AM...STS-107: EI+16:44 - End peak heat

06:30:30 AM...1st left-to-right roll reversal
06:36:16 AM...Velocity less than mach 2.5
06:38:23 AM...Velocity less than mach 1
06:39:06 AM...Shuttle on the HAC
06:42:35 AM...Landing

Keeping tabs on uncertain weather, the Discovery astronauts closed the shuttle's 60-foot-long cargo bay doors today and geared up for a day-late landing at the Kennedy Space Center to close out an extended two-wek mission.

Today's forecast is similar to Monday's, with uncertainty about the possible development of low clouds and rain. By closing Discovery's payload bay doors at 1:20 a.m., the crew preserved the option of a 5:07 a.m. landing at the Florida spaceport, the first of a half-dozen landing opportunities in Florida, New Mexico and California.

But flight director LeRoy Cain held open the possibility of waving off the first FLorida opportujnity depending on how the weather plays out.

"OK, Eileen, once again the word of the evening for weather at the Cape is instability," astronaut Ken Ham radioed shuttle commander Eileen Collins around 1:15 a.m. "Right now, we're watching some cells popping up within the 30 mile circle over the water, probably in the vicinity of 20 to 25 miles off the coast and Rommel's (chief astronaut Kent Rominger is) reporting that they're building up fairly high, up in the 30s, and they do have some electrical activity associated with them.

"We're watching all that. Our optimism isn't quite as high as it was earlier," Ham said. "Obviously, we're going to ask you to keep going along the timeline and we're going to look real hard at the decision for fluid loading with respect to this rev. We understand the situation we're in for you and the whole day ahead of you. How copy?"

"Discovery copies all that," Collins replied. "Thanks for the update and we'll be ready for whatever decision we make."

Fluid loading refers to a requirement for returning astronauts to take salt tablets and drink several quarts of water to restore lost fluids, a protocol that eases their re-adaptation to gravity. Cain won't ask the crew to fluid load until he's reasonably confident the astronauts have a realistic shot at landing.

Here are timelines for the first two opportunities to land in Florida and the first opportunity for a landing at Edwards Air Force Base, Calif. NASA also is staffing a third landing site near White Sands, N.M., but a landing there is considered extremely unlikely. The forecast for Edwards calls for excellent weather.

NOTE: Times for major events during Columbia's descent two-and-a-half years ago are included for reference. Those times are synched with entry interface, the point when the shuttle reaches the discernible atmosphere, and show when events would have happened had Columbia's re-entry time matched Discovery's. All times in EDT:

Rev 217 Deorbit to KSC

01:20 AM......Payload bay doors closed
01:30 AM......Mission control 'go' for OPS-3 entry software load
01:40 AM......OPS-3 transition
02:05 AM......Entry switchlist verification
02:15 AM......Deorbit maneuver update
02:20 AM......Crew entry review
02:35 AM......Collins/Kelly don entry suits
02:52 AM......IMU alignment
03:00 AM......Collins/Kelly strap in; Mission specialists suit don
03:00 AM......Fluid loading begins
03:17 AM......Shuttle steering check
03:20 AM......APU hydraulic power unit start
03:27 AM......Toilet deactivation
03:35 AM......Vent doors closed for entry
03:40 AM......Mission Control 'go' for deorbit burn
03:46 AM......Mission specialists seat ingress
03:35 AM......TDRS-West acquisition of signal
03:55 AM......Single APU start

04:00:32 AM...Deorbit ignition (dT: 2:42; dV: 187 mph)
04:03:44 AM...Deorbit burn complete

04:35:18 AM...Entry interface: altitude 400,000 feet (75.7 miles)
..............Range to landing: 5,084 statute miles

04:39:48 AM...STS-107: EI+04:30 - 1st unusual data
04:40:08 AM...STS-107: EI+04:50 - 1st unusual heating

04:40:17 AM...1st roll command to left

04:42:02 AM...STS-107: EI+06:44 - Peak heating start
04:42:09 AM...STS-107: EI+06:51 - WLE temps 2,650
04:42:23 AM...STS-107: EI+07:05 - Plasma into wing
04:44:09 AM...STS-107: EI+08:51 - WLE temps 2,800
04:48:09 AM...STS-107: EI+12:51 - WLE temps 2,900

04:48:30 AM...1st left-to-right roll reversal

04:49:09 AM...STS-107: EI+13:51 - WLE temps 2,880
04:50:05 AM...STS-107: EI+14:47 - Tire alarm
04:50:09 AM...STS-107: EI+14:51 - WLE temps 2,850
04:50:41 AM...STS-107: EI+15:23 - Initial loss of data
04:51:14 AM...STS-107: EI+15:56 - Last valid data
04:52:02 AM...STS-107: EI+16:44 - End peak heating

05:00:38 AM...Velocity less than mach 2.5
05:02:47 AM...Velocity less than mach 1
05:03:09 AM...Shuttle on the HAC
05:07:02 AM...Landing

--------------------------------------------------

Rev 218 Deorbit to KSC

05:17 AM......Mission Control 'go' for deorbit burn
05:23 AM......Mission specialists seat ingress
05:32 AM......Single APU start

05:37:30 AM...Deorbit ignition (dT: 2:43; dV: 188 mph)
05:40:13 AM...Deorbit burn complete

06:10:45 AM...Entry interface: altitude 400,000 feet (75.7 miles)
..............Range to landing: 5,004 statute miles

06:15:15 AM...STS-107: EI+04:30 - 1st unusual data
06:15:35 AM...STS-107: EI+04:50 - 1st unusual heating

06:15:43 AM...1st roll command to left

06:17:29 AM...STS-107: EI+06:44 - Start peak heating
06:17:36 AM...STS-107: EI+06:51 - WLE temps 2,650
06:17:50 AM...STS-107: EI+07:05 - Plasma into wing
06:19:36 AM...STS-107: EI+08:51 - WLE temps 2,800
06:23:36 AM...STS-107: EI+12:51 - WLE temps 2,900
06:24:36 AM...STS-107: EI+13:51 - WLE temps 2,880
06:25:32 AM...STS-107: EI+14:47 - Tire alarm
06:25:36 AM...STS-107: EI+14:51 - WLE temps 2,850
06:26:08 AM...STS-107: EI+15:23 - Initial loss of signal
06:26:41 AM...STS-107: EI+15:56 - Last valid data
06:27:29 AM...STS-107: EI+16:44 - End peak heating

06:30:30 AM...1st left-to-right roll reversal
06:36:16 AM...Velocity less than mach 2.5
06:38:23 AM...Velocity less than mach 1
06:39:06 AM...Shuttle on the HAC
06:42:35 AM...Landing

--------------------------------------------------

Rev 219 Deorbit to EDW

06:46 AM......Mission Control 'go' for deorbit burn
06:52 AM......Mission specialists seat ingress
07:01 AM......Single APU start

07:06:27 AM...Deorbit ignition (dT: 2:42; dV: 187 mph)
07:09:09 AM...Deorbit burn complete

07:40:11 AM...Entry interface: altitude 400,000 feet (75.7 miles)
..............Range to landing: 5,024 statute miles

07:44:41 AM...STS-107: EI+04:30 - 1st unusual data
07:45:01 AM...STS-107: EI+04:50 - 1st unusual heating

07:45:11 AM...1st roll command to left

07:46:55 AM...STS-107: EI+06:44 - Start peak heating
07:47:02 AM...STS-107: EI+06:51 - WLE temps 2,650
07:47:16 AM...STS-107: EI+07:05 - Plasma into wing
07:49:02 AM...STS-107: EI+08:51 - WLE temps 2,800

07:52:27 AM...1st left-to-right roll reversal

07:53:02 AM...STS-107: EI+12:51 - WLE temps 2,900
07:54:02 AM...STS-107: EI+13:51 - WLE temps 2,880
07:54:58 AM...STS-107: EI+14:47 - Tire alarm
07:55:02 AM...STS-107: EI+14:51 - WLE temps 2,850
07:55:34 AM...STS-107: EI+15:23 - Initial loss of signal
07:56:07 AM...STS-107: EI+15:56 - Last valid data
07:56:55 AM...STS-107: EI+16:44 - End peak heating

08:05:30 AM...Velocity less than mach 2.5
08:07:40 AM...Velocity less than mach 1
08:08:45 AM...Shuttle on the HAC
08:11:48 AM...Landing

In several previous status reports, I reported that shuttle landings at Edwards Air Force Base, Calif., typically costs NASA about $5 million for ground processing and a 747 ferry flight back to the Kennedy Space Center in Florida. The actual cost is roughly $1 million. I have corrected all references in previous status reports posted in the STS-114 archive. My apologies for the error.

Uncertainty about dynamic weather at the Kennedy Space Center forced entry flight director LeRoy Cain to pass up two landing opportunities today and to keep the shuttle Discovery's crew in orbit an extra 24 hours.

An initial landing opportunity was passed up because of low clouds near the Shuttle Landing Facility and while conditions appeared to improve while Discovery went back around the planet, forecasters could not guarantee acceptable conditions for the second landing opportunity at 6:22 a.m.

Less than 15 minutes before the 5:15 a.m. rocket firing that would have committed commander Eileen Collins and her crew to re-entry, Cain decided to wave off for the day.

"As you can imagine from our silence down here, there continues to be a low layer (of clouds) varying between 300, 400 and 500 feet, it's been few and scattered all evening and again, the one word that describes all this all night has been 'unstable,' astronaut Ken Ham radioed Collins from Houston.

"Our current observed weather is actually 'go,' it's few at 500, broken at 15,000 and the forecast is in the same neighborhood, 800 scattered, broken at 15,000, which is technically 'go' as well. However, we just can't get comfortable with the stability of the situation for this particular opportunity. So we're going to officially wave you off for 24 hours."

"OK, Houston, we copy that," Collins replied. "It will be a wave off for today."

Cain said Sunday that if Discovery failed to get back to Florida today he would activate NASA's backup landing site at Edwards Air Force Base, Calif. - and possibly White Sands, N.M. - and bring the shuttle down, on one coast or the other, Tuesday.

The forecast for Kennedy calls for just a few clouds at 2,000 feet and 10,000 feet, scattered clouds at 25,000 feet with a slight chance of showers within 30 nautical miles. Winds will be out of 220 degrees at three knots with peaks to five knots, well within NASA's crosswind limits.

At Edwards, the outlook is for few clouds at 10,000 feet, scattered clouds at 25,000 feet and winds from 220 degrees at four knots peaking to seven knots. White Sands is expecting a slight chance of showers and broken cloud decks at 10,000 and 25,000 feet.

Here are all possible landing times for Tuesday and Wednesday (in EDT):

ORBIT...TIG..........LANDING....SITE

Tuesday, Aug. 9

217.....04:01 AM.....05:07 AM...Kennedy Space Center
218.....05:33 AM.....06:39 AM...White Sands, NM
218.....05:37 AM.....06:43 AM...Kennedy Space Center
219.....07:06 AM.....08:12 AM...Edwards AFB, CA
219.....07:09 AM.....08:13 AM...White Sands, NM
220.....08:44 AM.....09:47 AM...Edwards AFB, CA

Wednesday, Aug. 10

232.....02:50 AM.....03:53 AM...Kennedy Space Center
233.....04:25 AM.....05:28 AM...Kennedy Space Center
234.....05:55 AM.....06:58 AM...Edwards AFB, CA
234.....05:57 AM.....06:59 AM...White Sands, NM
235.....07:31 AM.....08:32 AM...Edwards AFB, CA
235.....07:33 AM.....08:34 AM...White Sands, NM

A broken deck of low clouds over the Kennedy Space Center forced entry flight director LeRoy Cain to delay the shuttle Discovery's re-entry today by at least one orbit in hopes conditions would improve enough to permit a landing at 6:21:39 a.m.

If Discovery can't get back to Florida today, the flight will be extended 24 hours. Cain said Sunday that in that case, NASA would activate the shuttle's backup landing site at Edwards Air Force Base, Calif., and he would bring Discovery down somewhere - in Florida or California - on Tuesday.

NASA wants to avoid a California landing if at all possible because it takes a week and costs some $1 million to ferry the craft back to the Kennedy Space Center. A California landing would all but eliminate a chance to launch the next shuttle in September. That flight already is in jeopardy because of problems with foam insulation on the shuttle's external tank. But even without foam problems, Discovery must be turned around to serve as a rescue vehicle for the September mission and a California landing would eat up a week of processing time.

Here's a timeline for today's final landing opportunity. Times for major events during Columbia's descent two-and-a-half years ago are included for reference. Those times are synched with entry interface, the point when the shuttle reaches the discernible atmosphere, and show when events would have happened had Columbia's re-entry time matched Discovery's.

DEORBIT TO THE KENNEDY SPACE CENTER ON ORBIT 202

04:55 AM......Mission control 'go' for deorbit burn
05:01 AM......Mission specialists seat ingress
05:10 AM......Single APU hydraulic power system start
	
05:15:10 AM...Deorbit ignition
05:18:19 AM...Deorbit burn complete
	
05:49:58 AM...Entry interface (the shuttle hits the discernible atmosphere)
05:54:28 AM...STS-107: EI+04:30 - 1st unusual data
05:54:48 AM...STS-107: EI+04:50 - 1st unusual heating

05:54:57 AM...1st roll command to left

05:56:42 AM...STS-107: EI+06:44 - Start peak heating

06:05:32 AM...1st left-to-right roll reversal
06:05:54 AM...STS-107: EI+15:56 - Last valid data
06:06:42 AM...STS-107: EI+16:44 - End peak heating

06:15:16 AM...Velocity less than mach 2.5
06:17:25 AM...Velocity less than mach 1
06:18:02 AM...Shuttle on the HAC
06:21:39 AM...Landing

The shuttle Discovery's cargo bay doors were closed for re-entry shortly after 1 a.m. today. There are no technical problems of any significance aboard the space shuttle and forecasters are predicting favorable weather for the first of two landing opportunities at 4:46:44 a.m. While a slight chance for rain within 30 nautical miles remains in the official forecast, the Spaceflight Meteorology Group at the Johnson Space Center in Houston believes that threat will ease as the morning wears on.

Aboard Discovery, commander Eileen Collins and her crew - pilot James Kelly, flight engineer Stephen Robinson, Andrew Thomas, Wendy Lawrence, Charles Camarda and Japanese astronaut Soichi Noguchi - are making final preparations for a three-minute and 12-second deorbit rocket firing beginning at 3:39:43 a.m. The transition to "OPS-3" entry software is expected in the next few minutes and a final update to Discovery's deorbit burn targets is expected around the top of the hour.

A formal go/no-go decision from mission control on whether to proceed with the rocket firing is expected about 20 minutes prior to ignition. If the weather or some other problem blocks an on-time descent, a second landing opportunity is available one orbit later. That opportunity requires a deorbit burn at 5:15:10 a.m. for a landing at 6:21:39 a.m.

Here is the timeline for today's activities. Times for major events during Columbia's descent two-and-a-half years ago are included for reference. Those times are synched with entry interface, the point when the shuttle reaches the discernible atmosphere, and show when events would have happened had Columbia's re-entry time matched Discovery's.

DEORBIT TO THE KENNEDY SPACE CENTER ON ORBIT 201

EDT...........EVENT

01:09 AM......Mission control 'go' for OPS-3 software transition
01:19 AM......OPS-3 entry software loaded
01:44 AM......Entry switchlist verification
01:54 AM......Deorbit maneuver update
01:59 AM......Crew entry review
02:14 AM......Collins and Kelly don pressure suits
02:31 AM......Inertial measurement unit alignment
02:39 AM......Collins and Kelly strap in; crewmates don pressure suits
02:56 AM......Shuttle steering check
02:59 AM......APU hydraulic power unit prestart
03:06 AM......Toilet deactivation
03:14 AM......Vent doors closed for entry
03:19 AM......Mission control 'go' for deorbit burn
03:25 AM......All other crew members strap in
03:35 AM......TDRS-West comsat acquisition of signal
03:34 AM......Single APU start
	
03:39:43 AM...Deorbit ignition (burn duration: 3:12; velocity change: 220 mph)
03:42:55 AM...Deorbit burn complete

04:15:07 AM...Entry interface (shuttle in discernible atmosphere)

04:19:37 AM...STS-107: EI+04:30 - 1st unusual data recorded
04:19:57 AM...STS-107: EI+04:50 - 1st unusual heating noted

04:20:07 AM...1st roll command to left

04:21:51 AM...STS-107: EI+06:44 - Start of peak heating
04:31:03 AM...STS-107: EI+15:56 - Last valid data
04:31:51 AM...STS-107: EI+16:44 - End of peak heating

04:33:26 AM...1st left-to-right roll reversal
04:40:22 AM...Velocity less than Mach 2.5
04:42:32 AM...Velocity less than Mach 1
04:43:26 AM...Shuttle banks and turns to line up on runway
04:46:44 AM...Landing


DEORBIT TO THE KENNEDY SPACE CENTER ON ORBIT 202

04:55 AM......Mission control 'go' for deorbit burn
05:01 AM......Mission specialists seat ingress
05:10 AM......Single APU start
	
05:15:10 AM...Deorbit ignition
05:18:19 AM...Deorbit burn complete
	
05:49:58 AM...Entry interface
05:54:28 AM...STS-107: EI+04:30 - 1st unusual data
05:54:48 AM...STS-107: EI+04:50 - 1st unusual heating

05:54:57 AM...1st roll command to left

05:56:42 AM...STS-107: EI+06:44 - Start peak heating

06:05:32 AM...1st left-to-right roll reversal
06:05:54 AM...STS-107: EI+15:56 - Last valid data
06:06:42 AM...STS-107: EI+16:44 - End peak heating

06:15:16 AM...Velocity less than mach 2.5
06:17:25 AM...Velocity less than mach 1
06:18:02 AM...Shuttle on the HAC
06:21:39 AM...Landing

Shuttle commander Eileen Collins and pilot James Kelly tested Discovery's re-entry systems today while their crewmates packed up for landing Monday at the Kennedy Space Center. With forecasters predicting good weather, entry flight director LeRoy Cain said his team is psyched up and ready for NASA's first shuttle re-entry since Columbia disintegrated over Texas two-and-a-half years ago.

"It's great to be here. We've looked forward to this for a long time, we've looked forward to this mission for a long time, we've had a great test flight, we've learned a lot about the vehicle and as you know, in the past couple of years we've learned a lot about ourselves," Cain told reporters early today.

"There's been a lot of good change. We're looking forward, we're not looking back. The team is in great spirits, we're all very excited and anxious. We will come in and we'll do the job tomorrow just like we'd do on any entry and landing day."

Aboard Discovery, Collins and Kelly fired up one of the shuttle's hydraulic power units early this morning, moved the ship's big rudder, checked the operation of cockpit displays and test-fired 38 maneuvering jets to make sure all of the shuttle's critical re-entry systems are operational.

A few hours later, flying into sunlight 220 miles above the still-dark Florida landing site, Discovery and the international space station put on a spectacular pre-dawn show, passing close by brilliant Mars with the shuttle leading the brighter lab complex by about 20 degrees, or the width of two fists held at arm's length.

Cain said Discovery's systems checked out normally and that he weather appeared ideal with scattered clouds, light winds and only a slight chance of off-shore rain.

Assuming the forecast holds up, Collins plans to fire Discovery's twin orbital maneuvering system braking rockets for three minutes and 12 seconds starting at 3:39:43 a.m. Monday, slowing the ship by 220 mph and dropping the far side of its orbit deep into the atmosphere over Florida's East Coast.

Flying along a southwest-to-northeast trajectory, Discovery will pass above Central America, the Gulf of Mexico and the west coast of Florida near Tampa as it glides toward touchdown at the Kennedy Space Center. Landing is expected around 4:46:44 a.m. Monday, closing out a 13-day voyaging spanning 5.3 million miles and 201 complete orbits since blastoff July 26.

Cain served as ascent-and-entry flight director for Columbia's final voyage. He was asked today whether he might be nervous during Monday's descent.

"Butterflies? If I came in to do an entry and landing on this flight or any flight and I didnŐt' have butterflies in my stomach, I'd probably turn around and go back outside and find somebody else to do the job, quite frankly," he said. "I consider that to be part of my own personal preparation process. There are a lot of things to think about, a lot of things to worry about and that's what I get paid to do, is to worry. And I do it a lot. So quite honestly, if I didn't feel that way I'd be concerned that I wasn't in the right frame of mind."

Asked if his flight control team might be distracted by memories of Columbia, Cain said "we're ready."

"We've practiced this hundreds of times, literally hundreds of times, and this is one of the most capable teams I've been associated with personally in my 17 years of flight control," he said. "I will let them know I have complete confidence in them to do the job and that we're ready to execute and we will be very focused, I can guarantee that."

Because the forecast for California's Edwards Air Force Base is good for several days, Cain does not plan to activate the backup landing site Monday and will focus instead on getting Discovery back on the ground in Florida.

Along with costing about $1 million in added work, a landing in California would eliminate any chance of launching the next shuttle mission in late September even if NASA solves its external tank foam problem. The shuttle Atlantis only has a four-day launch window and if Discovery is diverted to California, workers likely would not have enough time to prepare that shuttle for launch on a possible rescue mission.

In any case, Collins and company have two Florida landing opportunities Monday, one at 4:47 a.m. and the other one orbit later at 6:22 a.m.

"We're just calling up KSC for tomorrow," astronaut Ken Ham radioed Discovery from mission control early today. "So barring any unforeseen failures, that's all we're looking at for tomorrow. The weather at the Cape, the official forecast is few (clouds) at 2,500 (feet), few at 10,000, scattered at 25,000 with good visibility. The winds are light and pretty much variable, mostly out of the west-northwest. There is a call out for a slight chance of rain showers within 30 (nautical miles) and those are mostly out over the Gulf stream and fairly light.

"The weather for tomorrow should be fairly similar to what we saw today and the day prior. If you were to have come home today, you would have been go for the deorbit burn. So we're looking like a pretty favorable condition for the Cape tomorrow. Additionally, for weather out west, Edwards looks good for now and for the foreseeable future, for the next few days anyway, looks fairly good in case we need to go that direction for Tuesday."

"We can't think of any questions," Collins replied. "Sounds like really great weather."

"Yeah, we're looking forward to it."

If the weather or some other problem blocks both Monday opportunities, Cain will keep Discovery in orbit an extra day, activate the backup landing site at Edwards and bring the ship down on one coast or the other Tuesday.

Cain said today many shuttle commanders prefer night landings over day entries because of the visibility of the brightly illuminated runway at Kennedy. He said it didn't matter one way or the other to Collins, who he described as "one of the most skilled and capable pilots that I've been associated with."

"She is very conscientious, she and I have sat down and talked a great deal about this entry and landing and so I am keenly aware of what her capabilities are as well as the other members of my team who are responsible in part for helping me figure out the best opportunities to present to the team from a deorbit and landing standpoint.

"I have complete confidence in Eileen's capability," he said. "Quite frankly, she's capable of landing the shuttle in conditions that are way beyond what I'm willing to put her in."

Here are all possible landing times for Monday through Wednesday, including targets for White Sands, N.M. (in EDT)

ORBIT...SITE....TIG..............LANDING

Monday, Aug. 8

201.....03:40 AM.....04:47 AM...Kennedy Space Center
202.....05:15 AM.....06:22 AM...Kennedy Space Center

Tuesday, Aug. 9

217.....04:05 AM.....05:08 AM...Kennedy Space Center
218.....05:37 AM.....06:39 AM...White Sands, NM
218.....05:41 AM.....06:43 AM...Kennedy Space Center
219.....07:11 AM.....08:13 AM...Edwards AFB, CA
219.....07:13 AM.....08:14 AM...White Sands, NM
220.....08:47 AM.....09:48 AM...Edwards AFB, CA

Wednesday, Aug. 10

232.....02:50 AM.....03:53 AM...Kennedy Space Center
233.....04:25 AM.....05:28 AM...Kennedy Space Center
234.....05:55 AM.....06:58 AM...Edwards AFB, CA
234.....05:57 AM.....06:59 AM...White Sands, NM
235.....07:31 AM.....08:32 AM...Edwards AFB, CA
235.....07:33 AM.....08:34 AM...White Sands, NM

Here is a detailed landing timeline for the first opportunity into Kennedy. Major milestones from Columbia's last re-entry are provided as a reference for readers who might not recall the timing of major events. Columbia entry data is keyed to minutes and seconds after entry interface, the moment the shuttle reaches the discernible atmosphere at an altitude of 400,000 feet. These data are provided for reference only.

EDT...........EVENT

11:39 PM......Begin deorbit timeline
11:54 PM......Payload bay radiator stow
12:04 AM......Mission specialists seat installation
12:10 AM......Computers set for deorbit prep
12:14 AM......Hydraulic system configuration
12:39 AM......Flash evaporator cooling system checkout
12:45 AM......Final payload deactivation
12:59 AM......Payload bay doors closed
01:09 AM......Mission control 'go' for OPS-3 software transition
01:19 AM......OPS-3 entry software loaded
01:44 AM......Entry switchlist verification
01:54 AM......Deorbit maneuver update
01:59 AM......Crew entry review
02:14 AM......Collins and Kelly don pressure suits
02:31 AM......Inertial measurement unit alignment
02:39 AM......Collins and Kelly strap in; crewmates don pressure suits
02:56 AM......Shuttle steering check
02:59 AM......APU hydraulic power unit prestart
03:06 AM......Toilet deactivation
03:14 AM......Vent doors closed for entry
03:19 AM......Mission control 'go' for deorbit burn
03:25 AM......All other crew members strap in
03:35 AM......TDRS-West comsat acquisition of signal
03:34 AM......Single APU start
	
03:39:43 AM...Deorbit ignition (burn duration: 3:12; velocity change: 220 mph)
03:42:55 AM...Deorbit burn complete

04:15:07 AM...Entry interface (shuttle in discernible atmosphere)

04:19:37 AM...STS-107: EI+04:30 - 1st unusual data recorded
04:19:57 AM...STS-107: EI+04:50 - 1st unusual heating noted

04:20:07 AM...1st roll command to left

04:21:51 AM...STS-107: EI+06:44 - Start of peak heating
04:31:03 AM...STS-107: EI+15:56 - Last valid data
04:31:51 AM...STS-107: EI+16:44 - End of peak heating

04:33:26 AM...1st left-to-right roll reversal
04:40:22 AM...Velocity less than Mach 2.5
04:42:32 AM...Velocity less than Mach 1
04:43:26 AM...Shuttle banks and turns to line up on runway
04:46:44 AM...Landing

Earlier today, Collins and her crewmates fielded questions from reporters on Earth in a final round of media interviews.

Wendy Lawrence, who operated the space station's robot arm during three spacewalks and who helped orchestrate the station's resupply, told CBS News the mission was "the busiest flight I've ever been on."

"I think this crew has done extremely well in that regard. And of course, there are thousands of people on the ground who made this mission successful. Thanks to them and their great planning, we've been able to pull off a very, very heavy workload. I've had some email from friends who said 'you all have made it look so easy.' But that has not been the case. It's been a challenging mission, a difficult one, and the real tribute is to the folks on the ground who helped us be very, very successful on orbit."

Said Charles Camarda, "I think as a test flight it was very successful. We evaluated several different (heat-shield damage detection) sensor systems, we've even done a repair, I think we far exceeded what we would have expected for this flight. We collected a lot of data, there's a lot more to be done and when we get back to the ground, I'm sure people will continue to work until we have (certified heat-shield) repair techniques and we feel comfortable that we've completely fixed the (foam on the external fuel) tank."

Discovery's flight took on a "Perils of Pauline" sort of tone with concern about foam debris shedding during launch, the potential threat posed by gap fillers extending from the shuttle's belly and last-minute worries about a damaged insulation blanket just below commander Eileen Collins' left window.

With every possible problem being subjected to detailed analysis and debate, some observers have suggested NASA may be faced with "analysis paralysis" and that the post-Columbia safety pendulum may have swung too far. But Kelly disagreed, saying "I think just the fact that we're here means we don't have paralysis by analysis."

"I think the folks on the ground have done an absolutely great job trying to take care of everything they possibly can take care (of) without flying the flight," he told CBS News. "The fact that we're up here now talking to you (shows that) did not happen and hopefully, that won't happen after this flight. We've got a lot of data we need to reduce and take care of and figure out and some change will have to be made before we fly STS-121 (the next shuttle flight). But I think we're on our way there."

Astronaut Andrew Thomas agreed.

"I hope the legacy will be that we've shown we've been able to return the vehicle back to safe, operational flight," he said. "I think we've also shown that we don't have a full, complete understanding of the aerothermal dynamics of what happens around the tank during the launch phase and it's shown us that we need to do that."

Japanese astronaut Soichi Noguchi told his family, "See you tomorrow. It's been a great trip and we'll come back with lots of good stories."

Here is the latest NASA TV schedule (rev. N) through the end of mission STS-114:

REV...EVENT..........................MET in DD/HH:MM...EDT........GMT

SUNDAY, AUGUST 7/FLIGHT DAY 13

184...RCS HOT FIRE..........................11/14:10...12:49 AM...04:49
186...ABC/CBS/CNN/FOX NEWS/NBC.......TDRE...11/17:27...04:06 AM...08:06
187...DISCOVERY CREW DEORBIT REVIEW.........11/19:20...05:59 AM...09:59
188...MISSION STATUS BRIEFING.........JSC...11/20:51...07:30 AM...11:30
189...KU BAND ANTENNA STOWAGE...............11/22:40...09:19 AM...13:19
191...DISCOVERY CREW SLEEP BEGINS...........12/02:00...12:39 PM...16:39
191...FLIGHT DAY 13 HIGHLIGHTS........JSC...12/02:21...01:00 PM...17:00
196...DISCOVERY CREW WAKE UP (begins FD 14).12/10:00...08:39 PM...00:39
198...DEORBIT PREPARATIONS BEGIN............12/13:05...11:44 PM...03:44

MONDAY, AUGUST 8

200.*.PAYLOAD BAY DOOR CLOSING..............12/14:21...01:00 AM...05:00
201...DEORBIT BURN..........................12/17:01...03:40 AM...07:40
202.*.KSC LANDING.....................KSC...12/18:08...04:47 AM...08:47

202.*.2ND KSC OPPORTUNITY DEORBIT BURN......12/18:36...05:15 AM...09:15
202.*.2ND KSC OPPORTUNITY LANDING...........12/19:43...06:22 AM...10:22 

The shuttle Discovery undocked from the international space station early today at 3:24 a.m., wrapping up a successful repair and resupply mission. With pilot James Kelly at the controls, Discovery pulled away directly in front of the lab complex and then flew a loop around the station for a spectacular photo survey.

In what may be the most stunning video ever shot of the lab complex, Discovery's cameras captured the station flying serenely against the backdrop of central Asia like some great bird, its huge solar arrays stretching across the scene like wings as the snow-capped Himalayas came into view below the limb of the Earth.

Earlier, in a brief farewell ceremony inside the station's Destiny lab module, shuttle commander Eileen Collins thanked the station's two-man crew "for being such great hosts."

"We thank them for being part of such a successful mission," she said of station commander Sergei Krikalev and flight engineer John Phillips. "We're so happy to have spent the time up here with them. ... And with that, we're going to say a short goodbye and do a fly around of their space station and get them some great pictures and do a photo survey and add some final memories."

Krikalev thanked the shuttle astronauts in turn along with ground support crews, saying "it's really a big event for us because we were waiting for this flight for several years, more than three years already. And fortunately, everything goes smooth. We want to say thank you and wish you a good, soft landing."

Phillips then took the microphone for a final word: "Thank you so much for being wonderful guests, it's really been a pleasure. And no, we're not glad to see you go. We'd love to have you stay a little longer. As Sergei says, have a great flight and a soft landing. We look forward to seeing you back in Houston."

The two crews exchanged hugs and handshakes before the shuttle astronauts floated out of the station and hatches between the two craft were sealed. Undocking occurred at 3:24 a.m. as the spacecraft sailed 223 miles above the south Pacific Ocean west of Chile.

Kelly then guided Discovery to a point 400 feet directly in front of the space station before beginning a slow loop around the outpost at 3:51 a.m., flying over the top of the lab then below it and back out in front again before departing the area.

Today's mission status/mission management team briefing is scheduled for 1 p.m. Here is NASA's latest television schedule (rev. L):

NOTE: VIEW WITH FIXED-WIDTH FONT

REV...EVENT..........................MET in DD/HH:MM...EDT........GMT

SATURDAY, AUGUST 6/FLIGHT DAY 12

169...DISCOVERY/ISS UNDOCKING...............10/16:45...03:24 AM...07:24
170...DISCOVERY FLYAROUND OF ISS BEGINS.....10/17:15...03:54 AM...07:54
170...FINAL SEPARATION FROM ISS.............10/18:29...05:08 AM...09:08
171...VTR PLAYBACK OF UNDOCKING.............10/18:35...05:14 AM...09:14
172...SHUTTLE CREW OFF DUTY PERIOD BEGINS...10/20:00...06:39 AM...10:39
175...DISCOVERY CREW SLEEP BEGINS...........11/02:00...12:39 PM...16:39
173.*.MISSION STATUS/POST MMT BRIEFING......11/02:21...01:00 PM...17:00
176.*.FLIGHT DAY 12 HIGHLIGHTS........JSC...11/03:21...02:00 PM...18:00
180.*.MISSION STATUS BRIEFING REPLAY..JSC...11/07:21...06:00 PM...22:00
181...DISCOVERY CREW WAKE UP (begins FD 13).11/10:00...08:39 PM...00:39
183...FCS CHECKOUT/CABIN STOWAGE BEGINS.....11/13:00...11:39 PM...03:39

SUNDAY, AUGUST 7/FLIGHT DAY 13

184...RCS HOT FIRE..........................11/14:10...12:49 AM...04:49
186.*.ABC/CBS/CNN/FOX NEWS/NBC INTERVIEWS...11/17:27...04:06 AM...08:06
187...DISCOVERY CREW DEORBIT REVIEW.........11/19:30...06:09 AM...10:09
188...MISSION STATUS BRIEFING.........JSC...11/20:51...07:30 AM...11:30
189...KU BAND ANTENNA STOWAGE...............11/22:40...09:19 AM...13:19
191...DISCOVERY CREW SLEEP BEGINS...........12/02:00...12:39 PM...16:39
191.*.FLIGHT DAY 13 HIGHLIGHTS........JSC...12/02:21...01:00 PM...17:00
196...DISCOVERY CREW WAKE UP (begins FD 14).12/10:00...08:39 PM...00:39
198...DEORBIT PREPARATIONS BEGIN............12/12:55...11:34 PM...03:34

MONDAY, AUGUST 8

200...PAYLOAD BAY DOOR CLOSING..............12/15:24...02:03 AM...06:03
201...DEORBIT BURN..........................12/17:04...03:43 AM...07:43
202...KSC LANDING.....................KSC...12/18:07...04:46 AM...08:46

A "tiger team" of NASA and contractor engineers is reviewing the manufacturing history of the shuttle Discovery's external fuel tank to find clues about what might have caused a chunk of foam insulation to pop off during launch July 26. NASA officials said today that foam in the area that pulled away was slightly damaged during the tank's processing, requiring a standard repair for relatively routine cracks and gouges.

The extent of the void was small - 0.2 inches by 0.6 inches - and it may have had nothing to do with the foam loss that marred Discovery's launch. But it has caught the interest of the engineering community because if the launch debris incident can be traced to this or any other one-time flaw that only affected Discovery's tank, NASA might be able to return the grounded shuttle program to flight in relatively short order without having to implement generic, fleet-wide modifications.

Time, however, is short. The next available launch window, which originally opened Sept. 9, has shrunk to just four days, now opening Sept. 22 and closing Sept. 25, primarily because Discovery was delayed getting off on its current mission. Whenever the shuttle Atlantis is cleared for the second post-Columbia mission, Discovery must be ready for launch on a rescue mission if major problems develop in orbit.

While most agency observers believe NASA has little chance of resolving the foam issue in time for Atlantis to blast off in the September window, NASA Administrator Michael Griffin has not given up hope.

"Until we run out of lead time to make the September window, we'll preserve it," Griffin told reporters today. "Because that's what the taxpayers pay us to do. When we no longer can make it, we'll tell you and we'll recycle for (the next launch window in) November."

The question of prior damage to the foam on Discovery's tank came up during a post-launch review following the insulation shedding observed during the shuttle's climb to space. A few seconds after the ship's solid-fuel boosters separated, a 0.9-pound chunk of foam ripped away from an aerodynamic ramp used to smooth the supersonic flow of air over external pressurization lines and electrical cables.

The so-called "protuberance air load" - PAL - ramp is sprayed on and shaped by hand. As such, it is subject to more inconsistencies than machine-sprayed foam. But the PAL ramp was not upgraded in the wake of the Columbia disaster because engineers believed its design was sound and because the last known incident of foam shedding from the ramp was in 1983.

A NASA official familiar with the incident said the same area of the ramp that broke free during launch suffered minor damage at some point during its processing. In such cases, red dye is applied that soaks into damaged foam and marks the extent of the problem. Technicians then sand the foam to remove the damaged material.

"There was a defect, a flaw or a gap of .02 by .06 inches on the PAL ramp," the NASA official said. "It's fairly small defect. As part of our normal procedures, we did a sand-and-blend (repair). We poured dye in the crack and chased the dye. We sand down until the dye is gone. In this case, it left a little bit of an indentation there, which is acceptable within our regulations. It was in the same region as the piece that came off."

One source described the defect as "crush" damage, but the NASA official said to his knowledge, "we don't have any indication of crushing damage. As far as we know, it was just that void there. It's like a crack for lack of a better term."

"It was fairly shallow and they just sanded it out," he said. "This is part of the fault tree, it's one of the boxes we have to check off. But there are more items we have to look at."

The Discovery astronauts, meanwhile, are gearing up to undock from the international space station early Saturday after an action-packed three-spacewalk mission to resupply and service the $30 billion outpost. Here is a timeline of upcoming activities (in EDT and mission elapsed time):

EDT........HH...MM...SS...EVENT

02:43 AM...10...16...04...Noon
03:08 AM...10...16...29...Sunset
03:24 AM...10...16...45...Discovery undocks from the space station
03:24 AM...10...16...45...Initial orbiter separation (+10 seconds)
03:26 AM...10...16...47...ISS holds current orientation
03:29 AM...10...16...50...Range: 50 feet
03:31 AM...10...16...52...Range: 75 feet
03:44 AM...10...17...05...Sunrise
03:53 AM...10...17...14...Range: 400 feet
03:54 AM...10...17...15...Start flyaround
04:05 AM...10...17...26...-R bar crossing (Discovery directly above ISS)
04:11 AM...10...17...32...Russian ground station LOS
04:12 AM...10...17...33...Noon
04:17 AM...10...17...38...-V bar crossing (shuttle behind station)
04:28 AM...10...17...49...+R bar crossing (shuttle below station)
04:40 AM...10...18...01...Sunset
04:41 AM...10...18...02...Separation burn (1.5 fps)
05:09 AM...10...18...30...Separation burn (3.0 fps)

Despite the launch-day foam incident, Griffin said Discovery's mission has been one of the most successful on record.

"People who think this has been a horribly troubled flight, not only for them is the glass not half full, there is no glass," he said earlier today. "This has been a great flight. We clearly, as I've said several times, I want to be honest, I want to be open, we made a mistake on the external tank. We have a special team looking at all that, we're going to try to find it and fix it.

"But almost everything we did with the external tank worked. Discovery is six times cleaner in flight than the average shuttle has been. Discovery is in great shape for landing. ... The flight crew did a magnificent job in removing those gap fillers, and so we strongly feel we will have a clean, very nominal re-entry."

Lead flight director Paul Hill said the Discovery astronauts transferred 23 pounds of nitrogen to the space station, 1,700 pounds of fresh water, 1,400 pounds of equipment stored on the shuttle's lower deck and another 3,000 pounds of supplies and equipment brought up in an Italian-built cargo module.

For the trip back to Earth, Discovery is now loaded with 6,300 pounds of no-longer-needed equipment, trash and other gear, including a broken control moment gyroscope that was replaced during one of the crew's spacewalks. NASA engineers want to get the gyro back to Earth to find out what went wrong and how to prevent similar problems in the future.

If all goes well, commander Eileen Collins and her crew mates will test Discovery's re-entry systems Sunday and pack up for the trip back to Earth. Two landing opportunities are available at the Kennedy Space Center - at 4:46 a.m. and 6:21 a.m. Monday - and two more at Edwards Air Force Base, Calif.

But NASA managers want to get Discovery back on the ground in Florida if at all possible and if the weather or some other problem blocks an on-time landing, entry flight director LeRoy Cain likely would extend the mission one day and bring Discovery back to Florida or California on Tuesday.

The official landing strategy has not yet been announced. But here are all of the possible landing opportunities for Discovery and its crew (all times EDT and subject to change; KSC - Kennedy Space Center; EDW - Edwards Air Force Base, Calif.):

ORBIT...SITE....DEORBIT BURN....LANDING

Monday, Aug. 8

201.....KSC.....03:43 AM........04:46 AM
202.....KSC.....05:19 AM........06:21 AM
203.....EDW.....06:49 AM........07:52 AM
204.....EDW.....08:25 AM........09:27 AM

Tuesday, Aug. 9

217.....KSC.....04:07 AM........05:09 AM
218.....KSC.....05:43 AM........06:45 AM
219.....EDW.....07:12 AM........08:15 AM
220.....EDW.....08:48 AM........09:50 AM

Wednesday, Aug. 10

232.....KSC.....02:54 AM........03:57 AM
233.....KSC.....04:29 AM........05:32 AM
234.....EDW.....05:59 AM........07:02 AM
235.....EDW.....07:35 AM........08:37 AM

WIth Discovery back on terra firma, the shuttle team's focus will shift to the external tank foam problems that developed during launch. Along with the PAL ramp foam, engineers also are concerned about at least two other areas where foam separated from the tank.

"The idea is to really look at it from an engineering standpoint and see what we've found out here," said Bill Gerstenmeir, space station program manager. "This flight was tremendous, we've got data now we've never had before. Now we can see when foam was liberated and exactly how the foam was liberated during ascent. So now you can take that back to maybe a defect that was present in the tank we didn't think about before, or wasn't there. Now you can see what that mechanism is and how it comes off.

"So we have a tremendous chance to learn from this exercise that we can really take our knowledge base of how we apply foam to tank, how we ensure that it doesn't come off or it comes off at a time when it's not a problem to us. We've got a tremendous engineering chance to learn from this test program, this flight we just flew."

Said Griffin, "The good thing is, almost all of the tank changes (made as a result of Columbia) worked. Some didn't. So what's the difference between the ones that did and the ones that didn't? That's what engineers do. And if we can extract that difference, then we can go and look at the next tank, or tanks. ... Now we have actual test data that we can use to see how well we did and in areas where we didn't do as well, why we didn't do as well. We've never had that before. Never."

Griffin said every external tank has "enormous portions of it that are all the same and every tank has areas where there are hand applications of foam or insulation because the automatic machinery isn't that sophisticated."

"And then every tank undergoes at some point in its career, minor damage," he said. "People touch it, or lean on it or whatever in the wrong way and things have to be patched and fixed. Those are the areas that will get a lot of attention.

"But we will never, as long as we manufacture these tanks or much of anything else, we will never be able to eliminate the fact that there is some custom work for each tank. So what we have to have coming out of these test flights is an understanding of which types of custom work are OK and which types of custom work that we do are not yielding good results. And we need to stop that and do something different."

NASA's mission management team today ruled out another shuttle repair spacewalk, this one to fix a damaged insulation blanket, saying a detailed analysis showed Discovery's crew can safely return to Earth as is.

"We have good news," astronaut Julie Payette radioed from mission control a few minutes past 1 p.m. "The MMT just got to the conclusion that the blanket underneath (the commander's) window is safe for return. There is no issue."

Concern about the damaged 20-inch-long, 4-inch-wide insulation blanket just below Discovery commander Eileen Collins' left cockpit window was the only remaining question mark about the shuttle's condition prior to re-entry and landing Monday at the Kennedy Space Center. Engineers earlier cleared the shuttle's heat-shield tiles and wing leading edge panels after an impromptu repair job Wednesday in which two protruding "gap fillers" were plucked from the ship's belly during the crew's third, and now final, spacewalk.

The blanket in question forms an interface of sorts between heat-shield tiles surrounding the cockpit windows and thermal insulation blankets that protect much of the shuttle's upper fuselage.

Engineers concluded earlier that the damaged blanket posed no threat during the high-speed, high-heating parts of re-entry. Rather, the concern was that the top layer of the blanket could rip away at lower velocities, when the shuttle has fallen into the thicker regions of the atmosphere, fly back and impact the back end of the space shuttle.

Deliberately damaged blankets similar to the one aboard Discovery were tested overnight in a wind tunnel at NASA's Ames Research Center in California to help engineers and aerodynamicists calculate when during entry debris might rip away, what sort of trajectory it might follow and whether an impact could cause serious damage to the shuttle's rear wing elevons, rudder/speed brake or aft rocket pods.

But the wind tunnel tests, along with additional analyses, showed the blanket posed no significant threat to Discovery's return to Earth.

"We have new analysis that shows debris transport would be no issue and we came to the same conclusion with the Ames (wind) tunnel tests," Payette told the crew today. "So basically, no EVA 4 (fourth spacewalk)."

"Thanks, no EVA 4," Japanese astronaut Soichi Noguchi replied from orbit. "That's, I would say, good news."

In Crawford, Texas, President Bush today praised the crew's repair work Wednesday, saying "like a lot of Americans, I was amazed at the procedures that took place to repair the craft. It was pretty remarkable."

"I believe that the mission is important," he said. "And I know that the mission directors will make the right decision about how to proceed. Ours is a country that values the safety of our citizens, particularly those we ask to take risks in space. And there will be a lot of deliberation, a lot of thought, that goes into the decision as to whether or not those brave souls should return on that vehicle."

Earlier today, the Discovery astronauts and their colleagues aboard the international space station paid tribute today to the 21 astronauts and cosmonauts who lost their lives in spacecraft mishaps, saying the benefits of space travel outweigh the risks and that America "must not be bridled by timidity."

Taking turns reading prepared scripts while downlinking video of a long pass across the Indian Ocean, the combined shuttle-station crews remembered the crews of Columbia, Challenger, Soyuz 1 and 11 and the Apollo 1 victims of a launch pad fire that forced NASA to redesign its Apollo moonships.

"Certainly, space exploration is not easy and there has been a human price that has been paid," said Wendy Lawrence. "As we step out into these new frontiers, we find that it is very unforgiving of our mistakes. The lives lost over 30 years ago with the earliest steps taken by the crews of Apollo 1, Soyuz 1 and Soyuz 11 showed us that and after, the crew of Challenger reaffirmed the need to be ever vigilant of the risk."

Charles Camarda then took over, saying "tragically, two years ago we came once more to realize that we had let our guard down. We became lost in our own hubris and learned once more the terrible price that must be paid for our failures."

"In that accident, we not only lost seven colleagues, we lost seven friends. Their families never shared any homecoming. Those seven were driven by the fire of the human spirit within, they believed in space exploration, they knew the risks, but they believed in what they were doing. They showed us that the power of the human spirit is insatiable. They knew that in order for a great people to do great things, they must not be bridled by timidity."

"To the crew of Columbia, as well as the crews of Challenger, Apollo 1, and Soyuz 1 and 11, and to those who have courageously given so much, we now offer our enduring thanks," said Discovery pilot James Kelly. "From you, we will carry the human spirit out into space and we will continue the explorations you have begun. We will find those new harbors that lie out in the stars of which you dreamed.

"We do this not just because we owe it to you, but we do it because we also share your dream of a better world. We share your dream of coming to understand ourselves and our place in this universe. And as we journey into space you will be in our thoughts and will be deeply missed."

Collins closed the brief memorial with a verse from Laurence Binyon's poem, "For the Fallen:

They shall grow not old, as we that are left grow old;
Age shall not weary them, nor the years condemn.
At the going down of the sun and in the morning
We will remember them.

For the record, here's a list of the 21 men and women who have lost their lives in spacecraft accidents:

• Apollo 1, 01/27/67: Launch pad fire

  Commander Virgil "Gus" Grissom
  Edward White
  Roger Chaffee

• Soyuz 1, 04/24/67: Re-entry parachute failure

  Vladimir Komarov

• Soyuz 11, 06/29/71: Capsule depressurization during entry

  Georgy Dobrovolsky
  Viktor Patsayev
  Vladislav Volkov

• Challenger, STS-51L: 01/28/86: Booster failure during launch

  Commander Francis "Dick" Scobee
  Pilot Michael Smith
  Mission Specialist Judith Resnik
  Mission Specialist Ronald McNair
  Mission Specialist Ellison Onizuka
  Payload Specialist Gregory Jarvis (private sector engineer)
  Spaceflight Participant Christa McAuliffe (school teacher)

• Columbia, STS-107: 02/01/03: Left wing plasma intrusion during entry

  Commander Rick Husband
  Pilot William McCool
  Mission Specialist Kalpana Chawla
  Mission Specialist Laurel Clark
  Mission Specialist Michael Anderson
  Mission Specialist David Brown
  Payload Specialist Ilan Ramon (Israel)
  

In an Op-Ed piece published by the Houston Chronicle, famed Apollo flight director Gene Kranz, known for the "Apollo 13" screenplay line "failure is not an option," echoed Camarda's concerns about timidity in the face of danger. In a strongly worded piece, Kranz decried critics who have suggested retiring the shuttle program in the wake of renewed concerns about external tank foam debris.

"For the risk-averse, the only acceptable thing to do now is retire the shuttle program immediately and wait for the divine arrival of the next generation of spacecraft," he wrote. "I am disgusted at the lack of courage and common sense this attitude shows.

"All progress involves risk. Risk is essential to fuel the economic engine of our nation. And risk is essential to renew America's fundamental spirit of discovery so we remain competitive with the rest of the world."

Kranz said Discovery's flight has been "remarkably successful" so far, despite the unexpected release of a large piece of foam insulation during launch. Overall, Kranz said, the shuttle tank shed 80 percent less foam than previous missions and "only in the news media, apparently, is an 80 percent improvement considered a failure. Rather than quit, we must now try to reduce even more the amount of foam that comes off the tank."

"There are many nations that wish to surpass us in space," he wrote. "Does the 'quit now' crowd really believe that abandoning the shuttle and the international space station is the way to keep America the pre-eminent space-fairing nation? Do they really believe that a new spacecraft will come without an engineering challenge or a human toll? The path the naysayers suggest is so out of touch with the American character of perseverance, hard work and discovery that they don't even realize the danger in which they are putting future astronauts - not to mention our nation."

President Bush did not directly address the risks of spaceflight, but he made it clear he believes his new moon/Mars initiative will reinvigorate the space program.

"It is important for our fellow citizens to understand that we're going to take the NASA mission beyond the current mission," he said. "The plan right now is to phase out the shuttle by 2010 and then begin to put a strategy in place that will use the moon as a launching spot for further exploration. ... The people I've talked to inside NASA are excited about the mission, the reinvigoration of the vision of exploration.

"And I appreciate the administrator (Michael Griffin) working on getting that strategy in place so that when the decision is made to finally get rid of this phase of exploration we'll be ready to take on the new phase. That's important for the American people to understand, that, one, exploration is important. Two, there'll be some good coming out of exploration. And three, that we've got a new vision embraced by NASA and its pioneers."

Here is the latest NASA television schedule (rev. J) for the rest of today and tomorrow (am * indicates a change from the previous schedule):

   REV...EVENT..........................MET in DD/HH:MM...EDT........GMT
   
   THURSDAY, AUGUST 4/FLIGHT DAY 10
   
   145...EXPEDITION 12 CREW NEWS CONFERENCE....09/03:21...02:00 PM...18:00
   145...DISCOVERY/ISS CREW SLEEP BEGINS.......09/03:30...02:09 PM...18:09
   145...FLIGHT DAY 10 HIGHLIGHTS........JSC...09/04:21...03:00 PM...19:00
   147.*.MISSION STATUS BRIEFING REPLAY..JSC...09/07:21...06:00 PM...22:00
   151...DISCOVERY CREW WAKE UP (begins FD 11).09/11:30...10:09 PM...02:09
   151...ISS CREW WAKE UP......................09/12:00...10:39 PM...02:39
   
   FRIDAY, AUGUST 5/FLIGHT DAY 11
   
   156...MPLM EGRESS AND DEACTIVATION..........09/14:30...01:09 AM...05:09
   156...SSRMS UNINSTALL MPLM FROM UNITY.......09/19:45...06:24 AM...10:24
   156...MPLM BERTH IN DISCOVERY PAYLOAD BAY...09/20:55...07:34 AM...11:34
   157...RENDEZVOUS TOOL CHECKOUT BEGINS.......09/21:30...08:09 AM...12:09
   157...RMS MOVES OBSS TO HANDOFF POSITION....09/21:45...08:24 AM...12:24
   157...MISSION STATUS BRIEFING.........JSC...09/21:51...08:30 AM...12:30
   158...SSRMS GRAPPLE OF OBSS FROM RMS........09/22:45...09:24 AM...13:24
   158...OBSS BERTH IN DISCOVERY PAYLOAD BAY...09/23:50...10:29 AM...14:29
   158...VIDEO FILE.......................HQ...10/01:21...12:00 PM...16:00
   161...FLIGHT DAY 11 HIGHLIGHTS........JSC...10/03:21...02:00 PM...18:00
   161...DISCOVERY CREW SLEEP BEGINS...........10/03:30...02:09 PM...18:09
   163...POST-MMT BRIEFING...............JSC...10/07:21...06:00 PM...22:00
   166...POST-MMT BRIEFING REPLAY........JSC...10/10:21...09:00 PM...01:00
   166...DISCOVERY CREW WAKE UP (begins FD 12).10/11:30...10:09 PM...02:09
   167...FCS CHECKOUT/CABIN STOWAGE BEGINS.....11/13:00...11:39 PM...03:39
   167...DISCOVERY/ISS CREW FAREWELL...........10/13:45...12:24 AM...04:24

With a successful spacewalk repair job today, engineers believe the shuttle Discovery's heat shield is in good shape for re-entry and landing Monday. But one question mark remains: What, if anything, to do about a damaged insulation blanket just below commander Eileen Collins' left cockpit window.

As the Discovery astronauts were unwinding from their third and presumably final spacewalk today, mission control called Collins with word that a fourth spacewalk, this one to repair the blanket, is a possibility, depending on how the damage assessment turns out.

"We wanted you to know that they are looking at the remote possibility, but yet if indeed the damage on the blanket warrants it, that they might have to plan for a fourth EVA," astronaut Julie Payette radioed from mission control in Houston. "This is a small probability, but we wanted you to know about their thinking, and also get any comments from your crew and especially from the EV (spacewalk) crew members about this idea so we can put that in the pot for planning."

"OK, thanks for the heads up," Collins replied.

Cameras on Discovery's robot arm were used to photograph the torn, puffed-up blanket today, providing zoomed-in views showing what looked like impact damage on the slender segment. At a change-of-shift news briefing, lead flight director Paul Hill said engineers hoped to complete a thorough damage assessment by Thursday morning.

Unlike the consequences of the protruding gap fillers removed by spacewalker Stephen Robinson earlier today, the blanket does not pose a threat during the high-speed, high-heating part of re-entry. Rather, the concern is that when Discovery reaches the denser, lower atmosphere, a piece of the blanket could rip away, possibly hitting the back of the orbiter.

"It's not an entry heating concern," Hill said. "The concern is if something comes off and then flies aft and hits some critical surface, whether its a control surface, the front of an OMS (rocket) pod or something like that. So we're taking a look at this as a debris problem, not an entry heating problem."

Hill said engineers are trying to assess "how significant it would be if we made impact with the amount of material that could come off in the worst case."

"There are also folks off doing statistical analysis on where the most likely areas are that could be hit," he added. "We'll have more data tomorrow. ... But we are definitely looking at various areas on the vehicle aft and what it would mean to us if we were to hit it with the predicted masses that could come off."

Mission management team chairman Wayne Hale likely will address the issue at his daily news briefing later today. This status report will be updated as warranted.

Like an anxious mom plucking out a splinter, astronaut Stephen Robinson gently pulled two dislodged gap fillers from the shuttle Discovery's belly early today in an unprecedented 223-mile-high repair job.

Floating under Discovery's nose on the end of the international space station's robot arm, Robinson reached out with his gloved hand and very carefully pulled the first gap filler out at 8:45 a.m.

"I'm grasping it, I'm pulling, it's coming out very easily," Robinson radioed. "OK, the offending gap filler has been removed."

Exactly 10 minutes later, after astronauts Wendy Lawrence and James Kelly guided the arm to the second protruding gap filler, Robinson completed the repair job with little fanfare.

"I'm ready for brakes," he called, asking the arm operators to stabilize the robotic crane.

"Brakes are on. You're go," Kelly replied from inside the Destiny laboratory module.

"Thank you. Here we go..."

Crystal clear video from Robinson's helmet camera showed the astronaut gently pushing on the gap filler to get a feel for it before working it out, pulling it free with seemingly little force. Reddish glue could be seen on the bottom of the stiff spacer as Robinson turned it this way and that.

"OK, I'd like to move away from the orbiter, body aft 5 feet please," he radioed.

"Body aft. We're taking the brakes off."

"Sounds great. OK, that came out very easily, probably even less force," Robinson commented. "It looks like this big patient is cured.

And with that, the Discovery astronauts pulled off one of the more memorable space shuttle repair jobs in program history, venturing below the underside of the shuttle for the first time to service an area of the spacecraft that is normally considered strictly off limits.

Spectacular television views from cameras mounted on the space station, Discovery's robot arm and Robinson's space suit allowed flight controllers and armchair astronauts around the world to follow along as the spacewalker worked alone against the backdrop of Discovery's tiled under belly.

Shots from fellow spacewalker Soichi Noguchi's helmet camera showed the entire space shuttle against the blue-and-white globe of Earth with Robinson visible in the distance on the end of the station's robot arm.

Contrary to news accounts that focused on what some viewed as an especially risky venture, Robinson had no problems at all, never came close to damaging any surrounding tiles and completed the repair work in the time needed to simply pluck the gap fillers out.

But the astronauts clearly took the work seriously, reviewing safety precautions while the arm slowly moved Robinson to the work sites below the shuttle.

"It goes without saying we don't want any inadvertent contact with tile or the belly of the orbiter," astronaut Andrew Thomas called from inside the shuttle-station complex.

"OK."

"You've got a lot of things still hanging onto you even though we cleaned you up, so try to keep an eye on where they are. ... And under the orbiter, we'll probably have comm drop outs, we may lose wireless video. So we'll need continuous communications protocol while you're doing the (job) so we can be assured it's going properly."

Thomas cautioned that if Robinson had to resort to a hacksaw to cut the gap fillers off, "the serrated edge is also going to be sharp so you need to watch that. If by any chance you do need to contact the tile with your hands, we would require only gentle hand reaction alone. We want you to distribute the load over several fingers or the backs of the fingers. How copy?"

"Copy all, particularly the hand touch," Robinson said as he approached the shuttle "My goal, of course, is not to touch the tiles at all, but I have touched the tiles at KSC with my work gloves on, so I know what to expect. I'll use a very gentle touch."

As it turned out, he didn't to.

"I can see it pretty well," he called at one point, referring the the first gap filler.

"How far are you from it, by the way?" Kelly asked.

I'm about eight feet, maybe seven feet, looking straight down on it. It looks to be about close to three inches on one side and about an inch and a half on the other side. The corner looks like it is bent over, presumably by air loads" during launch.

A few moments later, he called Kelly: "Vegas, I'm ready to go get it if you are. ... I'm ready to go."

"It's your show, Steve, take it away," Thomas said."

After the repair work was over, Robinson took a moment to enjoy the spectacular view and to chat with flight controllers in Houston.

"You guys did a great job," astronaut Mike Massimino called from mission control.

"Thank you, Mike," someone replied.

"We trained for four years," Robinson joked. "We're going to spend the next four years signing autographs! ... I'm getting the best view. Oh my goodness... just beautiful."

The 61st spacewalk devoted to station assembly and maintenance began at 4:48 a.m. and ended at 10:49 a.m. for a duration of six hours and one minute. Robinson and Noguchi logged 20 hours and five minutes of EVA time during their three excursions, pushing the station total to 368 hours and 20 minutes.

Earlier today, Robinson and Noguchi installed a large tool storage platform on the side of the Quest airlock module. Noguchi also mounted an experiment package on the top of the station's P6 solar array truss. But mission planners earlier decided to defer installation of a camera mount to a future spacewalk and after the gap filler fix, they opted to also defer retrieval of a radiator coupling.

Engineers are still debating what, if anything, might need to be done about a damaged insulation blanket just below commander Eileen Collins' left cockpit window. Astronauts at the Johnson Space Center worked overnight to develop possible remedies, but no final decisions have been made.

During a news briefing Tuesday evening, Wayne Hale, chairman of NASA's mission management team, said he hoped to make a final decision later today or tomorrow, after the engineering analysis is complete.

"Right now, we know that in terms of the local area, it's OK," Hale said. "This is just a question of could it fly back and hit something on the after part of the vehicle? And, in fact, the biggest work going on, I think, is to determine whether or not it's even possible the blanket could come off."

The Discovery astronauts used a camera on the end of a sensor boom attached to the shuttle's robot arm today to collect additional, close-up pictures of the blanket to help engineers assess the damage.

Astronauts Soichi Noguchi and Stephen Robinson installed a large tool and spare parts storage facility today after working around initial alignment problems. Noguchi then floated 60 feet above the shuttle Discovery's cargo bay to install an experiment package atop the station's P6 solar array truss. Wendy Lawrence, meanwhile, operating the station's robot arm, repositioned the space crane to support an unprecedented shuttle repair job. Flight controllers said that repositioning work went faster than expected and they told Robinson to delay his next job, retrieving a radiator component, and to press ahead instead with the shuttle gap filler repair work.

Based on when the astronauts floated out of Discovery's airlock this morning, the unusual repair job was expected to begin around 8:30 a.m., all other things being equal. It is now the next item on Robinson's agenda.

Astronauts Stephen Robinson and Soichi Noguchi, floating in the shuttle Discovery's airlock, switched their spacesuits to internal power at 4:48 a.m. to officially begin the third spacewalk of the first post-Columbia mission. A few moments later, they floated into the shuttle Discovery's cargo bay.

This is the 61st spacewalk devoted to station assembly and maintenance and the third for Robinson and Noguchi, who logged 14 hours and four minutes of spacewalk time in two previous excursions. The cumulative total for the previous 60 spacewalks is 362 hours and 19 minutes by 53 astronauts and cosmonauts.

The primary goals of today's planned seven-hour excursion are to attach a massive external tool box to the space station's Quest airlock module; to mount an experiment package on the top of the station's solar array truss; and to remove two "gap fillers" sticking up between heat shield tiles on Discovery's belly.

The impromptu repair job was added to today's spacewalk because of uncertainty over what sort of damage might result if the gap fillers were left in place for re-entry. The concern was that the gap fillers, sticking up into the hypersonic wind flow across the belly of the shuttle, could trigger an early onset of turbulence that would result in higher downstream heating.

To remove the gap fillers, Robinson will ride the space station's robot arm, operated by astronaut Wendy Lawrence, down below the right side of the shuttle and under its belly. Parked a foot or so away, he will attempt to pluck the gap fillers out with his gloved hand. If that doesn't work, he will use forceps to apply additional force, up to about 10 pounds. If that doesn't work, he will use a hacksaw blade to cut the fillers off at the surface of the tiles.

Here is a timeline of today's activities based on the actual start time of the EVA. Soichi's call sign is EV-1 while Robinson is EV-2 (times subject to change):

EDT........HH...MM...EVENT

04:48 AM...00...00...Airlock egress
05:18 AM...00...30...EV-1/EV-2: External stowage platform (ESP-2) installation
06:03 AM...01...15...EV-1: MISSE 5 experiment group installation
06:03 AM...01...15...EV-2: Grapple fixgture removal
06:18 AM...01...30...SSRMS: ESP-2 ungrapple
06:23 AM...01...35...Station robot arm (SSRMS) walkoff to mobile transporter
06:48 AM...02...00...Shuttle robot arm (SRMS): TPS sample box scan
07:03 AM...02...15...EV-1: Radiator rotary joint coupler removal
07:03 AM...02...15...EV-2: Gap filler too prep
08:03 AM...03...15...EV-1: Attach foot restraint to SSRMS
08:03 AM...03...15...EV-2: Attach tethers, ingress SSRMS
08:18 AM...03...30...EV-1: Close TPS sample box; monitor EV-2
08:18 AM...03...30...EV-2: gap filler removal
08:28 AM...03...40...SSRMS moves EV-2 to work site 1
09:08 AM...04...20...SSRMS moves EV-2 to work site 2
10:18 AM...05...30...IV egress for shuttle EVA
11:03 AM...06...15...EV-1/EV-2: Payload bay cleanup and tool stow
11:48 AM...07...00...Airlock repressurization

The repair job has generated widespread attention, in part because no astronaut has ever attempted to work on the belly of the space shuttle and because any slipups by Robinson or Lawrence could result in damage to the tiles needed to protect Discovery from the heat of re-entry. But engineers say the worry is misplaced and that the work is straight-forward.

"I have every confidence the EVA is going to go well to remove the gap fillers as well as do the other tasks," said mission management team chairman Wayne Hale. "We've given ourselves three different ways to deal with the gap filler. Yes, we have the capability to perform another EVA. If something happens, either we didn't get the gap fillers where we wanted or something else needs to be worked on, then we will attack that and go deal with it. But right now, I am very confident we'll take care of the gap fillers."

Lead flight director Paul Hill said once the gap fillers are removed, engineers will have no concerns about heat damage during Discovery's return to Earth Monday.

"Once we pull the gap fillers, there's no concern in the TPS (thermal protection system) community about the bottom of the vehicle during deorbit," Hill said. "These particular gap fillers are there not for entry heating, but to protect from buffeting on the way up hill because of the turbulent flow between the external tank and the bottom of the orbiter. So once we pull them out, no issue.

"Now, if for some reason we damage the area while we're pulling them out, they're going to want some pictures and bring down some data to show them what the area around the work site looks like. We're not expecting there to be any damage. If for some reason we have to cut the gap fillers flush, then there is a greater likelihood of scuffing up or scratching the outside RCG coating on the tiles."

It is that presumed sensitivity to damage that initially made flight controllers nervous about attempting any sort of repair.

"The reaction across the community is that we'd be nuts for trying to put a crew member that close to glass-covered tiles where if we drop a tool or a boot comes out of a foot restraint, we could kick the tile and damage it worse. So there is that generic sense already. We don't want to get close to TPS and risk damaging it and making a bad problem worse."

But after an exhaustive analysis, the team was convinced the risk of damage due to the repair was less than the risk of possibly significant heat damage during re-entry.

This status report will be updated throughout the morning. In the meantime, here is today's status report from mission control.

STS-114
Report #16
Tuesday, Aug. 3, 2005 - 12:00 a.m. EDT
Mission Control Center, Houston, Texas

The Space Shuttle Discovery crew begins their ninth day in space with preparations for the third spacewalk of the mission. This extravehicular activity (EVA) was a preplanned activity for the mission, but now includes a new task repair of two protruding gap fillers between tiles on the bottom the Shuttle.

Mission Specialists Steve Robinson and Soichi Noguchi are scheduled to begin their third spacewalk at 4:14 a.m. EDT as they exit out of the Space Shuttle airlock. The two will be assisted by Andy Thomas, serving as the intravehicular officer overseeing the spacewalk from inside, as well as Pilot Jim Kelly and Mission Specialists Wendy Lawrence and Charlie Camarda who will be supporting various robotic arm activities throughout the day.

The spacewalk is scheduled to last about 7 hours. The first task entails Kelly and Lawrence maneuvering the External Stowage Platform-2 (ESP-2), via the Station's robotic arm, which they pulled from Discovery's payload bay earlier today, onto the Station. As the ESP-2 reaches its final position, Robinson and Noguchi will guide the structure and secure it into place. With that task complete, Lawrence and Kelly will conduct a "walk off" maneuver of the Station robotic arm, by attaching the "free" end to the Mobile Base System and releasing the other end from the Destiny Laboratory module to where it will be needed as a platform for Robinson later in the EVA.

The two spacewalkers will move on to individual tasks, with Noguchi installing the Materials International Space Station Experiment-5 (MISSE-5), a materials experiment that will study the degradation of solar cell samples in the space environment. He'll then remove the Rotary Joint Motor Controller from the Space Station truss before proceeding to a support position to assist Robinson in his final tasks.

Meanwhile, Kelly will work with Camarda, using the Orbiter Boom Sensor System to inspect repair demonstration tiles inside the Shuttle's payload bay. Later, Camarda will also work with Krikalev and Phillips to continue stowing supplies and equipment inside Discovery and the Station. Discovery Commander Eileen Collins will monitor and supervise all the activities.

Robinson, now attached to the Station robotic arm, will attempt to repair two tile gap filler protrusions located on the underside of Discovery. He will first try to gently pull out the protruding material, and if need be, remove by trimming with a hacksaw.

Gap fillers are used in areas to restrict the flow of hot gas into the gaps between Thermal Protection System components. They consist of a layer of coated Nextel fabric and are normally about 0.020-inch thick. These protrusions were identified from photos taken during the rendezvous pitch maneuver conducted on flight day three, as Discovery approached the orbiting Space Station.

The crews are scheduled to go to sleep about 3:09 p.m. EDT.

Amid planning for an unprecedented shuttle heat-shield repair spacewalk Wednesday, engineers also are assessing the potential threat posed by a damaged insulation blanket just below commander Eileen Collins' left cockpit window. While engineers say the blanket poses no threat of heat damage during entry, there's a chance a small portion of the blanket could rip away in the lower, denser atmosphere and hit the shuttle's aft section.

Wayne Hale, chairman of NASA's mission management team, said today engineers earlier concluded "that even if that blanket completely came off during entry we're perfectly safe from the local area thermal effects, that is not a problem at all for us in that local area."

"The folks who have been looking at that, of course, have been heavily involved in many of the aspects of the gap filler discussion (see below), the aerodynamics and so forth. ... They're now looking at where that blanket might go if it actually comes off during entry."

The analysis is expected to be complete by Thursday.

"Right now, we know that in terms of the local area, it's OK," Hale said of the damaged blanket. "This is just a question of could it fly back and hit something on the after part of the vehicle? And, in fact, the biggest work going on, I think, is to determine whether or not it's even possible the blanket could come off. "It appears to be well adhered in terms of the underlying RTV glue," he said. "And the stitching holding it down to the edges of that looks like it's in good shape."

Hale said the blanket, which measures 20.4 inches long and 3.8 inches wide, appears to have been hit by something earlier in the mission. Impact damage of some sort is the presumed culprit, but that is merely an assumption at this point. Unlike the gap filler problem, the blanket poses a threat when the shuttle is much closer to Earth, and flying much slower. In relative terms, of course.

"This would not be a concern if it came off at high Mach number, above Mach 6," Hale said, referring to a velocity six times greater than that of sound. "You know, once you get to Mach 6 with the shuttle, you're just kind of putzing around. At lower Mach numbers, where the air is thicker, then there is some transport mechanism that folks are going to go off and look at.

"They're concerned about a physical impact," he said. "The heating concerns rapidly go away later in the trajectory. So, this would not be a high-heating, high-Mach-number kind of concern. This would be a low Mach number, impact-if-any kind of concern."

He said engineers believe the largest piece that could rip away would weigh just 0.05 pounds or so - eight tenths of an ounce. This issue does not appear to be serious. But given the way Discovery's flight has been proceeding, readers are urged to stay tuned.

Editor's Note...
The following discussion was uplinked to the Discovery astronauts overnight as part of a lengthy daily "execute package" from mission control. It is fairly technical, but given the recent discussion about protruding gap fillers and how they might affect the shuttle Discovery's aerodynamics during re-entry, it provides insight into how managers reached the decision to order a spacewalk repair. Acronyms are spelled out for clarity, but the reader is assumed to be familiar with the general thrust of the discussion.

SOURCE: NASA
The major discussion and decision point from the MMT (mission management team) today was with regard to performing the EVA (spacewalk) task to remove the 2 protruding gap fillers. The MMT decided that the prudent course of action was to add this task to EVA 3 to remove the uncertainty and potential concerns for flying entry with the gap fillers in place. As a result we are sending along more details of the task and timeline, and the overall plan for EVA 3 with this task inserted.

The MMT decision resulted after careful deliberation and extensive discussion on the potential thermal and structural effects of leaving the gap fillers in place and experiencing early boundary layer transition (BLT), as compared to the relative risk aspects of this particular EVA task. The following outlines some of those particulars, and of course weŐre also sending along the presentation material for your consideration.

The aero/thermal team outlined their approach to predicting the point at which early transition could occur, and they then discussed the resulting potential thermal implications for both the RCC (reinforced carbon carbon leading edge panels) and TPS (thermal protection system) tile. As you know there are many challenges in this area, not the least of which is that the wind tunnel and CFD (computational fluid dynamics) data bases do not cover this Mach regime, and of course the only flight test data we have is that for Shuttle entry, since no other winged vehicles have flown in this regime. We have had 2 documented cases of early BLT at about M (Mach) 18, STS-28 and STS-73.

We have had several cases of early transition and ABLT (asymmetric boundary layer transition) at lower Mach numbers, the majority of them in the M 12-15 region. For reference, transition normally occurs in the M 8-12 region. As a result, the team used this limited flight data for the M 18 early BLTŐs to extrapolate on the currently accepted and certified model for BLT.

The extrapolation was necessary in order to consider the effects of having early BLT with our flight conditions, primarily because these gap fillers are further forward relative to our data base, and among other things the BL is thinner in this area over these forward nose tiles. The end item answer is that because of the further forward position and the amount of protrusion that we have, approximately 1 inch, the resulting best estimate for early BLT is about M 21.5 (+/- 2.5M).

The team then evaluated the potential effects of an early BLT in this Mach regime, and presented the results for both the RCC and the TPS tile area. It was noted that as it relates to the RCC, we have no documented flight evidence or reason to believe that we have ever had early transition that affected the RCC. Said differently, we have no evidence that we have ever had turbulent flow wash onto the RCC in these regions of higher heating.

However, given an early BLT at M 21.5 and the resulting flow due to these gap fillers being off-centerline, the analyses show that we could have turbulent flow wash onto the RCC. It was noted that these analyses were overall best estimate (conservative in some aspects and best estimate in other aspects). The magnitude of the resultant heating could be on the order of the heating rates that we expect on a TAL (trans-Atlantic landing) for example, where we go to the single use RCC limits of ~ 3250 deg F. This is compared to the nominal EOM (end of mission) multi-flight reuse limits that we design to of about 2950 deg F.

If the early BLT occurred at the high end of the uncertainty at about M 24 the heating rate could be even higher than our TAL limits by about 100 deg F. For the tile areas, the analyses showed that this thermal profile could result in negative margins on the mid-body and aft fuselage structures from the normal 1.4 FS (factor of safety). The results ranged from slightly negative margins to as much as a 30% decrease in the FS (0.98 FS) depending on the exact tile locations.

As a note, the aero and flight control communities also reviewed these conditions and determined that there would be no concerns. As youŐll recall the WRAP DAP (digital autopilot software) was designed to handle early transition and ABLT, so these satisfactory results were expected. As such, the MMT opted to not review those specific details, although the technical community did the necessary rigor for completeness.

The team acknowledged that there is high uncertainty in the analyses for determining just how early BLT could occur, as well as for determining the resulting potential thermal implications for the RCC and tile. It is possible that weŐve flown with these conditions before, however we do not have data to show that we have and therefore we cannot prove that it would be a lower risk than the EVA task.

As such, given the relative risk trade between the potential for these heating conditions as compared to the EVA task to remove the gap fillers, the MMT determined that the EVA was the prudent approach. As for the proposed EVA 3 task, the first and preferred option is to remove the gap fillers by pulling them out with the gloved fingers, or the forceps if required. The downmode will be to cut them off to the lowest level feasible, with the hacksaw or scissors. If they are left in place the desire is to get them to a height of no more than 29 ~ 0.4 Ň. The general plan would be to pre-position ESP-2 on FD8, and then access the area from the starboard side with an APFR on the SSRMS after completing the ESP-2 installation task.

The current estimates are for a 7:15 duration EVA, with 1:15 dedicated to the gap filler task. We are sending the details of this task as well as the overall EVA 3 plan for your consideration and look forward to your comments/questions.

The Discovery astronauts said today they were initially concerned about the safety of a proposed spacewalk repair job Wednesday to remove two protruding "gap fillers" sticking out from heat-shield tiles on the shuttle's belly. But they are now convinced it's not only safe, but a relatively simple task that will eliminate any lingering concern about unwanted re-entry heating.

After discussing the spacewalk details with her crew mates today, Discovery commander Eileen Collins told flight controllers they had agreed to add the repair work to an already-planned spacewalk Wednesday, the crew's third and final planned outing.

"The crew has talked about the EVA (spacewalk repair) and we are go for EVA on flight day nine," Collins called down. "Go for EVA tomorrow."

"Eileen, we copy. Go for EVA on flight day nine," replied Canadian astronaut Julie Payette in mission control. "This will be exciting."

In a morning news conference, astronaut Andrew Thomas told reporters that when the crew first heard about the planned repair work, "I think a number of us did have misgivings. We were concerned about it, we were concerned about the implications of it and what was motivating it."

"The ground has been very good in sending us up a lot of information about it," he said. "We understand some of the physics behind what would happen if (the gap fillers) weren't there. We know if it were to remain there, even under a worst-case scenario, it wouldn't present a threat to the orbiter. There might be some minor structural damage that might require post-flight attention, but it wouldn't be a threat to us personally.

"However, it's a lot better for a number of reasons, performance reasons, if you can remove this material and that justifies doing it, considering that every indication is that the removal of the material should be straight forward and pretty easy."

The protruding gap fillers, two of thousands in use across the underside of the shuttle, are not associated with foam debris shed by Discovery's external fuel tank during launch. The stiff, ceramic cloth material is inserted between heat shield tiles to ensure smooth air flow and to prevent adjacent tiles from rubbing as the shuttle's aluminum skin flexes during launch or due to temperature swings.

During Discovery's approach to the international space station last week, commander Eileen Collins piloted the ship through a 360-degree pitch maneuver that allowed the station crew to photograph the ship's normally unseen belly. Image analysts quickly spotted two gap fillers that had been dislodged. According to documentation of a similar incident during an earlier mission, engineers believe gap fillers typically dislodge as the shuttle goes supersonic during launch.

One gap filler seen on Discovery protrudes a full inch above the surface of the surrounding tiles while the other extends 0.6 inches.

The concern is that one or both gap fillers in question could "trip the boundary layer" during re-entry, that is, disrupt the smooth, laminar flow of supersonic air across the belly of the shuttle and create eddies of turbulence that, in turn, result in higher downstream heating.

The issue is when that transition to turbulent flow might occur. It occurs normally when the shuttle's velocity has dropped to around eight to 10 times the speed of sound, starting toward the back of the heat shield and moving forward. But a protruding gap filler in a 1995 shuttle mission tripped the boundary layer at Mach 18, causing significant tile damage during entry.

A gap filler also can trigger an asymmetric boundary layer transition, changing the aerodynamics and causing the shuttle's flight computers to compensate by firing rocket thruysters or adjusting the ship's elevons.

That's not thought to be an issue with Discovery. Rather, engineers were concerned that the protruding gap fillers could disrupt the boundary layer at speeds higher than Mach 20, thus exposing downstream tiles to higher heating for an even longer period. In the end, mission managers decided not to risk any such damage to the shuttle and asked the astronauts to remove the offending gap fillers during the crew's third spacewalk.

The primary objective of the spacewalk is to attach a large tool caddy and spare parts kit called an external stowage platform to the side of the station's Quest airlock module. As soon as it's locked in place, the station's robot arm will disengage and lock its free end onto a mobile transporter mounted on rails along one side of the station's main solar array truss. Moving like slow-moving inchworm, the other end of the arm will then release a grapple fixture on the Destiny laboratory module, freeing it to grasp an astronaut foot restraint.

From its new perch on the truss, the robot arm will be able to move astronaut Stephen Robinson down the starboard side of the space shuttle, giving him access to the work sites on the far side of the orbiter. Television cameras on the station arm and the shuttle's own robot arm will let flight controllers monitor his position to help ensure he stays well away from any inadvertent contact with Discovery's heat shield.

The most significant concern is any such inadvertent contact that might damage one or more tiles.

"Like all kinds of repairs, it's conceptually very simple but it has to be done very, very carefully," Robinson said today. "The tiles as we all know are fragile and an EVA (spacewalk) crew member out there is a pretty large mass. I'll have to be very, very careful, but the task is extremely simple and we predict it won't be too complex."

Arm operators Wendy Lawrence and pilot James Kelly will take extra care moving Robinson into position.

"We're ready to do this in a very careful manner," Robinson said. "And besides, it's not just me. We'll also have a camera ... looking at me and trying to look at the clearance between me and the orbiter's belly. So we'll have lots of ways to be very, very conservative. It's going to be like watching grass grow. Nothing's going to happen fast."

The two gap fillers are located relatively close to the shuttle's nose. Robinson first will try to simply pull the spacers free.

"There won't be any yanking going on at all," he said. "It will be a gentle pull with my hands and if that doesn't work, I have some forceps, I'll give a slightly more than gentle pull and if that doesn't work, I'll saw it off with a hacksaw. No yanking."

Robinson, identified during spacewalks as EV-2, will be joined by Japanese astronaut Soichi Noguchi, EV-1. Here is an updated timeline of Wednesday's spacewalk activities (in EDT and elapsed time):

EDT........HH...MM...EVENT

04:14 AM...00...00...Airlock egress
04:44 AM...00...30...EV1/EV2: External stowage platform (ESP 2) installation
05:29 AM...01...15...EV1: MISSE 5 experiment package installation
05:29 AM...01...15...EV2: Grapple fixture removal and stowage
05:44 AM...01...30...Station arm (SSRMS): ESP 2 ungrapple
05:49 AM...01...35...SSRMS: walk off to S0 truss mobile transporter
06:14 AM...02...00...Shuttle arm (SRMS): TPS sample box scan
06:29 AM...02...15...EV1: Rotary joint motor coupler removal
06:29 AM...02...15...EV2: Gap filler tool prep
07:29 AM...03...15...EV1: Attach foot restraint to SSRMS
07:29 AM...03...15...EV2: Attach tethers, ingress SSRMS
07:44 AM...03...30...EV1: Close TPS sample box; stand by
07:44 AM...03...30...EV2: gap filler removal process begins
07:54 AM...03...40...SSRMS: move EV2 to work site 1
08:34 AM...04...20...SSRMS: move EV2 to work site 2
09:14 AM...05...00...SSRMS cleanup/egress
09:44 AM...05...30...IV egress for shuttle EVA
10:29 AM...06...15...EV1/EV2: Payload bay cleanup and tool stow
11:14 AM...07...00...Airlock repressurization

The space station's robotic arm will move Robinson down the right side of Discovery and then under its nose. He will not be visible from the shuttle's crew cabin or the space station. But he will be in view of the robot arm cameras and in constant UHF radio contact with his crew mates and flight controllers in Houston through the station's communications system.

Robinson said his major concern will be making sure his helmet doesn't bump into the shuttle's fragile tiles as he nears the orbiter. As for the actual repair work, "I've got several old airplanes at home that I've had for many, many years, so I'm pretty comfortable with using tools very carefully."

"But no doubt about it, this is going to be a very delicate task. But as I say, a simple one. The hacksaw is really a contingency device. The idea is to just pull out this thin gap filler, either by hand or with a pair of forceps, and we'll use the hacksaw only if necessary. But I think it's a great solution ... and it should be pretty safe."

Here is the remainder of today's activities and NASA's latest television schedule (rev. I) for tomorrow:

REV...EVENT..........................MET in DD/HH:MM...EDT........GMT

TUESDAY, AUGUST 2/FLIGHT DAY 8

106...DISCOVERY/ISS TRANSFERS RESUME........06/15:30...02:09 AM...06:09
106.*.EVA #3 PROCEDURE REVIEW...............06/16:00...02:39 AM...06:39
107.*.EVA #3 TILE GAP FILLER TOOL ASSEMBLY..06/17:00...03:39 AM...07:39
108...JOINT CREW NEWS CONFERENCE.....TDRW...06/19:20...05:59 AM...09:59
109...RUSSIAN PAO EVENT............ISS Ku...06/20:06...06:45 AM...10:45
111...RUSSIAN PAO EVENT REPLAY........JSC...06/20:36...07:15 AM...11:15
111.*.SSRMS ESP-2 GRAPPLE & UNBERTH.........06/21:00...07:39 AM...11:39
111...JOINT CREW NEWS CONFERENCE REPLAY.....06/21:51...08:30 AM...12:30
111.*.EVA #3 REVIEW.........................07/00:00...10:39 AM...14:39
111...MISSION STATUS BRIEFING.........JSC...07/00:51...11:30 AM...15:30
112...EVA PRE-BREATHE.......................07/00:55...11:34 AM...15:34
112...SHUTTLE/ISS HATCHES CLOSED............07/01:30...12:09 PM...16:09
112...SHUTTLE CABIN DEPRESS TO 10.2 PSI.....07/01:55...12:34 PM...16:34
113...VIDEO FILE.......................HQ...07/02:21...01:00 PM...17:00
114...DISCOVERY/ISS CREW SLEEP BEGINS.......07/04:30...03:09 PM...19:09
115...FLIGHT DAY 8 HIGHLIGHTS.........JSC...07/05:21...04:00 PM...20:00
116...POST-MMT BRIEFING...............JSC...07/07:21...06:00 PM...22:00
119...POST-MMT BRIEFING REPLAY........JSC...07/11:21...10:00 PM...02:00
119...DISCOVERY CREW WAKE UP (begins FD 9)..07/12:30...11:09 PM...03:09
120...ISS CREW WAKE UP......................07/13:00...11:39 PM...03:39

WEDNESDAY, AUGUST 3/FLIGHT DAY 9

121...EVA #3 PREPARATIONS BEGIN.............07/14:30...01:09 AM...05:09
123...EVA #3 BEGINS.........................07/17:35...04:14 AM...08:14
123...DISCOVERY/ISS TRANSFERS RESUME........07/17:55...04:34 AM...08:34
123.*.SHUTTLE/ISS HATCHES OPEN..............07/18:00...04:39 AM...08:39
123.*.SSRMS ESP-2 INSTALLATION ON ESPAD.....07/18:25...05:04 AM...09:04
124.*.MISSE 5 INSTALL/FRGF REMOVAL..........07/19:10...05:49 AM...09:49
124.*.OBSS SURVEY OF TILE BOARD.............07/19:55...06:34 AM...10:34
125.*.GAP FILLER TOOL PREP/RJMC REMOVAL.....07/20:10...06:49 AM...10:49
125.*.TILE GAP FILLER REMOVAL...............07/21:25...08:04 AM...12:04
126.*.SHUTTLE/ISS HATCHES CLOSED............07/22:50...09:29 AM...13:29
127.*.SHUTTLE/ISS HATCHES OPEN..............07/23:35...10:14 AM...14:14
127.*.EVA #3 ENDS...........................08/00:25...11:04 AM...15:04
127...MISSION STATUS BRIEFING.........JSC...08/01:21...12:00 PM...16:00
128...VIDEO FILE.......................HQ...08/02:21...01:00 PM...17:00
130...DISCOVERY/ISS CREW SLEEP BEGINS.......08/04:30...03:09 PM...19:09
130...FLIGHT DAY 9 HIGHLIGHTS.........JSC...08/05:21...04:00 PM...20:00
132...POST-MMT BRIEFING...............JSC...08/07:21...06:00 PM...22:00
134...POST-MMT BRIEFING REPLAY........JSC...08/11:21...10:00 PM...02:00
135...DISCOVERY CREW WAKE UP (begins FD 10).08/12:30...11:09 PM...03:09
135...ISS CREW WAKE UP......................08/13:00...11:39 PM...03:39

Earlier today, President Bush called the astronauts from the White House.

"Thank you for taking my phone call," he said. "I just wanted to tell you all how proud the American people are of our astronauts. I want to thank you for being risk takers for the sake of exploration. I want to welcome our Japanese, Australian and Russian friends and wish you God speed in your mission. I know you've got very important work to do ahead of you and we look forward to seeing a successful completion of this mission. Obviously, as you prepare to come back, a lot of Americans will be praying for a safe return. So it's great talking to you. Thanks for being such great examples of courage for a lot of our fellow citizens."

"Well thank you very much, Mr. President," Collins replied. "We want to tell you we really enjoy what we're doing, we really believe in our mission and we believe in space exploration and getting people off the planet and seeing what's out there. The steps that we're taking right now are really worth it, we want everybody to know that. Thank you very much for taking the time out of your busy schedule to talk to us."

"Well listen," Bush replied, "I want to thank you, commander, and thank your fellow astronauts there. I agree with you, I think what you're doing is really important. You've got a strong supporter for your mission here in the White House. ... We're with you and wish you all the very best. Thanks for taking my phone call. Now get back to work!"

A Discovery astronaut, working on the end of the space station's robot arm, will attempt to remove two protruding "gap fillers" sticking up from protective heat-shield tiles on the belly of the shuttle Discovery Wednesday during an already planned spacewalk, officials said today.

NASA's mission management team decided to order the repair work after a long meeting in which aerodynamicists said they could not guarantee a problem-free re-entry with the gap fillers sticking out as is. The stumbling block was uncertainy about high-altitude, high-speed aerodynamics and how turbulence, caused by the extended gap fillers, might affect heat loads on the orbiter.

Wayne Hale, chairman of the management team, told reporters late today that estimates of possible consequences ranged from no problems of any significance to exceeding the shuttle's design limits and safety margin. While the worst-case scenarios might or might not trigger a catastrophic failure, serious tile damage could result.

"Today at the mission management team meeting we had a very long discussion about aerodynamics," Hale said. "I went in with a very simple question: Did we have the engineering knowledge and analysis that would, without a shadow of a doubt, allow us to be 100 percent confident the vehicle could fly safely during entry?

"We investigated that at length, the team has been working for three days, they came in with a very long report, the management team asked them a lot of detailed questions and at the end of the day, the bottom line is there is large uncertainty because nobody has a very good handle on aerodynamics at those altitudes at those speeds. Given that large degree of uncertainty, life could be normal during entry or some bad things could happen.

"Then we examined our options to set our minds at rest and to make sure we didn't stay up late nights worrying about bad things happening, the EVA (spacewalk) team has ... put together a very simple plan, with good safety precautions and mitigations of many hazards that will allow the crew member to go out and remove those two gap fillers. And so when we looked at the unknown versus what we do know about EVA, it was a very easy decision to add the task to EVA number three, to go remove the two little gap fillers."

Please see the stories immediately below for a full explanation of the gap filler issue, early and asymmetric boundary layer transitions and other related topics. A revised mission flight plan will be posted here early tomorrow, along with a preliminary look at the spacewalk timeline.

Here is the timeline for tomorrow's activities (from revision H of the NASA television schedule; an * indicates a change from the previous schedule):

NOTE: VIEW WITH FIXED-WIDTH FONT

REV...EVENT..........................MET in DD/HH:MM...EDT........GMT

TUESDAY, AUGUST 2/FLIGHT DAY 8

106...DISCOVERY/ISS TRANSFERS RESUME........06/15:30...02:09 AM...06:09
108...JOINT CREW NEWS CONFERENCE.....TDRW...06/19:20...05:59 AM...09:59
109...RUSSIAN PAO EVENT............ISS Ku...06/20:06...06:45 AM...10:45
112...RUSSIAN PAO EVENT REPLAY........JSC...06/20:36...07:15 AM...11:15
109...JOINT CREW NEWS CONFERENCE REPLAY.....06/21:51...08:30 AM...12:30
111...EVA #3 PROCEDURE REVIEW...............07/00:00...10:39 AM...14:39
111...MISSION STATUS BRIEFING.........JSC...07/00:51...11:30 AM...15:30
112...EVA PRE-BREATHE.......................07/00:55...11:34 AM...15:34
112...SHUTTLE/ISS HATCHES CLOSED............07/01:30...12:09 PM...16:09
112...SHUTTLE CABIN DEPRESS TO 10.2 PSI.....07/01:55...12:34 PM...16:34
113...VIDEO FILE.......................HQ...07/02:21...01:00 PM...17:00
114...DISCOVERY/ISS CREW SLEEP BEGINS.......07/04:30...03:09 PM...19:09
115...FLIGHT DAY 8 HIGHLIGHTS.........JSC...07/05:21...04:00 PM...20:00
116...POST-MMT BRIEFING...............JSC...07/07:21...06:00 PM...22:00
119.*.POST-MMT BRIEFING REPLAY........JSC...07/11:21...10:00 PM...02:00
119...DISCOVERY CREW WAKE UP (begins FD 9)..07/12:30...11:09 PM...03:09
120...ISS CREW WAKE UP......................07/13:00...11:39 PM...03:39

NASA engineers are refining plans for an unprecedented but relatively straight forward spacewalk repair job Wednesday to remove two protruding "gap fillers" from the shuttle Discovery's underside heat shield tiles. While no spacewalking astronaut has ever been asked to work under the shuttle, out of direct view, engineers say the gap filler fix is not technically difficult or any more risky than other more routine spacewalk activities.

"It should be a simple task," said Cindy Begley, lead spacewalk officer for Discovery's mission. "It could be just as easy as grabbing it with his fingers and pulling it out. And we hope that's all it's going to be."

A formal decision on whether or not to add the repair task to an already planned spacewalk Wednesday is expected by the end of the day, after NASA's mission management team receives a briefing on the aerodynamic effects of the protruding gap fillers and an assessment of the threat they might pose.

Gap fillers do exactly what their name implies: They fill gaps between adjacent heat-shield tiles to prevent heat intrusion and side-to-side rubbing as the shuttle's aluminum skin flexes. Photographic inspection of Discovery's under side showed that two of the thousands of gap fillers in place had shaken or partially pulled loose during launch or after the shuttle reached orbit. As a result, portions of the two gap fillers in question stick up above the surface of the tiles. One extends about an inch above the tiles while the other protrudes about six-tenths of an inch.

The concern is that the gap fillers could disrupt the smooth flow of supersonic, super-heated air across the belly of the shuttle early in re-entry - the so-called boundary layer - creating turbulent flows that, in turn, could lead to extreme heating that might damage surrounding tiles or even wing leading edge panels. The early creation of turbulent airflow is known as "tripping the boundary layer."

Shuttles have re-entered with extended gap fillers before, suffering tile damage as a result, but this is the first time NASA engineers have had a chance to see one in advance and consider possible fixes. That's because of a post-Columbia safety program that now includes a shuttle pitch-around maneuver during approach to the space station that provides an opportunity to photograph the belly of the orbiter in enormous detail.

Over the weekend, a tiger team of veteran spacewalkers and engineers began "evaluating several methods for removing the gap filler or cutting the gap filler," Begley said "They're evaluating how to get access to the area, which arm to get on and go down there and we have plans in work for a new EVA timeline if we need to do that.

"So we're getting everything staged and ready to go. We're even going to send some information to the crew so they can be looking at that if we end up going in that direction. ... As always, we try to get all our ducks in a row ahead of time."

Assuming the astronauts get the go ahead as expected, the station's robot arm would be "walked off" the Destiny laboratory module in inchworm fashion to reach a mounting point on the station's main solar array truss. Astronaut Stephen Robinson, equipped with forceps, scissors and a hacksaw-like tool, then would lock his boots in a foot restraint and the arm would be maneuvered to carry him to the gap filler work sites.

"The basic part of the task is to get to the worksite and that's something we've never done before, put an EV crew member underneath the vehicle," Begley said. "The task itself, pulling out the gap filler, they're going to have to be very careful of the area not to damage anything while they're there. We're making sure we're taking as many tools off of him as we can and holding the safety tethers back behind him.

"The first attempt is going to be to pull it out," she said. "We don't expect that to take a lot of force. If it seems to be taking a lot of force, then we're going to look at cutting it off. And we have a number of tools (available). We have a forceps you can lock on to the gap filler and gives him a tether point to hang on to it as he's cutting it off to get it out. Those are the things we're looking at. Doing the actual cutting may give us a little bit of debris to look at but otherwise, I think it's a fairly simple task, making sure we're not going to hit the vehicle when we're doing that."

Asked about what worst-case scenario NASA would be protecting against by ordering the repair work, mission operations representative Phil Engelauf said "I'm going to hedge a little bit here because that's going to be the topic of discussion at the (mission management team).

"The aerothermal team is going to come in and present the summation of the analysis they've been working on for a couple of days," Engelauf said. "The preliminary indications are that we're not going to be able to give a very definitive answer and that, in fact, is probably what will drive the discussion one way or the other.

"There are not a lot of vehicles that fly in the flight regime that the shuttle operates in and so our testing and real flight experience with boundary layer transition in the Mach 18 to 20 regions is a pretty thin data base and it's not well supported by analytical models because we've never had anything to validate those models against. The guys have been working furiously to try to understand that.

"Their considerations are bulk local heating as well as control," Engelauf said. "We think we've pretty well cleared the control issues. ... We're really down to local aero heating and there are two different categories of issues. These two pieces of gap filler are located one near the centerline of the vehicle, far forward by the nose landing gear door, and then a second one a little bit farther back but somewhat off to the side of the vehicle.

"For the most forward one, the biggest concern is the far forward early boundary layer transition, early in the flight regime. ... They're worried about transitions above Mach 20, which is earlier than the design point for the vehicle. That would be primarily (a) localized heating situation. But primarily for the tile. With the one over closer to the side, again if you had an asymmetrical transition you could potentially have localized high heating on the RCC (leading edge panels) because of anything wrapping around the vehicle and up over the leading edge of the wing."

Shuttles have returned from space with protruding gap fillers before "without detriment but also somewhat in ignorance because we've never had the opportunity to observe the way we have now. This was not, frankly, one of the things we spent a lot of time working on the past two years."

"There's a fair amount of conservatism prevailing in the community that until we can satisfy ourselves, maybe the better course of action is to go out here and remove these if that's the right thing to do."

Astronauts Soichi Noguchi and Stephen Robinson staged a successful seven-hour 14-minute hour spacewalk today, installing a critical replacement gyroscope for the international space station's stabilization and orientation system.

The 60th spacewalk devoted to station assembly and maintenance began at 4:42 a.m. and ended at 11:56 a.m., giving 53 astronauts and cosmonauts a cumulative total of 362 hours and 19 minutes of station spacewalk time. Robinson and Noguchi have now logged 14 hours and four minutes of EVA time in two excursions with a third on tap Wednesday.

"It's been great working with you guys today, it's been a real pleasure for the entire team down here," Mike Massimino radioed the astronauts from mission control.

"OK, Mike, thank you very much for your patience," astronaut Andy Thomas replied from Discovery.

Two spacewalkers installed a new motion control gyroscope on the international space station today, but initial electrical checks failed. Japanese astronaut Soichi Noguchi then was asked to disconnect and reseat three critical electrical cables and, after finding one that was a bit loose, the gyroscope powered up normally.

"The last few actions that Soichi did on the connectors worked," Mike Massimino radioed from mission control in Houston. "We have three good connectors and a healthy CMG. Thanks a lot."

The installation of a replacement control moment gyroscope was the primary objective of today's spacewalk. Before stowing tools and cleaning up the shuttle Discovery's cargo bay, Noguchi and fellow spacewalker Stephen Robinson were asked to carry out a few other "get-ahead" tasks, two of them to support possible repair work Wednesday.

Mission managers are debating asking Robinson or Noguchi to remove two troublesome "gap fillers" sticking up between heat shield tiles on the belly of the space shuttle. NASA managers have not yet approved the work, although sources say it appears likely the gap filler fix will be added to crew's third and final spacewalk later this week.

Noguchi and Robinson were asked to remove a foot restraint from the end of the space station's robot arm so the arm, if the repair work is approved, can "walk off" the laboratory module Destiny, using its free end to lock onto a support fixture on the station's main solar array truss. The arm must be repositioned to give one of the spacewealkers access to Discovery's underside.

Noguchi and Robinson also were asked to retrieve a tool stowed in an externally mounted took box for possible use during the proposed gap filler repair and to reposition yet another foot restraint as a get-ahead task for the next station assembly crew.

At 6:15 a.m., spacewalkers Soichi Noguchi and Stephen Robinson successfully unbolted and removed a massive control moment gyroscope from the international space station, the first step in a lengthy replacement job.

"OK... a 600-pound mass..." observed Robinson as the astronauts began pulling the gyroscope from its housing.

"Here we go," Noguchi replied.

Noguchi, riding on the end of the space station's robot arm, held the massive gyro as astronaut Wendy Lawrence, operating Canadarm2, backed him away from the station's Z1 truss.

"It's not easy for me to hold this rascal," Robinson joked. After making a few final checks, he told Noguchi: "You've got a ticket to ride."

Spectacular television shots from the space statoin shows Noguchi, holding the black gyroscope in front of him, floating against the dark of space as the shuttle-station complex passed 220 miles above Europe.

"Soichi, you look fantastic," Robinson said, adding that the crew's training closely matched what they were experiencing in space.

"Oh, the view is priceless," Noguchi radioed a few moments later.

Floating in the shuttle Discovery's airlock, astronauts Stephen Robinson and Soichi Noguchi switched their spacesuits to internal battery power at 4:42 a.m. to officially begin a high-priority spacewalk to replace one of the international space station's gyroscopes.

"All right! Let's get started," said Robinson as the two headed out into the shuttle's cargo bay.

This is the 60th spacewalk devoted to space station assembly and maintenance by 40 U.S. astronauts, 10 Russian cosmonauts, one Frenchman, one Canadian and one Japanese (Noguchi). Going into today's excursion, station spacewalk time totaled 355 hours and five minutes.

Here is an approximate timeline of today's activities, zeroed to the official start time of the spacewalk:

EDT.........EVENT

04:42 a.m...Egress and setup; control moment gyroscope No. 1 (CMG-1) thermal shroud removal; robot arm foot restraint setup

05:52 a.m...Remove CMG-1: Demate connectors; loosen bolts; unbolt CMG-1

06:32 a.m...Remove new gyro: Noguchi, on the station's robot arm, is moved back down to the shuttle's cargo bay to temp stow CMG-1; the new gyro is unbolted and temp stowed to make room for CMG-1, which will be returned to Earth

08:02 a.m...CMG-1 is mounted in Discovery's cargo carrier; Noguchi, on the station arm, carries the new CMG up to the Z1 truss where it will be installed; Robinson cleans up cargo bay work site and joins Noguchi at the Z1 truss

09:02 a.m...Install new CMG: remate electrical connectors; re-attach thermal shroud

10:12 a.m...Cleanup and airlock ingress

The space station uses four massive control moment gyroscopes to maintain the lab's orientation in space without having to tap into limited supplies of on-board rocket fuel. They are housed in the Z1 truss, which was attached to the Unity module's upward-facing, or zenith hatch - hence the name - during shuttle mission STS-92 in October 2000.

Along with saving fuel, the 800-pound gyros, spinning at 6,600 rpm, allow station crews and flight controllers to reorient the outpost and keep it stable without using rocket firings that would jar sensitive microgravity experiments.

But on June 8, 2002, CMG-1 suffered a malfunction and shut down. Station astronaut Carl Walz reported hearing an unusual noise inside the Unity module. He said the noise appeared to be coming from the module's zenith area. Mission control then told Walz engineers were working an issue with a spin bearing in CMG No. 1. Walz said the noise was quite noticeable inside the module.

"We're hearing a pretty loud, audible noise, kind of a growling noise, from inside the node," Walz reported.

"It looks like we have a mechanical failure of the spin bearings on CMG-1," an astronaut in mission control replied. "It's currently spinning down right now. The growling noise is undoubtedly due to vibration."

The station's orientation, or attitude, can be controlled by just two CMGs in a worst-case scenario. And indeed, a second gyro, CMG-2, was knocked off line last year because of trouble with a circuit breaker. The circuit breaker was replaced during a station-based spacewalk, but the new unit malfunctioned in March, taking CMG-2 off line once again. During their first spacewalk Saturday, Robinson and Noguchi wired around the faulty breaker to restore CMG-2 to service.

The replacement CMG was carried into orbit on a support structure at the back of Discovery's cargo bay. Robinson and Noguchi, on the end of the station's robot arm, first will float up to the Z1 truss, unfasten thermal blankets, disconnect electrical cables and remove CMG-1 by loosening six bolts. They will carry it back down into the cargo bay and mount it on a temporary support fitting called a ball stack. The new CMG then will be unbolted and mounted on a different ball stack while CMG-1 is locked into the payload bay support carrier for return to Earth.

At that point, Wendy Lawrence and James Kelly, operating the space station's robot arm, will move Noguchi and the new CMG up to the Z1 truss for installation. After Robinson and Noguchi complete electrical connections and re-fasten the thermal blankets, engineers in mission control will begin preparations for spinning up the new gyro.

"When you describe something like this it sounds very easy," Robinson said in a NASA interview. "It turns out itŐs a little more delicate and difficult work than that, but conceptually is very simple. We bring up a new one in the payload bay of the shuttle. Soichi and I come out of the airlock, we go up to the broken one, unbolt it. Soichi is on the robot arm at this point, on the space station robot arm being operated by Jim Kelly and Wendy Lawrence, and I will help unbolt it and weŐll get the big shroud out of the way, and, and pull the broken gyro out of the space station.

"Soichi will just hang onto it with his hands, and in space you can actually move around this 600-pound mass, very carefully with your hands Of course heŐll be tethered to it, will climb down to the payload bay and Soichi will be moved down to the payload bay on the robot arm, and we will temporarily stow the gyro in the payload bay, pick up the new one, reverse the whole process, and put the new one back into the station."

Asked if the replacement work was complicated, Robinson told CBS News it was more a matter of precision than complexity.

"This thing weighs 660 pounds mass and it has to be put into this little slot with a couple of thousandths (of an inch) clearance by hand and you can't see very much of where you're putting it in. So you have to work together. This is going to be a very delicate task and we're going to have to have a very feather like touch at all times."

The gyroscopes are critical to station operation. Here's a description from a NASA press kit:

The motion control subsystem (MCS) hardware launched as part of the Z1 element includes the CMGs and the CMG assemblies.

The CMG assembly consists of four CMGs and a micrometeorite/orbital debris shield. The four CMGs, which will control the attitude of the ISS, have a spherical momentum storage capability of 14,000 ft-lb/sec, the scalar sum of the individual CMG wheel moments. The momentum stored in the CMG system at any given time equals the vector sum of the individual CMG momentum vectors.

To maintain the ISS in the desired attitude, the CMG system must cancel, or absorb, the momentum generated by the disturbance torques acting on the station. If the average disturbance torque is nonzero, the resulting CMG output torque is also nonzero, and momentum builds up in the CMG system. When the CMG system saturates, it is unable to generate the torque required to cancel the disturbance torque, which results in the loss of attitude control.

The CMG system saturates when momentum vectors have become parallel and only momentum vectors change. When this happens, control torques perpendicular to this parallel line are possible, and controllability about the parallel line is lost.

Russian segment thrusters are used to desaturate the CMGs.

An ISS CMG consists of a large flat wheel that rotates at a constant speed (6,600 rpm) and develops an angular momentum of 3,500 ft-lb/sec about its spin axis. This rotating wheel is mounted in a two-degree-of-freedom gimbal system that can point the spin axis (momentum vector) of the wheel in any direction.

At least two CMGs are needed to provide attitude control. The CMG generates an output reaction torque that is applied to the ISS by inertially changing the direction of its wheel momentum. The CMG's output torque has two components, one proportional to the rate of change of the CMG gimbals and a second proportional to the inertial body rate of the ISS as sensed at the CMG base. Because the momentum along the direction of the spin axis is fixed, the output torque is constrained to lie in the plane of the wheel. That is why one CMG cannot provide the three-axis torque needed to control the attitude of the ISS.

Each CMG has a thermostatically controlled survival heater to keep it within thermal limits before the CMGs are activated on Mission 5A. The heaters are rated at 120 watts and have an operating temperature range of -42 to -35ˇF.

Editor's Note...
For readers interested in additional details about space shuttle re-entry aerodynamics, here's part of a recent paper co-authored by Wayne Hale, chairman of NASA's mission management team:

OPERATIONAL EXPERIENCE WITH HYPERSONIC FLIGHT OF THE SPACE SHUTTLE

N. Wayne Hale, Jr., Nicole O. Lamotte, and Timothy W. Garner

Under normal conditions, boundary layer transition from laminar to turbulent flow occurs approximately at the same time on both sides of the vehicle (left/right), usually around Mach 8 for a space shuttle entry. However, it occasionally occurs early (as early as Mach 19 on STS- 73). Transition causes a sharp rise in the heating rate and the thermal protection system (TPS) temperatures. This is magnified when transition occurs early, which has resulted in slumped tiles and other damage to the shuttle TPS. The higher heating has also aggravated the effect of ascent debris impacts. This TPS damage has required additional TPS refurbishment work during the turnaround for the next flight.

Transition can also begin on one side of the vehicle, which temporarily causes asymmetric drag, and rolling and yawing moments. The primary consequence of this rolling and yawing moment disturbance is an increase in RCS propellant usage at the onset of asymmetry, followed by a second, smaller increase when transition occurs on the other side of the vehicle, which eliminates the asymmetry. In vehicles with limited aileron trim capability, this could also contribute to aileron trim saturation in the presence of airframe and center of gravity asymmetries.

The time of transition is affected by the angle of attack and the surface roughness. Transition can be triggered slightly early by a roll reversal, which temporarily increases the angle of attack in response to the decrease in drag caused by the increasing altitude rate. However when transition occurs very early, the cause is usually a protruding gap filler. There are gaps between the TPS tiles to allow for thermal expansion. The gap fillers are installed between the tiles to reduce hot gas flow to the filler bars underneath these gaps. They occasionally become dislodged and protrude in the flow, which can trigger early, and often asymmetric, transition. This phenomenon caused some surprise when it was first analyzed after STS-28.

Visual inspection of the tiles and temperature measurements permitted reconstructing the spread of turbulent flow from a small area at the back of the left wing, to the entire left wing and finally to both sides of the vehicle. The responsible gap fillers could be seen during the postlanding vehicle inspection. However, occasionally no protruding gap filler can be seen, and no cause can be determined for the asymmetric transition. It is hypothesized that in such a case the gap filler has fallen off during entry. Improved TPS installation techniques seem to have decreased the incidence and severity of ABLT recently.

The modeling of ABLT has evolved over time. The model currently used in the statistical simulations, which is based on flight experience, predicts that early transition (before Mach 10.9) will occur in 20% of the flights, and be asymmetric in 60% of those flights. However, if transition occurs after Mach 10.9, it only has a 10% probability of being asymmetric. This reflects the high correlation between early and asymmetric transition.

The occurrence of the earliest transition, and the hottest entry, on the 72nd flight (as well as the occurrence of the most severe density shears on the 48th and the 56th flights) illustrate the importance of maintaining sufficient safety margins in the design and operation of hypersonic vehicles, even after the end of the test flights. Another lesson learned is the desirability of installing a sufficient number of thermocouples on both sides of the vehicle. Every flight is an opportunity to refine the understanding of aerothermodynamics.

Engineers are considering what, if anything, to do about two protruding "gap fillers" on the belly of the space shuttle Discovery that could trigger increased re-entry turbulence and localized, potentially dangerous, heating if they are left as is.

Lead flight director Paul Hill said today a team of engineers is studying the aerodynamic threat posed by the exposed gap fillers while another team is looking into possible spacewalk repair fixes. Any such repair work likely would be added to a spacewalk already planned for Wednesday.

This is not a new issue and it's not related to the foam debris that fell of Discovery's tank during launch. Engineers spotted the protruding gap fillers in photos shot by the space station's crew last week as commander Eileen Collins flipped the shuttle about before docking to permit a close examination of the shuttle's underside heat shield.

"We have a team of folks that are working aggressively at options to go and make that gap filler safe if we decide it's an issue," Hill said at a morning news briefing. "We expect to have final results on aero heating and a decision on whether we need to do anything about the gap fillers on Monday."

Engineers may opt to do nothing if the analysis shows Discovery can safely return to Earth as is. If not, "we have various options that range from pulling the gap filler out, to trimming the gap filler, to putting it back down into the gap," Hill said. "And those are actively being worked right now. I couldn't tell you yet where it's going."

Brent Jett, scheduled to command the third post-Columbia mission, told CBS News today astronaut troubleshooters are leaning toward recommending that one of Discovery's spacewalkers use pliers to simply pull the offending gap fillers out if a repair job ultimately is deemed necessary.

"There's a team off right now looking at that from an EVA perspective," Jett said. "There are three astronauts on that team - Joe Tanner, Jim Reilly and Dave Wolf. I spoke to Joe this morning and he said as of right now, the leading candidate is to remove it, with the second option being to trim it. But that could change between now and tomorrow."

NASA managers have said in recent days that Discovery appears to be in good shape and that engineers expected to be able to clear the shuttle for re-entry as is during a final meeting Monday. Minor dings to the shuttle's heat-shield tiles have, in fact, been cleared and engineers are expected to clear the ship's wing leading edge panels later today.

The protruding gap fillers are not associated with foam debris shed by Discovery's external fuel tank during launch. The ceramic cloth material is inserted between heat shield tiles on the belly of the shuttle to ensure smooth air flow and to prevent adjacent tiles from rubbing as the shuttle's aluminum skin flexes during temperature swings.

One gap filler protrudes a full inch above the surface of the surrounding tiles while the other extends 0.6 inches. The largest known case of a gap filler protrusion toward the front of the shuttle is 0.25 inches.

The concern is that one or both gap fillers in question could "trip the boundary layer" during re-entry, that is, disrupt the smooth, laminar flow of supersonic air across the belly of the shuttle and create eddies of turbulence that, in turn, would result in higher downstream heating.

Wayne Hale, chairman of NASA's mission management team, said late today a final decision on how to proceed will be made Monday, after a thorough review by aerodynamicsts and spacewalk planners.

"Initially, when we saw this problem I thought well, gee, gap fillers, we've seen that before, flown through assymetric boundary layer transitions before, very little damage on the bottom of the orbiter and the only real effect we've seen from these aero transitions in previous flights has been to tiles that were already damaged," Hale said.

"So my immediate knee-jerk reaction was that we can live with this. On the other hand, this is bigger than we've seen before, the EVA guys have gone out and are putting together a plan that we'll hear about tomorrow. And if it's relatively simple, I mean, why worry? Why would you not just go take care of it if you had a simple plan to deal with it. So it really depends on how the plan comes out tomorrow."

It is not unusual - or necessarily unsafe - for gap fillers to slip out of position and extend into the airflow, as long as they don't extend too far. If they do, they can trigger an early transition to turbulent airflow, exposing downstream tiles to higher heating for longer periods.

After a mission by the shuttle Columbia in 1995, technicians found an exposed gap filler roughly the same size as the larger gap filler seen protruding from Discovery's tiles. During Columbia's re-entry, a few downstream tiles were damaged by higher heat loads. Peak heating on the underside of the shuttle typically runs about 2,300 degrees Fahrenheit, but turbulent flow associated with a boundary layer transition can reach 2,800 degrees depending on where and when it occurs.

"Both of these gap fillers exceed our generic constraint, or kind of our conventional wisdom," Hill said. "This is about a quarter inch protrusion in a forward location. Both of these are well above a quarter of an inch. That's why we have to depend on site-specific aero analysis. ... Boundary layer transition at these Mach numbers is such a tricky thing, we can't speculate. We have to wait for the data."

At the risk of making more out of this development than it might warrant, here is a brief description of the boundary layer and how it works with the space shuttle from "Comm Check: The Final Flight of Shuttle Columbia" by this writer and Michael Cabbage of the Orlando Sentinel:

During re-entry, the shuttle compresses the thin air in front of it creating two shock waves. Those shock waves intersect around RCC (wing leading edge) panel 9, subjecting panels in that area to the most extreme heating. But the compression of the air in front of the shuttle forms a so-called boundary layer, a region just a few inches thick that resists further compression and acts as a natural insulator. Just a few inches away from the leading edge, just beyond the boundary layer, atoms are torn apart and temperatures can exceed 10,000 degrees. But the boundary layers keeps temperatures on the leading edge RCC panels at around 3,000 degrees.

A smooth surface is essential for the boundary layer to form and it is crucial to a shuttle's survival during the plunge to Earth. If the boundary layer is disturbed for any reason, its insulating effect can be compromised by high-temperature turbulence, subjecting the shuttle's tiles and RCC panels to much more heat than they were designed to handle.

"You have this big massive thing coming through, slamming into the molecules, more and more as you get farther into the atmosphere, and that collision basically disassociates these molecules into their constituents," explained James Hallock, a member of the Columbia Accident Investigation Board. "And that's where all this heating is coming from. [The shuttle] uses aerodynamic braking, basically. So you've got to get down to where the molecules are to slow it down. That's really what's slowing it down, running into these molecules and dissipating a lot of the energy of speed into heat. É Once this boundary layer forms, it really protects you pretty well."

During all re-entries, the smooth flow of air in the boundary layers eventually transitions to turbulent flow. That transition normally occurs at velocities between eight and 12 times the speed of sound. Anything above Mach 13 is considered an early transition and results in 15 to 25 percent higher heating.

"We do worry in general about an early boundary layer transition, especially a far forward early boundary layer transition," Hill said. "The farther forward we go, the more severe the entry heating problem is if we trip that boundary layer early or if we start seeing turbulent flow at a far forward location.

"It increases the heating, both at the area where we tripped, say, where these gap fillers are protruding, and it increases the heating on all the areas downstream of it. So that's our general concern. Our biggest concern ... is going to be that exact location right around where we trip the boundary layer. We'll see a significant increase in heating there.

"I can tell you that if we have thin tiles, or we already had damage right there, it would be something we would be very worried about with a protrusion as far as an inch at a forward location like that," Hill said. "So I am very anxious to see this aero heating analysis tomorrow."

Aboard Discovery, the astronauts plan to stage the second of three planned spacewalks Monday, this one to replace one of the international space station's four stabilizing gyroscopes. Here is the latest timeline from NASA (CMG: control moment gyroscope; EVA: spacewalk; MMT: mission management team):

REV...EVENT..........................MET in DD/HH:MM...EDT........GMT

MONDAY, AUGUST 1/FLIGHT DAY 7

89...EVA #2 PREPARATIONS BEGIN..............05/14:30...01:09 AM...05:09
91...EVA #2 BEGINS..........................05/17:35...04:14 AM...08:14
92...SHUTTLE/ISS HATCHES OPEN...............05/18:00...04:39 AM...08:39
92...CMG #1 SHUTDOWN........................05/18:41...05:20 AM...09:20
92...ISS CMG REMOVAL AND REPLACEMENT........05/18:45...05:24 AM...09:24
95...NEW CMG CHECKOUT.......................05/22:36...09:15 AM...13:15
95...NEW CMG STARTUP........................05/22:51...09:30 AM...13:30
95...SHUTTLE/ISS HATCHES CLOSED.............05/23:15...09:54 AM...13:54
96...EVA #2 ENDS............................06/00:05...10:44 AM...14:44
96...SHUTTLE/ISS HATCHES OPEN...............06/00:15...10:54 AM...14:54
96...MISSION STATUS BRIEFING..........JSC...06/00:51...11:30 AM...15:30
97...VIDEO FILE........................HQ...06/02:21...01:00 PM...17:00
98...DISCOVERY/ISS CREW SLEEP BEGINS........06/04:30...03:09 PM...19:09
99...FLIGHT DAY 7 HIGHLIGHTS..........JSC...06/05:21...04:00 PM...20:00
100...POST-MMT BRIEFING...............JSC...06/07:21...06:00 PM...22:00
103...POST-MMT BRIEFING REPLAY........JSC...06/11:21...10:00 PM...02:00
104...DISCOVERY CREW WAKE UP (begins FD 8)..06/12:30...11:09 PM...03:09
104...ISS CREW WAKE UP......................06/13:00...11:39 PM...03:39

The mission management team already approved a one-day extension for Discovery's flight to let the shuttle crew assist in repairs to station exercise equipment as well as supply stowage and transfers. Discovery now is targeted for landing at the Kennedy Space Center at 4:37 a.m. on Aug. 8. Here are updated deorbit and landing times for Aug. 8 and 9 (in EDT):

   EDT........HH...MM...SS...EVENT

   Monday, Aug. 8

   03:35 AM...12...16...56...Deorbit Burn
   04:37 AM...12...17...58...Landing at KSC (planned)
   05:10 AM...12...18...31...Deorbit
   06:12 AM...12...19...33...Landing at KSC

   Tuesday, Aug. 9

   03:53 AM...13...17...14...Deorbit Burn
   04:56 AM...13...18...17...Landing at KSC
   05:25 AM...13...18...46...Deorbit Burn
   06:31 AM...13...19...52...Landing at KSC

NASA's mission management team today extended the shuttle Discovery's flight by one day, giving the astronauts more time to assist and resupply the international space station's two-man crew, and concluded the shuttle's heat-shield tiles and insulation blankets are fit for a normal re-entry Aug. 8.

But management team chairman Wayne Hale said engineers planned to take one more day to fully assess the health of the orbiter's wing leading edge panels and another 24 hours after that to fully analyze two other minor problems to make sure they won't affect Discovery's handling and heating during re-entry.

"The imagery analysis from the launch phase has been completed," Hale told reporters late today. "And the damage assessment on the orbiter is in full swing. Today, a day ahead of our schedule, I'm pleased to report the tile and (insulation) blankets have been formally cleared by the engineering team. A few pock-marked tiles, a little gouge at the base of the nose landing gear door, a thermal blanket that had billowed out on the side below the commander's window, all of those items have been formally assessed through rigorous engineering models in great detail and found to be acceptable to fly home as is."

Hale said he expects engineers to formally clear the shuttle's reinforced carbon carbon wing leading edge panels Sunday, leaving only one issue to be resolved: The status of two so-called "gap fillers" seen protruding between tiles on the underside of the shuttle.

Gap fillers are inserted between tiles to provide a protective cushion. They can occasionally shake free during launch and end up extending beyond the surface of the surrounding tiles. During re-entry, the shuttle depends on a smooth, or laminar, flow of air over the underside tiles and anything that extends up into the flow can trigger turbulence. Turbulence, in turn, can cause higher heating across downstream tiles and affect the handling of the shuttle.

Engineers do not believe the gap fillers in question pose a threat to Discovery, Hale said, they are simply being thorough in their analysis.

"It's been a great day today," Hale told reporters. "We had an outstanding EVA (spacewalk), the crew is performing in just an awesome manner, it's admirable how well theyŐre doing. The orbiter is performing nearly flawlessly, we have no new funnies on our list to look at.

"Today we tested in space, in orbit, the thermal protection system, two of the methods of (heat-shield) repair we've developed ... and both of them performed very well. I got very good reports from the designers who watched those tests today on the ground and evaluated the outcome. They worked very well, we're very pleased and it bodes well.

"It is one thing to do tests on the ground, it is another thing entirely to be able to do a test actually in space and get real data in the real environment on how these materials will perform."

Hale said the astronauts had conserved enough electrical power, primarily by limiting the use of a booster fan, "to formally declare we have the capability to extend the mission a day. ... This will allow the crews to get more work done, more (equipment and supply) transfers."

The shuttle's three fuel cells produce electricity by combining liquid hydrogen and oxygen. A by product of the reaction is water, which is transferred to the space station. By staying an additional day, Discovery's crew will generate enough extra water to supply the station crew for 20 days.

The Discovery astronauts are also going to leave a few laptop computers behind and provide additional assistance to their station counterparts.

A Russian Progress supply ship is scheduled for launch Sept. 8 but NASA's plans to launch another shuttle supply mission in mid September are in limbo because of the external tank foam insulation problems seen during Discovery's launching.

NASA only has two shuttle launch windows left this year, one in September and a short four-day window in November.

Space station program manager Bill Gerstenmeir said the space station's current crew, and replacements expected to arrive in October aboard a Russian Soyuz spacecraft, will have more than enough supplies to make it through the rest of the year even if the shuttle stays grounded into 2006.

"The plan is we're going to have a Progress at the first part of September, Sept. 8, and that looks fine and it's on track and then there's another Progress later," he said. "We're fine from a consumables standpoint all the way through the end of the year. We're in very good shape. ... We're going to get a lot of water from the shuttle here, which will put us in very good shape."

Astronauts Stephen Robinson and Soichi Noguchi wrapped up a smooth spacewalk today, testing heat-shield repair techniques, hot wiring one of the space station's gyroscopes and mounting an attachment device that later will hold a large external tool kit and spare parts box.

"You did a great job today," astronaut Mike Massimino radioed the crew from mission control. "It's just been a pleasure for us down here to work with you. Awesome views, great job, everything was just perfect. ... We'll look forward to another two great EVAs and a great flight continuing."

"Mike, thanks for your support, we appreciate it very much," replied astronaut Andy Thomas aboard Discovery.

The spacewalk began at 5:46 a.m. and ended at 12:36 a.m., for a total time of six hours 50 minutes. It was the 59th spacewalk devoted to station assembly and maintenance, pushing the cumulative total to 355 hours and five minutes by 40 NASA astronauts, 10 Russian cosmonauts, one Canadian, one Frenchman and now, one Japanese.

All of the objectives of the crew's first spacewalk were met, along with a few additional tasks. Noguchi was asked to photograph an apparently loose insulation blanket near Discovery's left-most cockpit window and Robinson retrieved two experiment packages mounted on the station's hull that were to have been brought back in next week.

"I can't even begin to tell you how excited and happy I am to welcome Steve and Soichi into the EVA hall of fame," said lead spacewalk planner Cindy Begley. "They have a total EVA time of six hours and 50 minutes today. They completed all of their scheduled tasks and they did all of them on the timeline and even some of them ahead of the timeline. ... I'm just more than happy with the performance today."

Flight controllers, meanwhile, reactivated control moment gyroscope No. 2, one of four used to maintain the station's orientation without the use of rocket fuel. CMG-2 was knocked off line earlier this year because of trouble with a circuit breaker. Robinson re-wired a patch panel today, bypassing the breaker and allowing controllers to spin it back up. CMG-1, which failed in 2002, will be replaced during a spacewalk Monday.

"The gyroscopes we have in the space station allow us to orient the space station without using any propellant, zero propellant, and if we got down to only one, we'd have to use the thrusters on the Russian segment in order to control our attitude," said station flight director Mark Ferring. "We really did not want to get down to a single gyroscope.

"With the recovery of the gyroscope here today, we're back up to three and after (a second spacewalk Monday), we're going to replace an entire gyroscope and we should be back up to a full count of four."

But gyro problems remain an issue. After CMGs 1 and 2 are spun up and put into operation, CMG-3 will be taken out of the control loop. While still functional, CMG-3 has been experiencing lubrication-related bearing issues while helping control the orientation of the massive station-shuttle complex.

Working the shuttle Discovery's cargo bay, astronauts Stephen Robinson and Soichi Noguchi tested potential heat-shield repair techniques today, a major milestone in NASA's recovery from the Columbia disaster.

Using a high-tech caulk gun, Robinson squeezed out dollops of a thick heat-resistant material known as NOAX and used trowels to work it into deliberately cracked and gouged samples of wing leading edge material. Multiple layers were applied and smoothed over the damage sites to build-up enough material to resist the heat of re-entry. The samples will be subjected to a battery of tests on Earth to find out how well the repairs might work in an actual re-entry.

"It seems to be well behaved," Robinson said of the thick goop. "I see just a very little bit of bubbling. ... It's about like pizza dough. Licorice-flavored pizza dough."

Noguchi floated nearby, using a toweled glove to clean the tip of the applicator and a variety of trowels.

"I would recommend if we were to do this for real to use lots of spatulas," Robinson said. "You can't clean it."

Because of time constraints, Robinson was told to skip one crack repair demonstration that engineers had planned to test in a high-temperature furnace back on the ground.

Noguchi then took center stage, using a different applicator to apply a dark "emittance wash" material to deliberately damaged heat-shield tiles. The material could prove useful fixing tiles with coating damage, improving their ability to reject heat.

"The idea of emittence wash is to apply a coat of a thick kind of dark gray paint to replace areas where the black tile coating has been cracked and removed," said Lora Bailey, a spacewalk planner at the Johnson Space Center. "Originally, it was intended for certain types of damages. However, the true extent of its use is being evaluated carefully by analysis and tests to determine the depth of damage that you can repair and also that is dependent on where the damage is on the vehicle."

Today's tests were completed about two-and-a-half hours into the planned six-and-a-half hour spacewalk.

"Everyone's smiling, great job," mission control radioed.

The astronauts now are pressing ahead with work to mount an attachment fitting to the space station's Quest airlock module where a large tool kit and spare parts box will be mounted during a spacewalk next week.

Astronauts Stephen Robinson and Soichi Noguchi began a planned six-and-a-half hour spacewalk today, a busy excursion highlighted by long-awaited tests of rudimentary tile and wing leading edge repair techniques that were developed in the wake of the Columbia disaster.

Floating in the shuttle Discovery's airlock, Robinson and Noguchi switched their spacesuits to internal battery power at 5:46 a.m., officially kicking off the 59th spacewalk devoted to station assembly and maintenance. Going into today's excursion, 39 U.S. astronauts, one Canadian, one Frenchman and 10 Russian cosmonauts had logged 348 hours and 15 minutes of spacewalk time servicing and assembling the space station.

Today's spacewalk has five primary goals:

1. To demonstrate rudimentary techniques for repairing damage to heat-shield tiles, using a paint-like material called "emittance wash;" and to test a material called NOAX that could be used to smooth over cracks in the reinforced carbon carbon wing leading edge panels;

2. To mount an attachment device that will be used later to hold a large external tool kit and spare parts box called the external stowage platform, to the station's Quest airlock module;

3. To replace a broken GPS antenna;

4. To make a wiring change to bypassing a faulty circuit breaker, restoring one of the station's four gyroscopes to normal operation;
5. To route a 50-foot-long secondary electrical cable to the external stowage platform attachment device on Quest.

While the repair demonstrations have generated widespread attention, the electronic bypass planned for control moment gyroscope No. 2 is a higher priority item for the engineering community.

The space station uses four CMGs to maintain the lab's orientation in space without having to tap into limited supplies of on-board rocket fuel. They are housed in the Z1 truss, which was attached to the Unity module's upward-facing, or zenith hatch - hence the name - during shuttle mission STS-92 in October 2000.

Along with saving fuel, the 800-pound gyros, spinning at 6,600 rpm, allow station crews and flight controllers to reorient the outpost and keep it stable without fuel-consuming, experiment-jarring rocket firings.

But on June 8, 2002, CMG-1 suffered a malfunction and shut down. The station's orientation, or attitude, can be controlled by just two CMGs in a worst-case scenario. And indeed, a second gyro, CMG-2, was knocked off line last year because of trouble with a circuit breaker. The circuit breaker was replaced during a station-based spacewalk, but the new unit malfunctioned in March, taking CMG-2 off line once again.

If all goes well, Robinson will re-route power to CMG-2 today and the astronauts will replace CMG-1 during their second spacewalk Monday.

But gyro problems remain an issue. After CMGs 1 and 2 are spun up and put into operation, CMG-3 will be taken out of the loop until after Discovery departs. While still functional, CMG-3 has been experiencing lubrication-related bearing issues while helping control the orientation of the massive station-shuttle complex. After Discovery departs, the gyro will be returned to service.

Columbia was brought down by a hole in the ship's left wing leading edge caused by the impact of external tank foam insulation during launch 16 days earlier. NASA originally planned for Robinson and Noguchi to test so-called cure in-place-ablator applicator - CIPAA - backpacks, loaded with a tile repair material known as STA-54, to fill in deliberately damaged tiles in Discovery's cargo bay.

But questions about the reliability of the procedure surfaced last year when engineers noticed the formation of air bubbles in the viscous STA-54 material as the two compounds that made it up were mixed together in the backpack. After extensive troubleshooting, engineers were able to reduce the bubbling but they could not eliminate it. The concern was that bubbles could migrate in weightlessness and form large voids as the material cured. Those voids could weaken the patch and its ability to shield against re-entry heating.

NASA's astronaut office opposed in-flight testing during Discovery's flight and tests were put on hold.

Another promising technique was a so-called overlay tile repair procedure in which damaged tiles would be covered with a thin, flexible sheet of heat-resistant carbon silicon-carbide. The sheet would be mounted atop a gasket and attached with fasteners similar to drywall bolts that would be screwed into surrounding tile.

Both CIPAA and the overlay technique are expected to be tested on a future shuttle flight. Robinson and Noguchi instead will test a tile repair technique known as "emittance wash" in Discovery's cargo bay.

Using a demonstration kit with deliberately damaged tiles, the spacewalkers will paint exposed surfaces with a material that will replace damaged or eroded coating and improve heat rejection.

NASA still has no way to repair the kind of leading edge damage that brought down Columbia, but Robinson and Noguchi will test a rudimentary technique in which a heat-resistant material known as NOAX will be smoothed over small cracks in RCC material.

NOAX, which stands for non-oxide adhesive experimental, will be squirted from a caulk gun-like device and then smoothed out with trowels.

A third repair procedure, aimed at fixing small holes in RCC panels, will be tested next week inside Discovery's crew cabin. It requires a flexible carbon silicon-carbide patch called a "plug" that would be inserted into a hole and held in place from behind by expansion bolts.

Between 20 and 30 different plugs, each with slightly different geometries, would be needed in a real repair kit to ensure a good fit virtually anywhere in the curving leading edge.

NASA Administrator Michael Griffin mounted a spirited defense of the shuttle program and the beleaguered external tank project today, saying virtually all of NASA's post-Columbia improvements to the huge tank worked as expected during Discovery's launching Tuesday. While at least three relatively large pieces of foam debris fell away from the tank during ascent, Griffin said he's confident engineers will develop a fix and that flights will resume sooner rather than later.

"Discovery is the cleanest bird we've seen," Griffin said, referring to the overall number of dents, dings and chips seen in the shuttle's heat-shield tiles. "Six times cleaner than the average across 113 (previous) missions. So the fact that we have three or four things that we still need to clean up from our first test flight in two-and-a-half years - I'm not a spin kind of a guy, you all know that - but in the world of engineering, we did pretty well."

Griffin fielded questions from a small group of reporters, starting off by joking about "late-breaking" news that had not made it into news reports.

"This is the cleanest flight practically that we've ever seen," he said. "The flight control team is executing above flawlessly, they haven't flown a shuttle for two-and-a-half years and the control team is doing better than perfect. The astronauts on orbit are executing better than perfect. (Commander) Eileen Collins performed a minimum-propellant rendezvous yesterday, did a perfect docking, all the equipment is in great shape. The orbiter has had, I think, one minor flaw, maybe a tape recorder (problem).

"Almost everything we did on the external tank to get it ready for flight has worked," Griffin said. "We expected and we have seen a dramatic reduction in the amount of debris that was generated. Looking at the photography we've seen so far on Discovery, we've had about 25 dings as opposed to a mission average of about 145 (in earlier flights). So the engineering work we did on the external tank has reduced scarring on the orbiter by a factor of about six. I thought all that was really kind of nice and you should know it."

During Discovery's launching, a large piece of foam peeled away from one of two so-called protuberance air load - PAL - ramps on the side of the tank. The ramps are in place to smooth the flow of air over pressurization lines and a cable tray when the spacecraft goes supersonic.

Two other larger-than-allowable chunks of foam separated from the area of the tank near the struts that support the nose of the shuttle. One of those divots came in an area where post-Columbia improvements had been implemented. The other came from an area that, like the PAL ramps, was unchanged.

The PAL ramp foam is sprayed in place by hand and engineers have long recognized it represents a potential source of debris. In the wake of Columbia's demise, managers considered ordering changes but in the end, decided it was safe to fly them as is. That rationale was based in part on flight history - only two early missions experienced known instances of PAL ramp foam loss - and because of non-destructive testing capable of finding the internal voids thought to contribut to shedding.

But given orbital lighting constraints, NASA only has post-launch photographic documentation of less than half the tanks launched to date and additional PAL ramp foam shedding might have gone undetected.

Richard Covey, co-chairman of a panel that assessed NASA's implementation of an accident board's return-to-flight recommendations, said early on, NASA identified the PAL ramps as "a potential source of large debris."

"They early on identified that and we, during the course of our subsequent fact finding, followed the agency through the extensive process they went through of analyzing the causes of foam loss, studying the flight histories that they had and looking at the non-destructive inspection techniques they could use to eventually develop rationale that said that changes to the PAL ramps were not required in order to eliminate critical debris.

"Now, we accepted that rationale and so when we talked in our report about the fact that critical debris itself had not been eliminated and we would expect debris to be liberated from the external tank, that led to me not being surprised that foam would come off the external tank. We fully expected that foam would. ... That being said, I am surprised as are other members of the task group that foam from the PAL ramp separated during this launch. We were surprised at both the fact that it was PAL ramp foam and the size of it."

He said NASA now will have to re-examine the physics behind foam loss and perhaps re-think the theory that voids in the foam are primarily responsible for separation in flight.

"The idea that voids were primarily the cause of separation, that you can see these voids in non-destructive inspection, has all been challenged by what happened on the launch," Covey said.

Griffin agreed NASA "missed" the threat posed by the PAL ramp foam, "but to extrapolate from that fact and say that we can't fix it, I think is just a bridge too far. I think we're going to fix it, I think we're going to fix it in short order, we're going to get back flying. All we ever said the other day was that we are not going to fly again until we fix it. And I think that's the right thing to do. But we don't expect this to be a long, drawn-out affair, to be honest with you. If that changes, we'll tell you, but that's what we're looking at now."

The PAL ramp issue is only part of NASA's problem. Foam also broke away from two other areas and "we clearly need to fix those," Griffin said.

"Now if we compare that performance to prior performances of the external tank, I don't need to tell you that the difference is huge," he said, coming back to his central point. "And that's what I was alluding to earlier. Almost everything we did to improve the external tank worked. We said at the start this was a test flight, we said that ... without putting this machine into flight, we said we would not be able to evaluate how well we'd done. Now we have some real flight data and we can go figure out what we need to do next."

NASA had hoped to launch the shuttle Atlantis by around Sept. 9 on another mission to resupply the international space station. Because of delays getting Discovery off that ground, Atlantis's launch target was expected to slip several days. Asked about the impact of a much longer delay, Griffin said "you've gone into speculation by saying we won't be able to fly the shuttle until early next year. At this point, we don't know that, we're not conceding that.

"We're putting together a NASA tiger team to look at foam remedies with emphasis on those that have not previously been considered or what we need to do to address the PAL ramp and the couple of other areas where we need to do better."

Asked how NASA could get another launch off this year with all the testing that now must be done, Griffin said "by being smart and working hard. If we can do those and are successful, then we'll capture one of those flight opportunities and if not, it will move. But we don't start out by assuming that we can't succeed."

Finally, Griffin was asked if the media had over-reacted to the foam problems experienced during Discovery's launch.

"I started out with what I thought was at least a little bit of humor, pointing out all the things that are going well on this mission precisely because I believe folks really have over reacted just a bit," he said. "There's no question that NASA's goal was to eliminate all significant foam shedding, debris shedding of any kind, from the tank. There is no question that we always said that while that was a goal, that perfection would be unattainable. So we had a size limit that we wanted to be below.

"There's no question that in maybe four places on the tank, pieces of foam bigger than what we wanted to see came off. So we weren't perfect. We said this was a test flight, we said that we because of the physics involved, the nature of the problem, we could not test this tank on the ground in a wind tunnel or in any other kind of facility, we had to put it back into flight to see how well we have done. So we did that.

"But this was a test flight," Griffin said. "It now has provided data that we can use going forward. The bad news is there were three or four things we didn't get. The good news is we hugely reduced, by a factor of six or more, we hugely reduced any damage to the orbiter through the engineering measures we took to improve the tank. We specifically said the return to flight test sequence was two test flights. I love it when stuff goes well and I know you guys do, too. We plan for the worst and we hope for the best and that's how we conduct business."

The shuttle Discovery's crew was surprised and disappointed to learn about foam insulation falling off their ship's external tank during launch. Commander Eileen Collins said today the shuttle program should remain grounded until the problem is fixed, but she said talk of retiring the winged spaceplanes is premature.

"This is something that has to be fixed," Collins told CBS Radio. "I don't think we should fly again unless we do something to prevent it from happening again. But I'd also like to point out, we're in the space shuttle Discovery right now, which is operating fantastically.

"I was expecting a lot more malfunctions or incidents with the equipment on Discovery because it's been so long since we've last flown. But it has done well. So I'm very confident. The shuttle should be retired eventually, but we've got more years in them and I think we need to get this problem licked with the external tank and keep working on it. I'm not ready to give up yet."

During launch Tuesday, a large 0.9-pound piece of foam debris flew off an aerodynamic "ramp" designed to smooth the airflow over a cable tray and two pressurization lines. While the foam did not hit the shuttle, it showed NASA had failed to solve the problem that doomed Columbia two and a half years ago.

As it turns out, the so-called PAL ramp was not part of NASA's post-Columbia redesign effort. Andrew Thomas, joining Collins on Discovery's flight deck, said engineers need to re-examine that aspect of the tank's construction and come up with a fix.

"I don't think it's time to retire the shuttle based on this alone," he said. "But I think if you look at where the agency wants to take the human spaceflight program, I think we need to continue the development of the space station, which requires the shuttle, and then after that I think then you should address the question of what to do with the shuttle. I think retirement is a viable option at that point. But I don't think it would be yet. It think the important thing now is to do the engineering that's necessary to fix this problem and make sure that it doesn't happen again."

Collins said the crew was "very surprised" when flight controllers informed them about the foam loss. "I did not expect any large pieces of foam to fall off the external tank. We thought we had that problem licked."

"Having thought about it for a while, in the end I'm disappointed this has happened, but it's something I believe we can fix," she said. "We didn't do a while lot of engineering work to that particular area of the tank so there is potential there to fix that and keep the shuttle flying." Said Thomas: "To all of us on the crew, it was a great sense of disappointment when we heard about that. It wasn't a concern to us because we felt somehow that entry might be under threat, it was a disappointment because we know so many good people who worked so hard on that problem to make sure the tank wouldn't liberate foam and here it happened on the first return-to-flight mission. That's a huge engineering disappointment. It's also a disappointment because we know it will now be necessary to keep the shuttles on the ground for a while longer while this problem gets the appropriate attention that it will deserve."

Asked if he viewed the foam incident as a close call personally, Thomas said "it's probably a bit dramatic to say that we dodged a bullet, although there's clearly some power in that metaphor."

"I do think it's important that we as an agency go back and look at this technically and try to understand what happened and understand why this particular area was not examined originally when the whole question of foam debris came up, as part of the post-flight analysis process, to make sure that this problem can be properly fixed."

The astronauts earlier today used the international space station's robot arm to pull a large Italian-built cargo module out of DiscoveryŐs cargo bay and then to attach it to a port on the station's Unity module. The cargo module will be unloaded over the next few days and repacked with no-longer-needed equipment and trash for return to Earth aboard Discovery.

In the meantime, shuttle pilot James Kelly and Charles Camarda positioned the shuttle's robot arm and a 50-foot-long sensor boom for so-called "focused inspections" of areas that suffered apparent damage during launch.

Nine target sites were identified by image analysts as areas of interest requiring additional examination, including a chipped heat-shield tile on the edge of a nose landing gear door, minor dings around an aft fuel tank attachment fitting and scuffs seen on a few wing leading edge panels.

"We do know that we have imagery that shows that that large piece of foam did not impact the orbiter," Collins said. "But we do have some other damage that is not significant. Remember, every shuttle flight does have a little bit of damage, there's no way to stop everything, the smallest pieces of foam from falling off the tank. So we do have areas we are going to go look at.

"The ground, Houston mission control, knows the areas from pictures taken from the space station yesterday, we know where to go look. So we're going to take the lasers and the camera, we'll be able to see ... the areas of potential damage and the depth of them. Again, we don't think any of them are significant but it's going to be good to go look at it. And it's also a good test of the system we put together."

Thomas said images of the shuttle taken from the station show Discovery "looks remarkably clean in terms of any damage to tiles. ... We were really quite impressed with the integrity of the orbiter."

With future shuttle flights on hold, NASA managers are considering the possibility of extending Discovery's mission by one day to let the astronauts do as much as possible to help out the space station's two-man crew.

"We're working on a major resupply here, we're bringing them water, logistics, we're taking home many of the things they don't need any more to give them more space to work," Collins said. "I think we'll be able to continue the space station in the configuration they're in, although it's not desired. We do need to get back to three or more crew members so we can do what we need to do on the space station for exploration."

But that will require additional shuttle flights and for now, future station visits are on hold.

The shuttle Discovery's crew might have dodged a bullet when a piece of foam debris broke away from an aerodynamic ramp on the side of the ship's external fuel tank during launch Tuesday. Had the foam broken away earlier, when the shuttle was deeper in Earth's atmosphere, the chunk could have hit the orbiter with potentially catastrophic results, engineers said today.

James Hallock, a member of the Columbia Accident Investigation Board, told CBS News today he agreed with NASA's decision to put shuttle flights on hold while engineers figure out what caused the external tank to shed foam and what might be needed to fix it.

"I personally think they made the right decision because obviously, they now can see they are getting these large pieces of foam coming off," Hallock said. "The small ones they'll probably never cure, but the large pieces are obviously the ones you really worry about."

Columbia was brought down by a piece of foam the size of a small suitcase that weighed an estimated 1.67 pounds. At the moment it broke free 81.7 seconds after launch, the foam and the shuttle were traveling at roughly 1,568 mph. A mere 0.161 seconds later, the low-density foam had slowed to about 1,022 mph. That's when Columbia ran into it, at a relative velocity of some 545 mph.

It was the foam's remarkably rapid deceleration in the airstream that gave it the high relative velocity and that was dependent on the thickness of the atmosphere at the time of separation.

During Discovery's climb to space, a few seconds after the shuttle's twin boosters separated, a large piece of foam measuring a yard across and weighing 0.9 pounds broke away from a so-called protuberance air load - PAL - ramp on the side of the external fuel tank. The ramp is there to smooth the airflow over a cable tray and two pressurization lines as the tank goes supersonic.

The debris tumbled away in the thin air below the shuttle's right wing and quickly disappeared from view. It did not hit the shuttle.

"The piece in question here was pretty close to the same size to the one that brought down Columbia," Hallock said. "The good news at this point is it occurred at such high altitude that it really didn't slow down much, you could see it on the camera, there weren't enough molecules to really slow it down so if that did hit something, it probably wouldn't do too bad.

"But if it had fallen off down around the 81 seconds that we had at Columbia, or anywhere around that time, there would be enough air molecules that that piece of foam would slow down rapidly, which would then make a very large difference in speed between it and whatever it hit."

Likewise, had the foam that doomed Columbia come off "30 seconds earlier or later, it may not have even hit the shuttle itself," Hallock said. "You need to be low enough into the atmosphere that there's enough molecules to slow it down enough to cause a problem. In Columbia, it slowed down so the difference was like 500 miles an hour. If you take a basketball, because that was what its size was, and throw it at 500 mph, you do get a sense that could do some damage. In this case, it did occur at higher altitudes, but it still says the problem is there."

John Shannon, a senior manager at the Johnson Space Center in Houston, said it didn't matter how close the foam that fell off Discovery's tank came to hitting the orbiter because engineers don't yet understand the failure mechanism. As such, it represents a potentially catastrophic threat.

"The systems engineering and integration guys are off and running what they call a debris transport analysis and they're trying to understand anywhere the PAL ramp could come off, or even at a specific point at any time during the ascent, what would that mean?" Shannon explained. "It's bad. I'll tell you that right now, it's bad. You could hit the orbiter and so that's why you've got to go fix it."

Depending on where - and equally important, when - the debris came off, "there's a large cone (of trajectories) that comes out that, depending on different times of release, because you've got different aerodynamics going around the vehicle as you go through different Mach numbers, or a different place on that PAL ramp, it could go in many different areas."

He said NASA currently does not have anything on the drawing board to replace the tank's PAL ramps.

"Originally what we thought is, let's just take the PAL ramp off because it's a big piece of manually poured foam, let's get rid of it, you don't need it," Shannon said. "And we ran some tests in the wind tunnel and the aero guys did a lot of computational fluid dynamics stuff and they said you can't do it. It's protecting these cable trays and those press (pressurization) lines back there as you go supersonic. So you've got to have something that keeps the air loads off of that area."

Commander Eileen Collins guided the space shuttle Discovery to a picture-perfect docking with the international space station today, a major milestone in a mission now overshadowed by a crisis of confidence in NASA after the grounding of the shuttle fleet Wednesday.

Docking occurred at 7:18 a.m. as the two spacecraft sailed 220 miles above the south Pacific Ocean. Working from Discovery's aft flight deck, Collins pulled off a textbook docking to cap a flawless two-day orbital chase.

"We have contact and capture," pilot James Kelly radioed as the shuttle's docking port gently contacted its counterpart on the front of the U.S. laboratory module, Destiny. After damping out post-docking oscillations, powerful latches locked the two spacecraft together.

One orbit later, at 8:50 a.m., a final hatch between Discovery and the station was cranked open and the shuttle crew floated into the Destiny module, welcomed aboard by station commander Sergei Krikalev and flight engineer John Phillips. Following naval tradition, Phillips rang a ship's bell to signal the shuttle crew's arrival.

The normally roomy laboratory module appeared cramped and crowded as Collins, Kelly, flight engineer Stephen Robinson, Andrew Thomas, Wendy Lawrence, Charles Camarda and Japanese astronaut Soichi Noguchi floated about, sharing smiles, handshakes and bear hugs with Krikalev and Phillips.

Krikalev passed out bread and salt, a traditional Russian gift for visitors, before giving his guests a safety briefing to familiarize them with evacuation routes and emergency procedures. Later today, Kelly and Lawrence, assisted by Phillips, will use the station's robot arm to pluck Discovery's orbiter boom sensor system - OBSS - out of the shuttle's cargo bay so it can be handed off to the shuttle's robot arm.

The OBSS was used Wednesday to inspect Discovery's nose cap and wing leading edge panels for signs of impact damage. The astronauts may use it for additional inspections later in the mission. But because of clearance issues with the shuttle now docked to the station, the lab's Canadian-built robot arm was required to remove it from the shuttle's cargo bay.

"We're on the space station right now, it looks absolutely fantastic," Collins radioed. "We're looking forward to several days of a lot of hard work getting the station in the best shape we can get it in."

Today's docking was the first since a linkup by the shuttle Endeavour in November 2002, the flight before Columbia blasted off on its final voyage Jan. 16, 2003.

Columbia was destroyed during re-entry 16 days later, victim of a launch-day external tank foam strike that blasted a hole in its left wing. NASA spent two-and-a-half years recovering from the disaster, focusing on fixing the external tank to make sure large pieces of foam insulation would not break off during launch.

But during Discovery's launching Tuesday, large pieces of potentially catastrophic foam broke away, graphically proving NASA and its contractor, Lockheed Martin, had failed to meet the No. 1 recommendation of the Columbia Accident Investigation Board.

It was a devastating blow to NASA, a major setback in the agency's plans to resume space station assembly. With the shuttle fleet now grounded indefinitely, some agency insiders worry the program is in danger of termination.

But NASA Administrator Michael Griffin, in an interview with CBS News earlier today, said shuttle flights will resume when the agency fixes the foam problem, however long that might take.

"Obviously, we didn't do as well as we needed to and we're not going to fly again until we're even more certain we have this problem fixed," he said. "We intend at this point to fix the foam problem and fly again as soon as we can when we are certain we've got it this time."

But he left little doubt NASA still plans to retire the shuttle fleet by 2010 as the agency transitions to a replacement rocket system intended to ferry astronauts to and from low-Earth orbit and eventually on to the moon.

"Our planning horizon at this point, per the president's direction, is to retire in 2010 at which time we believe the international space station will be complete and it will be time for the us to move on to a new human-rated system and new voyages of discovery to the moon."

Despite the concern about the shuttle program's immediate future, Discovery's crew has pressed ahead with a near flawless mission, executing a trouble-free rendezvous and docking early today.

During final approach, with the shuttle flying nose forward a few hundred feet directly below the space station, Collins manually pitched the nose up in a spectacular 360-degree maneuver that allowed Krikalev and Phillips to photograph Discovery's underbelly.

About a minute after launch, part of a heat-shield tile at the edge of a nose landing gear door cracked and ripped away, exposing the white interior of the insulator. Krikalev and Phillips, using 400- and 800-millimeter lenses, photographed the underside of the shuttle to give flight controllers a chance to evaluate the chipped tile and whatever other damage might be present.

While it will take photo analysts time to fully evaluate the pictures, "neither of us saw anything alarming," Phillips radioed mission control.

"They showed us some of the shots of the orbiter and from what we could tell, it looks like it's in great shape," Collins radioed later from the station.

Asked if he was confident Discovery's thermal protection system is healthy enough for a normal re-entry Aug. 7, Griffin told CBS News "we're not 100 percent sure at this point, no, but we have looked at the data from the wing leading edge impact sensors and we have not seen any evidence of an actual strike."

"Our video footage, it's very clear that that large piece of foam missed Discovery," he said. "We continue to examine the data but at this point, we think Discovery is a clean bird."

Revision C of the NASA television schedule is posted below. Here is the timeline for today and tomorrow:

REV...EVENT..........................MET in DD/HH:MM...EDT........GMT

THURSDAY, JULY 28/Flight Day 3

32...HATCH OPENING AND HANDSHAKE............01/22:40...09:19 AM...13:19
32...OBSS GRAPPLE BY SSRMS..................01/23:15...09:54 AM...13:54
32...DISCOVERY/ISS TRANSFERS BEGIN..........01/23:30...10:09 AM...14:09
32...MISSION STATUS BRIEFING..........JSC...01/23:51...10:30 AM...14:30
33...SSRMS MOVES OBSS TO HANDOFF POSITION...02/00:10...10:49 AM...14:49
33...SSRMS HANDOFF OF OBSS to RMS...........02/01:05...11:44 AM...15:44
33.*.SHUTTLE VTR PLAYBACK OF DOCKING........02/01:10...11:49 AM...15:49
35...VIDEO FILE........................HQ...02/03:21...02:00 PM...18:00
35...GOES-N Pre-Launch News Briefing..KSC...02/04:21...03:00 PM...19:00
36...DISCOVERY/ISS CREW SLEEP BEGINS........02/05:00...03:39 PM...19:39
36...FLIGHT DAY 3 HIGHLIGHTS..........JSC...02/05:21...04:00 PM...20:00
37...POST-MMT BRIEFING................JSC...02/07:21...06:00 PM...22:00
40...POST-MMT BRIEFING REPLAY.........JSC...02/11:21...10:00 PM...02:00
41...DISCOVERY CREW WAKE UP (begins FD 4)...02/13:00...11:39 PM...03:39

FRIDAY, JULY 29/Flight Day 4

41...ISS CREW WAKE UP.......................02/13:30...12:09 AM...04:09
42...SSRMS GRAPPLE OF MPLM..................02/15:05...01:44 AM...05:44
43...FLIGHT DIRECTOR UPDATE...........JSC...02/15:21...02:00 AM...06:00
43...MPLM UNBERTH FROM DISCOVERY............02/15:35...02:14 AM...06:14
43...MPLM INSTALLATION ON UNITY BEGINS......02/16:30...03:09 AM...07:09
44...DISCOVERY/ISS TRANSFERS RESUME.........02/18:00...04:39 AM...08:39
45...SSRMS UNGRAPPLE OF MPLM................02/18:45...05:24 AM...09:24
45...SSRMS WALKOFF TO MBS BEGINS............02/19:00...05:39 AM...09:39
45...AP RADIO/NPR/CBS RADIO.........A/G-2...02/19:40...06:19 AM...10:19
46...OBSS SURVEY OF DISCOVERY TPS...........02/20:30...07:09 AM...11:09
46...MPLM ACTIVATION........................02/22:06...08:45 AM...12:45
48...MISSION STATUS BRIEFING................02/23:21...10:00 AM...14:00
48...EVA #1 PROCEDURE REVIEW................02/23:40...10:19 AM...14:19
48...MPLM INGRESS...........................03/00:10...10:49 AM...14:49
49...EVA PRE-BREATHE/SAFER CHECKOUT.........03/01:40...12:19 PM...16:19
50...SHUTTLE CABIN DEPRESS TO 10.2 PSI......03/02:05...12:44 PM...16:44
50...VIDEO FILE........................HQ...02/02:21...01:00 PM...17:00
52...DISCOVERY/ISS CREW SLEEP BEGINS........03/05:00...03:39 PM...19:39
52...FLIGHT DAY 4 HIGHLIGHTS..........JSC...03/05:21...04:00 PM...20:00
53...POST-MMT BRIEFING................JSC...03/07:21...06:00 PM...22:00
...*.GOES-N SATELLITE LAUNCH COVERAGE..................06:15 PM...22:15
56...POST-MMT BRIEFING REPLAY.........JSC...03/11:21...10:00 PM...02:00
57...DISCOVERY CREW WAKE UP (begins FD 5)...03/13:00...11:39 PM...03:39

In a major setback for NASA, senior managers today grounded the shuttle fleet, saying no more missions will be launched until engineers figure out why large, potentially catastrophic pieces of foam insulation broke away from the shuttle Discovery's external fuel tank during launch Tuesday.

"Until we're ready, we won't go fly again," said shuttle program manager Bill Parsons. "I don't know when that might be, I'll state that up front. We're just in the beginning of this process of understanding. This is a test flight. This is a flight we had to go off and try to get as much information as we could and see if the changes that we had made to the tank (in the wake of the Columbia disaster) were sufficient. Obviouysly, we have some more work to do.

"I don't know if that's a month, I don't know if that's three months. We've got a lot of work in front of us to go figure that out."

The unexpected problem, coming on the heels of what had appeared to be a triumphant return to space for Discovery and its crew, put a planned September flight by the shuttle Atlantis in clear doubt and raised the possibility of an extended hiatus that would put additional strain on the international space station and along with it, the nation's confidence in NASA.

The largest pieces of foam did not strike Discovery and engineers believe the ship's seven-member crew will be able to safely return to Earth Aug. 7 after a long-awaited mission to deliver supplies and equipment to the space station. If all goes well, commander Eileen Collins will guide Discovery to a linkup with the unfinished $30 billion outpost early Thursday.

But the foam problem took the luster off an otherwise successful mission, and for good reason. Deputy shuttle program manager Wayne Hale said if the largest piece of debris had fallen off earlier, while Discovery was still in the lower reaches of the atmosphere, "we think this would have been really bad. So it's not acceptable, OK?"

Discovery blasted off from the Kennedy Space Center at 10:39 a.m. Tuesday, two-and-a-half years after Columbia disintegrated during re-entry. The disaster was caused by damage to the ship's left wing that resulted from the impact of external tank foam insulation during launch 16 days earlier.

NASA's top priority in the wake of the accident - and the No. 1 recommendation of the Columbia Accident Investigation Board - was to redesign the way the foam is applied to the tank to minimize so-called foam shedding. Engineers were confident Discovery's tank would shed nothing larger than a marshmallow, repeatedly stating their belief that the new tanks were the safest ever built.

But during Discovery's climb to space, a few seconds after the ship's solid-fuel boosters separated two minutes and five seconds into flight, a relatively large, pillow-size piece of foam ripped away from an aerodynamic ramp intended to prevent turbulent airflow around cable trays and pressurization lines.

A new camera mounted on the external tank to monitor the tank's performance showed the debris lifting off and quickly tumbling away in the thin, supersonic airflow. Earlier, part of a heat shield tile near the nose landing gear door cracked off and fell away. That incident is still under study, but it appears relatively minor.

The tank debris passed under the shuttle's right wing and did not appear to come close to actually striking the orbiter. But the size of the yard-long piece of debris was a shock.

In addition, camera footage shot by Discovery's crew after the tank was jettisoned showed two other relatively large divots in the foam near the area where two struts held the shuttle's nose to the tank.

"We saw a couple of divots in areas that frankly are not satisfactory to us," Hale said. "We spent a lot of time working on what we call the liquid hydrogen interface ring, the LH2 interface, to make sure we would not loose pieces of foam and in fact, we see about a six- or seven-inch piece of foam that came off right there at that interface, which is unsatisfactory and we've got to work on that."

In the aftermath of Columbia, engineers discussed the possibility of eliminating the so-called protuberance air load - PAL - ramp, a long, hand-applied foam dam used to improve the tank's aerodynamics. Here is a description from NASA's STS-114 press kit:

The top-to-bottom assessment of the External TankŐs Thermal Protection System led to re-evaluation of other areas in which foam insulation is prone to loss. One of those was the existing design of the Protuberance Air Load (PAL) ramps designed to prevent unsteady air flow underneath the tankŐs cable trays and pressurization lines.

Two PAL ramps exist on each External Tank. One is near the top of the liquid oxygen tank, close to the nose cone, while the other is below the intertank, near the top of the liquid hydrogen tank. They consist of thick, manually sprayed layers of foam, which could provide a source of debris.

The forward 10 feet of the LH2 PAL ramp was removed and replaced to allow access to the LH2 intertank flange area to implement foam closeout improvements. An extensive evaluation that included careful dissections to collect data on existing PAL ramps determined location, size and frequency of any voids which are known to promote foam separation due to expansion of trapped air or gases during ascent.

Over the course of six months, seven enhancement sprays were performed on high-fidelity mockups to develop a new spray process, which proved through non-destructive evaluation that not only were fewer voids formed, but those that did form fell into the acceptable range for flight safety.

After comparing all of the data gathered and analyses performed, it was decided that the first two tanks PAL ramps are safe to fly using the current application process with detailed scrutiny of evaluation applied. An enhanced spray process is in work for future tanks, as well as continued work in developing redesign options including elimination of the ramps; reducing the rampsŐ sizes by two thirds; or building a trailing edge ŇfenceÓ on the back side of the cable tray, which would act like a nozzle throat and prevent unsteady flow in that area.

"Our expectations when we went into this flight that we wouldn't have an unexpected debris event," Parsons said. "The program throughout the last two and a half years, we reviewed many areas on this tank and ways we can improve that. The PAL ramp was one of those areas we reviewed, whether we should make modifications to that or whether we had enough technical data that we felt it was OK to fly as is.

"There were some concerns about this ramp, that is a large piece of foam. So the community was very diligent about looking at this. We did realize that eventually one day we needed to put together a program to remove this PAL ramp if at all possible. But at the time, we didn't have enough data where we could technically do that and be safe.

"In the end, we had enough data to show we had had very few problems with the PAL ramp and we decided it was safe to fly as is. Obviously, with the event that we've had, we were wrong. ... We did not expect the PAL ramp to have the issue that it had, but it did."

Parsons said it likely will take engineers a fair amount of time to understand what caused the foam to separate and to come up with a fix.

"This is a test flight," he said. "We went off to take a good look at this vehicle and see how it would perform. And unfortunately, it didn't perform as well as we'd like it to perform. I can't say what the imapct of this is until we get some good evaluation of this and try to understand what caused this to happen.

"We are going to go through and do a thorough evaluation and then we'll determine when it's safe to fly. Obviously, we can't fly with PAL ramps the size of this ramp coming off the way it did. I mean, obviously, we have to go fix this."

While the shuttle fleet will remain grounded while engineers troubleshoot the latest problem, Parsons said the nation should not lose confidence in NASA or the space shuttle.

"We think this vehicle is safe," Parsons said. "We think we can fly this vehicle and we think we can make this vehicle safe for the next flight. We feel very, very confident in our ability to make this vehicle safe."

Discovery's mission, meanwhile, continues. Here is the crew's schedule for Thursday:

   
WEDNESDAY, JULY 27/Flight Day 2
   
25...DISCOVERY CREW WAKE UP.................01/13:00...11:39 PM...03:39

THURSDAY, JULY 28/Flight Day 3

27...RENDEZVOUS OPERATIONS BEGIN............01/15:00...01:39 AM...05:39
27...PLAYBACK OF Ku/OBSS CLEARANCE VIDEO....01/15:15...01:54 AM...05:54
27...RENDEZVOUS TOOLS CHECKOUT CONTINUES....01/15:30...02:09 AM...06:09
27...PLAYBACK OF CREW TANK VIDEO............01/15:35...02:14 AM...06:14
27...FLIGHT DIRECTOR UPDATE...........JSC...01/15:51...02:30 AM...06:30
29...TI RENDEZVOUWS BURN....................01/18:01...04:40 AM...08:40
30...DISCOVERY RPM MANUEVER.................01/19:29...06:08 AM...10:08
30...DISCOVERY/ISS DOCKING..................01/20:39...07:18 AM...11:18
31...ISS CREW RPM PHOTOGRAPHY PLAYBACK......01/20:55...07:34 AM...11:34
32...DISCOVERY/ISS CREW HATCH OPENING ......01/22:40...09:19 AM...13:19
32...OBSS GRAPPLE BY SSRMS..................01/23:15...09:54 AM...13:54
32...DISCOVERY/ISS TRANSFERS BEGIN..........01/23:30...10:09 AM...14:09
32...MISSION STATUS BRIEFING..........JSC...01/23:51...10:30 AM...14:30
33...SSRMS MOVES OBSS TO HANDOFF POSITION...02/00:10...10:49 AM...14:49
33...SSRMS HANDOFF OF OBSS to RMS...........02/01:05...11:44 AM...15:44
33...VTR PLAYBACK OF DISCOVERY/ISS DOCKING..02/01:20...11:59 AM...15:59
35...VIDEO FILE........................HQ...02/03:21...02:00 PM...18:00
35...Delta 4/GOES-N News Conference...KSC...02/04:21...03:00 PM...19:00
36...DISCOVERY/ISS CREW SLEEP BEGINS........02/05:00...03:39 PM...19:39
36...FLIGHT DAY 3 HIGHLIGHTS..........JSC...02/05:21...04:00 PM...20:00
37...POST-MMT BRIEFING................JSC...02/07:21...06:00 PM...22:00
40...POST-MMT BRIEFING REPLAY.........JSC...02/11:21...10:00 PM...02:00
41...DISCOVERY CREW WAKE UP (begins FD 4)...02/13:00...11:39 PM...03:39
41...ISS CREW WAKE UP.......................02/13:30...12:09 AM...04:09

Debris seen falling away from the shuttle Discovery's external fuel tank during launch Tuesday was a large piece of foam insulation from a so-called "ramp" used to prevent turbulent airflow around cable trays and pressurization lines, NASA officials said today. The loss of foam debris led to the shuttle Columbia's destruction and while the debris that fell from Discovery's tank did not strike the orbiter, the incident is a potentially serious setback for the shuttle program.

Seconds after Discovery's solid-fuel boosters separated two minutes and five seconds after launch Tuesday, a relatively large piece of debris could be seen peeling away from the tank and quickly tumbling away in the supersonic airflow. Earlier, part of a heat shield tile near the nose landing gear door cracked off and fell away, but that incident appears relatively minor.

The tank debris passed under the shuttle's right wing and did not appear to come close to actually striking the orbiter. But sources told CBS News earlier today the size of the debris was a shock. NASA's return to flight in the wake of the Columbia disaster has been driven by an accident board recommendation to minimize, if not eliminate, foam debris shedding. The loss of a relatively large piece of foam from Discovery's tank indicates the agency failed to meet its No. 1 post-disaster objective.

NASA hopes to launch the shuttle Atlantis in September, but the timing of that flight and subsequent missions is in doubt pending the outcome of troubleshooting to figure out what caused the foam to fall away from Discovery's tank. And what might be needed to fix the problem.

NASA plans a news conference within the hour. This page will be updated as soon as possible thereafter.

The Discovery astronauts have completed their laser scans of the shuttle's right wing leading edge and reinforced carbon carbon nose cap. The crew currently is scanning the left wing, wrapping up a detailed survey of the areas that experience the most extreme heating during re-entry.

So far, no obvious signs of any significant damage have been seen, but engineers will carry out a detailed assessment before reaching any conclusions. Wayne Hale, chairman of NASA's mission management team, will brief reporters on the progress of the mission, and efforts to inspect the shuttle for potential impact damage, at 6 p.m.

Image analysts are continuing to assess the potential significance of at least three debris events spotted during Discovery's launch Tuesday.

The shuttle's external tank suffered a bird hit seconds after liftoff and the apparent loss of a relatively large piece of foam or some other type of debris just after the ship's solid-fuel boosters were jettisoned two minutes and five seconds into flight.

A minute or so earlier, imagery from a new camera mounted on the external tank showed a piece of a black heat shield tile on the edge of a nose landing gear door cracking off and flying away. That ding appears relatively minor, at least to the untrained eye.

But the external tank debris could have implications for downstream flights it it turns out to involve foam shedding. Mission operations manager John Shannon said Tuesday it would take engineers several days to complete their assessments of the shuttle's condition and that it was premature to speculate about whether any one of the events might, or might not, turn out to be significant.

For readers interested in looking ahead, here is a preliminary timeline for Discovery's docking Thursday with the international space station (times in EDT and mission elapsed time):

EDT........DD...HH...MM...EVENT

July 28

04:40 AM...01...18...01...ISS maneuver start
04:40 AM...01...18...01...TI rendezvous rocket firing
04:45 AM...01...18...06...ISS in the proper orientation
05:16 AM...01...18...37...Sunset
05:39 AM...01...19...00...Range: 10,000 feet
05:47 AM...01...19...08...Range: 5,000 feet
05:51 AM...01...19...12...Sunrise
05:53 AM...01...19...14...Range: 3,000 feet
05:57 AM...01...19...18...MC-4 rendezvous burn
06:01 AM...01...19...22...Range: 1,500 feet
06:03 AM...01...19...24...Rotational pitch maneuver start window open
06:06 AM...01...19...27...Range: 1,000 feet
06:09 AM...01...19...30...KU antenna to low power
06:10 AM...01...19...31...+R bar arrivall; Discovery directly below ISS
06:15 AM...01...19...36...Range: 600 feet
06:17 AM...01...19...38...Russian ground station acquisition of signal
06:17 AM...01...19...38...Start pitch maneuver
06:23 AM...01...19...44...US solar arrays feathered
06:25 AM...01...19...46...End pitch-up maneuver
06:27 AM...01...19...48...RPM full window close
06:28 AM...01...19...49...Initiate pitch up maneuver
06:32 AM...01...19...53...Russian solar arrays feathered
06:32 AM...01...19...53...Last Russian ground station loss of signal
06:39 AM...01...20...00...+V bar arrival; range: 300 feet
06:40 AM...01...20...01...Range: 300 feet
06:47 AM...01...20...08...Range: 250 feet
06:48 AM...01...20...09...Sunset
06:48 AM...01...20...09...Range: 200 feet
06:51 AM...01...20...12...Range: 170 feet
06:53 AM...01...20...14...Range: 150 feet
06:57 AM...01...20...18...Range: 100 feet
07:00 AM...01...20...21...Range: 75 feet
07:04 AM...01...20...25...Range: 50 feet
07:07 AM...01...20...28...Range: 30 feet; start stationkeeping
07:12 AM...01...20...33...End stationkeeping; push to dock
07:17 AM...01...20...38...Range: 10 feet
07:18 AM...01...20...39...DOCKING
07:22 AM...01...20...43...Sunrise

The Discovery astronauts geared up for a detailed inspection of the shuttle's reinforced carbon carbon nose cap and wing leading edge panels today. The post-Columbia safety survey is designed to spot any entry-critical damage to the areas of the shuttle that experience the most extreme heating during the return to Earth.

The astronauts also plan to inspect tiles around the upper part of the shuttle's crew cabin and aft orbital maneuvering system rocket pods, along with checking out the spacesuits that will be used later in the mission during three planned spacewalks at the international space station.

Image analysts on the ground, meanwhile, continued assessing the potential significance of at least three debris events spotted during Discovery's launch Tuesday.

The shuttle's external tank suffered a bird hit seconds after liftoff and the apparent loss of a relatively large piece of foam or some other type of debris just after the ship's solid-fuel boosters were jettisoned two minutes and five seconds into flight.

A minute or so earlier, imagery from a new camera mounted on the external tank showed a piece of a black heat shield tile on the edge of a nose landing gear door cracking off and flying away. Mission operations manager John Shannon said Tuesday it would take engineers several days to complete their assessments of the shuttle's condition and that it was premature to speculate about whether any one of the events might, or might not, turn out to be significant.

Lead flight director Paul Hill, speaking well before launch, said he fully expected to see tile damage during Discovery's mission.

"We will likely have coating loss or chips on the bottom of the vehicle that are well within the capability of the (thermal protection system) to bring us to the ground," he said. "It is the nature of having a glass-covered thermal protection system on the bottom of the orbiter. But a we've demonstrated on more than 100 flights, chipped coating on tile, it still performs like a champ. That material will bring us down to the ground, it's very robust. We understand what types of damage change that equation."

With post-Columbia imagery upgrades, Hill said, "we are now going to have all these fabulous pictures of the bottom of the vehicle. We're going to see a lot of white dings, many or all of which I expect we will have in our flight history as cases we have landed with and were absolutely no impact. But we have talked about this a lot, now we're going to be presented with all of this and there is this concern that folks will over react, that we see little things that clearly are within the capability of the vehicle to bring us to the ground but now that we can see it, we may over react.

"There's been a lot of concern about whether or not we'll over react and that would paralyze us for making all of the right decisions during the flight or maybe jumping the gun and repairing a vehicle when we didn't need to repair the vehicle."

But Hill said based on the team's performance during multiple simulations, "we're not going to over react, we are capable of seeing a lot of damage and crossing out the ones that are clearly within our capability and then focusing on the ones that are either right on the edge and we need more data or we're just not happy with it.

"I think we've demonstrated that the community is capable of hearing that story, assessing the data and making good, technical decisions and not making emotion-driven decisions."

Today's inspection activities are unrelated to the debris events seen during launch. Rather, it is the direct result of a recommendation made by the Columbia Accident Investigation Board to "provide a capability to obtain and downlink high-resolution images of the underside of the orbiter wing leading edge and forward section of both wings' thermal protection system."

Columbia was brought down during re-entry two-and-a-half years ago by a hole in the leading edge of its left wing. The Discovery astronauts plan to spend a large part of their second day in space carefully inspecting the leading edges of both their wings, along with the reinforced carbon carbon nose cap of the shuttle, using a new 50-foot-long boom mounted on the starboard side of the payload bay known as the orbiter boom sensor system, or OBSS.

Engineers on the ground, meanwhile, will assess data from new sensors mounted directly behind the leading edge panels that are capable of registering the force of an impact. The wing leading edge sensor data was downlinked overnight.

Astronaut Andrew Thomas will use the shuttle's 50-foot-long robot arm to pick up the equally long OBSS. Starting around 5:30 a.m., Thomas, assisted by pilot James Kelly and Charles Camarda, will maneuver the boom back and forth, using an OBSS television camera and a laser sensor to inspect the wing leading edges and the nose cap.

"Getting the boom, lifting it up out of the payload bay and positioning it to start the survey is all done essentially manually," Thomas told CBS News in a pre-launch interview. "I'll be doing that. Berthing it will be done manually. But once you've got it in the initial position to start the survey, then we invoke computer control sequences which will drive it automatically. And we kind of need to. This thing is 50 feet long, so a small deflection at the end of the arm would be a huge deflection at the end of the boom and you have the risk of hitting structure. So that parts all automated."

Asked what the survey might show, Thomas said "I don't think we'll see too much."

"You know, we have to do this," he said. "They've done all this work on the tank and you have to confirm that what they've done on the tank is as expected. Any debris that's released from the tank is understood and controlled. And the only way you can do that is by these detailed surveys. This flight and the next flight are the ones that are largely going to do that. So we have to do it.

"You point out a risk, though, that there will be sites that will be false alarms, false positives which will exercise the management structure that's on the ground to look at damage sites, compare them to pre flight and make judgments about their importance or otherwise. That's actually not a bad thing to do, though. Because this is the first time it's being done and we need to exercise those procedures.

"I can see, for example, that we will get a call for some targeted inspections of certain sites. ... And I think that's a good thing to do, too, because it exercises the whole organization chain."

Given the launch day debris events, that seems like a safe bet. Here is an updated timeline of today's events (in mission elapsed time and EDT)

07/27/05
Wed  03:09 AM...00...16...30...Centerline camera installation
Wed  03:39 AM...00...17...00...Ergometer setup
Wed  03:54 AM...00...17...15...Orbiter boom survey system (OBSS) unberth
Wed  04:09 AM...00...17...30...Laptop computer setup completed
Wed  04:09 AM...00...17...30...KU-band antenna deploy
Wed  04;39 AM...00...18...00...OMS pod photo survey
Wed  05:19 AM...00...18...40...Rendezvous tools checkout
Wed  05:24 AM...00...18...45...OBSS: Starboard wing LDRI survey begins
Wed  05:49 AM...00...19...10...SAFER jet backpack checkout
Wed  06:19 AM...00...19...40...Lawrence exercises
Wed  06:19 AM...00...19...40...Spacewalk power tool checkout
Wed  06:49 AM...00...20...10...Crew meals begin
Wed  06:49 AM...00...20...10...OBSS: Nosecap survey begins
Wed  07:49 AM...00...21...10...Airlock prep
Wed  08:29 AM...00...21...50...OBSS: Nosecap survey complete
Wed  08:39 AM...00...22...00...EMU (spacesuit) checkout
Wed  08:54 AM...00...22...15...Docking ring extension
Wed  09:24 AM...00...22...45...OBSS: Port wing LDRI survey begins
Wed  10:24 AM...00...23...45...EMU checkout continues
Wed  10:49 AM...01...00...10...OBSS: Port wing survey complete
Wed  10:49 AM...01...00...10...OBSS berthing
Wed  11:44 AM...01...01...05...SRMS survey
Wed  12:59 PM...01...02...20...LDRI downlink
Wed  01:04 PM...01...02...25...NC-3 rendezvous burn
Wed  03:39 PM...01...05...00...STS crew sleep begins
Wed  11:39 PM...01...13...00...STS crew wakeup
Wed  11:39 PM...01...13...00...ISS crew wakeup

A year ago, Hill said in an earlier interview with CBS News, engineers thought entry critical damage to the leading edge panels "required a penetration of the RCC, not just coating damage or even small damage to the substrate on the outside."

"More recent arc jet testing has us worried that coating damage alone, if it's large enough and if we had internal damage - delamination - between the layers, that the combination of those two could be entry critical."

In the early years of the shuttle program, Hill said, tests indicated the leading edge RCC panels could tolerate penetrations a quarter of an inch across. But that testing was with a clean hole punched in the panel, which is what one would expect with a hypervelocity impact in space. But during launch, impact velocities would be much lower and any resulting penetrations would be more ragged.

Engineers then began wondering if lower-velocity impacts might be entry critical. Subjecting RCC panels to re-entry heating in an arc jet furnace, engineers discovered that as long as a protective coating remained intact, internal delamination would not cause any major problems.

"But if the coating is gone and underneath that coating you're delaminated, then picture the RCC itself from a side view like a cross section. Now you've got this bubble or this void in between layers," Hill explained. "What you've done is, you've significantly reduced the density of this RCC that's exposed to the heat load. So it burns faster.

"So now instead of being this more solid material that's hard to light, kind of like if you take a piece of hard wood like oak and you hold a match to that oak, it won't light typically. But if you shave off some splinters of that oak, you can get them to flash. Damned if that's not what we found in a handful of RCC runs for uncoated RCC."

For the tests, engineers deliberately damaged an RCC panel by pushing on it with a metal cylinder. After confirming the panel developed delamination as a result, "they put that bad boy under the arc jet and it burned like there was no tomorrow. The whole area that covered the delamination burned off like a fuse."

If it is credible that the shuttle could take an impact that has enough energy to cause delamination and loss of coating, "then that does not have to be very big to be catastrophic," Hill said. "From an RCC damage perspective, that looks like a penetration. So now the question is, do we believe that testing? Have we done enough of those tests to be sure that is an entry critical damage form? And then, is it credible for us to take an impact that could cause that kind of damage?"

One OBSS laser sensor, known as the laser dynamic range imager, or LDRI, dwill be used to inspect the wing leading edge panels and the shuttle's nose cap. A second sensor, known as the laser camera system, or LCS, may be used later to focus on a suspect area or to collect additional data.

"If you think about the laser, the way we're recording the data is similar to recording video," Hill said. "Imagine standing on the side of a soccer field watching one of your kids play soccer with a camcorder. And you're panning that camera real fast so you can watch him run down the field. But when you play it back at home, you can't make out hide nor hair because everything is blurred. That's the problem we've got. We've got a translation constraint. If we move too fast, we blur the image, which directly affects the resolution and we can't see the small stuff we're looking for."

Here today's NASA television schedule (rev. A):

REV...EVENT..........................MET in DD/HH:MM...EDT........GMT

WEDNESDAY, JULY 27/Flight Day 2

10...DISCOVERY CREW WAKE UP (FD-2)..........00/14:00...12:39 AM...04:39
12...WB-57 LAUNCH VIDEO REPLAY........KSC...00/16:21...03:00 AM...07:00
12...RMS CHECKOUT...........................00/16:30...03:09 AM...07:09
12...FLIGHT DIRECTOR UPDATE...........JSC...00/16:51...03:30 AM...07:30
12...RMS GRAPPLE & UNBERTH OF OBSS..........00/17:15...03:54 AM...07:54
12...Ku BAND ANTENNA DEPLOYMENT.............00/17:30...04:09 AM...08:09
13...OMS POD PHOTOGRAPHY....................00/18:00...04:39 AM...08:39
13...RENDEZVOUS TOOLS CHECKOUT BEGINS.......00/18:40...05:19 AM...09:19
13...RMS/OBSS SURVEY BEGINS.................00/18:45...05:24 AM...09:24
13...SAFER CHECKOUT.........................00/19:10...05:49 AM...09:49
15...AIRLOCK PREPARATION....................00/21:10...07:49 AM...11:49
15...EMU CHECKOUT...........................00/22:00...08:39 AM...12:39
15...ODS RING EXTENSION.....................00/22:15...08:54 AM...12:54
17...OBSS BERTH.............................01/00:10...10:49 AM...14:49
17...MISSION STATUS BRIEFING..........JSC...01/00:21...11:00 AM...15:00
17...RMS SURVEY OF DISCOVERY CABIN..........01/00:55...11:34 AM...15:34
18...VTR PLAYBACK OF OBSS VIDEO.............01/02:30...01:09 PM...17:09
20...VIDEO FILE........................HQ...01/04:21...03:00 PM...19:00
20...DISCOVERY CREW SLEEP BEGINS............01/05:00...03:39 PM...19:39
20...FLIGHT DAY 2 HIGHLIGHTS..........JSC...01/05:21...04:00 PM...20:00
22...POST-MMT BRIEFING................JSC...01/07:21...06:00 PM...22:00
24...POST-MMT BRIEFING REPLAY.........JSC...01/11:21...10:00 PM...02:00
25...DISCOVERY CREW WAKE UP.................01/13:00...11:39 PM...03:39

By WILLIAM HARWOOD
CBS News

KENNEDY SPACE CENTER, Fla. - The shuttle Discovery, carrying seven astronauts, critical space station supplies and the hopes of a nation, rocketed into orbit today in a nerve-wracking bid to revive America's human space program two-and-a-half years after the Columbia disaster. But at least two pieces of debris seen falling from the shuttle shortly after launch brought back haunting memories of Columbia, prompting intensive analysis to determine what, if any, threat they might pose.

In one incident, part of a heat shield tile next to one of Discovery's nose landing gear doors - an area that experiences extreme heating during re-entry - apparently cracked under stress and broke away about a minute and five seconds after launch. One minute later, a relatively large piece of debris appeared to break away from the shuttle's external fuel tank, passing well beneath the right wing without striking the shuttle.

But a major goal of NASA's post-Columbia recovery effort was to minimize the amount of foam, ice or tile debris that can shake off during launch, possibly hitting and damaging the orbiter. The incidents seen today were captured by a new camera mounted on the external tank.

What might have caused the landing gear door tile to break, or how significant it might turn out to be, was not immediately clear. Likewise, engineers had no immediate explanation for the debris that appeared to originate from the fuel tank.

"This is a test flight and we're seeing areas of the vehicle working in flight regions that we have never seen before," said John Shannon, mission operations manager at the Johnson Space Center. "We've never seen the underside of the orbiter like that."

Closer to the ground, launch pad cameras caught a bird hitting the tip of the external tank a few seconds after blastoff. But it was a relatively low-speed collision and while it was no doubt a significant event for the bird, it caused no obvious damage to the shuttle.

In any case, Shannon said it would take several days to fully analyze all the available imagery to determine what, if anything, might need to be done about the tile damage or any other areas of potential concern.

"I don't have any conclusions because we're in the first half day of a six-day activity," he said. "We're going to get better data."

Commander Eileen Collins was advised of the incidents and thanked flight controllers for their work getting Discovery into space.

"We know the folks back on the planet Earth are just feeling great right now and our thanks to everybody for all of the super work that's been down over the past two-and-a-half years to get us flying again," she radioed after reaching orbit. "That was by far the smoothest ascent ... that we've ever experienced. (You) couldn't ask for a better flight."

Later, addressing the flight control team, Collins paid tribute to "the great ship Columbia and her inspiring crew."

"We miss them and we are continuing their mission," she said. "God bless them tonight and God bless their families. Good night."

With Collins and pilot James "Vegas" Kelly at the controls, Discovery's three hydrogen-fueled main engines shuddered to life at 10:39 a.m., throttling up to full power with a rush of flame and thunder.

Six-point-six seconds later, with the spacecraft straining against massive hold-down posts, the shuttle's two solid-fuel boosters ignited with a crackling roar, eight explosive bolts detonated and the spaceplane majestically vaulted skyward from launch pad 39B.

Climbing straight up atop twin jets of bright-orange 5,000-degree flame, Discovery rolled about its vertical axis and lined up on a trajectory paralleling the East Coast, the crew in a heads-down position beneath the ship's huge external tank.

It was a thrilling moment for thousands of area residents, tourists and countless engineers and technicians who worked virtually around the clock through weekends and holidays to recover from the Feb. 1, 2003, Columbia disaster and, more recently, problems with a fuel tank sensor that blocked a July 13 launch attempt.

Despite extensive troubleshooting, engineers were never able to pin down exactly what went wrong. But based on the results of the testing and additional checks built into today's countdown, NASA managers were prepared to waive a launch commit criterion to permit a launch with three operational sensors.

Debate about changing the rule in the heat of a flight campaign reminded many observers of schedule-driven management failures cited by the Columbia Accident Investigation Board. But top shuttle managers said safety was never at risk in today's campaign and that the decision to proceed was based on a solid understanding of the problem.

As it turned out, all four sensors worked flawlessly today, the launch rule was not changed and Discovery was cleared for flight as is.

Even so, memories of Columbia's Feb. 1, 2003, destruction - triggered by the impact of foam debris 81.7 seconds after launch 16 days earlier - were never far from mind as Discovery roared away through a partly cloudy sky. The shuttle's powerful booster rockets were jettisoned as planned two minutes and five seconds after liftoff, falling back to Earth as the shuttle smoothly accelerated toward space.

The debris incidents generated quite a bit of interest at the Johnson Space Center, but Shannon said it was premature to draw any conclusions. He said the tile damage appeared about an inch-and-a-half across, but analysts do not yet have any information on how deep the damage might be and thus, whether or not it poses any threat to the space shuttle.

As for the debris seen flying off the external tank a little more than one minute later, Shannon said it was not yet clear whether a piece of foam broke away, whether the event was associated with the booster separation sequence or whether it represented an unknown phenomenon that has not been previously seen.

But minimizing debris shedding was a major goal of NASA's post-Columbia recovery and more than 112 television cameras, high-speed film and movie cameras and powerful radars were focused on Discovery as it climbed away, giving engineers unprecedented views of the craft.

Two WB-57 jet aircraft, flying to either side of the shuttle's ground track at an altitude of 60,000 feet, used nose-cone mounted telescopes and high-definition TV cameras to monitor the shuttle through booster separation. The video offered a unique, never-before-seen perspective of a shuttle launch, but the debris events under discussion were not obvious in a video replay.

Earlier today, NASA Administrator Michael Griffin urged reporters to give the image analysis team time to do its work.

"Our guys are going to take a really serious look at the end-to-end footage and in fact, a danger would be possibly seeing something that's very large and visible that we haven't had a chance to look at and ignore something that would be smaller and more significant.

"The guys are going to take a professional look at every frame of footage that we have from every camera that we have. Because as I've said, these are test flights right now. The primary object under test is the external tank and all of the design changes we made to that tank so we'd never have a repeat of (Columbia)."

It will take engineers several days to review all the launch imagery, data from new impact sensors in Discovery's wings and to conduct extensive inspections in orbit using a new TV/laser scanning device and cameras aboard the international space station.

Engineers fully expected to see signs of impact damage even though the shuttle's external tank has been redesigned to minimize ice formation and foam shedding. But if any major damage is, in fact, detected, flight controllers are spring loaded to carry out additional inspections to precisely characterize the damage before considering what, if anything, might need to be done.

Unlike Columbia's ill-fated crew, the Discovery astronauts are equipped with rudimentary repair equipment that will be tested during the first of three planned spacewalks after docking with the international space station Friday. But it is doubtful NASA would ask a crew to rely on untried repair procedures if serious damage is, in fact, detected.

In a worst-case scenario, Collins and company more likely would be asked to move into the space station to await rescue by another shuttle crew. In that scenario, Discovery would be undocked by remote control from the Johnson Space Center in Houston and guided to an unmanned re-entry and breakup over the Pacific Ocean.

But NASA managers - and the astronauts - were confident the tank redesigns would be up to the task, allowing them to focus on the goals of the long-awaited mission.

"There will be debris, there will be some damage, I'm convinced of that," Discovery's flight engineer, Stephen Robinson, told CBS News before launch. "If there isn't, that'll be great but I'll sure be surprised. I would be very surprised if it's critical damage, damage that won't allow us to fly home on. But here's the thing. We'll know it. We won't have to wonder. We'll know it.

"We'll have the technology now for the first time on this mission to take a look at it with all the cameras and sensors. This is the way we verify all the engineering that's been done. So we'll get to look at our bird before we come home.

"Then, on top of that, if the worst on worst on worst happens and we do have critical damage, the space station will (be available for safe haven), we won't have to risk our lives coming back through the atmosphere. This is what gives me tremendous confidence and makes me feel very lucky I'm flying now."

Joining Collins, Kelly and Robinson aboard Discovery were Andrew Thomas, Wendy Lawrence, Charles Camarda and Japanese astronaut Soichi Noguchi. The goal of the 114th shuttle mission is to deliver critical supplies and equipment to the space station; to bring no-longer-needed hardware and trash back to Earth; to test rudimentary tile and wing leading edge repair techniques; and to install a new gyroscope in the space station's orientation control system.

The gyro installation, heat shield repair tests and work to attach tools needed for upcoming station assembly flights will be carried out during three spacewalks later in the mission.

But the first item on the agenda after reaching orbit was for Noguchi to photograph Discovery's empty external fuel tank as it drifted away to find out if any foam insulation came off during the climb to space.

Data collected from scores of impact sensors mounted behind the shuttle's reinforced carbon carbon wing leading edge panels was downlinked to flight controllers late in the day for detailed analysis.

The astronauts plan to spend all day Wednesday inspecting the exterior of the leading edge panels, the RCC nose cap and heat-shield tiles around the crew cabin for any signs of impact damage, either due to ice or foam.

Columbia was destroyed during re-entry because of a hole in an RCC panel on its left wing, which allow super-heated air to burn its way inside as the ship plunged back into Earth's atmosphere. The wing melted from the inside out, triggering Columbia's breakup.

Lost in the disaster were commander Rick Husband, pilot William "Willie" McCool, flight engineer Kalpana Chawla, David Brown, Michael Anderson, Laurel Clark and Israeli astronaut Ilan Ramon.

The Columbia Accident Investigation Board blamed the disaster on the separation of a suitcase-sized chunk of foam from an aerodynamically shaped ramp intended to prevent ice from building up around the struts holding the shuttle's nose to the external tank. A large piece of insulation from the same area broke away from a shuttle two missions earlier. But NASA managers concluded it did not represent a safety-of-flight issue and cleared the next two missions for launch while a fix was designed.

The accident board faulted that decision, saying NASA managers never recognized the threat posed by foam debris and instead had come to accept debris shedding as normal. Minor foam impact damage was seen on every flight even though NASA's original design criteria called for a debris-free fuel tank.

The accident board also found fault with how NASA responded to the launch-day strike on Columbia, saying poor communications and the agency's institutional "culture" prevented concerns from lower-level engineers from reaching senior managers. Those managers concluded Columbia could safely re-enter "as is" and never ordered spy satellite photographs that might have revealed the full extent of the damage.

Most engineers believe Columbia's crew could not have been saved even if those steps had been taken, but many fault agency management for not giving the team a chance to come up with a solution.

The Columbia Accident Investigation Board made 29 recommendations for improving management and flight safety, including 15 that had to be implemented before the shuttle program could resume missions. An independent panel of experts ultimately concluded NASA had fully implemented 12 of the recommendations but had fallen short on the three most critical requirements: eliminating all debris shedding from the external tank; initiating a program to strengthen the tiles and RCC panels to make them more resistent to impacts; and developing reliable tile and RCC repair techniques to cope with any damage that might occur.

But over the past two-and-a-half years, NASA has learned that it is impossible to eliminate all debris from the tank given its current design. A program to strengthen the leading edge panels was canceled after President Bush ordered the agency to retire the shuttle fleet by 2010. And in a sort of "Catch-22," NASA was unable to develop reliable repair procedures without first testing them in space.

But the review panel did not suggest NASA should not launch Discovery, merely that the recommendations, as written, were not fully implemented. Retired Adm. Harold Gehman, chairman of the Columbia Accident Investigation Board, agreed NASA had met the intent of the recommendations and "I know of no reason why they should not proceed with the launch."

For his part, Griffin said NASA had done everything humanly possible to fix the shuttle program's shortcomings and that any additional delay would only produce incremental improvements in safety.

"If we ground the shuttle fleet, we're not going to be able to complete station assembly, we're not going to be able to do other things that we want to do," he said before launch. "If, of course, we believe that all debris sources have been reduced to a level low enough that the shuttle outer mold lines won't be damaged, then the tile repair issue becomes kind of moot.

"We're in that gray area where we believe we have greatly reduced the risk due to debris, foam and ice, but not so much we're completely comfortable with it. So the STS-114 crew ... will be lifting off in the face of a known risk. In that vein I want to point out this is a test flight. In a sense, they're all test flights."

Here is the flight plan for the rest of this evening and Thursday (acronyms: OBSS - orbiter boom sensor system; RMS - remote manipulator system, or robot arm; OMS POD - orbital maneuvering system rocket pod; SAFER - simplified aid for EVA resuce; a jet backpack):

REV...EVENT..........................MET in DD/HH:MM...EDT........GMT

    July 26
    
....LAUNCH............................KSC...00/00:00...10:39 AM...14:39
....MECO....................................00/00:08...10:47 AM...14:47
1...LAUNCH REPLAYS....................KSC...00/00:17...10:56 AM...14:56
1...POST LAUNCH NEWS CONFERENCE.......KSC...00/01:06...11:45 AM...15:45
2...PAYLOAD BAY DOOR OPENING................00/01:27...12:06 PM...16:06
3...RMS CHECKOUT............................00/03:05...01:44 PM...17:44
3...FLIGHT CONTROL TEAM VIDEO REPLAY..JSC...00/03:21...02:00 PM...18:00
3...RMS IMAGING OF KU ANTENNA CLEARANCE.....00/03:55...02:34 PM...18:34
4...LAUNCH ENGINEERING REPLAYS........KSC...00/05:21...04:00 PM...20:00
5...DISCOVERY CREW SLEEP BEGINS.............00/06:00...04:39 PM...20:39
5...MISSION STATUS BRIEFING...........JSC...00/06:21...05:00 PM...21:00
5...FLIGHT DAY 1 HIGHLIGHTS...........JSC...00/07:21...06:00 PM...22:00
6...VIDEO FILE.........................HQ...00/08:21...07:00 PM...23:00

     July 27
     
10...DISCOVERY CREW WAKE UP.................00/14:00...12:39 AM...04:39
12...WB-57 VIDEO REPLAY...............KSC...00/16:21...03:00 AM...07:00
12...RMS CHECKOUT...........................00/16:30...03:09 AM...07:09
12...FLIGHT DIRECTOR UPDATE...........JSC...00/16:51...03:30 AM...07:30
12...RMS GRAPPLE & UNBERTH OF OBSS..........00/17:15...03:54 AM...07:54
12...Ku BAND ANTENNA DEPLOYMENT.............00/17:30...04:09 AM...08:09
13...OMS POD PHOTOGRAPHY....................00/18:00...04:39 AM...08:39
13...RENDEZVOUS TOOLS CHECKOUT BEGINS.......00/18:40...05:19 AM...09:19
13...RMS/OBSS SURVEY........................00/18:45...05:24 AM...09:24
13...SAFER CHECKOUT.........................00/19:10...05:49 AM...09:49
15...AIRLOCK PREPARATION....................00/21:10...07:49 AM...11:49
15...EMU CHECKOUT...........................00/22:00...08:39 AM...12:39
15...ODS RING EXTENSION.....................00/22:15...08:54 AM...12:54
17...OBSS BERTH.............................01/00:10...10:49 AM...14:49
17...MISSION STATUS BRIEFING..........JSC...01/00:21...11:00 AM...15:00
17...RMS SURVEY OF DISCOVERY................01/00:55...11:34 AM...15:34
18...VTR PLAYBACK OF OBSS VIDEO.............01/02:30...01:09 PM...17:09
20...VIDEO FILE........................HQ...00/04:21...03:00 PM...19:00
20...DISCOVERY CREW SLEEP BEGINS............01/05:00...03:39 PM...19:39
20...FLIGHT DAY 2 HIGHLIGHTS..........JSC...01/05:21...04:00 PM...20:00
22...POST-MMT BRIEFING................JSC...01/07:21...06:00 PM...22:00
24...POST-MMT BRIEFING REPLAY.........JSC...01/11:21...10:00 PM...02:00

Discovery commander Eileen Collins and her crew - pilot James Kelly, flight engineer Stephen Robinson, Andrew Thomas, Wendy Lawrence, Charles Camarda and Japanese astronaut Soichi Noguchi - began strapping in today for launch at 10:38:59 a.m.

Forecasters are now predicting an 80 percent chance of good weather and tests show all of the engine cutoff - ECO - sensors in Discovery's hydrogen tank appear to be working properly.

Discovery was grounded July 13 when hydrogen ECO sensor No. 2 did not respond properly to a pre-launch test. Despite a week-and-a-half of exhaustive troubleshooting, engineers were never able to recreate the problem and the issue remained an "unexplained anomaly" going into today's countdown.

But the testing indicated the problem was not generic and NASA managers were prepared to amend a launch commit criterion requiring four-of-four sensors to permit a launch. As of this writing, no such discussion has been required.

The weather appears ideal and there are no technical problems of any significance. NASA's ice inspection team has just returned from pad 39B to brief launch director Mike Leinbach on the condition of the shuttle and and its external fuel tank.

During the July 13 launch try, ice was seen in two areas of the tank - at the base near a propellant feedling and near the top of the tank below an umbilical that supports a heated nitrogen gas purge. The cold temperatures in the tank can cause the nitrogen to condense, creating icing conditions around the umbilical connection. No word yet from NASA on whether either area has ice today.

Engineers began pumping a half million gallons of rocket fuel into the shuttle Discovery's external tank at 12:48 a.m. today, setting the stage for a launch attempt at 10:38:59 a.m. All four hydrogen main engine cutoff - ECO sensors - are now submerged in liquid hydrogen as are all four liquid oxygen sensors.

The sensors responded normally and all eight registered "wet" when the tank was 5 percent full. A few minutes later, engineers sent commands to simulate a dry tank and again, all eight sensors responded properly.

The simulation commands are part of NASA's strategy to detect any sensors that might fail in the wet state like hydrogen ECO sensor No. 2 did during Discovery's first launch attempt July 13. So far, all of the sensors are responding normally.

Engineers will cancel the so-called "sim dry" commands around 7:15 a.m. and let the sensors return to the wet state. At that point, the countdown computer system will make a series of checks to confirm the sensors change state as required. A second test is planned during a final hold at the T-minus nine-minute mark.

Forecasters continue to predict a 60 percent chance of favorable weather for today's launch attempt. Discovery's crew is scheduled to begin strapping in around 7:20 a.m. to await launch.

This status report will be updated throughout the morning as conditions warrant. In the meantime, here are today's countdown highlights at a glance (in EDT and HH:MM to launch)

HH...MM...SS...EDT...........EVENT

09...34...00...01:00:00 AM...Crew breakfast
06...50...00...03:44:00 AM...Begin 3-hour built-in hold
06...50...00...03:44:00 AM...Closeout crew to white room
06...45...00...03:49:00 AM...External tank in stable replenish mode
06...35...00...03:59:00 AM...Crew cabin comm checks
06...34...00...04:00:00 AM...Final crew medical checks
06...05...00...04:29:00 AM...Pre-ingress switch reconfig
05...34...00...05:00:00 AM...Crew photo opportunity
04...25...00...06:09:00 AM...Astronauts begin donning pressure suits
03...50...00...06:44:00 AM...Resume countdown
03...45...00...06:49:00 AM...Crew departs O&C building
03...19...00...07:15:00 AM...ECO sensor test (time approximate)
03...15...00...07:19:00 AM...Crew ingress begins
02...25...00...08:09:00 AM...Astronaut comm checks
02...00...00...08:34:00 AM...Hatch closure
01...25...00...09:09:00 AM...White room closeout
01...10...00...09:24:00 AM...Begin 10-minute built-in hold
01...08...00...09:26:00 AM...NASA test director's countdown briefing
01...00...00...09:34:00 AM...Resume countdown
00...59...00...09:35:00 AM...Backup flight computer check
00...55...00...09:39:00 AM...KSC area clear to launch
00...49...00...09:45:00 AM...Begin final built-in hold
00...49...00...09:49:00 AM...RTLS abort runway verification
00...44...00...09:54:00 AM...NTD launch status verification
00...29...00...10:10:00 AM...Final ECO sensor checks (time approximate)

00...09...00...10:29:59 AM...Resume countdown
00...07...30...10:31:29 AM...Orbiter access arm retraction
00...05...00...10:34:00 AM...Launch window opens
00...05...00...10:33:59 AM...Hydraulic power units started
00...04...55...10:34:04 AM...Oxygen replenish terminated
00...04...00...10:34:59 AM...Purge sequence 4
00...04...00...10:34:59 AM...Inertial measurement units to inertial
00...03...55...10:35:04 AM...Aerosurface profile
00...03...30...10:35:29 AM...Main engine steering test
00...02...55...10:36:04 AM...Oxygen tank pressurized
00...02...35...10:36:24 AM...Fuel cells to internal reactants
00...02...30...10:36:29 AM...Clear caution-and-warning memory
00...02...00...10:36:59 AM...Crew closes visors
00...01...57...10:37:02 AM...Hydrogen tank pressurization
00...00...50...10:38:09 AM...Booster joint heater deactivation
00...00...31...10:38:28 AM...Shuttle computers take over countdown
00...00...21...10:38:38 AM...Booster steering test
00...00...07...10:38:52 AM...Main engine start (T-6.6 seconds)
00...00...00...10:38:59 AM...SRB ignition (LAUNCH)

Assuming Discovery gets off the ground today, here is the crew's timeline in orbit for today and tomorrow from the current NASA television schedule (in EDT and mission elapsed time):

REV...EVENT..........................MET in DD/HH:MM...EDT........GMT

....July 26
....LAUNCH............................KSC...00/00:00...10:39 AM...14:39
....MECO....................................00/00:08...10:47 AM...14:47
1...LAUNCH REPLAYS....................KSC...00/00:17...10:56 AM...14:56
1...POST LAUNCH NEWS CONFERENCE.......KSC...00/01:06...11:45 AM...15:45
2...PAYLOAD BAY DOOR OPENING................00/01:27...12:06 PM...16:06
3...ROBOT ARM (RMS) CHECKOUT................00/03:05...01:44 PM...17:44
3...FLIGHT CONTROL TEAM VIDEO REPLAY..JSC...00/03:21...02:00 PM...18:00
3...RMS IMAGING OF KU ANTENNA CLEARANCE.....00/03:55...02:34 PM...18:34
4...LAUNCH ENGINEERING REPLAYS........KSC...00/05:21...04:00 PM...20:00
5...DISCOVERY CREW SLEEP BEGINS.............00/06:00...04:39 PM...20:39
5...MISSION STATUS BRIEFING...........JSC...00/06:21...05:00 PM...21:00
5...FLIGHT DAY 1 HIGHLIGHTS...........JSC...00/07:21...06:00 PM...22:00
6...VIDEO FILE.........................HQ...00/08:21...07:00 PM...23:00

.....July 27
10...DISCOVERY CREW WAKE UP.................00/14:00...12:39 AM...04:39
12...WB-57 VIDEO REPLAY...............KSC...00/16:21...03:00 AM...07:00
12...RMS CHECKOUT...........................00/16:30...03:09 AM...07:09
12...FLIGHT DIRECTOR UPDATE...........JSC...00/16:51...03:30 AM...07:30
12...RMS GRAPPLE & UNBERTH OF OBSS..........00/17:15...03:54 AM...07:54
     SENSOR BOOM
12...Ku BAND ANTENNA DEPLOYMENT.............00/17:30...04:09 AM...08:09
13...OMS ROCKET POD PHOTOGRAPHY.............00/18:00...04:39 AM...08:39
13...RENDEZVOUS TOOLS CHECKOUT BEGINS.......00/18:40...05:19 AM...09:19
13...RMS/OBSS SURVEY OF HEAT SHIELD.........00/18:45...05:24 AM...09:24
13...JET BACKPACK CHECKOUT..................00/19:10...05:49 AM...09:49
15...AIRLOCK PREPARATION....................00/21:10...07:49 AM...11:49
15...SPACESUIT CHECKOUT.....................00/22:00...08:39 AM...12:39
15...DOCKING RING EXTENSION.................00/22:15...08:54 AM...12:54
17...OBSS BERTH.............................01/00:10...10:49 AM...14:49
17...MISSION STATUS BRIEFING..........JSC...01/00:21...11:00 AM...15:00
17...RMS SURVEY OF DISCOVERY................01/00:55...11:34 AM...15:34
18...VTR PLAYBACK OF OBSS VIDEO.............01/02:30...01:09 PM...17:09
20...VIDEO FILE........................HQ...00/04:21...03:00 PM...19:00
20...DISCOVERY CREW SLEEP BEGINS............01/05:00...03:39 PM...19:39
20...FLIGHT DAY 2 HIGHLIGHTS..........JSC...01/05:21...04:00 PM...20:00
22...POST-MMT BRIEFING................JSC...01/07:21...06:00 PM...22:00
24...POST-MMT BRIEFING REPLAY.........JSC...01/11:21...10:00 PM...02:00

The shuttle Discovery's countdown continues to tick smoothly toward blastoff Tuesday. There are no technical problems of any significance at pad 39B an if all goes well, engineers will begin pumping a half-million gallons of liquid oxygen and liquid hydrogen rocket fuel into the shuttle's external tank around 12:15 a.m. For a launch attempt at 10:39 a.m. Tuesday.

For readers interested in such details, the launch window has been modified very slightly based on the latest tracking of the international space station. Discovery's launch window now opens at 10:33:59 a.m. and closes at 10:44:00 a.m. The preferred launch time, when Earth's rotation carries the launch pad into the plane of the station's orbit, is 10:38:59 a.m., one second earlier than NASA's previous estimate.

Forecasters are continuing to predict a 60 percent chance of acceptable weather, although there is concern about off-shore rain showers and detached anvil clouds moving into the launch area or within 20 nautical miles of the shuttle's emergency runway. The forecast remains 60 percent "go" Wednesday and 70 percent favorable Thursday.

Should launch be delayed 24 hours, Discovery's window will be shortened to five minutes and liftoff will be targeted for 10:11:29 a.m. The window will close at 10:16:02 a.m. The shortened window is the result of the station's orbit and a requirement to dock with the station two days after launch (flight day 3).

Here are remaining countdown highlights at a glance (in EDT and HH:MM to launch; times rounded to the nearest minute until T-minus 9 minutes):

HH...MM...SS...EDT...........EVENT

Monday, July 25
25...34...00...09:00:00 AM...Flight crew weather briefing
21...04...00...01:30:00 PM...Pad gantry to park position
19...04...00...03:30:00 PM...Crew sleep begins
16...50...00...05:44:00 PM...Resume countdown after a built-in hold
14...50...00...07:44:00 PM...Pad clear of non-essential personnel
14...20...00...08:14:00 PM...Booster joint heater activation
14...05...00...08:29:00 PM...Fuel loading weather briefing
11...50...00...10:44:00 PM...Begin 2-hour built-in hold
11...00...00...11:34:00 PM...Mission management team meeting

Tuesday, July 26
10...34...00...12:00:00 AM...NASA television coverage begins
10...04...00...12:30:00 AM...Crew wakeup
09...50...00...12:44:00 AM...Resume countdown
09...50...00...12:44:00 AM...Fueling begins (transfer line chilldown)*
09...05...00...01:29:00 AM...Hydrogen ECO sensors "wet"
06...50...00...03:44:00 AM...Begin 3-hour built-in hold
05...34...00...05:00:00 AM...Crew photo opportunity
03...50...00...06:44:00 AM...Resume countdown
03...45...00...06:49:00 AM...Crew departs O&C building
03...19...00...07:15:00 AM...Hydrogen ECO sensor test
03...15...00...07:19:00 AM...The astronauts begin strapping in
01...10...00...09:24:00 AM...Begin 10-minute built-in hold
01...00...00...09:34:00 AM...Resume countdown
00...49...00...09:45:00 AM...Begin T-minus 9-minute hold; ECO sensor test

00...09...00...10:29:59 AM...Resume countdown at T-minus 9 minutes
00...00...00...10:38:59 AM...Launch

* Fueling is expected to begin a half-hour earlier than listed if all
preparations are completed in time.

A more detailed countdown timeline is available below.

NASA managers today cleared the shuttle Discovery for another launch try Tuesday, weather permitting, on a critical flight to service and resupply the international space station. The decision came after senior managers agreed on a strategy that would permit blastoff even if - and only if - the shuttle experiences a fuel sensor problem like the one that grounded the ship July 13.

"I've delved into this situation as thoroughly as my intellect permits," NASA Administrator Michael Griffin told reporters late today. "And I'm quite comfortable with where we are. ... Even if (a sensor failure) does recur, we're still two-failure tolerant, so it's not a safety of flight issue."

Asked if he was concerned about any public perception NASA had "launch fever" and was not giving the sensor issue the attention it might deserve, Griffin said "I think what you want coming out of the Columbia accident and the loss of Columbia and the soul-searching examination that NASA has undertaken since then, what you want of NASA is that we make the right technical decisions, that we do the right thing to the extent that we can figure that out. Which is hard.

"We can't restrict the range of our options to those things which are going to present well. We have to figure out the right thing and try to do that and then work hard to try to explain to you why it is the right thing. And these are rather arcane matters, I would admit. They're rather difficult. And sometimes they don't always present well. (But) you want us doing what's right, not what's necessarily obvious or popular."

Wayne Hale, chairman of NASA's mission management team, put it like this: "I wake up every day and I ask myself are we pushing too hard, are we doing this thoroughly, have we done the right technical things, have we asked the right people, have we built the tests properly? I am committed, and I think the whole team is committed, to doing this in a safe manner.

"I think we're all still struggling a little bit with the ghost of Columbia and therefore we want to make sure we do it right. This has been a very vexing problem. ... I would tell you that based on the last 10 days worth of effort, the huge number of people and the tremendous number of hours that have been spent in testing and analysis, I think we're coming to the right place."

Forecasters continue to predict a 60 percent chance of acceptable weather during Discovery's five-minute launch window. The window opens at 10:39:00 a.m. - the same time Columbia blasted off on its final mission two-and-a-half years ago - and closes at 10:43:56 a.m. The forecast is 60 percent "go" Wednesday and 70 percent favorable Thursday.

But the real question mark is how Discovery's fuel sensors will operate when the shuttle's external tank is loaded with supercold rocket fuel early Tuesday.

Despite exhaustive, around-the-clock tests, engineers were never able to find the problem that caused hydrogen main engine cutoff - ECO - sensor No. 2 to "fail wet" during the July 13 launch attempt. The ECO sensors are part of a backup system that ensures the shuttle's main engines shut down normally before the tank runs out of fuel. NASA launch rules require all four ECO sensors to be operating before a countdown can proceed.

But in a routine pre-launch test during Discovery's first launch try, ECO sensor No. 2 continued indicating it was wet after computer commands were sent to simulate a dry tank. The countdown was called off and engineers mounted a major effort to track down the cause of the problem.

They initially were hopeful that electrical interference caused by subtle grounding problems might explain why sensor No. 2 failed to respond properly. The grounding problems were fixed, but troubleshooters were never able to duplicate the unexpected sensor behavior.

Even so, they believe the testing completed to date proves the problem, whatever the cause, is not a generic defect and as such, they are confident it will not affect the other sensors.

"We have literally run every check that we can think of that people could suggest to us to try to find this problem and so far, no repeat," said Hale. "So we have developed a plan that says we have to go to cryogenic temperature to find out what's going on next.

"We also need to turn on all the equipment in the orbiter and the launch pad area to see if there's any electromagnetic interference that we could not check piece wise earlier. So both of those things together say we are ready to go the launch countdown configuration to see our next level of checking."

Engineers plan to begin pumping liquid oxygen and hydrogen rocket fuel into Discovery's external tank around 12:30 a.m. Tuesday. Two minutes after the hydrogen ECO sensors are submerged in super-cold hydrogen, engineers will begin sending commands to simulate a dry tank to make sure none of the sensors have "failed wet."

Then, just before the crew straps in around 7:19 a.m., the commanding will stop and the main countdown computer will carry out a series of checks to verify the sensors change state, from wet to dry and back again, on command. A final check is planned about a half-hour before liftoff.

"We are going to continuously check the validity of the signal from the sensor all the way to the orbiter computers during the tanking (process) so if at any time any one of these circuits fails, we will know about it, we'll be able to isolate time-wise when it happened, we'll be able to know what was going on in the ship, either electrically or thermally, so that will give us a clue to where the problem might lie," Hale explained.

To help isolate the problem during fuel loading, engineers swapped the wiring between ECO sensors 2 and 4. If a problem shows up during fueling with sensor No. 4, engineers will have high confidence the problem is in the sensor itself or somewhere in the wiring between the sensor and the point sensor box. If sensor No. 2 misbehaves, they will have high confidence the problem is in the point sensor box.

Because electrical interference could still be a contributing factor, engineers will conduct checks during the final hours of the countdown to isolate any such signatures.

Here's the ECO sensor launch strategy at a glance:

• All four ECO sensors work normally: Launch
  
• More than one hydrogen ECO sensor malfunctions: Scrub
  
• Any liquid oxygen ECO sensors fail: Scrub
  
• Hydrogen ECO sensor 1 or 3 malfunctions: Scrub
  
• Hydrogen ECO sensor 2 fails wet: Launch
  
• Hydrogen ECO sensor 4 fails wet: Launch
  

Because of the wiring swap, "if the problem recurs, it will give us an indication of whether the problem is in our famous point sensor black box or in the wiring or the sensor itself," Hale said. "So we'll know that. And we've defined a very rigid set of requirements and tests to be done if this problem re occurs. We also know that we've done a lot of moving around in the aft, we've mated and demated connectors, we've wiggled a lot of wiring, we've tested the point sensor box extensively and it is possible that we have caused whatever the problem was to go away.

"So if the problem doesn't recur, we feel we have good redundancy to go fly and that's why we are prepared, following the tests to ensure all these sensors are working right ... to load the crew up and go fly."

But if sensors 2 or 4 act up, "then we're going to do some more tests just to make sure we understand what's causing that to happen," Hale said. "And if we're comfortable that we have a good understanding of the cause, then we can go fly for those specific two cases. If anything else happens - if we have any of the liquid oxygen sensors fail, if we have a hydrogen sensor that fails to a dry state instead of the wet state, if we have a sensor that fails on channel 1 or 3 or multiple sensor failures, anything like that happens - we're going to stop because that says we really need to do more testing."

But launching with three operational sensors would violate NASA's launch commit criterion requiring all four to be working at launch.

The agency's original launch rule required three operational ECO sensors for a countdown to proceed. But in the wake of the 1986 Challenger disaster, the LCC was amended to four-of-four because of concerns two sensors could be knocked out by a single failure in an upstream electronic black box known as a multiplexer-demultiplexer. The single-point failure was corrected during Discovery's last overhaul, but the four-of-four launch rule remained on the books.

And that rule will be in place Tuesday. But the mission management team is prepared to sign an exception to the rule - permitting Discovery to launch with three of four ECO sensors - depending on what happens after the tank is loaded with fuel.

Detailed background on how the sensors work and a chronology of sensor-related problems affecting Discovery's launch processing flow, is available on the CBS News/Spaceflight Now ECO sensor page.

The shuttle Discovery's countdown remains on track today with forecasters continuing to predict a 60 percent chance of acceptable weather at launch time Tuesday. NASA's mission management team will meet this afternoon to assess the status of launch processing and to discuss the team's strategy for managing any fuel sensor problems that might crop up Tuesday. Senior NASA managers will brief reporters during a late afternoon news conference (no earlier than 4:30 p.m.) carried on NASA television.

Earlier today, NASA test director Jeff Spaulding said the launch team is not tracking any significant technical issues at pad 39B and that engineers closed the shuttle's aft engine compartment yesterday after winding up work to test the ship's liquid hydrogen engine cutoff - ECO - sensors.

Discovery was grounded July 13 when one of four fuel sensors in the shuttle's external hydrogen tank failed to operate properly during a pre-launch test. Despite exhaustive tests and electrical checks, engineers were unable to pin down the cause of the sensor glitch and it remains an "unexplained anomaly."

NASA managers are optimistic all four sensors will operate properly when the tank is loaded with fuel Tuesday. If not, the team has developed a plan that, assuming senior managers concur, would permit a launch with just three operational sensors if final checks confirm the problem is not generic. See the July 23 status report for a recap of the sensor stategy. ECO sensor background and a chronology of recent sensor problems is available here.

"We have had a great many challenges preparing the space shuttle Discovery and her crew to get ready for this historic return-to-flight mission," Spaulding said. "And certainly, that's (resulted in) creating the safest shuttle to date and I'm proud to report that the work that we're doing has created that shuttle. Our flight and ground systems are ready, our launch teams are ready, our flight crew is ready to begin this mission of returning our shuttle to flight and bringing our crew safely home."

Forecasters are predicting a 40 percent chance of showers and detached anvil clouds from offshore storms that could delay blastoff. The Florida forecast for Wednesday is again 60 percent "go," improving to 70 percent Thursday.

This morning, liquid hydrogen and oxygen are being pumped aboard the space shuttle to power its three electricity producing fuel cells.

If all goes well, a protective gantry will be rollwed away from Discovery around 1:30 p.m. Monday and the shuttle's external tank will be loaded with rocket fuel starting around 12:30 a.m. Tuesday. Launch is targeted for 10:39 a.m., the same time shuttle Columbia blasted off on its final voyage two-and-a-half years ago.

Here are countdown highlights at a glance (in EDT and HH:MM to launch):

HH...MM...SS...EDT........EVENT

Sunday, July 24
46...34...00...12:00 PM...Fuel cell oxygen load complete
46...34...00...12:00 PM...Fuel cell hydrogen loading begins
44...04...00...02:30 PM...Fuel cell hydrogen loading complete
43...04...00...03:30 PM...Pad open; ingress white room
43...04...00...03:30 PM...Crew sleep begins
42...34...00...04:00 PM...Begin 4-hour built-in hold
38...34...00...08:00 PM...Countdown resumes
38...34...00...08:00 PM...Main engine preps
36...04...00...10:30 PM...Gantry seal deflation tile inspection
35...04...00...11:30 PM...Crew wakeup

Monday, July 25
30...34...00...04:00 AM...Begin 13-hour 44-minute hold
29...34...00...05:00 AM...Collins/Kelly practice landing procedures
25...34...00...09:00 AM...Flight crew weather briefing
21...34...00...01:00 PM...Pad gantry to park position
19...04...00...03:30 PM...Crew sleep begins
16...50...00...05:44 PM...Resume countdown
14...50...00...07:44 PM...Pad clear of non-essential personnel
14...20...00...08:14 PM...Booster joint heater activation
14...05...00...08:29 PM...Fuel loading weather briefing
11...50...00...10:44 PM...Begin 2-hour built-in hold
11...00...00...11:34 PM...Mission management team meeting

Tuesday, July 26
10...34...00...12:00 AM...NASA television coverage begins
10...04...00...12:30 AM...Crew wakeup
09...50...00...12:44 AM...Resume countdown
09...50...00...12:44 AM...Fueling begins
09...05...00...01:29 AM...Hydrogen ECO sensors "wet"
06...50...00...03:44 AM...Begin 3-hour built-in hold
05...34...00...05:00 AM...Crew photo opportunity
03...50...00...06:44 AM...Resume countdown
03...45...00...06:49 AM...Crew departs O&C building
03...19...00...07:15 AM...Hydrogen ECO sensor test
03...15...00...07:19 AM...The astronauts begin strapping in
01...10...00...09:24 AM...Begin 10-minute built-in hold
01...00...00...09:34 AM...Resume countdown
00...49...00...09:45 AM...Begin final built-in hold; second ECO sensor test
00...09...00...10:30 AM...Resume countdown
00...00...00...10:39 AM...Launch

A more detailed countdown timeline is available below.

NASA restarted the shuttle Discovery's countdown today, pressing for a Tuesday launch after extensive troubleshooting and a wiring change that engineers hope will resolve, if not fix, a fuel sensor problem that scrapped a July 13 launch try.

"We believe our flight systems and ground support hardware are ready, we know our flight crew and support teams are ready and we're all eagerly anticipating and looking forward to a successful launch and mission," said NASA test director Pete Nickolenko.

With forecasters predicting a 60 percent chance of good weather, NASA's shuttle team started a fresh countdown at noon, setting up a launch attempt at 10:39 a.m. Tuesday, the same time shuttle Columbia blasted off on its final mission two-and-a-half years ago.

Shuttle forecaster Kathy Winters said the primary concern will be the development of a sea breeze Tuesday and showers that might move into the launch area from offshore. The outlook for Wednesday is 60 percent "go," improving to 70 percent should the flight slip to Thursday.

"In general, we've been keeping a close eye on tropical storm Franklin, but we're starting to get more confident in this northeasterly turn," Winters told reporters today . "For launch, basically what we're concerned about is the placement of that sea breeze and the timing of the development of those showers."

Discovery 's launch window, based on the international space station's orbit and a requirement to launch in daylight to verify post-Columbia fuel tank modifications, extends through July 31. If the weather or some other problem delays launch Tuesday, NASA can make attemps July 27, 29 and 31 or July 27, 30 and 31, depending on the weather and work to service the shuttle's electrical generators.

Engineers were hopeful earlier this week that electrical interference caused by subtle grounding problems might explain why one of four hydrogen fuel sensors in Discovery's external tank failed to respond properly to a pre-launch test July 13.

At it turned out, the three grounding problems were very minor - 0.2 milli-ohms of resistance or less - and engineers were never able to duplicate the unexpected behavior of ECO sensor No. 2. As such, the problem remains an "unexplained anomaly."

But NASA managers believe they have developed a strong rationale for proceeding with the 114th shuttle mission after an exhaustive battery of tests and work to swap the electrical cables routing commands and data from sensors 2 and 4 to an electronic component called a point sensor box.

If a problem shows up during fueling with sensor No. 4, engineers will have high confidence the problem is somewhere in the wiring between the point sensor box and the sensor itself, and not a generic problem that could affect the other three sensors. In that case, NASA's mission management team could consider making an exception to a launch commit criterion that calls for four operational sensors before a countdown can proceed.

The ECO sensor system is a backup that protects against other failures that might result in running the shuttle's main engines long enough to drain the external tank. Complete details on sensor operation and logic are available on the CBS News/Spaceflight Now ECO sensor page.

NASA's original flight rule, in place for the first 25 shuttle missions, only required three of four sensors because the system is redundant - three of the four sensors would have to "fail wet" to drain the tank after some other problem required their use in the first place - and to protect against the possibility of a faulty sensor, which cannot be easily replaced.

But after Challenger, engineers discovered a single-point failure mode in an electronics black box upstream of the point sensor box that could take out two fuel sensors at once. That failure mode was corrected several years ago, but the four-of-four LCC was never changed back to three of four.

If sensor No. 4 - the one now connected to the wiring that originally went to sensor No. 2 - acts up Tuesday, NASA may be forced to sign an exception to the LCC to permit Discovery to take off with three of four operational ECO sensors. If any other sensors misbehave, the launch will be called off and troubleshooters will go back to the drawing board.

"Certainly, if we get anything else new, that would certainly be a cause for a scrub condition," said NASA test director Pete Nickolenko. "If we were to see (problems with sensor No. 4), we can reasonably conclude it was related to that circuitry that was downstream of the point sensor box and then we could entertain that 3-of-4 flight rationale, which we have been working on developing. But anything other than that might implicate the point sensor box ... or something else in the wiring."

NASA's mission management team will meet Sunday afternoon to assess Discovery's readiness to launch, the status of the ECO sensor troubleshooting and whether or not to adopt a three-of-four strategy if sensor No. 4 does, in fact, act up. It's not yet clear whether the MMT has consensus to make an exception to the LCC or whether new NASA Administrator Mike Griffin will go along.

In any case, the sensors will be tested around 7:15 a.m. Tuesday as the astronauts are heading to the launch pad to strap in. The sensors will be tested again during a final hold at the T-minus nine-minute mark.

Engineers believe they have done everything possible to ensure all four sensors will work properly.

"The battery of testing and analysis that we've done so far leads us to believe we are confident that we've got good sensors," Nickolenko said. "The true proof will be when we perform the tanking operation for the launch attempt Tuesday morning. But so far, based on what I understand, we've got good sensor paths, we've got a good point sensor box, we've tested it as exhaustively as we possibly can."

The grounding problems discovered earlier this week were minor, but they were fixed anyway. The resistance across one ground was .2 milli-ohms, Nickolenko said, when the specification called for .1 milli-ohms. Two other grounds measured around .11 and .14 milli-ohms. All three were disconnected, the mating surfaces were sanded and the wires reconnected and bonded in place. Measurements showed all three were back within specifications.

Engineers said earlier this week that even subtle grounding problems could result in electromagnetic interference that might affect the signals to and from the ECO sensors. But again, engineers were not able to duplicate the failure signature and the problem remains an unexplained anomaly.

"We performed the EMI checks and we saw no anomalous indications and after reviewing that data, the troubleshooting team gave us the concurrence to proceed with the electrical ground repairs," Nickolenko said. "We're optimistic we're going to see all good sensors."

The shuttle Discovery's crew returned to the Kennedy Space Center today to prepare for blastoff Tuesday on the first post-Columbia mission. Commander Eileen Collins, making the crew's first comments on NASA plans to possibly launch the shuttle with a known fuel sensor problem, said extensive testing and analysis had given the astronauts "a lot of confidence" and "we think they've got a great plan."

Collins, pilot James Kelly, flight engineer Stephen Robinson, Andrew Thomas, Wendy Lawrence, Charles Camarda and Japanese astronaut Soichi Noguchi arrived at the Shuttle Landing Facility around 11:30 a.m. after staggered flights from Houston in T-38 jet trainers.

Greeting reporters under a bright sun and partly cloudy sky, Collins said "we're hoping this weather holds through all of next week, whatever day we launch."

"I hope we get this nice, dry weather and with an early morning launch, we hope we can give you a good show," she said. "We're really excited about getting this launch off, we're very prepared, and we'll be talking to you from space."

Earlier concern about tropical storm Franklin has abated and the shuttle's countdown is scheduled to begin at noon Saturday for a launch attempt at 10:39:00 a.m. Tuesday. Here are countdown highlights (in EDT: a detailed countdown is posted below):

Saturday, July 23

........................10:00 AM...Countdown status briefing on NTV
........................11:30 AM...Call to stations
70...34...00...07/23/05 12:00 PM...Countdown begins

Sunday, July 24

54...34...00...07/24/05 04:00 AM...Begin 4-hour built-in hold
53...34...00...07/24/05 05:00 AM...Collins/Kelly suited STA flights
50...34...00...07/24/05 08:00 AM...Resume countdown
49...04...00...07/24/05 09:30 AM...Fuel cell oxygen loading begins
........................10:00 AM...Countdown status briefing on NTV
42...34...00...07/24/05 04:00 PM...Begin 4-hour built-in hold
........................04:30 PM...MMT news briefing (no earlier than)
38...34...00...07/24/05 08:00 PM...Countdown resumes

Monday, July 25

30...34...00...07/25/05 04:00 AM...Begin 13-hour 44-minute hold
29...34...00...07/25/05 05:00 AM...Collins/Kelly unsuited STA flights
........................10:00 AM...Countdown status briefing on NTV
21...34...00...07/25/05 01:00 PM...RSS to park position
16...50...00...07/25/05 05:44 PM...Resume countdown
14...05...00...07/25/05 08:29 PM...Fuel loading weather briefing
11...50...00...07/25/05 10:44 PM...Begin 2-hour built-in hold
11...20...00...07/25/05 11:14 PM...ET ready for fueling
11...00...00...07/25/05 11:34 PM...MMT tanking meeting

Tuesday, July 26

10...34...00...07/26/05 12:00 AM...NASA television coverage begins
10...04...00...07/26/05 12:30 AM...Crew wakeup
09...50...00...07/26/05 12:44 AM...Resume countdown; fueling begins
09...05...00...07/26/05 01:29 AM...Hydrogen ECO sensors "wet"
06...50...00...07/26/05 03:44 AM...Begin 3-hour built-in hold
06...50...00...07/26/05 03:44 AM...Closeout crew to white room
05...34...00...07/26/05 05:00 AM...Crew photo opportunity
03...50...00...07/26/05 06:44 AM...Resume countdown
03...45...00...07/26/05 06:49 AM...Crew departs O&C building
03...19...00...07/26/05 07:15 AM...ECO sensor sim test to "dry"
03...15...00...07/26/05 07:19 AM...Crew ingress
02...00...00...07/26/05 08:34 AM...Hatch closure
01...10...00...07/26/05 09:24 AM...Begin 10-minute built-in hold
01...08...00...07/26/05 09:26 AM...NTD countdown briefing
01...00...00...07/26/05 09:34 AM...Resume countdown
00...49...00...07/26/05 09:45 AM...Begin final built-in hold
00...44...00...07/26/05 09:55 AM...NTD status; ECO sensor test (time TBD)
00...09...00...07/26/05 10:30 AM...Resume countdown
00...00...00...07/26/05 10:39 AM...Launch

Collins and her crewmates were in the process of strapping in aboard Discovery for a launch attempt July 13 when problems with one of the four hydrogen engine cutoff - ECO - sensors in the shuttle's external tank failed to operate properly during a routine test.

The ECO senors are part of a backup system that ensures the shuttle's main engines shut down properly and don't run too long in the event of some other problem that might prevent an on-time cutoff. A main engine likely would tear itself apart if it simply ran out of gas and NASA's current launch rules call for all four ECO sensors to be operational for a countdown to proceed.

NASA and contractor engineers have been wrestling with ECO sensor issues since a fueling test in April. See the CBS News/Spaceflight Now ECO sensor page for complete details.

Since the problem with ECO sensor No. 2 cropped up last week, engineers have been working around the clock to figure out what went wrong and how to fix it. Suspicion has focused on the sensor itself, the wiring that leads to it and on a so-called point sensor box in Discovery's engine compartment that routes sensor data to the ship's flight computers. But so far, troubleshooters have found nothing wrong other than a few subtle grounding issues. They had hoped those grounding issues might explain the sensor anomaly, but they were unable to recreate the failure signature at the launch pad.

In the meantime, NASA managers ordered engineers to swap the wiring used to connect sensors 2 and 4 to the point sensor box. If sensor 4 acts up during fueling Tuesday, troubleshooters will have strong evidence the problem is, in fact, in the wiring and not a generic problem that could affect the other sensors.

Under that scenario, NASA's mission management team likely would sign an exception to the launch commit criterion calling for four-of-four sensors and allow Discovery to launch with three. For the ECO sensor system to let the engines drain the tank, all three of the other sensors would have to "fail wet," indicating the tank still had fuel when, in fact, it was empty. The odds of that happening are considered remote.

NASA's mission management team will meet Sunday to review launch preparations and to consider the three-of-four scenario on the assumption sensor No. 4 might act up. If any of the other sensors misbehave, launch will be called off.

Speaking at the shuttle runway today, Collins gave the mission management team a vote of confidence and thanked the teams of engineers who have been troubleshooting the sensor issue since last week.

"We've ... been keeping track of the events that have been taking place and the developments of the plan to test and analyze what happened to ECO sensor 2," she said. "My crew has been listening to the technical meetings and the management meetings and that's very important for us to do that, to really understand the problem.

"It's very important for us to say that we're very proud of the work the engineers and the managers and the technicians have done over the past week and a half trying to find out what's going on with this very elusive problem. And we have a lot of confidence in what they're doing and we think they have a great plan that they're going forward with."

She said the astronauts hope to blast off Tuesday but "the launch date to us isn't that important. What's important to us is that we get through this process and that we do it right."

"We have a fantastic team of people working on this, they've been putting in very, very long hours and they've been really working hard," Collins said. "They are very dedicated people, very dedicated to the space program and getting the shuttle flying again. And we're very, very proud of the work they're doing. That really comes form the bottom of our hearts."

Posted immediately below are updated launch windows through the end of the official July 31 launch period. Because the international space station's orbit was adjusted earlier to optimize the front end of Discovery's July launch period, some of the remaining launch opportunities require relatively large rendezvous rocket firings and shorter windows. The window for July 27, for example, is less than four minutes long. The optimum window on July 29 is just 11 seconds long. Mike Wetmore, director of shuttle processing at the Kennedy Space Center, said Wednesday the current plan calls for launch attempts July 26 and 27, after which the team would stand down for 48 hours to service the shuttle's electrical generators. Two more attempts could be made on July 29 and 31 if necessary, with fuel cell servicing in between.

NASA managers are debating a launch period extension July 31, but as of this writing, no such decisions have been made.Note: To optimize ascent performance and to improve margins for East Coast aborts, NASA will target the "in-plane" launch time if possible (times in EDT):

DATE.......OPEN..........IN PLANE......CLOSE.........RENDEZVOUS

07/26/05...10:33:57 AM...10:39:00 AM...10:43:56 AM...Flight Day 3

07/27/05...10:12:26 AM...10:12:26 AM...10:16:12 AM...FD-3

07/28/05...09:45:42 AM...09:50:42 AM...09:55:42 AM...FD-3

07/29/05...09:23:11 AM...09:28:11 AM...09:28:22 AM...FD-3

07/30/05...08:57:27 AM...09:02:27 AM...09:07:27 AM...FD-3

07/31/05...08:34:56 AM...08:39:56 AM...08:44:56 AM...FD-3

08/01/05...08:09:13 AM...08:14:13 AM...08:19:12 AM...FD-3

08/02/05...07:46:41 AM...07:51:41 AM...07:56:41 AM...FD-3

08/03/05...07:20:58 AM...07:25:58 AM...07:30:57 AM...FD-3

08/04/05...06:58:26 AM...07:03:26 AM...07:08:26 AM...FD-3

SpaceCalc, a collection of Excel spreadsheets loaded with mission specific information, has been updated with the latest flight plan, launch windows, revised ascent summary, etc. An updated edition of the CBS News Space Reporters Handbook will be posted shortly.

NASA managers monitored a fast-developing tropical storm southeast of Florida today but pressed ahead with plans to restart the shuttle Discovery's countdown Saturday for a Tuesday morning launch attempt. That's assuming engineers can complete fuel sensor troubleshooting in time and that senior NASA managers sign off on a strategy that might include relaxing a long-standing launch rule if the sensor problem can't be fixed.

Engineers have been working at pad 39B to find out whether subtle electrical grounding problems discovered earlier this week contributed to the malfunction of an external tank engine cutoff - ECO - sensor last week. Based on reports earlier today, troubleshooters had not been able to recreate the failure "signature" at the launch pad. But testing was expected to continue through the night (see the CBS News/Spaceflight Now ECO sensor page for complete details).

Regardless of the outcome of the troubleshooting effort, engineers plan to swap the wiring between suspect ECO sensor No. 2 and No. 4, which operated normally during Discovery's countdown last Wednesday. If sensor No. 4 acts up when the tank is loaded for launch Tuesday, engineers will have strong evidence the problem is in the wiring.

In that scenario, NASA managers could sign an exception to a launch rule requiring four-of-four ECO sensors on the assumption the problem is not generic and because of the system's built-in redundancy. If any of the other sensors act up, the launching would be called off.

Forecasters, meanwhile, are closely monitoring a tropical depression southeast of Florida that may develop into a hurricane over the next few days. The 5 p.m. forecast from the National Hurricane Center showed the storm moving northwest toward Florida until Friday afternoon, when it is expected to turn north and then loop around to the east and eventually back to the southwest. The predicted turn will occur roughly due east of the Kennedy Space Center.

The storm does not yet pose a direct threat to the Florida coast, but it likely will affect the state's weather. Stay tuned.

The STS-114 countdown and flight plan, which now includes the latest rendezvous plan and updated station docking, undocking, deorbit and landing times, have been revised and are posted below. Final updates of the CBS Space Reporters Handbook and SpaceCalc, a collection of Excel worksheets loaded with mission-specific timelines and other information, are in work and will be posted here shortly.

Troubleshooters may have found the cause of the fuel sensor malfunction that grounded the shuttle Discovery last week. Additional tests are planned, but the analysis to date, along with a proposed wiring modification, prompted NASA managers today to forego another fueling test and to press ahead instead for a launch attempt Tuesday at 10:39 a.m.

"The decisions we made today were to continue with our troubleshooting tonight, and that will go on through the evening and into the first part of tomorrow," said shuttle program manager Bill Parsons. "We have some very specific things we're going to go perform and it will help us completely close this issue out."

An attempt to launch Discovery on the first post-Columbia flight was called off July 13 when one of four engine cutoff - ECO - sensors in the shuttle's external hydrogen tank failed a routine test. NASA launch rules require all four sensors to be operational for a countdown to proceed (see the CBS News/Spaceflight Now ECO sensor page for complete details).

Since then, engineers have been working through an exhaustive series of tests to isolate and correct the problem. At the same time, mission managers have been debating the possibility of amending the launch rule to permit a flight with three of four operational sensors if troubleshooting failed to pin the problem down but could at least show it was not a generic issue.

Testing to date has been carried out under ambient conditions at the launch pad, using laboratory analysis and software to mimic the launch-day environment. Some engineers favored another tanking test prior to any additional launch attempts to collect data in the actual operating environment. But the mission management team today ruled out that option and agreed to press on into a countdown.

The decision came after a lengthy meeting in which engineers reviewed the progress of troubleshooting to date, including the rationale for amending the four-of-four launch commit criterion.

In the end, the rule was not amended, in part because engineers believe they will be able to prove the problem is not generic; and because they may have found a possible "smoking gun" that might explain why engine cutoff sensor No. 2 acted erratically during Discovery's countdown last Wednesday.

"We have the most probable causes that we've listed in a fault tree analysis," Parsons said. "We have work to do to close that fault tree analysis out, that's all part of this troubleshooting plan. We will go and close out all the common causes we think this could be.

"When we get to that point, then we've done everything we can. That's kind of where we're going to get to, we've done everything we can and we've eliminated the most probable cause. ... We believe the best way to go through this is to do a countdown.

"If the sensors work exactly like we think they will, then we'll launch on that day," Parsons said. "If anything goes not per the plan that we've laid out in front of us, then we'll have a scrub and we'll have to talk about it and either we can fix that and do a quick turnaround or we'll have other issues on our plate. But right now, we believe we've eliminated all the common causes that we think could do this and we've done everything we possibly could on the vehicle."

Extensive testing turned up subtle grounding problems in the orbiter that could permit electrical interference that, in turn, could cause a sensor to "fail wet," which is what happened to sensor No. 2 during a pre-launch test last week.

The ECO sensors are part of a backup system that would be used to shut down the main engines in the event of some other major malfunction that might cause the powerplants to run too long, draining the external tank. Engineers believe a main engine would tear itself apart if it suddenly ran out of fuel while operating. But in some remote scenarios, that could happen if three of the four ECO sensors failed wet, meaning they continued showing fuel in the tank when, in fact, it was dry.

Engineers plan extensive troubleshooting to identify possible sources of interference and its effects in an electronics unit called a point sensor box, which routes sensor data to the shuttle's computers. Candidates include the circuitry for new post-Columbia heaters added to the shuttle's external tank to prevent ice formation.

The grounding problems will be fixed regardless. If the heaters are, in fact, the source of the interference, Discovery would be good to go because the heaters are turned off before launch. But additional testing is required to prove this hypothesis and it may or may not pan out.

"This has been a very, very thorough effort that we've been through," said John Muratore, the engineer leading the troubleshooting. "We've used every kind of analysis technique and test technique that we can find.

"In the end, analysis of the circuitry suggested that grounding might be a problem. Testing of the (point sensor) box ... in the lab in Houston suggested electromagnetic interference could interfere with the box. We went looking in the vehicle and we found a discrepancy in the grounding of the vehicle.

"We're going to go in tonight and try to replicate as best we can the electromagnetic environment of launch to see if we can trap the signature of it," Muratore said. "Even if we can't trap the signature, we're going to go make the system good. And then the best way to go and further understand the problem is ... to load the tank up and observe its operation and that's best accomplished in the environment of a launch countdown."

He said the grounding problem was extremely subtle, adding "we are talking in fractions of a milli-ohm (of resistance).

"That doesn't seem like very much," he said. "But it matters from the point of view of the box. If the box has these different channels of electronics running through it and it's got power and grounding for each of those separately and if they start to get off a little differently from each other, that's when the circuits can get tripped up. So a very small amount of resistance can cause those grounds to flip up and down and that can happen when you get a large electrical transient" in another system.

Like an electrical heater turning on or off, or the operation of some other system.

"This isn't a case of running around seeing what's not plugged in," Muratore said. "It isn't that kind of thing. It's where you go look at it and you say that looks like a good bond. If you didn't use a very, very careful precision meter, it would read as a perfect structural ground. But in these very complex electrical systems, you can get into that kind of problem."

Regardless of how the grounding/interference issue is resolved, engineers plan to swap the electrical cables leading from the point sensor box to sensors 2 and 4. The system will be tested Tuesday before the crew climbs aboard.

If another problem shows up with sensor No. 2, engineers will be back to square one and the launch will be scrubbed. But if sensor No. 4 acts erratically, they will have solid evidence the fault is in the electrical circuit and not a generic issue that could affect the other three sensors.

In that case, NASA's mission management team likely would sign an "exception" to the current launch commit criterion requiring four operational sensors for launch and allow Discovery to take off three of four.

"We're going to hook sensor 4 up to the electronics for sensor 2 and vice versa," Muratore said. "That way, if the problem is with the sensor or the associated wiring it's going to show up now as a sensor 4 problem. If the problem is in the point sensor box, it's going to show up as a sensor 2 problem. That will enable us to determine which side of the system it is. That assumes the problem recurs exactly the way it happened before. But we think that will give us an important insight if the problem does reoccur as to what the potential cause is."

Parsons explained it this way: "The scenarios would be, all four sensors work and you're good to go; sensor 4 shows a failure ... that would say there's a strong possibility we understand this completely and we're good to go because it's a wiring problem, an open in a wire, and we still have three sensors that are good.

"But if it came back as a sensor 2 problem, I think everybody would have to say wait a minute, whatever we've done hasn't really solved our issues and we need to step back and take another hard look at this. So what we've done is given ourselves another safety net, another way to take a look at this and make sure we really do understand this system and it is operating exactly like we think it should operate."

But he said engineers "expect to have four of four sensors" operating Tuesday.

"We have talked in great detail about a rationale for flying with three of four sensors," he said. "We are not complete with that. But in fact, if we had a failure of the sensor and we could understand the failure and it was a known failure that we expected based on the pin swap, then we might very well be willing to go fly with three of four sensors. There's good flight rationale behind that."

But that remains an open question because not everyone believes NASA should change an LCC in the heat of a countdown. James Voss, a former astronaut and a CBS News consultant, reflected those views, saying "even though my personal feeling is it's probably safe, they can find ways to launch and it would still be a safe launch, with an anomaly that's unexplained like this, I don't think it's the right thing to do from a philosophical standpoint."

Mike Wetmore, director of shuttle processing at the Kennedy Space Center, said the current plan calls for launch attempts July 26 and 27, after which the team would stand down for 48 hours to service the shuttle's electrical generators. Two more attempts could be made on July 29 and 31 if necessary, with fuel cell servicing in between.

Discovery's launch window closes July 31. But mission managers are studying the possibility of extending it a few more days if necessary by giving up optimum lighting in orbit to photograph the external tank after it separates from the shuttle.

NASA managers, engineers and their contractor counterparts face a busy day of meetings to discuss troubleshooting to resolve a hydrogen fuel sensor problem that grounded the shuttle Discovery last week. Earlier this week, engineers discovered a possible electrical grounding problem aboard Discovery, but it's not yet known whether that issue is still on the table or whether it has any bearing on the behavior of the engine cutoff - ECO - sensor that triggered the launch delay.

NASA's mission management team plans to meet today to assess the troubleshooting to date and to make decisions about how to proceed. The agency continues to target next Tuesday as a "no-earlier-than" launch date, but the team is now discussing the possibility of another tanking test Monday to collect additional data.

A news conference is planned late today and this page will be updated as required. For detailed background on how the ECO sensors operate, graphics showing their location and a chronology of Discovery's sensor problems to date, see the CBS News/Spaceflight Now ECO sensor page.

In the meantime, SpaceCalc has been updated (to rev. K) to reflect the July 26 launch target. The revised flight plan is posted below, along with a preliminary look at the countdown.

But readers should use the flight plan for general guidance only. It is being updated by NASA to reflect changes in how the robot arms aboard the shuttle and the space station will be used due to yesterday's relocation of a Soyuz spacecraft and those changes are not yet incorporated in SpaceCalc or in the detailed flight plan below.

The countdown start time also will change, but the launch day activities and events are accurate as is. Again, updates will be posted here as new information becomes available.

All that said, here's a quick look at major mission events assuming an on-time launch Tuesday (times in EDT/MET):

DATE.EDT........HH...MM...SS...EVENT

07/26/05
Tue  10:39 AM...00...00...00...STS-114 Launch
Tue  10:48 AM...00...00...09...Main engine cutoff
Tue  11:17 AM...00...00...38...OMS-2 rocket firing
Tue  04:39 PM...00...06...00...STS crew sleep begins

07/27/05
Wed  12:39 AM...00...14...00...STS crew wakeup
Wed  05:24 AM...00...18...45...OBSS thermal protection system survey
Wed  10:49 AM...01...00...10...OBSS berthing
Wed  03:39 PM...01...05...00...STS crew sleep begins
Wed  11:39 PM...01...13...00...STS crew wakeup
Wed  11:39 PM...01...13...00...ISS crew wakeup

07/28/05
Thu  01:29 AM...01...14...50...Rendezvous timeline begins
Thu  04:29 AM...01...17...50...TI rendezvous rocket firing
Thu  07:16 AM...01...20...37...Discovery docks with ISS
Thu  09:19 AM...01...22...40...Handshake/Welcome
Thu  03:39 PM...02...05...00...STS/ISS crew sleep begins
Thu  11:39 PM...02...13...00...STS crew wakeup

07/29/05
Fri  12:09 AM...02...13...30...ISS crew wakeup
Fri  03:09 AM...02...16...30...cargo module installation begins (time change expected)
Fri  03:39 PM...03...05...00...STS/ISS crew sleep begins
Fri  11:39 PM...03...13...00...STS crew wakeup

07/30/05
Sat  12:09 AM...03...13...30...ISS crew wakeup
Sat  04:34 AM...03...17...55...EVA-1: Airlock depress
Sat  11:14 AM...04...00...35...EVA-1: Airlock repress
Sat  03:39 PM...04...05...00...STS/ISS crew sleep begins
Sat  11:39 PM...04...13...00...STS crew wakeup

07/31/05
Sun  12:09 AM...04...13...30...ISS crew wakeup
Sun  03:09 PM...05...04...30...STS/ISS crew sleep begins
Sun  11:09 PM...05...12...30...STS crew wakeup
Sun  11:39 PM...05...13...00...ISS crew wakeup

08/01/05
Mon  04:04 AM...05...17...25...EVA-2: Airlock depress
Mon  10:44 AM...06...00...05...EVA-2: Airlock repress
Mon  03:09 PM...06...04...30...STS/ISS crew sleep begins
Mon  11:09 PM...06...12...30...STS crew wakeup
Mon  11:39 PM...06...13...00...ISS crew wakeup

08/02/05
Tue  06:04 AM...06...19...25...Crew news conference
Tue  03:09 PM...07...04...30...STS/ISS crew sleep begins
Tue  11:09 PM...07...12...30...STS crew wakeup
Tue  11:39 PM...07...13...00...ISS crew wakeup

08/03/05
Wed  04:04 AM...07...17...25...EVA-3: Airlock depress
Wed  10:14 AM...07...23...35...EVA-3: Airlock repress
Wed  03:09 PM...08...04...30...STS/ISS crew sleep begins
Wed  11:09 PM...08...12...30...STS crew wakeup
Wed  11:39 PM...08...13...00...ISS crew wakeup

08/04/05
Thu  07:54 AM...08...21...15...Cargo module berthing (time change expected)
Thu  03:09 PM...09...04...30...STS/ISS crew sleep begins
Thu  11:09 PM...09...12...30...STS crew wakeup
Thu  11:39 PM...09...13...00...ISS crew wakeup

08/05/05
Fri  01:09 AM...09...14...30...Farewell ceremony
Fri  01:24 AM...09...14...45...Egress and hatch closure
Fri  04:17 AM...09...17...38...Discovery undocks from ISS
Fri  01:39 PM...10...03...00...STS crew sleep begins
Fri  09:39 PM...10...11...00...STS crew wakeup

08/06/05
Sat  12:39 AM...10...14...00...Flight control system checkout
Sat  12:39 AM...10...14...00...Cabin stow begins
Sat  01:39 PM...11...03...00...STS crew sleep begins
Sat  09:39 PM...11...11...00...STS crew wakeup

08/07/05
Sun  12:49 AM...11...14...10...Deorbit timeline begins
Sun  04:50 AM...11...18...11...Deorbit to KSC (rev. 186)
Sun  05:54 AM...11...19...15...Landing at KSC

NASA managers today told the shuttle launch team to gear up for a possible attempt to launch Discovery next Tuesday if troubleshooters can either fix the fuel sensor problem that grounded the ship last week or gather enough data to prove it won't affect other sensors this time around.

Engineers have not yet found a "smoking gun" that might explain why fuel sensor No. 2, one of four in Discovery's liquid hydrogen tank, acted erratically last Wednesday, forcing launch managers to scrub the countdown a little more than two hours before blastoff. NASA's launch rules require all four sensors to be operating properly for a countdown to proceed.

Ed Mango, a senior NASA manager helping oversee the troubleshooting work, said engineers are about two thirds of the way through a detailed series of tests and inspections at the launch pad, work that should be wrapped up by Wednesday. Other teams are wrapping up reviews of past sensor performance, failure modes, possible fixes and the rationale behind NASA's current "four-of-four" sensor launch rule.

If engineers track down the problem in the next two days, they will either make repairs or, if that's not possible, attempt to prove the flaw will not affect any of the other sensors. Under the former scenario, NASA could stick with the current four-of-four sensor rule and launch Discovery at 10:39 a.m. Tuesday.

Under the latter scenario, agency managers could amend the sensor rule to permit a launching with three-of-four operational sensors. The sensors would be checked or even tested during the fueling process and, if the other three sensors operated normally, Discovery's crew could be cleared for launch.

Engineers also could opt to run a stand-alone tanking test Tuesday, recycle the countdown and, depending on the results, make a launch attempt the next day or the day after that.

As of this writing, there are simply too many variables to predict how the next launch campaign might play out. And in any case, NASA managers are still holding out hope it won't come to that.

"Our number one goal here is to find this problem and fix this problem," said Wayne Hale, deputy shuttle program manager and chairman of NASA's mission management team. "To date we have not found it, so we're not able to fix it.

"The next step, if we are unfortunate in the next 24 to 48 hours, is is to consider going to cryogenic temperatures, that is to say, reload the external tank and see what happens at those cryogenic temperatures. We're going to let the team concentrate on the ambient testing and see if they can find something in the next couple of days."

But if nothing turns up, "then we need to go to the next level and that's the cryogenic-level testing," Hale said.

It is possible that the ultra-low temperatures of the supercold propellants played a role in the sensor problem, perhaps by causing a wire or a connector to contract enough to open a circuit. That is the type of failure that might not show up during tests at ambient temperatures. But it might show up during a fueling test.

"Today at the mission management team, we reviewed the timelines to support that and we think the next opportunity to tank the vehicle would be no earlier than Tuesday, the 26th," Hale said. "There is some debate as to whether or not we could, in fact, do the kinds of tests we need to do at cryo temperatures in a launch countdown and go ahead and launch that day, or whether we need to do a test, detank, recycle and think about the data.

"So that decision is before us," Hale said. "But the next tanking would be no earlier than Tuesday. Again, hopefully in the next 24 or 48 hours we will find the glitch that's got us all confused, or frustrated, or find your adjective, and be able to fix it and go forward."

The four hydrogen engine cutoff - ECO - sensors are part of a backup system that ensures the shuttle's main engines shut down properly before running out of fuel. An operating shuttle main engine likely would tear itself apart if it simply ran out of gas (for detailed background on how the sensors operate, graphics showing their location and a chronology of Discovery's sensor problems to date, see the CBS News/Spaceflight Now ECO sensor page).

While the consequences are extreme, multiple failures would be required for the worst-case scenario to play out: A major malfunction of some sort would have to affect engine performance and then three of the four ECO sensors would have to fail "wet," indicating the presence of rocket fuel when, in fact, the tank was dry.

The odds of such multiple failures are remote, unless some sort of generic problem is lurking in the system. But in the absence of an obvious culprit, that's a major question mark as this week's testing continues.

Another major question mark is how NASA, in the wake of the Columbia disaster, might ultimately justify changing a launch commit criteria in the heat of a launch campaign, when managers are under considerable self-imposed pressure to get the shuttle back into space as soon as possible.

Discovery's launch window is defined primarily by the international space station's orbit and a requirement to launch the first two post-Columbia flights in daylight. NASA wants good lighting at launch and when the external tank separates half a world away for photography showing the condition of the shuttle and the external tank.

The current launch window closes July 31. The next window opens Sept. 9 and the year's final window opens in November. Agency managers are looking into the possibility of extending the July window through Aug. 4 by relaxing the orbital lighting requirements for photographing the tank after it separates from the shuttle.

Hale said today he was aware some observers might get the impression the options under discussion show NASA has "go fever," especially given the discussion about re-evaluating the rationale behind the ECO sensor launch rule and talk of extending the launch window. But he insisted safety is the agency's top priority and that it only made sense to preserve launch options while troubleshooting continues.

As it turns out, NASA's original launch commit criteria called for three-of-four operational ECO sensors. But in the wake of the Challenger disaster, a review of shuttle systems revealed that two of the sensors were powered or controlled by a single component in an electronic black box "upstream" of the sensor system. Because the failure of that component could take down two sensors, the rule was changed to require four-of-four at launch.

A modification was proposed several years ago and implemented during Discovery's last major overhaul, eliminating the single-point failure mode. This will be the first flight for the modification. In the downtime after the 2003 Columbia disaster, engineers discussed changing the LCC back to three-of-four, but ultimately decided to let the rule stand as is.

"The discussion which we had briefly before getting into the launch count is, are we ready to go back to three-of-four as the launch commit criteria?" Hale explained today. "And folks said, we're really busy trying to return to flight, we've got a lot of work on our plate that requires some serious thought, let us not make that change for the first flight. We'll think about it downstream.

"Well, now everybody's interested in that. So we're thinking about that."

In the absence of an obvious problem with an equally obvious solution, NASA may be forced to amend the four-of-four rule to get Discovery off the ground during the current launch window. For now, however, the launch team is simply keeping its options open while the testing plays out at pad 39B.

"What we're doing right now is, we're preserving the opportunity that if we found something in the troubleshooting that's ongoing today, if something jumped out and we said there's the problem, then that tanking test would actually be the launch countdown and we would go on the 26th," said shuttle program manager Bill Parsons.

"What we're saying, though, is if we haven't found anything by then and this technical community comes back and says look, we need to make a few changes, or we want to put instrumentation in the orbiter and we think we need to do a tanking test, then the 26th becomes that tanking test.

"What we're doing is, we're preserving schedule," Parsons said. "We're trying to keep this on schedule as much as we can right now and preserve our options until the last opportunity. The tanking test we would do would be in a countdown configuration. We understand that process."

After a long weekend of troubleshooting, engineers have not found an obvious problem that might explain why one of four hydrogen fuel sensors failed to operate properly during the shuttle Discovery's aborted countdown Wednesday.

Testing and inspections continued into the night Sunday, but initial hopes of finding an obvious, easy-to-fix problem faded with each passing hour. NASA's mission management team, which has met every day since Discovery was grounded, will meet again on Monday to review the weekend's progress, the results of ongoing analyses and to consider possible courses of action.

As of this writing, launch remains targeted for no earlier than late this week. Whether or not that target remains even theoretically possible past Sunday, NASA managers have not given up on getting Discovery off the ground before its current launch window closes July 31, assuming the ongoing tests eventually isolate the problem that grounded Discovery in the first place.

Among a variety of options now on the table, sources said, is another fueling test, either a stand-alone exercise or a tanking test conducted as part of an actual launch countdown, to further isolate the problem. But as of this writing, those are merely options on the table for discussion and no such decisions have been made.

Discovery's launch on the first post-Columbia mission was aborted July 13 a little more than two hours before the planned liftoff when engine cutoff - ECO - sensor No. 2 failed to respond properly to an automated test. The cutoff sensors are part of a backup system that ensures the shuttle's main engines don't run too long, draining the tank dry, if some other problem prevents a normal, on-time shut down. An engine that ran out of fuel while running at full throttle likely would tear itself apart and NASA's launch commit criteria require all four ECO sensors to be operational for a countdown to proceed.

The ECO sensor system is simple in concept but complex in operation. Rather than repeat those details here, please see the CBS News/Spaceflight Now ECO sensor page for graphics, details about the system's operation and a chronology of the sensor problems that have plagued Discovery since its initial tanking test April 14.

While engineers are leaving no stone unturned trying to isolate the problem and fix it, NASA managers have directed one of the troubleshooting teams to study the rationale for amending the launch rules to permit a takeoff with just three of four operational ECO sensors. That's assuming, of course, engineers can successfully demonstrate that whatever prevented ECO sensor No. 2 from operating properly during Discovery's countdown won't affect other sensors after launch.

Several NASA sources familiar with the discussions pointed out the ECO sensor system is a backup that only comes into play if some other major problem affects engine performance or fuel usage. On top of that, the ECO system is internally redundant. Even if another problem required the ECO system to work, three of the four sensors would have to fail "wet" - falsely indicating they were still submerged in liquid hydrogen - before the system would allow the engines to drain the tank. On top of all that, the shuttle is launched with more propellant than it needs.

But in the wake of the Columbia disaster and the recommendations of the Columbia Accident Investigation Board, it may be difficult for NASA to change an LCC "on the fly," regardless of how technically sound the engineering rationale might be. And in any case, deputy shuttle program manager Wayne Hale, chairman of NASA's mission management team, is on record saying he likely would oppose such a move.

"Going down the logic path, one of our safety requirements on this vehicle is that we are two-fault tolerant in our electronics," Hale said Friday. "We can take two failures and can continue to keep on flying safely. And anytime you step away from that standard, you incur risk and you'd better make sure you have an airtight story to step away from that posture.

"If we get to the end of all this troubleshooting and everything's working fine, we may come around to the discussion of 'what if.' But we're not ready to go there yet."

Many engineers believe a tanking test of some sort will be required if the team fails to find anything wrong in the electronics aboard Discovery that route data from the fuel tank sensors to the shuttle's flight computers.

If a test ultimately is approved, engineers may swap the wires leading from a control box to ECO sensors 1 and 2. If the No. 2 sensor malfunctioned again, engineers would have evidence the sensor itself, and not the wiring, is suspect. If the No. 1 sensor acted up, it would indicate a wiring problem. It's also possible all four sensors would operate normally. Either way, engineers would have additional data to help make an informed decision about how to proceed and whether or not amending the four-of-four ECO sensor launch constraint could be justified.

It's not yet clear, however, whether a tanking test is possible before Discovery's launch window runs out July 31. The end of the window is defined by the lighting in space when the shuttle's external tank separates. NASA managers want good lighting so cameras on the shuttle can photograph the tank for signs of foam insulation loss.

Extending the launch window to Aug. 4 would result in a bit more shade on parts of the tank as seen by a camera mounted in the belly of the space shuttle. Initial studies indicate the camera could still achieve the desired results, but again, no final decisions have been made.

The window cannot be extended past Aug. 4 under any scenario because the lighting for the shuttle launch itself would not be sufficient.

Engineers working inside the shuttle Discovery's cramped engine compartment this weekend have not yet found any obvious signs of trouble that might explain what caused one of four fuel sensors to operate improperly during last Wednesday's aborted countdown.

NASA managers are holding open the possibility of launching Discovery late next week, but that assumes the problem is found in the next few days, that a quick fix is possible and that engineers can demonstrate the problem won't crop up again.

Initial inspections carried out late Friday and today show the wiring in Discovery's engine compartment that routes data from the external tank fuel sensors to an electronic "black box" in avionics bay No. 5 appears to be in good shape with no obvious defects. But additional tests are planned and troubleshooters remain hopeful they will eventually find the culprit.

The engine cutoff - ECO - sensors serve as a critical safeguard in the event of other problems that could cause a main engine to shut down early or run too long. All four must be operational for a countdown to proceed (ECO sensor background, graphics and a chronology of Discovery's sensor problems are available from Spaceflight Now and CBS News.

Earlier today, NASA released a statement from Discovery commander Eileen Collins, who said the astronauts are "keeping in close touch with the troubleshooting plan."

"We have confidence that the best people are working it," she said. "While the launch delay is disappointing, we have strong confidence that the mission will launch safely and successfully."

Engineers are working around the clock and through the weekend, putting on a "full-court press" to recreate, isolate and eliminate the fuel sensor problem that grounded the shuttle Discovery Wednesday," NASA officials said late today.

If troubleshooters get lucky and find an obvious problem with a quick fix, Discovery could be ready for another launch attempt by late next week, according to deputy shuttle program manager Wayne Hale. If not, mission managers are studying the possibility of extending Discovery's launch window from July 31 to Aug. 4. Either way, the next launch window opens Sept. 9.

"We are not in any sense of the word becoming pessimistic about making the July launch window," Hale said. "We are here for the duration, we're committed to giving this the good old college try until we get the problem resolved."

Discovery was grounded Wednesday when one of four hydrogen fuel sensors at the bottom of the shuttle's external tank failed a pre-flight test. The sensors serve as a critical safeguard in the event of other problems that could cause a main engine to shut down early or burn too long. All four must be operational for a countdown to proceed.

NASA ran into problems with the engine cutoff - ECO - sensors and associated electronics earlier this year, trigging extensive tests and troubleshooting (see below for details about sensor operation and logic). Engineers never succeeded in duplicating the problem or identifying what caused it. Instead, they replaced virtually all of the cables, connectors and the so-called point sensor box in the shuttle's engine compartment that relays sensor data to the ship's computers.

Still, NASA went into Discovery's countdown with the sensor problem characterized as an "unexplained anomaly." As the countdown cutoff demonstrated, the problem was aptly named (see below for a chronology of the ECO sensor issue with Discovery).

John Muratore, a senior manager and troubleshooter based at the Johnson Space Center in Houston, said 12 teams of experts are studying the problem "very diligently to try to understand what are the set of conditions that are causing this problem so we can recreate it, isolate it and eliminate it from the system."

"To do that, we have 12 teams of people working across the country, they're going to work all weekend," he said. "We have engineering tag-ups scheduled Saturday, Sunday, Monday, and we're planning to go basically every day until we have got the problem identified and isolated and a solution in work."

Asked if he was optimistic about a quick fix in light of the team's failure to solve the original sensor problem, Hale said he remains "very hopeful because we're taking this troubleshooting to a significantly higher level than we took it the first time."

"The first time through, we didn't involve nearly as many folks, we didn't have the data as well coordinated as we have it now," he said. "I feel very confident that we will get a solution to our problems. Now it's not out of the woods yet, I don't want to mislead you. But I'm very hopeful."

Over the weekend, engineers plan to re-enter the shuttle's aft engine compartment to gain access to avionics bay No. 5 where Discovery's point sensor box is located. Wiring will be inspected and various connectors will be checked to make sure the system's electrical continuity is intact and there are no obvious shorts. They also plan to use a test device to simulate sensor data to gain additional insights into the behavior of the point sensor box electronics.

"We are putting a full-court press on this to resolve this anomaly," Hale said.

The ECO sensor system is ingenious, complex and until this point, very reliable. Here's a brief overview:

ECO SENSOR BACKGROUND
Twenty four propellant sensors are used in the shuttle's external tank, 12 each in the oxygen and hydrogen sections. Eight are used in each tank to measure the amount of propellant present before launch. Four in each tank, known as engine cutoff - ECO - sensors, are part of a backup system intended to make sure the ship's engines don't shut down too early, resulting in an abort, or run too long, draining the tank dry with potentially catastrophic results. All four ECO sensors in each tank are required to be operational for a countdown to proceed.

The hydrogen ECO sensors are located at the very bottom of the tank near the entrance to the pipe that carries hydrogen into the shuttle's engine compartment.

The cutoff sensors are armed late in the ascent when a relatively small amount of rocket fuel remains in the tank. Once armed, the shuttle's computer system checks the status of each sensor, which is still immersed in cryogenic propellant, to make sure it is "wet." To protect against a faulty sensor, the first "dry" indication from any one of them is discarded.

During normal operations, the shuttle's flight computers continuously calculate the orbiter's position and velocity, using that data to figure out when the engines should be shut down to achieve the desired target. As a backup, the computers also monitor the ECO sensors as the tank empties to protect against unexpected problems that might affect the performance of the propulsion system.

The shuttle is launched with more fuel than it needs and in normal operation, the ECO sensors would never be "dry" before the normal guidance-based engine shutdown sequence begins. But if a problem does occur, and the computers detect two "dry" sensors, they will shut the engines down to keep from running the tank dry. As long as at least three sensors indicate "wet," however, fuel is assumed to be in the tank and the engines will keep running.

Once the system is armed, two sensors must fail "dry" to trigger an inadvertent engine shutdown. Before arming, three sensors must fail "dry." If three sensors fail "wet," the engines could run the tank empty.

The odds of such multiple failures are "extremely remote," according to internal NASA documents describing earli er problems. In fact, no cutoff sensors have failed in flight since the sixth shuttle mission in 1983 when the design was changed.

But the consequences of an early or late engine shutdown are extreme. A premature shutdown could prevent a crew from reaching orbit while a late shutdown could result in an engine fire or explosion. Even though the cutoff sensor system is considered a backup to the shuttle's flight computers, NASA's launch commit criteria require four operational cutoff sensors in each tank to provide multiple layers of redundancy.

The engine cutoff sensor system has been put to the test only two times in the history of the shuttle program.

During the shuttle Challenger's launching July 29, 1985, on mission STS-51F, a main engine shut down five minutes and 43 seconds after blastoff because of an internal temperature sensor failure. The fuel consumption of the two engines that kept running was affected and the end result was an ECO sensor engine cutoff.

The only other such shutdown in shuttle history occurred during Discovery commander Eileen Collins' last flight, mission STS-93, when a hydrogen leak in the coolant tubes making up main engine No. 3's nozzle caused more oxygen to be consumed than expected. In that case, oxygen ECO sensors went "dry," triggering engine shutdown.

In both cases, the shutdowns happened late in the ascents and both shuttle crews were able to complete their missions (Challenger's crew ended up in a lower-than-planned orbit due to the.

At today's briefing, Hale was asked why NASA required four operational sensors at launch given the seemingly small chance that multiple failures could occur on any given flight.

"Going down the logic path, one of our safety requirements on this vehicle is that we are two-fault tolerant in our electronics," he said. "We can take two failures and can continue to keep on flying safely. And anytime you step away from that standard, you incur risk and you'd better make sure you have an air-tight story to step away from that posture. If we get to the end of all this troubleshooting and everything's working fine, we may come around to the discussion of what if. But we're not ready to go there yet."

Discovery ECO sensor chronology
NASA has encountered a string of problems in recent weeks with the ECO sensor system in Discovery, glitches that have proven to be surprisingly difficult to resolve. The problems began during a tanking test in April when ECO sensors 3 and 4 operated intermittently.

Engineers removed an electronic controller, called a point sensor box, from Discovery and replaced wiring to the two sensors in question (wiring to sensors 1 and 2 wasn't touched). But the controller checked out OK and troubleshooters were unable to trace the cause of the problem A point sensor box from the shuttle Atlantis was installed and a second fueling test was conducted.

This time around, the sensors worked normally. But during additional post-test troubleshooting, the replacement sensor controller box malfunctioned. It was replaced by one taken from the shuttle Endeavour. NASA already had decided to replace Discovery's tank to address ice debris issues. With a fresh controller, replacement cabling, a new tank and solid test results, NASA managers decided to treat the sensor issue as an "unexplained anomaly" that presumably had been fixed.

But during Wednesday's countdown, the No. 2 low-level hydrogen sensor failed to switch from "wet" to "dry" during a test in which computers send signals to simulate a dry tank. When the tank was drained, the other three sensors changed from wet to dry as expected. The No. 2 sensor remained "wet" for another three hours before switching back to "dry."

A major concern for NASA is whether the problem affecting Discovery is generic or an isolated issue. The shuttle Atlantis is being prepared for launch in September and it also will serve as a rescue craft in case of problems with Discovery once it reaches orbit. The status of its cutoff sensor system is equally critical.

"Until you understand what the problem is, the potential exists for a generic or fleet-wide problem," Hale said. "That's one of the reasons why we want to resolve this, not just fix the particular problem that happened with the tank and orbiter combination we've got."

Said Muratore: "The bottom line is, we don't know if we're having a problem in the tank, we're having a problem in the wiring, we're having a problem in the electronics box."

"We don't know if the equipment is fine and it's just the environment that we're operating in is somehow subtly different, or we don't know if there's a problem in the equipment," he said. "And until we know that, everything is suspect. When we clear items by test and analysis, then we'll move on and decide whether we've got to deal with problems on other vehicles or if it's just limited to this one vehicle."

Launch of the shuttle Discovery is on hold until late next week at the earliest, NASA officials said today, to give engineers time to fully troubleshoot the fuel level sensor problem that grounded the spaceplane Wednesday.

The launch team at the Kennedy Space Center had been maintaining Discovery in an extended countdown "hold" while NASA managers debated their options. But earlier today, deputy shuttle program manager Wayne Hale said the work needed to find and fix the problem that grounded Discovery will take several days at least and that launch is out of the question until late in the week at the earliest.

And that assumes the problem can, in fact, be resolved by then.

Discovery's crew, meanwhile - commander Eileen Collins, pilot James Kelly, flight engineer Stephen Robinson, Andrew Thomas, Wendy Lawrence, Charles Camarda and Japanese astronaut Soichi Noguchi - was cleared to fly back to the Johnson Space Center to continue training and to await further developments.

The shuttle must get off the ground by July 31 or the flight most likely will slip into September. The launch window is defined by a complex combination of factors, including the shuttle's abiilty to reach the international space station; the nature of the station's orbit; and a NASA-imposed requirement to launch in daylight and to jettison the external tank half a world away with enough light to see potential foam insulation loss.

NASA managers are studying the possibility of extending the current launch to Aug. 4, but that would require giving up the lighting required to photograph the tank with a camera mounted in an umbilical cavity in the belly of the shuttle where propellant lines enter the engine compartment. That imagery considered a high-priority item and it's not clear that senior managers would be willing to give those pictures up.

Assuming the launch window is not extended, NASA would have find the problem, fix it, verify the fix and close up Discovery's aft compartment by around July 25 to preserve the final three days of the launch period. If the problem is resolved faster, of course, the countdown could be restarted earlier.

Early today, engineers unloaded hydrogen and oxygen used by the shuttle's electrical generators before opening the ship's aft compartment to gain access to the fuel sensor electronics that route sensor data to Discovery's flight computers.

Discovery was grounded Wednesday two-and-a-half hours before blastoff when a routine computerized test revealed problems with one of the four hydrogen fuel engine cutoff - ECO - sensors in the shuttle's external fuel tank.

The ECO sensor system is ingenious and relatively complex, as is the history of problems experienced by the shuttle Discovery in the weeks and months leading up to Wednesday's launch attempt. Readers already up to speed on these issues can skip the following explanation, which covers the operation of the system and what engineers have discovered since Discovery's first tanking test in April.

ECO SENSOR BACKGROUND
Twenty four sensors are used in the shuttle's external tank, 12 each in the oxygen and hydrogen sections. Eight are used in each tank to measure the amount of propellant present before launch. Four in each tank, known as engine cutoff - ECO - sensors, are part of a backup system intended to make sure the ship's engines don't shut down too early, resulting in an abort, or run too long, draining the tank dry with potentially catastrophic results. All four ECO sensors in each tank are required to be operational for a countdown to proceed.

The hydrogen ECO sensors are located at the very bottom of the tank by the entrance to the pipe that carries hydrogen into the shuttle's engine compartment. The cutoff sensors are armed late in the ascent when a relatively small amount of rocket fuel remains in the tank. Once armed, the shuttle's computer system checks the status of each sensor, which is still immersed in cryogenic propellant, to make sure it is "wet." To protect against a faulty sensor, the first "dry" indication from any one of them is discarded.

During normal operations, the shuttle's flight computers continuously calculate the orbiter's position and velocity, using that data to figure out when the engines should be shut down to achieve the desired target. As a backup, the computers also monitor the ECO sensors as the tank empties to protect against unexpected problems that might affect the performance of the propulsion system.

The shuttle is launched with more fuel than it needs and in normal operation, the ECO sensors would never be "dry" before the normal guidance-based engine shutdown sequence begins. But if a problem does occur, and the computers detect two "dry" sensors, they will shut the engines down to keep from running the tank dry. As long as at least three sensors indicate "wet," however, fuel is assumed to be in the tank and the engines will keep running.

Once the system is armed, two sensors must fail "dry" to trigger an inadvertent engine shutdown. Before arming, three sensors must fail "dry." If three sensors fail "wet" at any time, the engines could run the tank empty.

The odds of such multiple failures are "extremely remote," according to internal NASA documents describing earlier problems. In fact, no cutoff sensors have failed in flight since the sixth shuttle mission in 1983. But the consequences of an early or late engine shutdown are extreme. Even though the cutoff sensor system is considered a backup to the shuttle's flight computers, NASA's launch commit criteria require four operational cutoff sensors in each tank to provide multiple layers of redundancy.

NASA has encountered a string of problems in recent weeks with the ECO sensor system in Discovery, glitches that have proven to be surprisingly difficult to resolve. The problems began during a tanking test in April when ECO sensors 3 and 4 operated intermittently.

Engineers removed an electronic controller, called a point sensor box, from Discovery and replaced wiring to the two sensors in question (wiring to sensors 1 and 2 wasn't touched). But the controller checked out OK and troubleshooters were unable to trace the cause of the problem A point sensor box from the shuttle Atlantis was installed and a second fueling test was conducted.

This time around, the sensors worked normally. But during additional post-test troubleshooting, the replacement sensor controller box malfunctioned. It was replaced by one taken from the shuttle Endeavour. NASA already had decided to replace Discovery's tank to address ice debris issues. With a fresh controller, replacement cabling, a new tank and solid test results, NASA managers decided to treat the sensor issue as an "unexplained anomaly" that presumably had been fixed.

But during Wednesday's countdown, the No. 2 low-level hydrogen sensor failed to switch from "wet" to "dry" during a test in which computers send signals to simulate a dry tank. When the tank was drained, the other three sensors changed from wet to dry as expected. The No. 2 sensor remained "wet" for another three hours before switching back to "dry."

The engineering teams assessing the problem are reviewing the history of the sensors, cabling and control electronics, changes made in the fuel tank since Columbia's ill-fated flight in 2003, changes made to Discovery since it last flew in 2002 and any procedural changes that might possibly have an impact.

At launch pad 39B, meanwhile, engineers plan to verify the cabling leading from the point sensor box to various connectors and ultimately, into the tank through a complex umbilical connection.

If a problem ultimately is found in the point sensor box, NASA would still be faced with explaining the sensor problems encountered during the tanking test in April, the original "unexplained anomaly" that NASA managers accepted going into Wednesday's countdown.

Most engineers believe the problem likely is related to one or more changes made in the wake of the Columbia disaster, but at this point, that remains speculation.

The shuttle Discovery's delayed launch on the first post-Columbia mission is off until at least Sunday, officials said today, and unless engineers find an obvious, simple-to-fix problem in the next day or so, the flight will be put on hold indefinitely.

"If we were to get extremely lucky, it is theoretically possible that we could still launch on Sunday," said Wayne Hale, deputy shuttle program manager and chairman of NASA's mission management team. "But I've got to tell you that this represents a really optimistic, good-luck scenario, which I think is not very credible. I wish I had better news.

"What we're more likely into is several days of troubleshooting and we will know tomorrow, when we get the integrated troubleshooting plan back at the mission management team, we'll be able to give you a more definitive timeline. That's the best I've got right now."

What sort of fix might be possible in time to support a Sunday launch attempt?

"I think the repair that might get us to Sunday is if we go in (the shuttle's aft compartment) and wiggle some of the wires and find a loose connection," Hale told reporters. "I mean, that's the one that comes to mind. You laugh, but that probably is the first step in any troubleshooting plan.

"Some technician is going to put his hands on the wires and connectors ... and start wiggling them. If we found a loose connection, OK, we might be back in business. I've got to tell you, the folks who put those wires together and those connections do a really good job. So the chances of that, not high."

NASA is keeping Discovery in a countdown configuration just in case engineers come up with a quick fix. But shuttle processing manager Mike Wetmore said the team can't maintain that level of readiness indefinitely and that in all likelihood, they will back out of the countdown configuration Friday. At that point, launch would be on indefinite hold pending the outcome of much more extensive troubleshooting.

Hale said 12 engineering teams had been established to troubleshoot the issue, a "vast engineering team" focused on getting the problem fixed and Discovery off the ground as soon as possible.

But the shuttle's launch window closes July 31. The next window runs from Sept. 9 through Sept. 24. If Discovery can't be quickly fixed, the flight could slip into September, pushing the second post-Columbia mission into a relatively tight November window that opens Nov. 7 and closes Nov. 10. NASA managers are already looking into what might be done to extend the November window if worse comes to worse.

But Hale said, "I'm not ready to give up on the July window at this point. We still have more than two weeks ahead of us, so that's the way we're headed."

Discovery was grounded Wednesday two-and-a-half hours before blastoff when a routine computerized test revealed problems with one of four hydrogen fuel sensors in the shuttle's external fuel tank.

The sensors are part of a backup system intended to make sure the ship's engines don't shut down too early or run too long, draining the tank dry with potentially catastrophic results. All four sensors are required for a countdown to proceed.

NASA has encountered a string of problems in recent weeks with the engine cutoff - ECO - sensor system in Discovery, glitches that have proven to be surprisingly difficult to resolve. The problems began during a tanking test in April when two of the four sensors operated intermittently.

Engineers removed an electronic controller, called a point sensor box, from Discovery and replaced wiring and connectors. But the controller checked out OK and troubleshooters were unable to trace the cause of the problem A point sensor box from the shuttle Atlantis was installed and a second fueling test was conducted.

This time around, the sensors worked normally. But during additional post-test troubleshooting, the replacement sensor controller box malfunctioned. It was replaced by one taken from the shuttle Endeavour. NASA already had decided to replace Discovery's tank to address ice debris issues. With a fresh controller, replacement cabling, a new tank and solid test results, NASA managers decided to treat the sensor issue as an "unexplained anomaly" that presumably had been fixed.

But during Wednesday's countdown, the No. 2 low-level hydrogen sensor failed to switch from "wet" to "dry" during a test in which computers send signals to simulate a dry tank. When the tank was drained, the other three sensors changed from wet to dry as expected. The No. 2 sensor remained "wet" for another three hours before switching back to "dry."

"It is properly indicating that it's dry right now," Hale said. "Could we talk ourselves into going, after all of this, without doing anything? No."

Asked if shuttle engineers had to definitively resolve the problem before Discovery could be cleared for flight, Hale said "it's an unfortunate part of the business, but sometimes you live with unexplained anomalies."

"What we've got to do, I think, is to do all the troubleshooting we can to try to find the solution to the problem," he said. "In other words, you don't jump to the UA rationale first. You do all the troubleshooting you can to try to solve the problem first. If at the end of the day you've done everything that you know to do and you've got an unexplained anomaly, then you have to ask yourself, why am I safe to proceed? Where do we go to define logic that says we're safe to proceed?

"You can imagine different scenarios where you'd talk about those kinds of things. But you really have to have clear and convincing rationale that says we're safe to proceed. And that only comes after you've done all the troubleshooting you possibly can."

Hale said historically, the sensors have been reliable and until Discovery's recent problems, "we've not had any low level sensors fail."

"Now we're beginning to think there may be something else going on," he said. "The cluster here is kind of interesting because it's all been hydrogen sensors. And even though they're the same sensor going to the same electronics box as the oxygen sensors, people are beginning to ask is there something peculiar that we've done in the wiring, perhaps, that go to the hydrogen sensors. So that's one of the threads the troubleshooting team is looking into."

Finally, Hale was asked what he would tell the public about NASA's efforts to resolve the problems with Discovery.

"I would tell you that going into space is right at the limits of human technology," he said. "We're doing something that's extremely difficult. This is not like going to the airport and getting in a commercial airliner. This is much more complicated and much more difficult. There are only a few nations and the history of the world that have this capability and they've all had to expend quite a bit of national treasure and quite a bit of effort.

"In the future, one hopes it will become more commonplace and perhaps it will become more like commercial airline travel, but we are not there today. ... If you think this is routine, you surely don't understand what it is we're trying to do here."

Shuttle engineers worked through the night to collect data that might help pinpoint why one of four critical hydrogen fuel sensors failed a test late in the shuttle Discovery's countdown Wednesday, forcing NASA managers to scrub the agency's long-awaited return to flight.

The first post-Columbia flight is now on hold until at least Saturday, and that assumes engineers get a break and quickly discover what caused the problem and find a relatively straight-forward fix. But given the elusive nature of recent, still-unresolved problems with the so-called engine cutoff - ECO - sensors and their associated electronics, a quick fix would seem unlikely.

"Unexplained anomalies are the worst ones," said deputy shuttle program manager Wayne Hale. "What you'd really like to do is find the problem and fix it. So when you have a problem that kind of comes and goes and you can't put your finger on it, that's a tough issue."

Hale was referring to recent ECO sensor system problems collectively even though Wednesday's test failure indicated an actual failure, not an intermittent glitch. In any case, launch director Michael Leinbach had no choice. NASA flight rules forbid a launch if all four fuel depletion sensors are not operating normally.

"The vehicle, the ECO sensors, for some reason did not behave today, so we are going to have to scrub this launch attempt," Leinbach radioed Discovery's crew at 1:32 p.m. "I appreciate all we have been through together, but this one is not going to result in a launch attempt today."

Recent problems with the ECO sensor system dates back to April when two of the four hydrogen depletion sensors operated intermittently during a fueling test. Engineers removed a "black box" from Discovery that is used to format and relay data from the sensors to the shuttle's flight computers and subjected it to detailed analysis. But it operated normally.

Because the box had been disassembled, a unit from the shuttle Atlantis* was installed aboard Discovery for a second tanking test to troubleshoot the ECO sensor issue and other, unrelated problems. This time around, the sensors worked normally but the point sensor box malfunctioned later, during additional troubleshooting to understand what happened during the first test in April.

NASA managers had already decided to haul Discovery off the launch pad and to attach it to a fresh tank and a new set of boosters, in part to ensure that fuel sensors in the original tank were not at fault. Engineers replaced virtually all of the electrical cabling in the ECO sensor system and installed yet another point sensor box, this one from Endeavour.

The Endeavour box, serial number 111, included eight transistors that were known to be from a suspect lot. Engineers did not expect any problems, however, because the unit passed a battery of tests. A third tanking test was not ordered because engineers believed they had replaced everything that could have been at fault.

"We have a new tank that's different from the tank we used in the (April) tanking test, we had changed out every wire, cable, connector, electronics box associated with these sensors. We had done a lot of tests on the boxes and the various components, we felt like we had a good system.

"Today, we had another anomaly. So we're going to sit back and think about what the cause of that problem was and how we're going to rectify it."

The fuel depletion sensors are a critical safeguard against potentially catastrophic failures. Here is a bit more background from a reference manual written by shuttle-builder Rockwell International:

"There are eight propellant-depletion sensors, four each for fuel (hydrogen) and oxidizer (oxygen). The fuel-depletion sensors are located in the bottom of the fuel tank. The oxidizer sensors are mounted in the orbiter liquid oxygen feed line manifold downstream of the feed line disconnect.

"During SSME (space shuttle main engine) thrusting, the orbiter general-purpose computers constantly compute the instantaneous mass of the vehicle due to the usage of the propellants. Normally, main engine cutoff is based on a predetermined velocity; however, if any two of the fuel or oxidizer sensors sense a dry condition, the engines will be shut down.

"The locations of the liquid oxygen sensors allow the maximum amount of oxidizer to be consumed in the engines, while allowing sufficient time to shut down the engines before the oxidizer pumps cavitate (run dry). In addition, 1,100 pounds of liquid hydrogen are loaded over and above that required by the 6-1 oxidizer/fuel engine mixture ratio. This assures that MECO (main engine cutoff) from the depletion sensors is fuel-rich; oxidizer-rich engine shutdowns can cause burning and severe erosion of engine components."

"They're there to protect us in case we run out of gas," Hale said. "Now we don't plan to run out of gas. As a matter of fact, we launch with some fairly comfortable propellant reserves in the external tank to allow for dispersions that might happen within the launch phase. So you have to have something go wrong to really need these sensors. That's the first thing you really need to understand.

"There are four of them and the flight software waits until it gets close to MECO ... close to the time when you are ready to be inserted into orbital velocity, you're almost there, and then starts looking at these sensors. There are four of them and if two of them go dry, that indicates to the computer it's time to shut down the main engines to provide a safety margin on the engines."

To cause the main engines to run the hydrogen tank dry, three of the four ECO sensors would have to fail "wet," that is, send indications they were immersed in cryogenic liquid hydrogen when in fact they were not. Likewise, three of the four would have to fail "dry" to trigger a premature shutdown.

During a test Wednesday to verify the sensor system was working properly aboard Discovery, three sensors responded normally and one appeared stuck on the wet setting. But that's all it took, Hale said, because "we have a very clear and unambiguous criteria that says all four of those sensors must work to provide us the kind of redundancy and reliability that's necessary for safe flight.

"So when one of those indictors started acting up today, we decided it was time to quit."

Asked if NASA might amend the flight rules to get Discovery off the ground, Hale said: "The answer to that is no. Every time we have reviewed the rules, we've come to the same conclusion. They're in place for good reason."

Steve Poulos, a senior shuttle manager at the Johnson Space Center, said engineers planned to re-examine the system from top to bottom and "hopefully by tomorrow (Thursday) evening, we'll have some better insight into what the potential concern is. I wouldn't guarantee at this point we'll know exactly what we have. But we'll certainly have more information tomorrow."

As it turns out, all of the transistors from the suspect lot were on a circuit board used to test the operation of the sensor in question. But Poulos refused to speculate about a possible connection, saying he needed more data before drawing any conclusions.

It took three hours or so to drain the propellants from Discovery's tank Wednesday and Leinbach said it would take another 21 to complete a long "safing" process. If all goes well, the launch pad will be re-opened for normal work around 3 p.m. Thursday.

"We're virtually certain we'll go into the aft of the orbiter and at least look at the box," he said. "Anything beyond that is fairly invasive. If we have to start thinking about getting into the external tank, that's an extremely large job to do and one we've said in the past we'd prefer to do back in the Vehicle Assembly Building. It's not out of the question to go into the aft of the hydrogen tank at the pad, we've never done it at the launch pad and we'd have to take a hard look at that."

Given the troubleshooting so far, Poulos said "there is a potential problem that might exist in the sensor box. We do not think it has (anything to do) with the power supply cards within the box. But there could be a concern in the signal conditioning system itself. And then from there, it could be an open circuit for this particular sensor somewhere beyond the box into the external tank and then ultimately to the sensor itself."

Editor's Note...
I originally reported suspect data from two of four ECO sensors. NASA now says data from just one sensor was suspect.

Launch director Mike Leinbach scrubbed today's planned launch of the shuttle Discovery two-and-a-half hours before takeoff time because of data indicating one of four hydrogen fuel flow sensors in the ship's external tank did not operate properly in a test.

NASA's flight rules require all four to be working normally for a countdown to proceed because the sensors control how the ship's main engines shut down when the shuttle reaches space. In certain failure scenarios, the engines could run the tank dry - or shut down early - leading to potential catastrophic failures.

It was not clear what caused today's problem, but the sensor system has encountered unusual trouble in recent weeks, possibly due to suspect transistors in an electronics "black box" in Discovery's aft compartment. The so-called "point sensor box" aboard Discovery included eight transistors from a suspect lot, sources said.

Discovery commander Eileen Collins and her six crewmates - pilot James Kelly, flight engineer Stephen Robinson, Andrew Thomas, Wendy Lawrence, Charles Camarda and Japanese astronaut Soichi Noguchi - were in the process of strapping in when Leinbach radioed the bad news at 1:32 p.m.

"The vehicle, the ECO sensors, for some reason did not behave today, so we are going to have to scrub this launch attempt," he said. "So once we develop our scrub-turnaround plan we'll get that back to you. I appreciate all we have been through together, but this one is not going to result in a launch attempt today."

A new launch target was not immediately announced.

It was a frustrating disappointment for Discovery's crew and for thousands of tourists, area residents and the shuttle launch team, which labored virtually around the clock for two-and-a-half years to ready the shuttle for NASA's first flight since the Columbia disaster. Throughout today's smooth running countdown, the primary concern had been the weather, with rain showers popping up across the Kennedy Space Center as launch time approached.

It was doubly frustrating to mission managers who thought they had resolved earlier problems with the engine cutoff - ECO - sensors that played a role in a decision to equip Discovery with a fresh fuel tank.

NASA originally hoped to launch Discovery in mid May, but the flight was put on hold in the wake of an April 14 fueling test. During that exercise, two of the four sensors in the shuttle's original tank failed to operate properly and the flight rules require four-of-four for launch.

The cutoff sensors are used to make sure the shuttle's three main engines do not run out of hydrogen while running, which would cause a potentially catastrophic oxygen-rich shut down.

"We really don't want the engine to be running at high speed and suddenly run out of fuel," astronaut David Wolf explained from the launch control center. "That can lead to a devastating breakdown of the engine, even uncontained failure of the engine if that should happen.

"Therefore, we have four sensors on the vehicle and it requires two of those to detect a low-level fuel in order to cut off the engines early before they would run out. This only comes into play in special conditions on the ascent, not a nominal ascent, so it's essentially a backup for a backup situation."

While the odds of needing this sensor in flight are low, "in the proper spirit of safety to any really feasible or projectable failure we want a full system, certainly at the point of launch." In the wake of the April 14 tanking test and a subsequent test in May, engineers decided to replace Discovery's external tank with one being prepared for use by the shuttle Atlantis in September. The decision was made because of the sensor issue, because of concerns about ice buildups on a liquid oxygen line and because of a suspect pressure relief valve.

Troubleshooting the ECO sensor problem, engineers replaced all of the electrical cabling in Discovery that routes data from the sensors to the shuttle's computers and tore down the point sensor box in the shuttle's engine compartment that routes the data to the computers.

The original box was taken out after the April tanking test and disassembled. A box from the shuttle Atlantis was installed for the second tanking test and while it worked normally, it experienced problems later. Engineers tested a sensor box removed from the shuttle Endeavour, which ultimately was installed aboard Discovery. One of the other units will be reassembled, tested and re-installed in Atlantis.

The original failure was categorized as an "unexplained anomaly," meaning engineers never fully resolved what triggered the failures. But they were confident the hardware aboard Discovery was ready to go.

During a news conference Tuesday, NASA Administrator Michael Griffin said "ECO sensor issues are not so much a matter of the box on Discovery, we think that's good because that box has been put through an entire (testing) cycle and it's good to go. We have had more failures on some other boxes than we are comfortable with, which does lead to questions involving aging orbiter avionics."

Engineers began pumping a half-million gallons of supercold liquid oxygen and liquid hydrogen rocket fuel into the shuttle Discovery's external tank early today, setting the stage for blastoff on the first post-Columbia mission at 3:51 p.m.

Fueling was delayed more than an hour and a half to give engineers time to troubleshoot a circuit controlling one of two heaters in the external tank. The heaters are used to warm nitrogen gas in an area between the oxygen and hydrogen sections of the tank, preventing the nitrogen from condensing due to the ultra-low temperatures of the rocket fuel.

NASA's launch commit criteria require one of the heaters to be operational to press ahead with fueling, but NASA managers opted to hold up the process to let engineers fix the problem. The repairs were made at the base of the launch pad and propellants began flowing at 7:11 a.m.

There are no other technical issues of any significance this morning and launch remains on track for 3:50:53 p.m. The biggest question as of this writing remains the weather, with forecasters predicting showers and thunderstorms in the area as launch time approach.

The official forecast calls for scattered clouds at 3,000, 8,000 and 25,000 feet with winds out of 120 degrees at 8 knots with gusts to 14. Afternoon winds are expected to push the storms inland, but it's not at all clear whether that will happen in time to permit Discovery's launching. The forecast remains 60 percent "go." In short, it's a typical mid-summer day in Florida. Here are countdown highlights for the remainder of the day (in EDT):

HH...MM...SS...DATE/EDT............EVENT

06...50...00...07/13/05 08:55 AM...Begin 3-hour built-in hold
06...50...00...07/13/05 08:55 AM...Closeout crew to white room
06...35...00...07/13/05 09:10 AM...Crew cabin comm checks
06...05...00...07/13/05 09:40 AM...Pre-ingress switch reconfiguration
05...30...00...07/13/05 10:15 AM...Crew breakfast photo op
04...55...00...07/13/05 10:50 AM...Crew ingress weather update
04...25...00...07/13/05 11:20 AM...Crew weather briefing
04...15...00...07/13/05 11:30 AM...Astronauts suit up for launch
03...50...00...07/13/05 11:55 AM...Resume countdown
03...45...00...07/13/05 12:00 PM...Crew departs O&C building
03...15...00...07/13/05 12:30 PM...Crew begins strapping in for launch
02...25...00...07/13/05 01:20 PM...Astronaut communications checks
02...00...00...07/13/05 01:45 PM...Hatch closure
01...25...00...07/13/05 02:20 PM...White room closeout
01...10...00...07/13/05 02:35 PM...Begin 10-minute built-in hold
01...08...00...07/13/05 02:37 PM...NASA test director countdown briefing
01...00...00...07/13/05 02:45 PM...Resume countdown
00...59...00...07/13/05 02:46 PM...Backup flight computer configured
00...55...00...07/13/05 02:50 PM...KSC area clear to launch
00...49...00...07/13/05 02:56 PM...Begin final built-in hold
00...49...00...07/13/05 03:01 PM...Return-to-launch-site runway verification
00...44...00...07/13/05 03:06 PM...NASA test director launch status poll
00...09...00...07/13/05 03:41 PM...Resume countdown

HH...MM...SS...DATE/EDT...............EVENT

00...07...30...07/13/05 03:43:23 PM...Orbiter access arm retraction
00...05...00...07/13/05 03:45:43 PM...Launch window opens
00...05...00...07/13/05 03:45:53 PM...Hydraulic power unit (APU) start
00...04...55...07/13/05 03:45:58 PM...Terminate LO2 replenish
00...04...00...07/13/05 03:46:53 PM...Purge sequence 4 hydraulic system check
00...04...00...07/13/05 03:46:53 PM...Inertial measurement units configured
00...03...55...07/13/05 03:46:58 PM...Aerosurface movement check
00...03...30...07/13/05 03:47:23 PM...Main engine steering test
00...02...55...07/13/05 03:47:58 PM...LO2 tank pressurization
00...02...35...07/13/05 03:48:18 PM...Fuel cells to internal reactants
00...02...30...07/13/05 03:48:23 PM...Clear caution-and-warning memory
00...02...00...07/13/05 03:48:53 PM...Crew closes visors
00...01...57...07/13/05 03:48:56 PM...LH2 tank pressurization
00...00...50...07/13/05 03:50:03 PM...SRB joint heater deactivation
00...00...31...07/13/05 03:50:22 PM...Shuttle computers take control of countdown
00...00...21...07/13/05 03:50:32 PM...SRB nozzle steering test
00...00...07...07/13/05 03:50:46 PM...Main engine start (T-6.6 seconds)
00...00...00...07/13/05 03:50:53 PM...Booster ignition

Here is the NASA television schedule for today:

REV...EVENT.........................MET in DD/HH:MM...EDT........GMT

WEDNESDAY, JULY 13
FD 1
...LAUNCH............................KSC...00/00:00...03:51 PM...19:51
...MECO....................................00/00:08...03:59 PM...19:59
1...NASA TV TO JSC...................JSC...00/00:10...04:01 PM...20:01
1...NASA TV TO KSC...................KSC...00/00:17...04:08 PM...20:08
1...LAUNCH REPLAYS...................KSC...00/00:17...04:08 PM...20:08
1...POST LAUNCH NEWS CONFERENCE......KSC...00/00:54...04:45 PM...20:45
2...PAYLOAD BAY DOOR OPENING...............00/01:27...05:18 PM...21:18
3...FLIGHT CONTROL TEAM VIDEO........JSC...00/03:09...07:00 PM...23:00
3...ROBOT ARM (RMS) CHECKOUT...............00/03:10...07:01 PM...23:01
3...RMS IMAGING OF KU ANTENNA/OBSS.........00/03:55...07:46 PM...23:46
4...LAUNCH ENGINEERING REPLAYS.......KSC...00/05:09...09:00 PM...01:00
5...DISCOVERY CREW SLEEP BEGINS............00/06:00...09:51 PM...01:51
5...FLIGHT DAY 1 HIGHLIGHTS..........JSC...00/06:09...10:00 PM...02:00

Engineers replaced a tile-covered closeout panel today on one of the shuttle Discovery's twin rocket pods after a protective cockpit window cover fell off, fell 60 feet and dinged two heat-shield tiles. The repair work went smoothly and NASA managers said it was not expected to impact plans to launch the shuttle Wednesday on the first post-Columbia mission.

The plastic cover was taped in place over one of Discovery's two overhead cockpit windows. It may have pulled off when an inflatable seal around the shuttle's fuselage was deflated prior to moving a protective gantry away from the orbiter.

The space shuttle's orbital maneuvering system rocket pods, one mounted on either side of the vertical stabilizer, are interchangeable across the shuttle fleet and so-called carrier panels are used to cover the fittings that secure the pods to the fuselage. The carrier panel in question featured three heat shield tiles, two of which were damaged by the falling window cover.

Discovery processing manager Stephanie Stilson described the repair procedure as "minor" and said the replacement carrier panel fit well. Just to be safe, engineers at the Johnson Space Center were asked to conduct an overnight analysis to make sure the forces involved in the impact were not high enough to cause any structural damage. Stilson said they were confident no such damage occurred.

NASA Administrator Michael Griffin, saying the space agency has done everything humanly possible to fix the problems that led to the Columbia disaster, told reporters today the shuttle Discovery is "go" for launch Wednesday, weather permitting, on an "utterly crucial" mission.

"I just came from a very interesting mission management team meeting," Griffin said after the management poll to clear Discovery for flight. "The net result of all that is we're go for launch tomorrow, pending weather. We're not really working any significant issues, just working through normal closeouts and hoping the weather gods are kind for tomorrow."

Late today, after Griffin's remarks, engineers were called to launch pad 39B to troubleshoot damage to protective heat shield tiles on Discovery's left-side orbital maneuvering system rocket pad. A protective plastic cockpit window cover somehow fell off and struck a tile-covered panel. The so-called carrier panel protects attachment fittings that hold the rocket pod to the shuttle's fuselage.

A replacement carrier panel was taken to the launch pad for installation, a job engineers said typically takes about an hour. As of this writing, the work was not expected to impact Discovery's launching Wednesday.

NASA's mission management team reviewed a handful of other open issues today, including concern about excessive heating on the strut fittings that hold Discovery's nose to its external fuel tank and concern about suspect components in one of the ship's electronic "black boxes."

Griffin said all the open issues were resolved to the management team's satisfaction and Discovery's launch on the 114th shuttle mission, the first since Columbia's destruction two-and-a-half years go, remained on target for 3:50:53 p.m. Wednesday. The launch window closes at 3:55:53 p.m.

On board will be commander Eileen Collins, pilot James Kelly, flight engineer Stephen Robinson, Andrew Thomas, Wendy Lawrence, Charles Camarda and Japanese astronaut Soichi Noguchi. The goal of the mission is to deliver critical supplies to the international space station; to bring trash and no-longer-needed gear back to Earth; to install a replacement gyroscope in the station's orientation control system; and to test rudimentary shuttle heat-shield repair techniques.

"I had a chance earlier this morning to walk down the orbiter and to meet with the crew and the crew families and I can tell you that the crew is just raring to go and they guys who are doing all the closeout work on the orbiter and at the pad are also raring to go," Griffin said. "They're pumped. So we're looking forward to tomorrow, as I'm sure you are, after two and a half years down."

Forecasters are calling for a 60 percent chance of good weather Wednesday through Friday. One long-range concern is tropical storm Emily and even though its current track carries it well to the south of Cuba and into the Gulf of Mexico, NASA managers are paying close attention.

"Hurricanes are always out there, weather's always out there, we'll just deal with that as it comes," Griffin said. "Our job for tomorrow - and the whole team knows this - is to figure out if it's OK to fly tomorrow."

Given the long-range weather prospects - and the fact that Discovery's launch window closes July 31 - it could be argued NASA is under a fair amount of pressure to get the shuttle off the ground as soon as possible. But Griffin said that was not the case.

"How do we know we're not getting 'go' fever? Because we're working through the process, we're asking all the questions and answering them appropriately. If we can't answer them, we'll stop. It's that simple."

Asked how important Discovery's flight was to NASA, Griffin said: "Obviously, it is utterly crucial for NASA, for the nation, for our space program to fly a safe mission."

"We take it seriously. We have done everything that we know to do," he said. "I think we got everything that everybody knows about out on the table. Can there be something that we don't know about that can bite us? Yeah. This is a very tough business. A very tough business. But everything we know about has been covered."

He said he viewed Discovery's launch and "every space launch we do" as a "tribute to all those who have gone before, the crews who have died as well as the crews who have lived."

"In the course of trying to conduct spaceflight activities, we in the United States have lost three crews," Griffin said. "Russia has lost two. It's a dangerous business, it will be for the foreseeable future. We work every time to make it less dangerous than the time before. ... But this is a matter to be regarded with the perspective of generations, and not weeks and months. So I think every launch is a tribute to all those who have chosen to risk their lives for the benefit of this nation's progress in space exploration."

Asked if Discovery's flight would be the safest in shuttle history, Griffin said the spaceplane has not flown enough times to provide the statistical data needed to answer the question. While the problem that doomed Columbia presumably has been fixed, "I don't think you can ever take the viewpoint that with a given launch or a given success that we have vindicated ourselves."

"There is no recovery from mistakes we've made, whether it goes back to the Apollo (launch pad) fire, the loss of Challenger or the loss of Columbia," Griffin said. "Going back even further, through a hundred years of aviation, the safety lessons that we who fly have learned and know are written in other people's blood.

"The minute we say we're good enough, we start getting bad again and we need not to do that. So we'll be looking at our management culture, our decision-making processes, our engineering processes aggressively as long as we hope to continue to fly safely."

He reiterated that NASA had done "everything that we know to do" to make Discovery as safe as possible.

"There is nothing that we know of that we have not addressed," he said. "Are there things out there that we don't know about? There may be. We sure hope not."

Here is the remainder of Discovery's countdown (in HH:MM to launch and EDT):

HH...MM...SS...DATE/EDT............EVENT

19...00...00...07/12/05 08:45 PM...Crew sleep begins
16...50...00...07/12/05 10:55 PM...Resume countdown

15...40...00...07/13/05 12:05 AM...Fuel cell activation
14...35...00...07/13/05 01:10 AM...Pad clear of non-essential personnel
14...20...00...07/13/05 01:25 AM...Booster joint heater activation
14...05...00...07/13/05 01:40 AM...Fuel loading weather briefing
13...50...00...07/13/05 01:55 AM...Final fueling preps
12...35...00...07/13/05 03:10 AM...Clear launch danger area
11...50...00...07/13/05 03:55 AM...Begin 2-hour built-in hold
11...50...00...07/13/05 03:55 AM...Safe-and-arm SRB pyro-initiator test
11...20...00...07/13/05 04:25 AM...External tank ready for fueling
11...05...00...07/13/05 04:40 AM...Mission management team meeting
10...15...00...07/13/05 05:30 AM...NASA television coverage begins
10...00...00...07/13/05 05:45 AM...Crew wakeup
09...50...00...07/13/05 05:55 AM...Resume countdown
09...50...00...07/13/05 05:55 AM...LO2, LH2 transfer line chilldown
09...40...00...07/13/05 06:05 AM...Main propulsion system chill down
09...40...00...07/13/05 06:05 AM...LH2 slow fill
09...10...00...07/13/05 06:35 AM...LO2 slow fill
09...00...00...07/13/05 06:45 AM...LO2 fast fill
08...50...00...07/13/05 06:55 AM...LH2 fast fill
08...45...00...07/13/05 07:00 AM...Crew medical checks
07...35...00...07/13/05 08:10 AM...LH2 topping
06...55...00...07/13/05 08:50 AM...LH2 replenish
06...50...00...07/13/05 08:55 AM...LO2 replenish
06...50...00...07/13/05 08:55 AM...Begin 3-hour built-in hold
06...50...00...07/13/05 08:55 AM...Closeout crew to white room
06...45...00...07/13/05 09:00 AM...Tank in stable replenish mode
06...35...00...07/13/05 09:10 AM...Crew cabin comm checks
06...05...00...07/13/05 09:40 AM...Pre-ingress switch reconfiguration
05...30...00...07/13/05 10:15 AM...Crew breakfast photo op
04...55...00...07/13/05 10:50 AM...Crew ingress weather update
04...25...00...07/13/05 11:20 AM...Crew weather briefing
04...15...00...07/13/05 11:30 AM...Astronauts suit up for launch
03...50...00...07/13/05 11:55 AM...Resume countdown
03...45...00...07/13/05 12:00 PM...Crew departs O&C building
03...15...00...07/13/05 12:30 PM...Crew ingress
02...25...00...07/13/05 01:20 PM...Astronaut comm checks
02...00...00...07/13/05 01:45 PM...Hatch closure
01...25...00...07/13/05 02:20 PM...White room closeout
01...10...00...07/13/05 02:35 PM...Begin 10-minute built-in hold
01...08...00...07/13/05 02:37 PM...NASA test director countdown briefing
01...00...00...07/13/05 02:45 PM...Resume countdown
00...59...00...07/13/05 02:46 PM...Backup flight computer configured
00...55...00...07/13/05 02:50 PM...KSC area clear to launch
00...49...00...07/13/05 02:56 PM...Begin final built-in hold
00...49...00...07/13/05 03:01 PM...RTLS runway verification
00...44...00...07/13/05 03:06 PM...NASA test director launch status poll
00...09...00...07/13/05 03:41 PM...Resume countdown

HH...MM...SS...DATE/EDT...............EVENT

00...07...30...07/13/05 03:43:23 PM...Orbiter access arm retraction
00...05...00...07/13/05 03:45:42 PM...(Planar window open)
00...05...00...07/13/05 03:45:53 PM...Hydraulic power unit (APU) start
00...04...55...07/13/05 03:45:58 PM...Terminate LO2 replenish
00...04...00...07/13/05 03:46:53 PM...Purge sequence 4
00...04...00...07/13/05 03:46:53 PM...Inertial measurement units to inertial
00...03...55...07/13/05 03:46:58 PM...Aerosurface movement check
00...03...30...07/13/05 03:47:23 PM...Main engine steering test
00...02...55...07/13/05 03:47:58 PM...LO2 tank pressurization
00...02...35...07/13/05 03:48:18 PM...Fuel cells to internal reactants
00...02...30...07/13/05 03:48:23 PM...Clear caution-and-warning memory
00...02...00...07/13/05 03:48:53 PM...Crew closes visors
00...01...57...07/13/05 03:48:56 PM...LH2 tank pressurization
00...00...50...07/13/05 03:50:03 PM...SRB joint heater deactivation
00...00...31...07/13/05 03:50:22 PM...Shuttle computers take control of countdown
00...00...21...07/13/05 03:50:32 PM...SRB nozzle steering test
00...00...07...07/13/05 03:50:46 PM...Main engine start (T-6.6 seconds)
00...00...00...07/13/05 03:50:53 PM...Booster ignition

The shuttle Discovery's countdown is ticking smoothly toward blastoff Wednesday, but forecasters said today the launch likely will hinge on whether afternoon showers move out of the area before the opening of the shuttle's short launch window. Liftoff is targeted for 3:50:52 p.m. Wednesday.

Shuttle forecaster Kathy Winters downgraded the forecast from 70 percent "go" to 60 percent Wednesday, Thursday and Friday. The primary concern is showers or thunderstorms within 20 nautical miles of the shuttle's emergency runway.

"As the sea breeze forms, we're going to see some showers and possibly even a thunderstorm develop during the countdown," Winters said. "But then as the sea breeze progresses to the west with the easterly flow, we should actually see an improvement in our weather at the launch pad.

"What our main concern is going to be is that these storms will move far enough to the west to be beyond the 20-nautical-mile ring around the Shuttle Landing Facility, which is our return-to-launch-site (abort) concern. So we are expecting to see some weather in the area, but it should progress to the west."

A shuttle main engine failure or some other major malfunction in the first few minutes of flight would force the crew to attempt an emergency return to the Kennedy Space Center. Because the shuttle makes a gliding approach and only has one shot at the runway, NASA flight runs require relatively clear sky, no rain and crosswinds of 15 knots or less. The 20-nautical-mile circle around the runway is intended to ensure good landing conditions a half-hour after launch when a returning crew would be on final approach.

NASA test director Jeff Spaulding said there are no technical problems of any significance at pad 39B, although NASA managers plan to meet later today to resolve a few lingering technical issues.

Overall, Spaulding said, "the vehicle is ready, the team's ready, I think our nation is ready and it seems like our weather will be ready so that we can begin our historic mission of returning the shuttle to flight and bringing our crew safely home."

Said payload manager Scott Higginbotham: "This has been a long journey for all of us, it's been a long story in many of our lives but we're all committed to seeing that we have a happy ending."

Spaulding said Discovery's crew - commander Eileen Collins, pilot James Kelly, flight engineer Stephen Robinson, Andrew Thomas, Wendy Lawrence, Charles Camarda and Japanese astronaut Soichi Noguchi - are in good spirits and ready to go.

"Their mood is one of excitement as is ours and they're anxious to get back to launching," he said. "They've been training for an awful long time and certainly, they're prepared and ready for this mission. That's their focus right now."

Discovery's crew plans to honor the memory of the Columbia astronauts in a ceremony aboard the shuttle on July 20, the 36th anniversary of the Apollo 11 landing on the moon in 1969. Earlier today, the families of the Columbia astronauts wished Discovery's crew well. Here is their statement:

July 12, 2005
A STATEMENT FROM THE COLUMBIA FAMILIES ON NASAŐS RETURN TO FLIGHT<P>

The following is a statement from the families of the Space Shuttle Columbia mission STS-107:

"As NASA prepares to launch the Shuttle Discovery, we, the Columbia Families, would like to show our support for the STS-114 crew and all the dedication and talent of those who supported this Return to Flight effort. We have had two and a half years to reflect daily on the loss of our loved ones as the Shuttle Columbia (STS 107) broke apart over Texas on February 1, 2003.

"In the aftermath of the Columbia tragedy we saw our nationŐs space program reinvent itself. The extraordinary efforts of local, state and national organizations involved in the recovery effort, the Columbia Accident Investigation Board, the Return to Flight Task Group and all the NASA and aerospace industry workforce implementing the Return to Flight effort have clearly done an exemplary job in defining and reducing the technical risk as much as possible. As the families of Apollo 1 and Challenger before us, we grieve deeply but know the exploration of space must go on. We hope we have learned, and will continue to learn, from each of these accidents, so that we will be as safe as we can be in this high risk endeavor.

"As important as solving the technical risk is, we must be vigilant to ensure the organizational and cultural issues that contributed to Apollo, Challenger, and Columbia are forever remembered. Under the leadership of the new NASA Administrator, we have every confidence that the sacrifice of our loved ones and those that preceded them, will be realized for the benefit of all humankind.

"Godspeed Discovery."

NASA's mission management team met today to assess the status of shuttle Discovery's launch processing and its readiness to take off Wednesday on the first post-Columbia mission. While the panel cleared most of the open items on the agenda, engineers were asked to work through the night to resolve a few final questions.

Discovery is scheduled for blastoff at 3:50:52 p.m. Wednesday and forecasters say they expect a 70 percent chance of good weather.

"I think we are ready to go fly," said Wayne Hale, chairman of NASA's mission management team. "There are risks in spaceflight and obviously as time goes on one hopes to improve the system and we will continue to improve the space shuttle system to reduce the risk. In the future, we will have less risk. That goes without saying."

But, Hale added, "there comes a point in time when you decide we have reached an acceptable level of risk to go carry out the mission we have."

"We're at that point," he said. "We're not going to be so much dramatically safer in the next short period of time that it's worth our while to delay. Now's the time to go fly."

During today's mission management team meeting, Hale said a "spirited" discussion covered a variety of open issues. One of them involved how hot the bipod strut fittings that attach the nose of the shuttle to its external fuel tank might get due to atmospheric friction as the ship rockets toward space.

The bipod attachment fittings used to be covered by foam insulation, the same insulation that broke away during the shuttle Columbia's external tank during launch in 2003. The bipod ramp foam, intended to prevent ice buildups before takeoff, was eliminated in the wake of the disaster in favor of heater strips. As a result, the fittings are exposed to the elements during launch.

"The balloons that we launch here at the Kennedy Space Center that we do analysis on to determine if the winds aloft are acceptable to fly ... there's one new test that we've added since the Columbia accident for heating on a certain part of the vehicle that we're having trouble passing with the balloons," Hale said.

"We think we may have put in an overly conservative constraint. Folks are off looking overnight to see if that's the case and if not, what are we going to do about it. So we have a couple of loose ends to tie up, but I wouldn't say they were major. I would say we do have to resolve these issues before we go fly."

Engineers are also working to close out open questions about an "unexplained anomaly" that occurred during a fueling test in April when two of four hydrogen fuel sensors failed to operate properly. All four must be working at launch and engineers were never able to pin down what went wrong.

Discovery ultimately was attached to a different tank, using different sensors, and Hale said all the cabling and the electronic "black box" in the shuttle that routes signals to and from the sensors were replaced.

But engineers are still assessing potential failure modes and final resolution is expected at the mission management team meeting Tuesday.

The shuttle Discovery's countdown ticked smoothly through its initial stages today as engineers geared up to load liquid hydrogen and oxygen to power the ship's electricity generating fuel cells. Forecasters, meanwhile, continue to predict a 70 percent chance off acceptable weather at launch time Wednesday.

Shuttle weather officer Kathy Winters said the major question mark at this point is the chance for showers and thunderstorms to develop within 20 nautical miles of the shuttle runway. That's where the crew would have to attempt a landing in the event of a major malfunction early in flight.

NASA flight rules forbid a launching if rain is predicted to move within 20 nautical miles of the runway a half-hour after launch when a returning crew would be on its final approach. Winters said crosswinds are expected to be within NASA's 15-knot limit Wednesday.

The forecast for Thursday and Friday remains 60 percent "go" both days. But conditions could deteriorate after that depending on the course of a tropical depression (No. 5) that is expected to turn into a hurricane in the next few days.

"Suffice it to say we're going to be looking at it quite closely," said NASA test director Pete Nickolenko.

Here are countdown highlights for the rest of the day (in EDT):

HH...MM...SS...DATE.....EDT........EVENT

July 11
53...20...00...07/11/05 10:25 AM...Space station/shuttle crew teleconference
52...45...00...07/11/05 11:00 AM...Orbiter pyro-initiator test
52...35...00...07/11/05 11:10 AM...Booster pyro-initiator test
51...45...00...07/11/05 12:00 PM...Flight control team/astronaut teleconference
51...35...00...07/11/05 12:10 PM...Master events controller test
51...00...00...07/11/05 12:45 PM...Wayne Hale/astronaut teleconference
50...15...00...07/11/05 01:30 PM...Mission management team meeting
49...45...00...07/11/05 02:00 PM...The countdown resumes after a 4-hour hold
48...15...00...07/11/05 03:30 PM...Fuel cell oxygen loading begins
48...14...30...07/11/05 04:30 PM...Pre-launch news conference on NTV
45...45...00...07/11/05 06:00 PM...Fuel cell oxygen loading complete
45...45...00...07/11/05 06:00 PM...Fuel cell hydrogen loading begins
43...15...00...07/11/05 08:30 PM...Fuel cell hydrogen loading complete
43...00...00...07/11/05 08:45 PM...Crew sleep begins
42...15...00...07/11/05 09:30 PM...Pad reopened for normal work
41...45...00...07/11/05 10:00 PM...Begin 4-hour built-in hold
41...45...00...07/11/05 10:00 PM...Crew module clean and vacuum
41...15...00...07/11/05 10:30 PM...Fuel cell loading umbilical demate

The remainder of the countdown is posted below.

"All our hardware and systems are performing nominally," Nickolenko said. "We're currently tracking no technical issues and our launch team (is) ready and looking forward to returning the shuttle Discovery to flight, to returning to the international space station and to safely return our astronauts back home."

NASA engineers made final preparations to start the shuttle Discovery's countdown later today, buoyed by forecasts calling for a 70 percent chance of acceptable weather during the ship's five-minute launch window Wednesday.

NASA test director Jeff Spaulding, who will oversee Discovery's countdown in the hours leading up to launch, told reporters today the launch team is not tracking any significant technical issues at pad 39B and that all systems are go for launch.

"A lot's happened over the last two-and-a-half years," he said. "Our focus during that timeframe has shifted from one of recovery and investigation to one of redesign, improvement and mission processing and now, to launch. Our launch team (is) well prepared and I know they're up to the task of returning our shuttle fleet to flight, of returning to the international space station and for returning our crew safely back home."

The countdown is scheduled to begin at 6 p.m. and if all goes well, Discovery will lift off on the 114th shuttle mission - the first since the Columbia disaster Feb. 1, 2003 - at 3:50:52 p.m. Wednesday. The ship's crew - commander Eileen Collins, pilot James Kelly, flight engineer Stephen Robinson, Andrew Thomas, Wendy Lawrence, Charles Camarda and Japanese astronaut Soichi Noguchi - arrived at the Kennedy Space Center Saturday night.

Forecaster Kathy Winters called for a 70 percent chance of acceptable weather Wednesday, although she said there was a chance inland thunderstorms could push into the launch area. The forecast is 60 percent "go" on Thursday and Friday should launch be delayed.

Winters said expected temperatures and humidity levels should help minimize the formation of ice on the shuttle's huge fuel tank, a major impact debris concern in recent weeks.

Spaulding said Discovery has enough on-board liquid hydrogen and oxygen, used by the ship's electrical generators, to make three launch attempts in four days. After that, the team would have to stand down for 72 hours to top off the tanks.

Asked about the mood at Kennedy as return to flight approaches, Spaulding said excitement is clearly building.

"Certainly for the last several months, it's been one where everybody has been pretty much having their head down, working very hard," he said. "We had a number of challenges over those months. And it's only recently, I think, that it's all come to fruition where we can see the light at the end of the tunnel.

"The excitement, I think has been building and growing," he added. "There's a great anticipation for launch ... and also maybe a quiet reserve as well just remembering where we've been. But we all do feel confident we've done it right."

Said Scott Higginbotham, payload manager for Discovery's mission: "It sure does feel good to be back in the saddle again."

"It's been too long, but here we are," he said. "I am very happy to report to you that all 28,000 pounds of international space station hardware that's in the payload bay of Discovery is ready to go."

The goal of Discovery's mission is to deliver critical supplies and equipment to the space station, along with a new stabilizing gyroscope that will be installed during the second of three spacewalks by Robinson and Noguchi. The spacewalkers also will test rudimentary heat shield repair techniques during their first excursion.

Assuming an on-time launch, Discovery is scheduled to land July 25 at 11:06 a.m.

Flying in one day early to dodge Hurricane Dennis, the shuttle Discovery's crew arrived at the Kennedy Space Center this evening to make final preparations for launch Wednesday on the first post-Columbia flight.

"This shuttle flight is the beginning of a new chapter in space exploration," commander Eileen Collins told a crowd of journalists at the Shuttle Landing Facility. "We are going to finish building the international space station, we're going to do the science up there that needs to be done in learning how to keep people going in space for long periods of time (and) leaving low earth orbit and going back to the moon.

"By exploring, we make this world a better place to live in and we continue to grow as a human species. That's important, not just for the United States but for all of us around the world."

Collins, pilot James Kelly, flight engineer Stephen Robinson, Japanese astronaut Soichi Noguchi, Andrew Thomas, Wendy Lawrence and Charles Camarda hope to strap in Wednesday for a launch attempt at 3:51 p.m., weather permitting. The countdown begins at 6 p.m. Sunday.

While Hurricane Dennis did not churn up any serious weather at the Kennedy Space Center Saturday, rain and thunderstorms are expected next week and with a short five-minute launch window, the weather could play a major role in when Discovery finally gets off on the long-awaited flight.

"To all the folks traveling out here to launch, especially with Hurricane Dennis bearing down on the Gulf Coast, hopefully they'll all make it here safely and later on this week, on Wednesday, we'll light the candle and head back up into space."

Collins and her crewmates appeared relaxed and in good spirits as they departed the SLF for crew quarters. All seven said they were eager to finally resume shuttle flights two-and-a-half years after the Columbia disaster grounded NASA's fleet.

"That's way too long," Thomas said of the hiatus. "It's definitely time we went back to flight and back to space. And having the responsibility of the return-to-flight mission certainly makes me feel that sense of privilege even more.

"To all the people down here in Florida who have prepared the vehicle and done so much work to make the flight possible, I say thank you. And to all the taxpayers and the members of the public who support the space program, I very much want to thank you for your support. I would like you to know that as we execute the mission, we will do our very best to live up to the great trust you have put in us."

Noguchi, after addressing Japanese journalists in his native tongue, summed up the crew's mood in English, exclaiming: "Let's go fly!"

NASA Finally Ready for Next Shuttle Flight

By WILLIAM HARWOOD
CBS News Space Consultant

KENNEDY SPACE CENTER, FL (CBS) - NASA is set to resume shuttle flights July 13, weather permitting with a three-spacewalk mission to repair the international space station's stabilization system, to deliver critical supplies and equipment and to prove the design defects that led to the Columbia disaster have been corrected.

Implementing an overlapping suite of approaches, NASA managers believe the chances of Columbia-like damage to Discovery during launch are minimal. But if damage does, in fact, occur, they are equally confident they will be able to detect it, determine if it is entry critical and, if it is, carry out at least rudimentary spacewalk repairs.

Those repair procedures are not fully tested and formally certified for use by Discovery's crew. But NASA managers say certified repair procedures are not required for flight because of the elimination of major debris, improved damage detection, a better understanding of the consequences of impacts and the crew's worst-case ability to use the space station as a "safe haven" until another shuttle, already prepped for flight, could be launched on a rescue mission.

"I believe in our flight rationale, which says we are fixing the vehicle," said LeRoy Cain, the ascent-entry flight director for Columbia's final mission and now, for return to flight. "We are eliminating critical debris from being liberated from the tank and the boosters, the launch pad, every source that we can think of, we think we're eliminating critical debris. That's number one for me.

"I really feel like the chances of us having something come off of the stack and create a problem for us in our flight are exceedingly low."

Low, but not zero. Even after two-and-a-half years of research, testing and analyses to minimize the amount of foam insulation or ice that can shake off the shuttle's external fuel tank during launch, NASA engineers say catastrophic impacts are still possible in worst-case scenarios.

Not to mention the ever-present possibility of a main engine or booster failure in a machine that weighs 4.5 million pounds at launch and accelerates from zero to more than 100 mph - straight up - in less than 10 seconds.

"People seem to have forgotten that when we fly the shuttle with the technology that we human beings own today - and there is none better - it's still not good enough," said new NASA Administrator Michael Griffin, a refreshingly candid rocket scientist with a doctorate and five master's degrees.

"This is a very risky venture," he said. "The people who are doing this are risking their lives in support of objectives the United States has in the pursuit of space exploration. Many people risk their lives on behalf of the United States and our flight crews, our astronauts, are in that group also. Everybody should understand that."

Shuttle commander Eileen Collins agreed.

"There are millions of things that have to go right on launch day and entry day, there are all kinds of sensors and transducers and wires and electrons and flow paths going all through the orbiter, any of which could break at any time," she said in an interview with CBS News. "People ask me, what worries you the most? It's really not what we know about but what we don't know about that worries me."

Even so, she said, "it's time for us to go fly."

"It's been over two years now and a huge effort has been put into getting the space shuttle back flying again," she said. "I think we're ready to do it. My only concern is that after my mission flies that we continue to make things better, we don't just drop it and say return to flight has happened and now we can go back to business as usual."

A "business as usual" attitude among senior NASA managers played a role in the decision to launch Columbia Jan. 16, 2003, despite a major foam debris strike two missions earlier.

During Columbia's launching, a piece of foam broke away from the same area of the ship's external tank and blasted a hole in the leading edge of the shuttle's left wing. During re-entry sixteen days later, on Feb. 1, 2003, super hot plasma entered the breach and melted the wing from the inside out, triggering the shuttle's destruction.

The Columbia Accident Investigation Board, or CAIB, made 29 recommendations to improve safety and management, including 15 that were to be implemented before the resumption of shuttle flights.

An independent panel charged with assessing that implementation concluded the agency had failed to fully address the three most critical recommendations to eliminate all debris sources; to make the shuttle's thermal protection system more resistant to impacts; and to develop repair techniques to fix any such damage that might occur.

The Return to Flight Task Group's conclusions rekindled a long-running debate among managers, engineers and even reporters about the intent of the CAIB given the deliberately vague wording used in some of the recommendations.

But the Task Group, led by former Apollo astronaut Thomas Stafford and former shuttle commander Richard Covey, adopted a literal interpretation of the recommendations. And by that standard, they concluded, NASA came up short on debris elimination, shuttle hardening and development of reliable repair techniques.

NASA managers did not argue with the panel's conclusions. But they pointed out that engineering reality had dramatically changed since the CAIB's initial report was released and that it had turned out to be impossible, given the shuttle's design and the time and funding available, to eliminate all debris.

While some tiles and seals around the landing gear doors were "hardened" to resist impacts, NASA managers called off efforts to toughen the shuttle's wing leading edges, concluding it did not make sense to mount such a major engineering effort given a recent presidential directive to retire the shuttle fleet by 2010. Only 18 to 20 shuttle flights are expected before the shuttle era ends.

As for repair techniques, NASA managers say they ended up in a "Catch-22:" To fully develop the required repair procedures, astronauts needed to test them in the space environment to make sure they could be trusted to bring a crippled ship home.

Instead, Discovery's crew will test two rudimentary leading edge repair techniques, one meant for cracks and one for small holes, and one procedure intended for use on tiles with minor impact damage. The results of the tests aboard Discovery, coupled with additional tests aboard the next shuttle flight in September, may provide the data needed to formally certify one or more repair techniques.

Then again, they may not.

"The fact is, several CAIB recommendations, taken word by word, are not implementable with the state of our knowledge today," Griffin said. "We do not know how to repair large holes in (reinforced) carbon carbon (leading edge panels) or even small holes, maybe.

"We are being as smart about this as we know how to be but we are up against the limits of our human knowledge. If someone wants more, they're going to have to find smarter humans. So the recommendations as they were written are not strictly speaking implementable, at least not all of them are, and the (Stafford-Covey Return to Flight Task Group) noted NASA was not able to implement them. That was not a surprise."

In the end, Griffin agreed with his engineers that NASA had done everything reasonably possible to improve shuttle safety and that only incremental improvements could be expected by keeping the fleet on the ground.

"If we ground the shuttle fleet, we're not going to be able to complete station assembly, we're not going to be able to do other things that we want to do," Griffin said. "If, of course, we believe that all debris sources have been reduced to a level low enough that the shuttle won't be damaged, then the tile repair issue becomes kind of moot.

"We're in that gray area where we believe we have greatly reduced the risk due to debris, foam and ice, but not so much we're completely comfortable with it. So the STS-114 crew ... will be lifting off in the face of a known risk."

For the record, Gehman said he believed NASA did, in fact, met the board's overall intent.

"It is our judgment that they've efforts have passed the criteria that we set up for them," Gehman said in an interview with CBS News. "But that doesn't mean they're allowed to give up on the repair. In our view, they have to keep working at it."

The critical recommendations, in his view, centered on four broad areas.

"First of all, you've got to understand foam creation and the creation of the hazard in the first place and you've got to do everything you can to prevent the creation of foam in the second place," Gehman said in the CBS interview. "The second thing you've got to do is, you've got to have much better pictures on launch and ascent to know whether or not there's been a foam event, or a debris event. You've got to know that. The third thing you've got to do is, you've got to essentially re-certify the orbiter to be ready to come back into the Earth's atmosphere. That translates into some kind of an inspection in orbit.

If serious damage is detected, "you have to have some minimal, practical kind of capability to do some kind or orbital repair, the best practicable kind of a repair. Knowing full well, depending on the size of the damage or what the nature of the damage was, there are some repairs that are beyond your capability to do in space.

"And it has been our unwritten policy ... and I told Stafford-Covey and asked Stafford and Covey to back me up on this and they have - and that is, you must attack all four of these things.

"Now you can do some better than others," Gehman said. "If you really think you've done a fabulous job of preventing the creation of debris in the first place, you've got some really good ways to take pictures to make sure your orbiter hasn't been struck or anything like that and you're really sure that it's in good condition, then you can do some of the other stuff to a lesser degree.

"But you do have to make an attempt at all four areas. Now, within those four areas, there are sometimes one, two, three or four things that you've got to do. But that was what our intent was."

In a recent teleconference with reporters, Gehman said "I know of no reason why they should not proceed with the launch. That's not to same thing as saying it's safe to go, that's a different story."

Asked to explain, he added: "I don't think the American people and the Congress of the United States realize how dangerous this is."

"We didn't realize how dangerous it was when we started this investigation," he said. "It was dangerous, it remains dangerous. We the country have got to replace this vehicle as soon as possible. ... I'm sure this next flight will be safer than the previous ones, but by any measure of 'safe,' this is not safe."

Discovery is targeted for launch on the 114th shuttle mission at 3:50:52 p.m. Wednesday, July 13. Joining shuttle veteran Collins on Discovery's flight deck will be pilot James Kelly, flight engineer Robinson, seated to Robinson's right, Japanese astronaut Soichi Noguchi.

Strapped in below on the shuttle's middeck will be Mir-veteran Andrew Thomas, Wendy Lawrence and Charles Camarda. All but Camarda and Noguchi are shuttle veterans.

The shuttle's primary cargo includes a refurbished control moment gyroscope to replace one that failed earlier aboard the space station; a tool kit and spare parts module that will be mounted on the station's airlock to enable future assembly work; and a pressurized logistics module loaded with space station equipment and supplies.

In an interview for the book "Comm Check: The Final Flight of Shuttle Columbia," by this author and Michael Cabbage, Collins was asked if she had any second thoughts about commanding mission STS-114.

"Absolutely not," she said. "In fact, I am more committed to flying this mission than I ever would have been. ... I am excited, I am going to be extremely confident because look at all this work that is being done, not just done because of (Columbia), but other things that we think are risky. I am so confident, I am so excited, I want to get our country back flying in space again, so I am not one blink of an eye worried about safety."

And her crewmates?

"It's time to go fly," Robinson told CBS News in a recent interview. "There will be debris, there will be some damage, I'm convinced of that. If there isn't, that'll be great but I'll sure be surprised. I would be very surprised if it's critical damage, damage that won't allow us to fly home on. But here's the thing. We'll know it. We won't have to wonder. We'll know it.

"We'll have the technology now for the first time on this mission to take a look at it with all the cameras and sensors. This is the way we verify all the engineering that's been done. So we'll get to look at our bird before we come home. Then, on top of that, if the worst on worst on worst happens and we do have critical damage, the space station will (be available for safe haven), we won't have to risk our lives coming back through the atmosphere. This is what gives me tremendous confidence and makes me feel very lucky I'm flying now."

LIMITED LAUNCH PERIODS

Columbia was brought down by a suitcase-size piece of foam insulation that broke away from an aerodynamically shaped ramp used to keep ice from forming on a strut connecting the shuttle's nose to its external fuel tank. The 1.67-pound piece of foam came off 81.7 seconds after liftoff and struck the underside of Columbia's left wing 0.2 seconds later, smashing into the lower side of a reinforced carbon carbon panel, one of 22 making up the wing's leading edge.

Ground cameras were unable to see the point of impact. One long-range tracker that might have shown the impact site was out of focus. And given the resolution of the cameras in place at the time, it's not clear obvious signs of damage would have been detected.

In any case, without the benefit of high-resolution video of the impact, mission managers were forced to rely on computer modeling and other indirect techniques for determining whether the foam strike could have caused any entry-critical damage. In the end, they wrongly concluded Columbia could safely re-enter as is.

Commander Rick Husband, pilot William McCool, Kalpana Chawla, Laurel Clark, David Brown, Michael Anderson and Israeli astronaut Ilan Ramon were killed Feb. 1, 2003, when hot gas burned its way into the interior of the left wing through a presumed breach on or near the underside of RCC panel No. 9. The wing melted and the shuttle broke up 37 miles above Texas.

In the wake of the disaster, the Columbia Accident Investigation Board ordered sweeping changes, including "an aggressive program to eliminate all external tank thermal protection system debris shedding at the source with particular emphasis on the region where the bipod struts attach to the external tank."

The bipod ramp was intended to keep ice from forming around the struts due to the ultra-low temperatures of the shuttle's liquid oxygen and hydrogen propellants. NASA's solution was to simply eliminate the ramps and to install 300-watt heaters on the strut attachment fittings to prevent ice buildups.

Tank engineers also implemented a variety of other changes to minimize foam shedding and the first upgraded tank was shipped to the Kennedy Space Center in the first week of January.

"Through all our testing, we believe the amount of foam that can come off the tank and not cause serious damage is .03 pounds," said Wayne Hale, deputy director of the shuttle program at the Johnson Space Center in Houston. "If you think about that, that is three one hundredths of a pound. That is something like six tenths of an ounce. So that's a pretty small piece of foam.

"All our investigations of the foam indicate we will not get a piece of foam coming off bigger than .008, eight one thousandths, or almost an order of magnitude smaller than the requirement."

It is not possible to eliminate all foam shedding, but "we're clearly moving toward an era where we expect to see much less damage in the tile and no critical damage that will require a repair," Hale said. "So that's our goal in this and it's beginning to look very positive that we'll be able to accomplish that level of control on the ET foam."

But to make absolutely sure, NASA must be able to inspect the tank and the space shuttle after launch with much greater precision than before. The CAIB recommended that NASA "upgrade the imaging system to be capable of providing a minimum of three useful views of the space shuttle from liftoff to at least solid rocket booster separation."

The panel also told NASA to look into putting cameras aboard ships and/or aircraft to provide additional coverage, to develop a capability to obtain high-resolution images of the tank after separation from the shuttle, to develop techniques for high-resolution imaging of the ship's underside and wing leading edges; and to make arrangements to obtain imagery from spy satellites if needed.

On its own, NASA managers decided to launch the first two post-Columbia shuttle flights in daylight to improve photo coverage and to time the launchings so the external tank, separating from the ship half a world away, also would be lighted by the sun.

To reach the space station, the shuttle must launch within a few minutes of when Earth's rotation carries the launch pad into the plane of the lab's orbit. And as a final complication, the shuttle can only visit the station when the "beta angle" - the angle between the plane of the station's orbit and the sun - ensures the shuttle-station stack will not get too hot.

Throwing all of those requirements together, NASA can only launch a shuttle to the station during relatively short windows.

NASA had hoped to launch Discovery in mid May, but the flight was delayed after a fueling test April 14 because of concern about ice buildups around a flexible liquid oxygen feedline bellows assembly. In addition, two of four hydrogen fuel sensors operated intermittently and a pressure relief valve in the hydrogen section of the tank cycled more often than expected.

At the same time, engineers were struggling to define the threat posed by ice during launch. While engineers had focused on foam impacts in the initial stages of the recovery program, it became clear earlier this year that ice posed a similar threat. Areas of concern centered on the feedline bellows and on so-called stand-off brackets that support the tank's externally mounted liquid oxygen line.

NASA managers ultimately ordered engineers to haul Discovery off the launch pad and back to the Vehicle Assembly Building for attachment to an external tank and boosters originally slated for the second post-Columbia mission. The new tank was equipped with a heater to minimize ice buildups around the feedline bellows.

Before the rollback, however, a second fueling test was conducted May 20, confirming the hydrogen vent valve's unusual behavior. Engineers believe the valve cycling was associated with a jet-like device called a diffuser that injects helium into the tank to help maintain the proper temperature and to pressurize the tank for flight. A new dual-screen diffuser was used in Discovery's original tank and managers decided to switch back to the original single-screen design for the new tank.

As for the hydrogen sensors, which operated normally the second time around, NASA managers decided June 6 to forego a third tanking test, saying they are confident the new tank will behave normally during Discovery's countdown.

And they ultimately concluded the threat posed by whatever ice does fall off the tank is an acceptable risk.

But unlike the threat posed by foam, agency managers refused to provide any numbers defining the smallest allowable ice or the odds of a catastrophic impact. They said the issue was too complex and that such numbers would be meaningless without a thorough understanding of the assumptions that went into the calculations.

"It's a really complex formulation, going from how much ice can you a have on the spacecraft and what velocities, at what vibration levels it shakes loose, how big are the pieces that shake loose and then how they transport through the aerodynamic environment," said John Muratore, the engineer in charge of the debris assessment.

"And then, what's the resistance of the tiles to ice impacts and finally, given an ice crater - which is different from a foam (insulation) crater - how well can we enter (the atmosphere) with regard to that?

"The uncertainties in each of those areas are significant," he said. "There are all sorts of numbers that are floating around. We have nine different estimates for ice on tile from one ice location. ... It is a very complex problem.

"So what we did was, we looked at the relative risks and we're convinced the ... the remaining risks due to ice is around an order of magnitude less than the ones that we fixed."

Muratore said ice represents more of a threat to the shuttle's tiles than it does to its reinforced carbon carbon leading edge panels and that even though engineers used "all the supercomputers at NASA to run it down ... it doesn't lend itself to a single number."

"Impact dynamics are a very complicated engineering discipline," he said.

While he would not discuss the underlying statistics, sources said some estimates of the likelihood of tile damage due to ice impacts - damage that would require repair or some other response - could be as high as 1-in-100 or so or as low as one in tens of thousands, depending on the assumptions that were made.

Engineers do not put much faith in either extreme because they do not have enough reliable test data to more precisely define the actual risk. And that won't change anytime soon. As one briefing chart put it, significant improvement in the understanding of the impact threat from ice and foam would require "another enormous undertaking."

The best way to improve that understanding in the near term, many engineers believe, is to collect actual flight data by launching Discovery as planned. They believe the studies carried out to date, plus NASA's experience in 113 previous shuttle missions, show the risk of a debris strike, what ever it might be, is acceptable.

"We believe it's an acceptable risk at this time," said shuttle program manager William Parsons.

UNPRECEDENTED VIDEO COVERAGE

From a mission control standpoint, ascent-entry flight director Cain said Discovery's flight will use the same rules and procedures in place for Columbia's launching with one major exception. NASA will use a new trans-Atlantic abort site in France that will replace Ben Guerir in Morocco.

While NASA will leave equipment at Ben Guerir for possible use in the future, security issues in the wake of the 9/11 terrorist attacks could not be ignored. The new landing site, known as Istres-le-Tube, is located northwest of Marseilles and is one of the largest military air bases in Europe.

"We're real excited about it," Cain said. "It's a great opportunity for us and the French air force and the air traffic control folks in southeastern France have been very helpful."

Barring an engine failure or some other malfunction that might trigger a TAL abort, Collins and Kelly will carry out the same ascent procedures as Rick Husband's crew aboard Columbia. But major changes have been implemented on the ground to document foam shedding from Discovery's external tank and any possible damage to the orbiter.

Bob Page, the engineer at the Kennedy Space Center who leads NASA's launch photo team, said Discovery's launching will be documented in unprecedented detail using a mixture of high-speed film cameras, high-definition TV cameras and even airborne sensors using two NASA WB-57 jets. More than 112 cameras are in place for tracking and analysis, including more than 20 HDTV cameras.

The high-definition video cameras are a major improvement over the standard television cameras in place when Columbia took off. Those operated at just under 30 frames per second, limiting how precisely engineers could track the debris from the fuel tank. The new HDTV cameras will operate at 60 frames per second and provide four times the resolution, or clarity.

But because the shuttle rolls into a "heads down" orientation shortly after launch, putting the belly on the other side facing upward, ground cameras, no matter how good, still will not be able to see certain areas of the shuttle's underside. Even so, any major debris strikes should be readily apparent.

"For tracking debris, for determining debris size, for determining debris velocity, for location of impact, for source location, for a lot of those things, I have much better data," Page said in a recent interview. "But what so far has been determined as a damage size that would be a threat to the vehicle, it is smaller than the resolution that I can work with at the time frames from 70 seconds out to SRB separation. The damage size that's a threat to the vehicle is a whole lot smaller than anything I can see."

That's where cameras aboard the shuttle will come into play. But if anything large breaks away, and certainly anything the size of Columbia's bipod ramp, Page's cameras will spot it, track it and help managers determine what damage it may have caused.

More than 50 high-speed film cameras mounted on the launch pad, running at 400 frames per second, will capture the initial seconds of flight from extreme close range. Three short-range tracking platforms around the perimeter of the pad, each one equipped with two film cameras and an HDTV camera, will capture imagery through the first minute or so of flight.

Six medium-range tracking platforms, each with film and HDTV cameras, will capture the view through the first 100 seconds and 10 long-range trackers, five of them north and five south of the shuttle's ground track, will cover the flight through the first 165 seconds, well after booster separation.

Because of the high frame rate, imagery from the HDTV cameras will be stored on hard drives. The engineering team that will analyze the footage will be able to access it across the agency from internal NASA web sites. The public will see normal-resolution television views from those tracking cameras in realtime over NASA's satellite network. Within six to eight hours, Page hopes to deliver processed HDTV imagery to public affairs for broadcast.

Along with switching to HDTV, Page also bumped up the frame rate of the film cameras to provide better temporal resolution.

"You've got to look at both the spatial and the temporal. Let's start with the film," he said. "We ran 35-millimeter film before and we're running it now. So the pixel, or grain, count is the same. But we've increased the frame rate from 64 frames per second to 100. So we have 50 percent more data points to track a piece of debris. As you're tracking a piece of debris as it moves down the stack during ascent, we have 50 percent more number of points, number of frames, to pinpoint where it is in three dimensional space. So that's important.

"From the video side, we're going from 30 frames per second interlaced, where every other line is scanned, and we're now going to 60 frames per second progressive scan, and that means every single line is scanned. So we have a hundred percent more data points, plus we have four times the resolution. We can measure the size better, we can see it better."

The smallest piece of debris or damage detectable by the ground cameras at the moment of solid-fuel booster separation is 15 to 16 inches across. To improve those numbers, NASA has equipped its two WB-57 jets with nose-mounted HDTV cameras and infrared sensors. The planes will fly 15 miles to the north and south of the shuttle's ground track at an altitude of 60,000 feet some 40 miles off shore. At booster separation, they will be 15 miles below the shuttle.

Each HDTV camera in the nose of each jet will be hooked up to a 4.2-meter focal length telescope. The telescopes should be capable of detecting debris or damage sites as small as six inches across at booster separation.

Page said the planes have two additional benefits for NASA: They can provide full video coverage of a return-to-launch-site abort and they will be used during re-entry to image the shuttle at altitude on its way home. But footage cannot be downlinked from the aircraft. It must be delivered to analysts after landing.

The CAIB recommended that NASA include the operational status of its tracking cameras in the agency's launch commit criteria, the set of rules used to determine whether a countdown can proceed or not. Page said the status of long-range trackers will be included in a management poll during a 10-minute hold at the T-minus 20-minute mark. A final poll of short-range camera status will be conducted during a hold at T-minus nine minutes. But the cameras are not part of the formal launch commit criteria because they do not directly affect flight safety.

No additional polling will be conducted after the T-minus nine-minute hold and any subsequent failures in the camera system will not stop a countdown. The high-speed launch pad cameras fire up at T-minus 10 seconds and Page said it would be more dangerous to abort a countdown at that point than it would be to continue without complete camera coverage.

"The LCC comes into play when you look at these cameras on the pad," he said. "Now I have a computer system that is controlling every single one of these. If I lose that computer system, I lose all of these views. So what I do is, I have an LCC down to T-minus nine minutes for the control system for these cameras, to make sure it is operating and it is stable. And then I can continue on from T-minus nine."

NASA is spending $40 million to upgrade the imaging system at the Kennedy Space Center and $9 million to modify the WB-57s. Through the end of the shuttle program, the agency expects to spend another $40 million on operations, pushing the total cost of the new system to nearly $90 million.

ON-BOARD CAMERAS PROMISE DRAMATIC VIEWS

Ground- and air-based shuttle imagery mark a clear improvement over what was in place for Columbia's launch. But it still isn't good enough to spot small areas of potential entry critical damage or damage to areas of the underside of the shuttle that cannot be seen from the ground.

To close that gap, NASA is installing cameras on the external tank of the shuttle, on the two solid-fuel boosters and in the recessed cavity where 17-inch propellant feed lines enter the belly of the orbiter. In addition, as soon as the shuttle reaches space, the astronauts will use a hand-held video camera to "shoot" the tank from close range before it drifts away.

"We have added some cameras on the tank and on the solids that are primarily looking at tank and at the bottom of the vehicle for the higher impact concern areas," said Paul Hill, the lead flight director for STS-114. "From a crew perspective, they're all hands off, almost passive operations. Once we separate from the ET, we have modified the ET separation maneuver, which will pitch us around so the crew can take pictures at about half the range we used to take pictures at.

"What we're more excited about from an ET photography perspective is the umbilical well camera. Because that camera is going to give us such a good shot of the ET foam in particular that's on the orbiter side and we're going to see that at a really close range. That will give us a really good idea of how the ET insulation performed during ascent."

Within a minute or so of separation, the crew will get a full view of the tank from Columbia's flight deck.

"We would definitely be able to see if we had large pieces of foam come off," Hill said. "My expectation is, we will have really good resolution because it is a still camera instead of video, and because of the close range. Because it's a digital still camera, we'll also be able to downlink (the images) instead of waiting until post-flight like we would a film camera."

NASA managers had hoped to downlink the imagery before the astronauts went to bed at the end of their first day in space. But in an embarrassing oversight, engineers discovered a clearance issue very late in the processing flow between the shuttle's high-speed KU-band television antenna and a new instrumented boom that will be used to help spot damage. Clearances between the antenna and the boom are so tight, NASA amended the flight rules to delay deployment of the KU-band antenna until the second day of the mission, after the boom is deployed.

During robot arm checkout during the first day of the flight, the astronauts will photograph the actual clearance between the boom and the antenna, providing data engineers can use to determine if the antenna can remain deployed after the sensor boom is stowed prior to space station undocking.

While mission managers will have to wait for the crew's tank separation video, the shuttle's computer system can use a slower antenna system to downlink data from new wing leading edge sensors that were added to the shuttle's wings as a post-Columbia upgrade.

Located on each wing's forward spar behind every RCC panel, the 132 accelerometers will provide data telling flight controllers whether anything struck the leading edges during launch. In fact, they may show engineers aspects of the shuttle they've never seen before.

Flight Day 1 highlights (all times in Eastern; a detailed flight plan is available on the Current Mission page):

   DAY.EDT.........DD...HH...MM...EVENT
   
   07/13/05
   Wed  03:51 PM...00...00...00...STS-114 Launch
   Wed  04:00 PM...00...00...09...Main engine cutoff
   Wed  04:29 PM...00...00...38...OMS-2 rocket firing
   Wed  04:41 PM...00...00...50...Post-insertion timeline
   Wed  06:21 PM...00...02...30...GIRA install; PGSC laptop setup (1)
   Wed  06:41 PM...00...02...50...Shuttle remote manipulator system (SRMS) powerup
   Wed  07:01 PM...00...03...10...SRMS checkout
   Wed  07:21 PM...00...03...30...Elevon park
   Wed  07:46 PM...00...03...55...SSRMS photographs KU-OBSS clearance
   Wed  07:51 PM...00...04...00...SEE setup
   Wed  08:06 PM...00...04...15...SSRMS powerdown
   Wed  08:53 PM...00...05...02...NC-1 rendezvous burn (166.1/122.8 nm)
   Wed  09:16 PM...00...05...25...Group B computer powerdown
   Wed  09:51 PM...00...06...00...STS crew sleep begins

The sensor system generates two types of data: Peak and detailed.

"Think about what a stereo equalizer looks like," Hill said. "You've seen these ones that, across the frequency band, as the signal bounces up and down, it leaves a hash mark. The system works kind of like that. It's recording very high rate frequency response data across the wing leading edge from all these accelerometers that are on the wing spar for every RCC panel. And it registers the peaks, the software pulls out where those little peaks are from T-0 all the way to after we've made it into orbit.

"The first thing we downlink is just the file that has all the peaks in it," Hill said. "That then tells us that we have a suspected impact somewhere and after we see that, then within an hour after the guys in the MER (mission evaluation room) see that and pick out the ones they think are potential impacts, then we put commands on board to downlink the detailed data around each one of those peaks."

Hill acknowledged that engineers worry "we're going to get data down that we don't understand, or because of the shake rattle and roll we'll get going up hill, we won't be able to interpret the data."

"What's in our favor on this is we've been flying accelerometers like this and the same data collection system in the aft compartment of the orbiter for years," he said. "Now we haven't had it on the wing leading edge and we weren't using it to detect impacts, but the hardware has been flying for some time and we have characterized a similar ascent vibe environment in the aft compartment."

In addition, the sensors have been used during impact tests at Southwest Research Institute in San Antonio, Texas, to collect actual data.

"So we have a certain amount of data on how the system will respond going up hill, we have other data to tell us what impact ought to look like," Hill said. "One thing's for sure, by the end of flight day one we'll have data on the ground and we'll know the answer to that question."

TAKING A CLOSE-UP VIEW OF THE WING LEADING EDGES

Even if no obvious signs of foam shedding or damage show up in the initial imagery or wing leading edge data, CAIB recommendation 3.4-3 calls for NASA to "provide a capability to obtain and downlink high-resolution images of the underside of the orbiter wing leading edge and forward section of both wings' thermal protection system." The Discovery astronauts are going to spend their second day in space carefully inspecting the leading edges of both wings, along with the RCC nose cap of the shuttle, using a new 50-foot-long boom mounted on the starboard side of the payload bay known as the orbiter boom sensor system, or OBSS.

Thomas will unlimber the shuttle's 50-foot-long robot arm and lock it onto the OBSS. Thomas, Kelly and Camarda then will spend the entire day maneuvering the boom back and forth along the leading edges of both wings, using an OBSS television camera and a laser sensor to inspect every square inch of the RCC panels. The astronauts also will use the boom to inspect the nose cap.

Maximum speed of the boom survey: 2 inches per second.

"If you think about the laser, the way we're recording the data is similar to recording video," Hill said. "Imagine standing on the side of a soccer field watching one of your kids play soccer with a camcorder. And you're panning that camera real fast so you can watch him run down the field. But when you play it back at home, you can't make out hide nor hair because everything is blurred. That's the problem we've got. We've got a translation constraint. If we move too fast, we blur the image, which directly affects the resolution and we can't see the small stuff we're looking for."

A year ago, Hill said, engineers thought entry critical damage "required a penetration of the RCC, not just coating damage or even small damage to the substrate on the outside."

"More recent arc jet testing has us worried that coating damage alone, if it's large enough and if we had internal damage - delamination - between the layers, that the combination of those two could be entry critical."

In the early years of the shuttle program, Hill said, tests indicated the leading edge RCC panels could tolerate penetrations a quarter of an inch across. But that testing was with a clean hole punched in the panel, which is what one would expect with a hypervelocity impact in space. But during launch, impact velocities would be much lower and any resulting penetrations would be more ragged.

Engineers then began wondering if lower-velocity impacts might be entry critical.

"What we found out in more recent arc jet tests, in the last couple or three months - this is the thing that's key - if (RCC coating is intact), our conventional wisdom is good, we don't care, internal delamination's not an issue.

"But if the coating is gone and underneath that coating you're delaminated, then picture the RCC itself from a side view like a cross section. Now you've got this bubble or this void in between layers. What you've done is, you've significantly reduced the density of this RCC that's exposed to the heat load. So it burns faster.

"So now instead of being this more solid material that's hard to light, kind of like if you take a piece of hard wood like oak and you hold a match to that oak, it won't light typically. But if you shave off some splinters of that oak, you can get them to flash. Damned if that's not what we found in a handful of RCC runs for uncoated RCC."

For the tests, engineers deliberately damaged an RCC panel by pushing on it with a metal cylinder. After confirming the panel developed delamination as a result, "they put that bad boy under the arc jet and it burned like there was no tomorrow. The whole area that covered the delamination burned off like a fuse."

If it is credible that the shuttle could take an impact that has enough energy to cause delamination and loss of coating, "then that does not have to be very big to be catastrophic," Hill said. "From an RCC damage perspective, that looks like a penetration. So now the question is, do we believe that testing? Have we done enough of those tests to be sure that is an entry critical damage form? And then, is it credible for us to take an impact that could cause that kind of damage?"

One OBSS laser sensor, known as the laser dynamic range imager, or LDRI, dwill be used to inspect the wing leading edge panels and the shuttle's nose cap. A second sensor, known as the laser camera system, or LCS, may be used later to focus on a suspect area or to collect additional data.

"With our increased knowledge of the transport model, there's only very few parts of the leading edge where we're really at risk of taking an impact that can do that," Hill said.

Flight Day 2 highlights:

   DAY..EDT........DD...HH...MM...EVENT
   
   07/14/05
   Thu  05:51 AM...00...14...00...STS crew wakeup
   Thu  08:06 AM...00...16...15...SRMS powerup
   Thu  08:08 AM...00...16...17...NC-2 rendezvous rocket firing (165.7/155.3 nm)
   Thu  08:21 AM...00...16...30...Centerline camera installation
   Thu  08:21 AM...00...16...30...SRMS checkout
   Thu  08:51 AM...00...17...00...Ergometer setup
   Thu  09:06 AM...00...17...15...Orbiter boom survey system (OBSS) unberth
   Thu  09:21 AM...00...17...30...PGSC setup (2)
   Thu  09:21 AM...00...17...30...KU-band antenna deploy
   Thu  10:31 AM...00...18...40...Rendezvous tools checkout (part 1)
   Thu  10:36 AM...00...18...45...OBSS thermal protection system (TPS) survey
   Thu  10:51 AM...00...19...00...Contingency water container (CWC) setup
   Thu  11:01 AM...00...19...10...SAFER jet backpack checkout
   Thu  11:31 AM...00...19...40...Lawrence exercises
   Thu  11:31 AM...00...19...40...Spacewalk power tool checkout
   Thu  12:01 PM...00...20...10...Crew meals begin
   Thu  01:01 PM...00...21...10...Airlock prep
   Thu  01:51 PM...00...22...00...EMU (spacesuit) checkout prep
   Thu  02:06 PM...00...22...15...Docking ring extension
   Thu  02:36 PM...00...22...45...OBSS survey resumes
   Thu  03:36 PM...00...23...45...EMU (spacesuit) checkout (MS1&2)
   Thu  04:01 PM...01...00...10...OBSS berthing
   Thu  04:46 PM...01...00...55...SRMS survey
   Thu  05:21 PM...01...01...30...NPC rendezvous rocket firing (165.8/155.1 nm)
   Thu  06:16 PM...01...02...25...NC-3 rendezvous rocket firing (165.8/155.1 nm)
   Thu  08:51 PM...01...05...00...STS crew sleep begins

RETURN TO THE SPACE STATION

The OBSS will survey RCC panels and the shuttle's reinforced carbon carbon nose cap. To look for signs of tile damage on the underside of the shuttle, including possible damage to critical seals around landing gear doors, Discovery's crew will rely on help from the crew of the space station.

Docking is targeted for flight day three. Collins will guide Discovery through a standard rendezvous profile, approaching the lab complex from behind and below.

On final approach, at a distance of about 600 feet directly below the station, Collins will carry out a slow 360-degree rendezvous pitch maneuver, or RPM, that will point the belly of the shuttle at the station.

As the shuttle's underside rotates into view, Expedition 11 commander Sergei Krikalev and flight engineer John Phillips, shooting through windows at opposite ends of the station, will photograph Discovery's belly with handheld digital cameras equipped with 400- and 800-millimeter lenses. During an earlier expedition, science officer Donald Pettit took test photographs of approaching Russian spacecraft to determine what the station's cameras could actually see.

"The 800 millimeter gives them one-inch resolution, which is what we're looking for around (landing gear) door seals," Hill said. "400 millimeters gives three inches, which is what they're looking for everywhere else."

Again, imagery from the station will be downlinked that day for detailed analysis.

After completing the RPM maneuver, Collins will position Discovery directly ahead of the space station with the shuttle's nose facing deep space and its cargo bay facing the lab complex. She then will guide the spacecraft to a docking with a pressurized mating adaptor attached to the Destiny lab module, the first shuttle linkup with the outpost since Nov. 25, 2002.

After leak checks, Krikalev and Phillips will welcome the shuttle crew aboard and provide a brief safety briefing before all nine astronauts get down to work.

Because of clearance issues after the shuttle is docked, Discovery's robot arm cannot unberth the OBSS for additional tile inspections. Instead, the space station's arm - the SSRMS - will pluck the sensor boom from the shuttle's cargo bay and hand it off to Discovery's arm a few hours after docking.

Flight Day 3 highlights:

   DAY..EDT........DD...HH...MM...EVENT
   
   07/15/05
   Fri  04:51 AM...01...13...00...STS crew wakeup
   Fri  04:51 AM...01...13...00...ISS crew wakeup
   Fri  06:21 AM...01...14...30...ISS daily planning conference
   Fri  06:26 AM...01...14...35...Group B computer powerup
   Fri  06:41 AM...01...14...50...Rendezvous timeline begins
   Fri  07:06 AM...01...15...15...ISS: Phillips exercises
   Fri  07:06 AM...01...15...15...Ku antenna clearance video downlinked
   Fri  07:21 AM...01...15...30...Rendezvous tools setup
   Fri  07:26 AM...01...15...35...ET video downlinked
   Fri  07:23 AM...01...15...32...NH rendezvous rocket firing (192.8/155.8 nm)
   Fri  07:51 AM...01...16...00...Noguchi exercises
   Fri  07:56 AM...01...16...05...ISS: Krikalev exercises
   Fri  08:15 AM...01...16...24...NC-4 rendezvous rocket firing (194.2/181.0 nm)
   Fri  08:51 AM...01...17...00...EMU removal from airlock
   Fri  09:21 AM...01...17...30...Thomas exercises
   Fri  09:41 AM...01...17...50...TI rendezvous rocket firing (194.6/185.2 nm)
   Fri  10:06 AM...01...18...15...Robinson exercises
   Fri  10:06 AM...01...18...15...ISS meal
   Fri  11:06 AM...01...19...15...Begin final approach
   Fri  11:10 AM...01...19...19...Discovery directly below ISS (+Rbar)
   Fri  11:31 AM...01...19...40...ISS crew films rotational pitch maneuver
   Fri  12:06 PM...01...20...15...ISS crew prepares PMA-2 for docking
   Fri  12:28 PM...01...20...37...Discovery docks with ISS (194.8/186.6 nm)
   Fri  12:56 PM...01...21...05...Leak checks
   Fri  12:56 PM...01...21...05...Camarda exercises
   Fri  01:26 PM...01...21...35...Shuttle airlock prepped
   Fri  01:31 PM...01...21...40...Group B powerdown
   Fri  01:31 PM...01...21...40...Post-rendezvous PGSC reconfig
   Fri  01:46 PM...01...21...55...Hatch opening
   Fri  02:31 PM...01...22...40...Handshake/Welcome
   Fri  02:41 PM...01...22...50...Safety Briefing
   Fri  03:06 PM...01...23...15...SSRMS OBSS grapple
   Fri  03:21 PM...01...23...30...Krikalev exercises
   Fri  03:21 PM...01...23...30...Collins exercises
   Fri  03:21 PM...01...23...30...EVA prep for transfer
   Fri  03:31 PM...01...23...40...OBSS unberth with SSRMS
   Fri  04:01 PM...02...00...10...OBSS handoff to SRMS
   Fri  05:11 PM...02...01...20...Lithium hydroxide exchange
   Fri  05:41 PM...02...01...50...SSRMS moves to Unity for MPLM inspection
   Fri  06:36 PM...02...02...45...ISS daily planning conference
   Fri  08:51 PM...02...05...00...STS/ISS crew sleep begins

The astronauts also will begin moving more than 1,000 pounds of station equipment stowed in the shuttle's middeck area over to the space station, along with tools that will be used in the upcoming spacewalks. The bulk of the supplies carried aloft aboard Discovery will be housed in the Italian-built multi-purpose logistics module mounted in the cargo bay. The 21,000-pound MPLM will be unberthed on Flight Day 4, using the station's robot arm, and attached, or mated, to the downward-facing port on the U.S. Unity module.

Once the MPLM is in place, the SSRMS will lock onto a mobile base system on the front side of the station's unfinished solar array truss to assist with additional tile inspections. Later that day, the station's arm will be moved back to its normal perch atop the Destiny module. The astronauts, meanwhile, will perform leak checks to make sure the MPLM is firmly mated, they will pressurize the vestibule between Unity and the supply module, activate critical system and then float inside to begin the process of moving supplies into the space station.

At roughly the same time, yet another shuttle tile survey will begin using the RMS-OBSS boom, with additional TV views provided by the SSRMS.

In addition, the tools that will be used for the upcoming spacewalks will be configured for use, two emergency jet backpacks will be moved aboard the station and the crew will spend an hour reviewing the procedures that will be used in the first spacewalk.

Flight Day 4 highlights:

   DAY..EDT........DD...HH...MM...EVENT
   
   07/16/05
   Sat  04:51 AM...02...13...00...STS crew wakeup
   Sat  05:21 AM...02...13...30...ISS crew wakeup
   Sat  06:51 AM...02...15...00...ISS daily planning conference
   Sat  06:56 AM...02...15...05...SSRMS grapples multi-purpose logistics module (MPLM)
   Sat  07:26 AM...02...15...35...SSRMS pulls MPLM from payload bay
   Sat  07:31 AM...02...15...40...Camarda exercises
   Sat  07:36 AM...02...15...45...ISS: Phillips exercises
   Sat  07:36 AM...02...15...45...ISS: Krikalev exercises
   Sat  08:01 AM...02...16...10...EVA tool config
   Sat  08:21 AM...02...16...30...MPLM installation begins
   Sat  08:56 AM...02...17...05...MPLM equipment setup
   Sat  09:26 AM...02...17...35...MPLM berthing mechanism (CBM) 1st torque
   Sat  09:26 AM...02...17...35...Crew meals begin (staggered)
   Sat  09:46 AM...02...17...55...MPLM CBS torquing (part 2)
   Sat  09:51 AM...02...18...00...Middeck transfers
   Sat  10:21 AM...02...18...30...Node 1 nadir CBCS removal
   Sat  10:36 AM...02...18...45...SSRMS ungrapples MPLM
   Sat  10:51 AM...02...19...00...SSRMS grapples mobile base system (MBS)
   Sat  11:31 AM...02...19...40...PAO A/G
   Sat  11:31 AM...02...19...40...MPLM vestibule pressurization
   Sat  11:36 AM...02...19...45...SSRMS ungrapples lab module
   Sat  11:51 AM...02...20...00...EVA tool config
   Sat  12:06 PM...02...20...15...SRMS/OBSS moved to tile survey point
   Sat  12:31 PM...02...20...40...MPLM vestibule preps and setup
   Sat  12:31 PM...02...20...40...Collins exercises
   Sat  12:36 PM...02...20...45...SRMS/OBSS docked survey
   Sat  12:36 PM...02...20...45...Middeck transfers
   Sat  01:31 PM...02...21...40...Thomas exercises
   Sat  02:01 PM...02...22...10...Robinson exercises
   Sat  02:31 PM...02...22...40...Noguchi exercises
   Sat  02:51 PM...02...23...00...ISS: Phillips exercises
   Sat  03:31 PM...02...23...40...EVA procedures review
   Sat  04:01 PM...03...00...10...MPLM ingress
   Sat  04:31 PM...03...00...40...SRMS/OBSS survey ends
   Sat  04:31 PM...03...00...40...MPLM EVA transfer
   Sat  05:31 PM...03...01...40...Transfer tagup
   Sat  05:31 PM...03...01...40...SAFER checkout
   Sat  05:31 PM...03...01...40...ISS daily planning conference
   Sat  05:46 PM...03...01...55...ISS: Krikalev exercises
   Sat  05:46 PM...03...01...55...STS crew leaves ISS
   Sat  05:56 PM...03...02...05...10.2 psi cabin depressurization
   Sat  08:51 PM...03...05...00...STS/ISS crew sleep begins
   

A LONG-AWAITED SPACEWALK TO TEST TILE REPAIR TECHNIQUES

After weeks of internal debate, testing and analyses, NASA managers met Feb. 10 selected three rudimentary tile and wing leading edge repair techniques to demonstrate during the first post-Columbia shuttle mission.

One of the repair techniques will be carried out inside the shuttle Discovery's crew cabin, a so-called "plug" procedure for repairing larger holes in wing leading edge panels. A less sophisticated tile repair technique, one intended for minor damage, will be demonstrated during the crew's first spacewalk, along with a technique for repairing small cracks in leading edge panels.

NASA originally planned for Robinson and Noguchi to use so-called cure in-place-ablator applicator - CIPAA - backpacks, loaded with a tile repair material known as STA-54, to fill in deliberately damaged tiles in Discovery's cargo bay.

But questions about the reliability of the procedure surfaced last year when engineers noticed the formation of air bubbles in the viscous STA-54 material as the two compounds that made it up were mixed together in the backpack. After extensive troubleshooting, engineers were able to reduce the bubbling but they could not eliminate it. The concern was that bubbles could migrate in weightlessness and form large voids as the material cured. Those voids could weaken the patch and its ability to shield against re-entry heating.

Chief astronaut Kent Rominger told CBS News his office opposed in-flight testing aboard Discovery's flight and sources said later that Discovery commander Eileen Collins agreed with that position.

A second option debated during the Feb. 10 meeting called for eliminating a repair demonstration spacewalk altogether. Instead, the crew would demonstrate a so-called overlay tile repair procedure in the shuttle's cabin, along with the plug technique for repairing small holes in leading edge panels. By eliminating the spacewalk, the crew would have more time for external tile inspections and logistics transfers to the international space station.

A third option, the one ultimately selected, was chosen because the techniques in question were the most technically mature and offered the best opportunity to collect useful in-flight data.

Robinson and Noguchi now plan to test a tile repair technique known as "emittance wash" in Discovery's cargo bay. Using a demonstration kit with deliberately damaged tiles, the spacewalkers will paint exposed surfaces with a material that will replace damaged or eroded coating and improve heat rejection.

Flight Day 5 highlights:

   DAY..EDT........DD...HH...MM...EVENT
   
   07/17/05
   Sun  04:51 AM...03...13...00...STS crew wakeup
   Sun  05:21 AM...03...13...30...ISS crew wakeup
   Sun  06:51 AM...03...15...00...EVA-1: EVA prep
   Sun  07:31 AM...03...15...40...ISS daily planning conference
   Sun  07:41 AM...03...15...50...SSRMS grapples lab module for EVA support
   Sun  07:51 AM...03...16...00...Collins exercises
   Sun  08:21 AM...03...16...30...Kelly exercises
   Sun  08:21 AM...03...16...30...Transfer review
   Sun  08:21 AM...03...16...30...EVA-1: EMU purge/prebreathe
   Sun  08:41 AM...03...16...50...SSRMS ungrapples mobile base station
   Sun  08:51 AM...03...17...00...Camarda exercises
   Sun  09:11 AM...03...17...20...External stowage platform
   								  attachment device (ESPAD) setup
   Sun  09:21 AM...03...17...30...Lawrence exercises
   Sun  09:46 AM...03...17...55...EVA-1: Airlock depress
   Sun  09:56 AM...03...18...05...EVA-1: Egress and setup
   Sun  09:56 AM...03...18...05...14.7 psi cabin repressurization
   Sun  10:31 AM...03...18...40...ISS: Phillips exercises
   Sun  10:31 AM...03...18...40...ISS: Krikalev exercises
   Sun  10:31 AM...03...18...40...ISS ingress
   Sun  10:46 AM...03...18...55...MPLM clothes transfer
   Sun  10:46 AM...03...18...55...Lithium hydroxide changeout
   Sun  11:01 AM...03...19...10...EVA-1: Tile/RCC repair demo
   Sun  12:01 PM...03...20...10...ISS/STS meals
   Sun  12:26 PM...03...20...35...SSRMS positioned for ESPAD installation
   Sun  12:41 PM...03...20...50...EVA-1/MS1: ESPAD cable routing
   Sun  12:41 PM...03...20...50...EVA-1/MS2, on SSRMS, removes ESPAD
   Sun  01:16 PM...03...21...25...Transfer operations
   Sun  01:26 PM...03...21...35...EVA-1/MS1/MS2: ESPAD installation on Quest
   Sun  01:56 PM...03...22...05...EVA-1/MS1: GPS antenna R&R
   Sun  02:11 PM...03...22...20...EVA-1/MS2: CMG get aheads
   Sun  02:21 PM...03...22...30...OBSS survey
   Sun  02:56 PM...03...23...05...EVA-1: MS1/MS2 secondary ESPAD cable routing
   Sun  03:21 PM...03...23...30...EVA-1: MS1: payload bay cleanup
   Sun  03:36 PM...03...23...45...STS crew departs ISS
   Sun  04:01 PM...04...00...10...ISS: Phillips exercises
   Sun  04:26 PM...04...00...35...EVA-1: Airlock repress
   Sun  04:51 PM...04...01...00...ISS: Krikalev exercises
   Sun  04:51 PM...04...01...00...CMG EVA setup
   Sun  06:21 PM...04...02...30...ISS daily planning conference
   Sun  07:06 PM...04...03...15...Daily transfer tagup
   Sun  08:51 PM...04...05...00...STS/ISS crew sleep begins

NASA still has no way to repair the kind of leading edge damage that brought down Columbia, but Robinson and Noguchi will test a rudimentary technique in which a heat-resistant material known as NOAX will be smoothed over small cracks in RCC material.

NOAX, which stands for non-oxide adhesive experimental, will be squirted from a caulk gun-like device and then smoothed out with trowels. The RCC will be heated prior to NOAX application and the patch itself will be heated for a half hour after that to cure the material.

The final repair procedure, aimed at fixing small holes in RCC panels, requires a flexible carbon silicon-carbide patch called a "plug." After fit checks and application of a sealant, a plug would be inserted into a hole and held in place from behind by expansion bolts.

Between 20 and 30 different plugs, each with slightly different geometries, would be needed in a real repair kit to ensure a good fit virtually anywhere in the curving leading edge.

The crew of the shuttle Atlantis now plans to test the CIPA technique during the second post-Columbia flight in September. The crew also may test a promising tile overlay technique that calls for spacewalkers to cover a panel of damaged tiles with a thin, flexible sheet of heat-resistant carbon silicon-carbide. The sheet would be mounted atop a gasket and attached with fasteners similar to drywall bolts that would be screwed into surrounding tile.

"The way I interpret the CAIB, I think a practicable repair technique is a requirement," said James Adamson, a member of the Stafford-Covey Return to Flight Task Group. "I don't believe it needs to be certified. It's an emergency technique for an emergency situation. I don't think it necessarily has to have completed all its testing. It has to be reasonable, doable and practicable. And I think NASA's going to have that."

A former shuttle astronaut, Adamson said it's possible "we might disagree that they have met the full intent, or goal, of the CAIB recommendation and still be OK with them deciding that it's OK to fly because of this over-arching reduction of risk.

"But it's really not our call to say the shuttle's safe to fly," he said. "We're looking at a very tiny subset of all the things NASA has to consider to fly again so we really can't be in a position of declaring the shuttle safe to fly. That's their call."

NASA originally planned to stage all three Discovery spacewalks from the space station's Quest airlock module. but concern about contamination in a system used to recharge the crew's spacesuits between outings forced the astronauts to use the shuttle's airlock instead.

The change had a major impact on flight planning, preventing Robinson an Noguchi from using a Quest system to help remove nitrogen from their blood, a requirement to prevent the bends when working in the 5 psi spacesuits. Using the shuttle airlock, the astronauts must close hatches between Discovery and the station before each spacewalk so the orbiter's cabin pressure can be reduced to 10.2 psi as part of the bends prevention process. Once a spacewalk is underway, the hatches can be reopened.

FIXING THE STATION'S GYRO SYSTEM

The Discovery astronauts will spend the day after the first spacewalk transferring more supplies and equipment to the station from the logistics module. Robinson and Noguchi will service their spacesuits and prepare the tools that will be needed for the second spacewalk to install the replacement control moment gyroscope. Two televised crew interviews are planned as the astronauts gear up for the critical station repair work.

Flight Day 6 highlights:

   DAY..EDT........DD...HH...MM...EVENT
   
   07/18/05
   Mon  04:51 AM...04...13...00...STS crew wakeup
   Mon  05:21 AM...04...13...30...ISS crew wakeup
   Mon  05:46 AM...04...13...55...Kelly exercises
   Mon  06:16 AM...04...14...25...Robinson exercises
   Mon  06:46 AM...04...14...55...Noguchi exercises
   Mon  07:01 AM...04...15...10...ISS daily planning conference
   Mon  07:21 AM...04...15...30...Transfer review
   Mon  07:51 AM...04...16...00...EVA tool cleanup and stow
   Mon  07:51 AM...04...16...00...Transfer operations
   Mon  07:51 AM...04...16...00...Lithium hydroxide exchange
   Mon  07:51 AM...04...16...00...Camarda exercises
   Mon  08:51 AM...04...17...00...Middeck preps
   Mon  09:06 AM...04...17...15...Lawrence exercises
   Mon  09:21 AM...04...17...30...EVA tools config
   Mon  10:21 AM...04...18...30...Collins exercises
   Mon  10:51 AM...04...19...00...EVA pistol grip tool setup
   Mon  10:51 AM...04...19...00...ISS: Krikalev exercises
   Mon  10:51 AM...04...19...00...ISS: Phillips exercises
   Mon  12:01 PM...04...20...10...PAO event (CDR, PLT, MS5)
   Mon  12:21 PM...04...20...30...Joint ISS/STS  meal
   Mon  01:21 PM...04...21...30...Transfer operations
   Mon  01:56 PM...04...22...05...PAO event (CDR, MS1, MS2, ISS: Krikalev)
   Mon  02:36 PM...04...22...45...ISS: Krikalev exercises
   Mon  03:21 PM...04...23...30...Crew choice television
   Mon  03:51 PM...05...00...00...EVA-2: Procedures review
   Mon  04:46 PM...05...00...55...Transfer review
   Mon  04:46 PM...05...00...55...ISS: Phillips exercises
   Mon  05:21 PM...05...01...30...ISS egress
   Mon  05:46 PM...05...01...55...10.2 psi cabin depressurization
   Mon  05:51 PM...05...02...00...Thomas exercises
   Mon  06:06 PM...05...02...15...ISS daily planning conference
   Mon  08:21 PM...05...04...30...STS/ISS crew sleep begins

The space station uses four massive control moment gyroscopes to maintain the lab's orientation in space without having to tap into limited supplies of on-board rocket fuel. They are housed in the Z1 truss, which was attached to the Unity module's upward-facing, or zenith hatch - hence the name - during shuttle mission STS-92 in October 2000.

Along with saving fuel, the 800-pound gyros, spinning at 6,600 rpm, allow station crews and flight controllers to reorient the outpost and keep it stable without using rocket firings that would jar sensitive microgravity experiments.

But on June 8, 2002, CMG-1 suffered a malfunction and shut down. Station astronaut Carl Walz reported hearing an unusual noise inside the Unity module. He said the noise appeared to be coming from the module's zenith area. Mission control then told Walz engineers were working an issue with a spin bearing in CMG No. 1. Walz said the noise was quite noticeable inside the module.

"We're hearing a pretty loud, audible noise, kind of a growling noise, from inside the node," Walz reported.

"It looks like we have a mechanical failure of the spin bearings on CMG-1," an astronaut in mission control replied. "It's currently spinning down right now. The growling noise is undoubtedly due to vibration."

The station's orientation, or attitude, can be controlled by just two CMGs in a worst-case scenario. And indeed, a second gyro, CMG-2, was knocked off line last year because of trouble with a circuit breaker. The circuit breaker was replaced during a station-based spacewalk, but the new unit malfunctioned in March, taking CMG-2 off line once again. During the first spacewalk, Robinson and Noguchi plan to wire around the faulty breaker to restore CMG-2 to service.

While the overall system remains fully operational, NASA wants to replace CMG-1 as soon as possible to provide additional redundancy in case of subsequent failures down the road.

Flight Day 7 highlights:

   DAY..EDT........DD...HH...MM...EVENT
   
   07/19/05
   Tue  04:21 AM...05...12...30...STS crew wakeup
   Tue  04:51 AM...05...13...00...ISS crew wakeup
   Tue  06:21 AM...05...14...30...EVA-2: Preps begin
   Tue  06:51 AM...05...15...00...ISS daily planning conference
   Tue  07:21 AM...05...15...30...Collins exercises
   Tue  07:26 AM...05...15...35...ISS: Phillips exercises
   Tue  07:51 AM...05...16...00...Transfer review
   Tue  07:51 AM...05...16...00...Kelly exercises
   Tue  07:51 AM...05...16...00...EVA-2: EMU pre-breathe
   Tue  08:21 AM...05...16...30...Camarda exercises
   Tue  08:21 AM...05...16...30...ISS: Krikalev exercises
   Tue  08:26 AM...05...16...35...ISS: Phillips exercises
   Tue  08:51 AM...05...17...00...Lawrence exercises
   Tue  09:16 AM...05...17...25...EVA-2: Airlock depress
   Tue  09:26 AM...05...17...35...EVA-2: Airlock egress and setup
   Tue  09:26 AM...05...17...35...14.7 psi cabin repressurization
   Tue  09:51 AM...05...18...00...ISS ingress
   Tue  09:51 AM...05...18...00...Transfer operations
   Tue  10:36 AM...05...18...45...EVA-2: CMG removal and replacement
   Tue  11:31 AM...05...19...40...Crew meals begin
   Tue  12:36 PM...05...20...45...Transfer operations resume
   Tue  02:56 PM...05...23...05...EVA-2: Cleanup and ingress
   Tue  03:06 PM...05...23...15...STS crew leaves ISS
   Tue  03:56 PM...06...00...05...EVA-2: Airlock repress
   Tue  04:21 PM...06...00...30...ISS: Krikalev exercises
   Tue  06:06 PM...06...02...15...ISS daily planning conference
   Tue  06:36 PM...06...02...45...Transfer tagup
   Tue  08:21 PM...06...04...30...STS/ISS crew sleep begins
   

The replacement CMG will be mounted on a carrier truss at the back of Discovery's cargo bay. Robinson and Noguchi first will float up to the Z1 truss, unfasten thermal blankets, disconnect electrical cables and remove CMG-1. They will maneuver it to a temporary stowage location and lock it in place. Then, using the SSRMS, the replacement CMG will be removed from the cargo bay truss and moved up to the Z1 truss for installation.

After Robinson and Noguchi complete electrical connections and re-fasten the thermal blankets, engineers in mission control will begin preparations for spinning up the new gyro. The spacewalkers, meanwhile, will move the old gyro back to the cargo bay truss and lock it down for return to Earth. If all goes well, the new unit will be spun up while they are still in the cargo bay.

The gyroscopes are critical to station operation. Here's a description from a NASA press kit:

The motion control subsystem (MCS) hardware launched as part of the Z1 element includes the CMGs and the CMG assemblies.

The CMG assembly consists of four CMGs and a micrometeorite/orbital debris shield. The four CMGs, which will control the attitude of the ISS, have a spherical momentum storage capability of 14,000 ft-lb/sec, the scalar sum of the individual CMG wheel moments. The momentum stored in the CMG system at any given time equals the vector sum of the individual CMG momentum vectors.

To maintain the ISS in the desired attitude, the CMG system must cancel, or absorb, the momentum generated by the disturbance torques acting on the station. If the average disturbance torque is nonzero, the resulting CMG output torque is also nonzero, and momentum builds up in the CMG system. When the CMG system saturates, it is unable to generate the torque required to cancel the disturbance torque, which results in the loss of attitude control.

The CMG system saturates when momentum vectors have become parallel and only momentum vectors change. When this happens, control torques perpendicular to this parallel line are possible, and controllability about the parallel line is lost.

Russian segment thrusters are used to desaturate the CMGs.

An ISS CMG consists of a large flat wheel that rotates at a constant speed (6,600 rpm) and develops an angular momentum of 3,500 ft-lb/sec about its spin axis. This rotating wheel is mounted in a two-degree-of-freedom gimbal system that can point the spin axis (momentum vector) of the wheel in any direction.

At least two CMGs are needed to provide attitude control. The CMG generates an output reaction torque that is applied to the ISS by inertially changing the direction of its wheel momentum. The CMG's output torque has two components, one proportional to the rate of change of the CMG gimbals and a second proportional to the inertial body rate of the ISS as sensed at the CMG base. Because the momentum along the direction of the spin axis is fixed, the output torque is constrained to lie in the plane of the wheel. That is why one CMG cannot provide the three-axis torque needed to control the attitude of the ISS.

Each CMG has a thermostatically controlled survival heater to keep it within thermal limits before the CMGs are activated on Mission 5A. The heaters are rated at 120 watts and have an operating temperature range of -42 to -35ˇF.

READYING THE STATION FOR CONTINUED ASSEMBLY

The day after installing the new gyroscope, the astronauts will enjoy a half-day off, share a joint crew meal and hold a traditional in-flight news conference. Krikalev and Noguchi also will participate in separate news conferences with reporters from their own countries. Supply and equipment transfers to and from the logistics module will continue and Robinson and Noguchi will configure their tools and service their spacesuits for their third and final excursion the following day.

Flight Day 8 highlights:

   DAY..EDT........DD...HH...MM...EVENT
   
   07/20/05
   Wed  04:21 AM...06...12...30...STS crew wakeup
   Wed  04:51 AM...06...13...00...ISS crew wakeup
   Wed  05:16 AM...06...13...25...Robinson exercises
   Wed  05:46 AM...06...13...55...Kelly exercises
   Wed  06:16 AM...06...14...25...Noguchi exercises
   Wed  06:21 AM...06...14...30...ISS daily planning conference
   Wed  06:51 AM...06...15...00...Transfer review
   Wed  07:21 AM...06...15...30...Lithium hydroxide exchange
   Wed  07:21 AM...06...15...30...Camarda exercises
   Wed  07:21 AM...06...15...30...Transfer operations
   Wed  07:51 AM...06...16...00...Lawrence exercises
   Wed  07:51 AM...06...16...00...EVA tools config
   Wed  08:21 AM...06...16...30...Collins exercises
   Wed  10:01 AM...06...18...10...Joint ISS/STS  meal
   Wed  11:01 AM...06...19...10...Crew photo
   Wed  11:16 AM...06...19...25...Crew news conference
   Wed  11:56 AM...06...20...05...Crew off duty time begins
   Wed  12:06 PM...06...20...15...Russian PAO event
   Wed  12:26 PM...06...20...35...ISS: booster fan DTO
   Wed  01:26 PM...06...21...35...ISS: Phillips exercises
   Wed  01:31 PM...06...21...40...ISS: Krikalev exercises
   Wed  03:51 PM...07...00...00...EVA-3: Procedures review
   Wed  04:46 PM...07...00...55...ISS: Phillips exercises
   Wed  05:01 PM...07...01...10...Transfer review
   Wed  05:21 PM...07...01...30...STS crew leaves ISS
   Wed  05:46 PM...07...01...55...10.2 psi cabin depressurization
   Wed  05:51 PM...07...02...00...Thomas exercises
   Wed  06:06 PM...07...02...15...ISS daily planning conference
   Wed  08:21 PM...07...04...30...STS/ISS crew sleep begins
   

The primary objective of the third spacewalk is installation of the external stowage platform, or ESP-2. Tipping the scales at 6,300 pounds, ESP-2 will be pre-packed with critical equipment needed for extensive station re-wiring during upcoming assembly missions when the lab's huge solar arrays will be attached to the currently unfinished boom. Robinson and Noguchi also will retrieve two materials science experiment packages used to expose various materials to the space environment and install a third.

The station's robot arm will be used to pull ESP-2 from its mounting in Discovery's cargo bay. The box then will be maneuvered up to the Quest airlock module on the starboard side of the Unity node for attachment by the spacewalkers. The SSRMS, meanwhile, will lock onto the logistics module attached to Unity's nadir port to set the stage for its detachment the next day.

Flight Day 9 highlights:

   DAY..EDT........DD...HH...MM...EVENT
   
   07/21/05
   Thu  04:21 AM...07...12...30...STS crew wakeup
   Thu  04:51 AM...07...13...00...ISS crew wakeup
   Thu  06:21 AM...07...14...30...EVA-3: EVA preps begin
   Thu  06:51 AM...07...15...00...ISS daily planning conference
   Thu  06:51 AM...07...15...00...RCC plug repair demo
   Thu  07:26 AM...07...15...35...ISS: Phillips exercises
   Thu  07:51 AM...07...16...00...Transfer review
   Thu  07:51 AM...07...16...00...Kelly exercises
   Thu  07:51 AM...07...16...00...EVA-3: EMU purge
   Thu  08:21 AM...07...16...30...Camarda exercises
   Thu  08:21 AM...07...16...30...ISS: Krikalev, Phillips exercise
   Thu  08:51 AM...07...17...00...Lawrence exercises
   Thu  09:16 AM...07...17...25...EVA-3: Airlock depress
   Thu  09:26 AM...07...17...35...EVA-3: Airlock egress and setup
   Thu  09:26 AM...07...17...35...14.7 psi cabin repressurization
   Thu  09:46 AM...07...17...55...Transfer operations
   Thu  09:56 AM...07...18...05...ISS ingress
   Thu  10:11 AM...07...18...20...EVA-3: Camera install CP9
   Thu  10:51 AM...07...19...00...SSRMS grapples ESP-2 (equipment stowage platform)
   Thu  11:21 AM...07...19...30...Crew meals begin
   Thu  11:36 AM...07...19...45...SSRMS unberths ESP-2
   Thu  12:21 PM...07...20...30...ISS: CPA installation
   Thu  12:31 PM...07...20...40...EVA-3: ESP-2 installation
   Thu  01:16 PM...07...21...25...EVA-3: MISSE 1 & 2 retrieval; MISSE 5 installation
   Thu  01:16 PM...07...21...25...SSRMS ESP-2 ungrapple
   Thu  01:31 PM...07...21...40...SSRMS maneuver to MPLM
   Thu  01:46 PM...07...21...55...EVA-3: MS2 FRGF removal
   Thu  02:01 PM...07...22...10...SSRMS grapples MPLM
   Thu  02:16 PM...07...22...25...EVA-3: Cleanup and airlock ingress
   Thu  02:26 PM...07...22...35...STS crew leaves station
   Thu  02:51 PM...07...23...00...ISS crew lock reconfigured
   Thu  03:26 PM...07...23...35...EVA-3: Airlock repress
   Thu  04:01 PM...08...00...10...Collins exercises
   Thu  04:51 PM...08...01...00...MPLM racks configured for entry
   Thu  05:51 PM...08...02...00...ISS daily planning conference
   Thu  06:36 PM...08...02...45...Transfer review
   Thu  08:21 PM...08...04...30...STS/ISS crew sleep begins
   

The next day, the astronauts will complete their final logistics module equipment transfers, deactivate the module's systems and remove it from Unity using the station's robot arm. After re-berthing the module in the shuttle's cargo bay, the SSRMS will lock onto the end of the OBSS boom, take it from the shuttle's arm and put it back in the payload bay for return to Earth. Engineers hope to eventually mount the OBSS boom permanently on the space station, after assembly reaches the point where clearance issues become a problem.

Flight Day 10 highlights:

   DAY..EDT........DD...HH...MM...EVENT
      
   07/22/05
   Fri  04:21 AM...08...12...30...STS crew wakeup
   Fri  04:51 AM...08...13...00...ISS crew wakeup
   Fri  06:01 AM...08...14...10...MPLM cleanup
   Fri  06:21 AM...08...14...30...Japanese PAO event (CDR, MS1)
   Fri  06:51 AM...08...15...00...ISS daily planning conference
   Fri  07:01 AM...08...15...10...Transfer review
   Fri  07:11 AM...08...15...20...Collins exercises
   Fri  07:16 AM...08...15...25...MPLM egress
   Fri  07:26 AM...08...15...35...Post EVA transfer/reconfig
   Fri  07:31 AM...08...15...40...MPLM deactivation
   Fri  07:51 AM...08...16...00...Vestibule preps for unmating
   Fri  07:51 AM...08...16...00...Kelly exercises
   Fri  08:36 AM...08...16...45...Lawrence exercises
   Fri  08:51 AM...08...17...00...Middeck transfers begin
   Fri  09:21 AM...08...17...30...Vestibule depressurization
   Fri  09:36 AM...08...17...45...Thomas exercises
   Fri  09:36 AM...08...17...45...Protein crystal growth experiment transfer
   Fri  10:21 AM...08...18...30...Noguchi exercises
   Fri  10:36 AM...08...18...45...PAO event (CDR, PLT, ISS FE)
   Fri  10:56 AM...08...19...05...ISS: Phillips exercises
   Fri  11:06 AM...08...19...15...SRMS positioned for MPLM demate
   Fri  11:36 AM...08...19...45...Crew meals begin
   Fri  11:51 AM...08...20...00...Node 1 CBM demate
   Fri  12:36 PM...08...20...45...MPLM uninstall
   Fri  01:06 PM...08...21...15...MPLM berthing in payload bay
   Fri  01:21 PM...08...21...30...3 water bags moved to station
   Fri  01:41 PM...08...21...50...Rendezvous tools checkout pt 1
   Fri  01:41 PM...08...21...50...MPLM ungrapple
   Fri  01:56 PM...08...22...05...SRMS maneuver for OBSS handoff to SSRMS
   Fri  02:56 PM...08...23...05...SSRMS grapples OBSS
   Fri  03:21 PM...08...23...30...ISS: Krikalev exercises
   Fri  03:21 PM...08...23...30...OBSS maneuver to clear UHF antenna
   Fri  04:01 PM...09...00...10...SSRMS berths OBSS
   Fri  04:01 PM...09...00...10...ISS: Phillips exercises
   Fri  04:21 PM...09...00...30...Robinson exercises
   Fri  04:41 PM...09...00...50...SSRMS ungrapples OBSS
   Fri  04:56 PM...09...01...05...RMS powerdown
   Fri  06:01 PM...09...02...10...ISS daily planning conference
   Fri  06:41 PM...09...02...50...Transfer review
   Fri  08:21 PM...09...04...30...STS/ISS crew sleep begins
   

DISCOVERY RETURNS TO EARTH

In what promises to be an emotional day for the shuttle-station teams in orbit and on the ground, Collins and her crewmates will bid farewell to Krikalev and Phillips during a departure ceremony shortly before Discovery undocks from the lab complex on the 11th day of the mission.

Flight Day 11 highlights:

   DAY..EDT........DD...HH...MM...EVENT

   07/23/05
   Sat  04:21 AM...09...12...30...STS crew wakeup
   Sat  04:51 AM...09...13...00...ISS crew wakeup
   Sat  06:21 AM...09...14...30...Farewell ceremony
   Sat  06:36 AM...09...14...45...Egress and hatch closure
   Sat  06:36 AM...09...14...45...Rendezvous tools checkout (part 2)
   Sat  06:36 AM...09...14...45...Kelly exercises
   Sat  07:06 AM...09...15...15...ISS daily planning conference
   Sat  07:06 AM...09...15...15...Orbiter docking system leak checks
   Sat  07:36 AM...09...15...45...Robinson exercises
   Sat  07:51 AM...09...16...00...Group B computer powerup
   Sat  07:51 AM...09...16...00...Centerline camera installation
   Sat  08:36 AM...09...16...45...Undocking timeline begins
   Sat  09:29 AM...09...17...38...Discovery undocks from ISS
   Sat  09:51 AM...09...18...00...Flyaround begins
   Sat  10:44 AM...09...18...53...Separation burn No. 1
   Sat  11:12 AM...09...19...21...Separation burn No. 2 (194.4/184.8 nm)
   Sat  11:21 AM...09...19...30...ISS crew meal
   Sat  11:26 AM...09...19...35...Group B computer powerdown
   Sat  11:41 AM...09...19...50...STS crew meal
   Sat  12:16 PM...09...20...25...PMA-2 depressurization
   Sat  12:41 PM...09...20...50...STS off-duty time begins
   Sat  12:41 PM...09...20...50...Lawrence exercises
   Sat  01:21 PM...09...21...30...ISS: Krikalev exercises
   Sat  02:21 PM...09...22...30...Camarda exercises
   Sat  02:41 PM...09...22...50...ISS: Phillips exercises
   Sat  02:46 PM...09...22...55...Collins exercises
   Sat  02:51 PM...09...23...00...Noguchi exercises
   Sat  03:21 PM...09...23...30...Thomas exercises
   Sat  06:51 PM...10...03...00...STS crew sleep begins
   

Collins and company will spend their final full day in space testing Discovery's re-entry systems and stowing loose gear for the return to Earth. Collins and Kelly also will practice landing procedures using a laptop-based shuttle flight simulator. Discovery's KU-band antenna will be stowed in the afternoon, ending normal television views from the orbiter.

Flight Day 12 highlights:

   DAY..EDT........DD...HH...MM...EVENT
      
   07/24/05
   Sun  02:51 AM...10...11...00...STS crew wakeup
   Sun  05:51 AM...10...14...00...Flight control system checkout
   Sun  05:51 AM...10...14...00...Cabin stow begins
   Sun  06:06 AM...10...14...15...Camarda exercises
   Sun  06:51 AM...10...15...00...Thomas exercises
   Sun  07:01 AM...10...15...10...Reaction control system hotfire test
   Sun  07:26 AM...10...15...35...Noguchi exercises
   Sun  07:46 AM...10...15...55...PILOT landing simulations (CDR, PLT, MS2)
   Sun  08:16 AM...10...16...25...Lawrence exercises
   Sun  08:46 AM...10...16...55...EVA hardware stowage
   Sun  09:46 AM...10...17...55...Collins exercises
   Sun  09:46 AM...10...17...55...Post EVA entry preps
   Sun  09:47 AM...10...17...56...Orbit adjust rocket firing (191.8/160.3 nm)
   Sun  10:46 AM...10...18...55...STS crew meal
   Sun  11:46 AM...10...19...55...PAO event (all)
   Sun  12:16 PM...10...20...25...Deorbit review
   Sun  12:46 PM...10...20...55...Cabin stow resumes
   Sun  01:16 PM...10...21...25...White Sands communications check
   Sun  01:21 PM...10...21...30...Kelly exercises
   Sun  02:01 PM...10...22...10...Robinson exercises
   Sun  02:51 PM...10...23...00...L-1 comm checks
   Sun  03:01 PM...10...23...10...Ergometer stow
   Sun  03:01 PM...10...23...10...PGSC FD-13 setup
   Sun  03:31 PM...10...23...40...KU-band antenna stow (assumes clearance OK)
   Sun  04:16 PM...11...00...25...L-1 comm checks
   Sun  06:51 PM...11...03...00...STS crew sleep begins
   

And then the stage will be set for the first shuttle re-entry since Columbia's fatal fall to Earth two-and-a-half years earlier.

"We're not changing anything as far as our trajectory planning or designing," Cain said. "We know we're right down the middle where we want to be. Of course, we've looked at all of that again. (But) we're going to continue to fly the way we've flown and what we consider to be the most benign entry profile we can do."

The shuttle's re-entry trajectory will carry the ship over the south Pacific Ocean, Central America, the Gulf of Mexico and then across Florida. The WB-57 jets will be in place west and east of the region of peak heating to document the shuttle's return. Engineers are hopeful the infrared sensors carried by the jets will help characterize the super-hot plasma around the spacecraft and perhaps improve understanding of at least some of the phenomena seen in amateur video of Columbia's descent.

But video from the modified WB-57 bombers will not be available until the day after landing.

In the wake of the Columbia mishap, NASA conducted studies to determine what risk a returning shuttle posed to the public in the event of another Columbia-class breakup at high altitude. Columbia's debris "footprint" was enormous, stretching almost all the way across Texas and into Louisiana. No one was injured by falling debris, but there were numerous close calls.

Based on population centers and the shuttle's ground track when returning from the space station, agency planners concluded the safest landing site, from a public risk perspective, was the Kennedy Space Center in Florida.

"We're going to plan to land at KSC, that's our prime landing site," Cain said. "Of course, our first line of defense and our prime rationale for flying to begin with is fixing the tank and the orbiter mods and the inspection and repair capability. We think that that rationale bolsters our ability to get back to KSC from a public risk standpoint.

If the weather or some other issue prevents a Florida landing, NASA will fall back on Edwards Air Force Base, Calif., and a backup site - Northrup Strip - at White Sands, N.M.

"The only difference is that if you look at the expectation of casualty information, there are some areas for Edwards and one specific area for Northrup where the public risk assessment is a little bit higher than the highest KSC entry approach. In other words, what was said fundamentally is any and all approaches into KSC are at a risk level that's equitable and acceptable from an agency policy standpoint. If you look at all the opportunities for all cross ranges to KSC and you look at all the areas that you're overflying during the entry ... it's all acceptable.

"When you go and plot that out for Edwards and Northrup, there are some areas that poke up above that line that defines the highest risk into KSC," Cain said. "You're going to have some cases where you're flying over the LA basin. And then even for Northrup, you've got a couple of cases where you fly over Mexico City or even the LA basin.

"So what we've said is, if I can't get into KSC for weather or whatever and I'm going to land at Edwards or Northrup, we're going to give consideration to not utilizing those approaches that have those higher public risk estimates that poke out above that line."

Flight Day 13 highlights:

   DAY..EDT........DD...HH...MM...EVENT
      
   07/25/05
   Mon  02:51 AM...11...11...00...STS crew wakeup
   Mon  05:06 AM...11...13...15...Group B powerup
   Mon  05:21 AM...11...13...30...Inertial measurement unit alignment
   Mon  05:21 AM...11...13...30...PGSC stow (part 2)
   Mon  06:01 AM...11...14...10...Deorbit timeline begins
   Mon  10:02 AM...11...18...11...Deorbit to KSC (rev. 186)
   Mon  11:06 AM...11...19...15...Landing at KSC
   
   BACKUP LANDING OPPORTUNITIES
   
   Mon  11:39 AM...11...19...48...Deorbit to KSC (rev. 187)
   Mon  12:41 PM...11...20...50...Landing at KSC
   Mon  01:08 PM...11...21...17...Deorbit to Edwards (rev. 188)
   Mon  02:10 PM...11...22...19...Landing at Edwards

"We've made a lot of progress in the last several months," Hale said. "We've been doing major work in a lot of different areas for return to flight. ... We are beginning, I think, to really converge on how to operate as a team and make effective decisions so that we can ensure we have a safe space flight."

Even so, Hale stressed that NASA would not catch "go fever" and launch Discovery before it's safe to fly.

"We are going to fly when we have determined that the vehicle is ready to fly, when it is safe to fly," he said. "We're not being driven by a calendar date, we're being driven by our readiness to go fly. So when we are convinced the external tank is in a good situation, when we are convinced we have the warning devices, the OBSS and all those other things, wing leading edge sensors all installed, checked out and ready to go, when we are convinced we have an adequate repair capability, then we'll go fly."

NASA engineers made final preparations to start the shuttle Discovery's countdown later today, buoyed by forecasts calling for a 70 percent chance of acceptable weather during the ship's five-minute launch window Wednesday.

NASA test director Jeff Spaulding, who will oversee Discovery's countdown in the hours leading up to launch, told reporters today the launch team is not tracking any significant technical issues at pad 39B and that all systems are go for launch.

"A lot's happened over the last two-and-a-half years," he said. "Our focus during that timeframe has shifted from one of recovery and investigation to one of redesign, improvement and mission processing and now, to launch. Our launch team (is) well prepared and I know they're up to the task of returning our shuttle fleet to flight, of returning to the international space station and for returning our crew safely back home."

The countdown is scheduled to begin at 6 p.m. and if all goes well, Discovery will lift off on the 114th shuttle mission - the first since the Columbia disaster Feb. 1, 2003 - at 3:50:52 p.m. Wednesday. The ship's crew - commander Eileen Collins, pilot James Kelly, flight engineer Stephen Robinson, Andrew Thomas, Wendy Lawrence, Charles Camarda and Japanese astronaut Soichi Noguchi - arrived at the Kennedy Space Center Saturday night.

Forecaster Kathy Winters called for a 70 percent chance of acceptable weather Wednesday, although she said there was a chance inland thunderstorms could push into the launch area. The forecast is 60 percent "go" on Thursday and Friday should launch be delayed.

Winters said expected temperatures and humidity levels should help minimize the formation of ice on the shuttle's huge fuel tank, a major impact debris concern in recent weeks.

Spaulding said Discovery has enough on-board liquid hydrogen and oxygen, used by the ship's electrical generators, to make three launch attempts in four days. After that, the team would have to stand down for 72 hours to top off the tanks.

Asked about the mood at Kennedy as return to flight approaches, Spaulding said excitement is clearly building.

"Certainly for the last several months, it's been one where everybody has been pretty much having their head down, working very hard," he said. "We had a number of challenges over those months. And it's only recently, I think, that it's all come to fruition where we can see the light at the end of the tunnel.

"The excitement, I think has been building and growing," he added. "There's a great anticipation for launch ... and also maybe a quiet reserve as well just remembering where we've been. But we all do feel confident we've done it right."

Said Scott Higginbotham, payload manager for Discovery's mission: "It sure does feel good to be back in the saddle again."

"It's been too long, but here we are," he said. "I am very happy to report to you that all 28,000 pounds of international space station hardware that's in the payload bay of Discovery is ready to go."

The goal of Discovery's mission is to deliver critical supplies and equipment to the space station, along with a new stabilizing gyroscope that will be installed during the second of three spacewalks by Robinson and Noguchi. The spacewalkers also will test rudimentary heat shield repair techniques during their first excursion.

Assuming an on-time launch, Discovery is scheduled to land July 25 at 11:06 a.m.

Flying in one day early to dodge Hurricane Dennis, the shuttle Discovery's crew arrived at the Kennedy Space Center this evening to make final preparations for launch Wednesday on the first post-Columbia flight.

"This shuttle flight is the beginning of a new chapter in space exploration," commander Eileen Collins told a crowd of journalists at the Shuttle Landing Facility. "We are going to finish building the international space station, we're going to do the science up there that needs to be done in learning how to keep people going in space for long periods of time (and) leaving low earth orbit and going back to the moon.

"By exploring, we make this world a better place to live in and we continue to grow as a human species. That's important, not just for the United States but for all of us around the world."

Collins, pilot James Kelly, flight engineer Stephen Robinson, Japanese astronaut Soichi Noguchi, Andrew Thomas, Wendy Lawrence and Charles Camarda hope to strap in Wednesday for a launch attempt at 3:51 p.m., weather permitting. The countdown begins at 6 p.m. Sunday.

While Hurricane Dennis did not churn up any serious weather at the Kennedy Space Center Saturday, rain and thunderstorms are expected next week and with a short five-minute launch window, the weather could play a major role in when Discovery finally gets off on the long-awaited flight.

"To all the folks traveling out here to launch, especially with Hurricane Dennis bearing down on the Gulf Coast, hopefully they'll all make it here safely and later on this week, on Wednesday, we'll light the candle and head back up into space."

Collins and her crewmates appeared relaxed and in good spirits as they departed the SLF for crew quarters. All seven said they were eager to finally resume shuttle flights two-and-a-half years after the Columbia disaster grounded NASA's fleet.

"That's way too long," Thomas said of the hiatus. "It's definitely time we went back to flight and back to space. And having the responsibility of the return-to-flight mission certainly makes me feel that sense of privilege even more.

"To all the people down here in Florida who have prepared the vehicle and done so much work to make the flight possible, I say thank you. And to all the taxpayers and the members of the public who support the space program, I very much want to thank you for your support. I would like you to know that as we execute the mission, we will do our very best to live up to the great trust you have put in us."

Noguchi, after addressing Japanese journalists in his native tongue, summed up the crew's mood in English, exclaiming: "Let's go fly!"

After assessing the projected path of Hurricane Dennis, NASA managers today ruled out rolling the shuttle Discovery back to the Vehicle Assembly Building, keeping next week's launch of the first post-Columbia mission on track.

NASA spokesman Bruce Buckingham, in a morning advisory, said "Hurricane Dennis has ended its eastward drift and the primary track is slightly more to the west."

"Landfall is currently projected at approximately the Florida/Alabama border," the advisory said. "The closest point to (Kennedy Space Center) is forecast to be 270 miles west at 5 p.m. on Saturday."

Forecasters are predicting a 15 percent chance of 40-knot winds at the launch pad and a 1 percent chance of winds at 70 knots. Heavy rain and winds in the 30- to 35-knot range are expected Saturday as Dennis passes to the northwest.

Friday evening, engineers disconnected ordnance on Discovery but suspend rollback preparations after a midnight meeting to assess the weather. The ordnance will be reconnected today with no impact on the July 13 launch target.

Discovery's crew, meanwhile, plans to fly to Kennedy Sunday morning for the 6 p.m. start of their countdown to blastoff on the 114th shuttle mission."

"The best weather next week is forecast to be on Wednesday and to a slightly lesser extent on Thursday," Buckingham said, "but deteriorating on Friday with considerably more afternoon thunderstorm activity."

Discovery's launch window extends through July 31.

Senior NASA managers met early today and decided to suspend preparations to roll the shuttle Discovery off the launch pad after forecasters projected a slightly more westerly track for Hurricane Dennis. Another meeting is now planned for 6:30 a.m. to assess the hurricane's path and a NASA spokeswoman said the shuttle could still be moved if necessary to protect it from high winds.

With Dennis threatening Florida, NASA managers decided late Thursday to begin making rollback preparations just in casethe storm veeed to the east and threatened the Space Coast. NASA spokeswoman Jessica Rye said early today that engineers had disconnected explosive ordnance as part of that prepartion and that rollback preps had been discontinued after a midnight management meeting to review the storm's progress and the status of launch processing.

By halting rollback preparations when they did, NASA managers preserved a July 13 launch opportunity for Discovery.

With Hurricane Dennis churning in the Caribbean, NASA managers this evening decided to begin preparing the shuttle Discovery for a possible roll back to the protection of the Vehicle Assembly Building should the storm take a turn to the east and threaten the Space Coast.

If the storm stays on its projected course and winds remain below NASA safety limits, the launch team will simply continue readying Discovery for blastoff Wednesday on the first post-Columbia mission. The ship's seven-member crew is scheduled to arrive Sunday for the start of a three-day countdown. But the storm could force the astronauts to change their travel plans even if it doesn't delay launch.

Engineers plan to meet at midnight and again at 7 a.m. Friday to assess the storm's path and their readiness to respond, with a noon meeting on tap to make a final decision. As of Thursday afternoon, the storm's projected track stayed well clear of the Kennedy Space Center, but NASA managers opted to play it safe just in case.

"We are beginning the rollback preparations," said NASA spokeswoman Jessica Rye. "They will not make a decision about rollback until tmorrow, probably about midday. The storm is continuing to take little trends to the right (east) and because of that they want to protect againt possible rollback by getting some preps done this evening."

If the storm does, in fact, take a turn toward Kennedy, Rye said the shuttle would be ready to roll by 8 p.m. Friday and back in the VAB by around 5 a.m. Saturday.

Discovery's launch period extends to July 31 and it's not clear as of this writing how many launch opportunities would remain if the ship had to be hauled off the pad. Rye said that from the point the shuttle returned to the pad, it would take engineers nine days or so to prepare the craft for flight and to run a countdown.

Under that scenario, it would appear Discovery's crew would still have a week or so to get off the ground before the July launch window expired. The next available window opens Sept. 9.

A shuttle is not allowed to remain at the pad if sustained 60-knot winds are expected. The limit for the move back to the VAB, when the shuttle is exposed to the elements, is 40 knots. NASA has moved shuttles off the launch pad 15 times in program history, four of them due to tropical storms or hurricanes.

Editor's Note...
Due to other commitments, I was unable to file this story until the day after the event. My apologies for the delay.

NASA managers wrapped up a two-day flight readiness review Thursday and formally cleared the shuttle Discovery for blastoff July 13 on the first post-Columbia shuttle mission. If all goes well, the countdown will begin at 6 p.m. July 10 for a launch attempt at 3:50:47 p.m. on July 13, weather permitting.

Shuttle program manager Bill Parsons said engineers still must close out open paper work and resolve a few last-minute issues, including questions about the age and certification of an actuator that drives the shuttle's body flap.

The body flap is used during re-entry to help control the shuttle's descent. But Parsons said he believes engineers will resolve the open issues in time to launch Discovery as planned.

"From an open paper standpoint, we have a number of exceptions to the certification of flight readiness that have to be worked off between now and (two days before launch)," Parsons said. "We have a program requirements control board scheduled for (July 7 and 8) and the majority of that paper will be closed out at those two PRCBs.

"And then, as we work on toward L-minus 2, there are a few other things that will be closed out. As far as the body flap actuator ... it looks like we'll be able to clear this for flight with no problems, but again, they have a little bit more work to do."

When all is said and done, the weather might pose the biggest problem for NASA. Thunderstorms are the rule, not the exception, in the summer months on Florida's east coast and they typically develop in the afternoon hours. During the news conference to announce the official launch date, a thunderstorm rumbled across Cape Canaveral, drenching the Florida spaceport.

But Discovery's launch window only extends through July 31 and NASA managers opted not to give up any opportunities at the front end of the window even though a one-week delay would have moved takeoff to the early afternoon.

"Somebody once said, 'it's been raining a long time, will it ever stop?' And the answer, of course, is it always has in the past," said launch director Michael Leinbach. "So the rain will let up and we hope it lets up on July 13th."

But, he added, "launching in the middle of the afternoon in the middle of July is going to be a challenge, let's face it."

"And so we'll deal with that real time," Leinbach said. "We have very strict criteria we will not violate and if we have acceptable conditions, you'll see Discovery fly. Simple as that."

Shuttle commander Eileen Collins, pilot James Kelly, flight engineer Stephen Robinson, Andrew Thomas, Wendy Lawrence, Charles Camarda and Japanese astronaut Soichi Noguchi plan to fly to the Kennedy space Center the morning of July 10 for the start of their countdown to blastoff.

"The crew is go for launch," NASA Administrator Michael Griffin said Thursday. "They want us to be go for launch. They don't want us to rush to flight, but they are ready to return to flight."

The goal of the 114th shuttle mission is to deliver a new stabilizing gyroscope to the international space station, to deliver needed supplies and equipment, to make preparations for upcoming assembly flights and to carry back trash and no-longer-needed equipment that has built up since the shuttle fleet was grounded in 2003. During one of three planned spacewalks, Robinson and Noguchi also plan to test rudimentary heat shield repair techniques.

Asked if he planned to celebrate Discovery's takeoff, Griffin said "I personally don't think we're going to be doing any celebrating until we have wheels stopped on the landing."

"There is an old saying among pilots, the flight's not over until the engine's off and the airplane's tied down," he said. "So we will go from the launch to looking at how our inspections go on orbit, carrying out our detailed test objectives for the flight, resupplying the space station, bringing up a new control moment gyro to replace the failed one, cleaning up some of the stuff that's accumulated on the space station for the last two-and-a-half years we've been unable to get to it. We have a lot of work to do."

Said Bill Readdy, associate administrator for space flight: "This isn't about weighing anchor, this is about when Discovery's back safe in harbor here at the Kennedy Space Center after a successful mission. At that point, I think there will be plenty of celebrating."

Earlier in the week, an independent panel of aerospace executives, academics and former astronauts held a final hearing to assess NASA's implementation of 15 return-to-flight recommendations made by the Columbia Accident Investigation Board in August 2003. The review panel, led by former Apollo astronaut Thomas Stafford and Richard Covey, a former shuttle commander, concluded NASA had failed to fully implement three of the CAIB's most critical recommendations.

But board members said that primarily was due to the literal nature of their interpretation of the recommendations and NASA's much improved understanding of the shuttle's susceptibility to debris impact damage.

Asked if he was disappointed by the results of the Stafford-Covey review, Griffin said to the contrary, he was "delighted."

"They were specifically tasked with assessing NASA's compliance with the literal wording of the Columbia Accident Investigation Board report," Griffin said. "Now it is an interesting question as to whether that should have been the tasking or not. That was two and a half years ago, I'll leave that aside. The fact is, they did what they were asked to do.

"The fact is, several CAIB recommendations, taken word for word, are not implementable with the state of our knowledge today. We do not know how to repair large holes in re-entry carbon carbon heat shields or even small holes, maybe. We're not even sure whether we know or don't know how to repair small holes. We are being as smart about this as we know how to be but we are up against the limits of our human knowledge.

"If someone wants more, they're going to have to find smarter humans. So the recommendations as they were written are not strictly speaking implementable, at least not all of them are, and ... that was not a surprise.

"The Stafford-Covey group also noted that the failure to meet those recommendations ... was not a constraint for flight in their opinion," Griffin continued. "If you read their report, on balance it contains really pretty much nothing but praise for the way NASA conducted itself in returning to flight."

Griffin said he agreed with former shuttle commander and moonwalker John Young, who views all rocket launches as test flights.

"That's exactly right," Griffin said. "The people who fly these things are taking risks on behalf of their country. Spaceflight is risky. As the years and the generations unroll, we will learn how to make it routine as we have done with airline travel after a hundred years. So we have a ways to go.

"All of that said, it is my assessment from the technical reviews that have been held over the last weeks and months that I've been involved is that the proximate causes of the loss of Columbia have been addressed. Many other things which could have been of concern or would have been a concern have also been addressed.

"We honestly believe this is the cleanest flight we have ever done. The only other flight that will ever be cleaner is the next one. So it's risky, we've done what we can do to minimize that based on the state of our knowledge today."

WASHINGTON - An independent panel charged with assessing NASA's implementation of post-Columbia safety upgrades said in its final hearing today that the agency has failed to fully implement three of the most critical safety upgrades recommended by the Columbia Accident Investigation Board.

But that failure, while potentially embarrassing to NASA, is as much a matter of semantics and the wording of the recommendations - and a much improved understanding of the shuttle's susceptibility to debris impact damage - as it is any lack of effort on NASA's part, panel members said.

In fact, the chairman of the Return to Flight Task Group, who piloted the shuttle Discovery on the first post-Challenger mission in 1988, said he would be willing to ride Discovery again next month when NASA hopes to resume shuttle flights after a two-and-a-half-year hiatus.

"I would not have a concern about flying," he said.

Said board member Joe Cuzzupoli: "We feel it is a safe vehicle to fly, based on their inputs."

NASA plans to hold a formal flight readiness review Wednesday and Thursday at the Kennedy Space Center to review mission preparations and to set an official launch date. The agency hopes to launch Discovery between July 13 and July 31 on a long-delayed service-and-supply mission to the international space station.

It will be the first manned American space flight since the shuttle Columbia was destroyed during re-entry Feb. 1, 2003, a disaster blamed on a chunk of foam debris that broke away from the ship's external fuel tank during launch. The foam blasted a hole in the leading edge of Columbia's left wing, triggering the ship's destruction in the fire of re-entry 16 days later.

The Columbia Accident Investigation Board made 29 recommendations for improving shuttle safety, including 15 that were to be implemented before launchings resumed.

In July 2003, former NASA Administrator Sean O'Keefe set up a panel of aerospace experts, former astronauts and academics to assess the agency's implementation of the 15 return-to-flight recommendations. O'Keefe said NASA would not question the recommendations, but would carry them out to the letter. Covey and former Apollo astronaut Tom Stafford headed up the Return to Flight Task Group.

The panel had hoped to present its final report a full month before the resumption of shuttle flights. But the report was held up pending NASA's completion of last-minute work to analyze the threat posed by ice shaking of the external tank during launch.

The results of that analysis were reviewed Friday and while engineers were unable to precisely characterize the threat posed by ice, senior managers agreed the risk was acceptable. They plan to make a formal "go" recommendation during a formal flight readiness review later this week at the Kennedy Space Center.

The last-minute analyses were key elements in two of the three open items being considered by the Stafford-Covey panel. The CAIB recommendations in question called for NASA to:

• Initiate an aggressive program to eliminate all External Tank Thermal Protection System debris-shedding at the source with particular emphasis on the region where the bipod struts attach to the External Tank.

• Initiate a program designed to increase the Orbiter's ability to sustain minor debris damage by measures such as improved impact-resistant Reinforced Carbon-Carbon and acreage tiles. This program should determine the actual impact resistance of current materials and the effect of likely debris strikes.

In the wake of the Columbia accident, NASA eliminated the bipod insulation directly responsible for the fatal impact and implemented a variety of other changes to minimize foam shedding. The debris that doomed Columbia was as big as a suitcase and weighed 1.67 pounds. Engineers believe the largest piece of foam that might be shed during Discovery's launch is on the order of a half ounce.

Ice was a thornier issue to resolve and NASA managers still are not able to precisely characterize the actual risk. Estimates for the possibility of damage that would require a response range from 1-in-100 or so to one in several tens of thousands.

But the RTF task group today concluded NASA had not met the literal intent of the two recommendations.

The agency did not initiate a program to eliminate "all" debris from the external tank, a feat now thought to be impossible due to the basic design of the tank. And while NASA improved the strength of the shuttle's heat shield system in some areas, it decided to stop efforts to beef up the wing leading edge panels after a decision by the Bush administration to retire the shuttle by 2010.

The third open item centered on a requirement for NASA to develop credible repair techniques to fix heat-shield damage that might occur despite the other improvements:

• For missions to the International Space Station, [NASA should] develop a practicable capability to inspect and effect emergency repairs to the widest possible range of damage to the Thermal Protection System, including both tile and Reinforced Carbon-Carbon, taking advantage of the additional capabilities available when near to or docked at the International Space Station.

Recommendation 6.4.1 also called for NASA to "accomplish an on-orbit Thermal Protection System inspection, using appropriate assets and capabilities, early in all missions. The ultimate objective should be a fully autonomous capability for all missions to address the possibility that an International Space Station mission fails to achieve the correct orbit, fails to dock successfully, or is damaged during or after undocking."

NASA has equipped Discovery with a 50-foot boom, laser sensors and TV cameras to carry out a detailed inspection of the shuttle on the second day of its mission. Additional inspections will be made by the space station's crew as Discovery approaches.

But NASA has been unable to develop certified repair techniques to fix entry critical damage to the shuttle's reinforced carbon carbon wing leading edge panels or its more fragile heat shield tiles. Repair techniques will be tested during Discovery's flight, but they are relatively immature and capable of repairing relatively minor damage.

NASA managers say agency and contractor engineers have done the best they could and that actual flight tests are required to find out how the techniques are affected by the space environment. Only then can they be certified and relied on for actual use in an emergency.

In the event Discovery suffers entry critical damage despite the other upgrades, NASA has made plans for the shuttle's crew to move into the international space station to await rescue by the shuttle Atlantis.

NASA managers believe the last-resort "safe haven" scenario, coupled with the redesigned tank, inspection procedures and other changes, make it reasonable to press ahead with launch.

The Return to Flight Task Group, after internal debate, adopted a rather strict interpretation of the recommendation in question, one that required any repair techniques to be thoroughly tested on the ground before launch to provide confidence they could be relied on in an emergency. And under that interpretation, NASA failed to meet the intent of the CAIB, the task group concluded.

But Adamson said the panel's interpretation was just that and that NASA had made strides in other areas that made up for any shortfall in the repair work.

"If you really get down to the heart of intent of CAIB, they were trying to break a chain of events," Adamson said. "What happened, a piece of debris gets liberated, it has the right transport mechanism, it impacts the orbiter in the right place, causes damage that needs to be repaired, there's no repair capability. And there's no ability to actually see and characterize the damage. All those things added up to conspire against Columbia.

"What the CAIB was really trying to tell us, I think, was that we've got to focus our effort in these four areas of reducing the debris, making the orbiter more impact resistant, be able to see and characterize the damage and then if you've got some, fix it," Adamson said.

"They wanted to see all NASA's efforts forcused in those four areas to reduce the likelihood that event could ever happen. And I think they've done that. That's exactly what they went off and did and the fact that in each one of these, taken independently, we might find some semantics in the wording that says we don't think they fully met the intent of this one (or that one) overall, they've significantly reduced the risk."

The actual "intent of the CAIB" has been a major question mark for NASA and the RTF Task Group because of the deliberately broad wording of recommendation 6.4.1.

For his part, Harold Gehman, the retired admiral who led the Columbia Accident Investigation Board, told CBS News in February that he was satisfied NASA had, in fact, met the CAIB's intent even though certified tile and leading edge repair techniques will not be available in time for Discovery's flight.

"It is our judgment that they're efforts have passed the criteria that we set up for them," Gehman said in a February interview.

Asked if he thought it was reasonable to fly without a certified repair technique given all the other improvements and safeguards implemented in the wake o the CAIB report, Gehman said: "That's correct."

"But that doesn't mean they're allowed to give up on the repair," he added. "In our view, they have to keep working at it."

The recommendations centered on four broad areas.

"First of all, you've got to understand foam creation and the creation of the hazard in the first place and you've got to do everything you can to prevent the creatino of foam in the second place," Gehman said in the CBS interview. "The second thing you've got to do is, you've got to have much better pictures on launch and ascent to know whether or not there's been a foam event, or a debris event. You've got to know that. The third thing you've got to do is, you've got to essentially re-certify the orbiter to be ready to come back into the Earth's atmosphere. That translates into some kind of an inspection in orbit.

If serious damage is detected, "you have to have some minimal, practical kind of capability to do some kind or orbital repair, the best practicable kind of a repair. Knowing full well, depending on the size of the damage or what the nature of the damage was, there are some repairs that are beyond your capability to do in space.

"And it has been our unwritten policy ... and I told Stafford-Covey and asked Stafford and Covey to back me up on this and they have - and that is, you must attack all four of these things.

"Now you can do some better than others," Gehman said. "If you really think you've done a fabulous job of preventing the creation of debris in the first place, you've got some really good ways to take pictures to make sure your orbiter hasn't been struck or anything like that and you're really sure that it's in good condition, then you can do some of the other stuf to a lesser degree.

"But you do have to make an attempt at all four areas. Now, within those four areas, there are sometimes one, two, three or four things that you've got to do. But that was what our intent was."

NASA managers met today and decided to press ahead with plans to launch the shuttle Discovery next month even though engineers have been unable to precisely define the threat posed by ice shaking off the ship's external tank during launch.

"At the end of the day, the recommendation ... was that we're in an acceptable risk posture and they recommended to me, the program manager, that we proceed on with the launch," said shuttle chief Bill Parsons. "I accepted that recommendation. And that's where we're headed now."

A final decision on whether to proceed toward a planned July 13 launch of the shuttle Discovery will be made late next week, after a formal flight readiness review and after a final report by an independent panel charged with assessing NASA's implementation of post-Columbia safety upgrades.

At issue in recent weeks has been the threat posed by ice falling from from the shuttle's external tank, especially any of five brackets that hold a 17-inch liquid oxygen feedline to the exterior of the huge tank.

Despite intensive computer modeling, experiments to duplicate the launch environment and statistical analyses based on past flights and other factors, engineers have been unable to precisely characterize the likelihood of ice debris damaging the shuttle's heat shield tiles or wing leading edge panels during launch.

During an afternoon news briefing, Parsons and John Muratore, the manager in charge of the debris assessment, refused to provide numerical odds of impact damage, saying the issue is too complex and that such numbers would be meaningless without a thorough understanding of the assumptions that went into the calculations.

"It's a really complex formulation, going from how much ice can you a have on the spacecraft and what velocities, at what vibration levels it shakes loose, how big are the pieces that shake loose and then how they transport through the aerodynamic environment," he said.

"And then, what's the resistance of the tiles to ice impacts and finally, given an ice crater - which is different from a foam (insulation) crater - how well can we enter (the atmosphere) with regard to that?

"The uncertainties in each of those areas are significant," he said. "There are all sorts of numbers that are floating around. We have nine different estimates for ice on tile from one ice location. ... It is a very complex problem.

"So what we did was, we looked at the relative risks and we're convinced the ... the remaining risks due to ice is around an order of magnitude less than the ones that we fixed."

Muratore said ice represents more of a threat to the shuttle's tiles than it does to its reinforced carbon carbon leading edge panels and that even though engineers used "all the supercomputers at NASA to run it down ... it doesn't lend itself to a single number."

"Impact dynamics are a very complicated engineering discipline," he said.

While he would not discuss the statistics underlying today's "go" recommendation, sources said some estimates of the likelihood of tile damage due to ice impacts - damage that would require repair or some other response - could be as high as 1-in-100 or so or as low as one in tens of thousands, depending on the assumptions that were made.

Engineers do not put much faith in either extreme because they simply do not have enough reliable test data to more precisely define the actual risk. And that won't change anytime soon. As one briefing chart put it, significant improvement in the understanding of the impact threat from ice and foam would require "another enormous undertaking."

The best way to improve that understanding in the near term, many engineers believe, is to collect actual flight data by launching Discovery as planned. They believe the studies carried out to date, plus NASA's experience in 113 previous shuttle missions, show the risk of a debris strike, what ever it might be, is acceptable.

"We believe it's an acceptable risk at this time," Parsons said.

Because Columbia was brought down during re-entry Feb. 1, 2003, by wing leading edge damage caused by an external tank foam strike during launch, NASA focused most of its intial return-to-flight efforts on characterizing the threat posed by foam and developing potential repair procedures.

The external tank's insulation was redesigned to eliminate the foam that was the direct cause of the Columbia disaster. Other improvements were implemented to minimize the size of foam debris that can pop off the tank due to thermal or aerodynamic stresses.

And while the astronauts will not have certified repair procesures on board, they will test a variety of repair techniques and carry out unprecedented external inspections. In the unlikely event unrepairable damage is detected, NASA has made plans for the crew to move into the international space station until a rescue mission could be launched.

The threat posed by ice debris came to light relatively recently, prompting a major effort to define the actual risk. The results of that analysis were presented to NASA managers today during a meeting at the Kennedy Space Center.

In the meantime, a panel of outside experts making up the Return to Flight Task Group plans to meet over the weekend to study today's results before holding a final hearing Monday in Washington.

The task group, chaired by former Apollo astronaut Thomas Stafford and former shuttle commander Richard Covey, was established by former NASA Administrator Sean O'Keefe to oversee NASA's implementation of recommendations made by the Columbia Accident Investigation Board. The CAIB made 29 recommendations to improve safety, 15 of which were to be implemented before the first post-Columbia shuttle flight.

NASA is not bound by the task group's assessment and the final decision to launch Discovery will be made by NASA Administrator Michael Griffin after next week's flight readiness review.

NASA had hoped to launch Discovery in mid May, but the flight was delayed after a fueling test April 14 because of concern about ice buildups around a flexible liquid oxygen feedline bellows assembly. In addition, two of four hydrogen fuel sensors operated intermittently and a pressure relief valve in the hydrogen section of the tank cycled more often than expected.

NASA managers ultimately ordered engineers to haul Discovery off the launch pad back to the Vehicle Assembly Building for attachment to an external tank and boosters originally slated for the second post-Columbia mission. The new tank is equipped with a heater to minimize ice buildups around the feedline bellows.

Before the rollback, however, a second fueling test was conducted May 20, confirming the hydrogen vent valve's unusual behavior. Engineers believe the valve cycling was associated with a jet-like device called a diffuser that injects helium into the tank to help maintain the proper temperature and to pressurize the tank for flight. A new dual-screen diffuser was used in Discovery's original tank and managers decided to switch back to the original single-screen design for the new tank.

As for the hydrogen sensors, which operated normally the second time around, NASA managers decided June 6 to forego a third tanking test, saying they are confident the new tank will behave normally during Discovery's countdown.

One wild card in the troubleshooting is the status of an electronics assesmbly in the shuttle's engine compartment that routes data from the fuel tank hydrogen sensors to the orbiter's flight computers. The original component was taken out after the April tanking test and disassembled for troubleshooting.

A box from the shuttle Endeavour was installed for the second tanking test and while it worked normally, it experienced problems later. Engineers currently are testing a sensor box removed from the shuttle Atlantis and if no problems are found, it will be installed aboard Discovery. One of the other units will be reassembled, tested and re-installed in Atlantis.

Assuming the flight stays on schedule, commander Eileen Collins and her six crewmates - pilot James Kelly, flight engineer Robinson, Soichi Noguchi, Andrew Thomas, Wendy Lawrence and Charles Camarda - will fly to the Kennedy Space Center on Sunday, July 10. The countdown is scheduled to begin at 6 p.m. that night.

The shuttle's primary cargo includes a refurbished control moment gyroscope to replace one that failed earlier aboard the space station; a took kit and spare parts module that will be mounted on the station's exterior to enable future assembly work; and a pressurized logistics module loaded with space station equipment and supplies.

The STS-114 edition of SpaceCalc, a collection of Excel worksheets covering a variety of space-related topics, is now in revision H (up to date through 6/17/05). SpaceCalc is written and maintained by William Harwood to aid reporters during shuttle missions. It is, by definition, a "work in progress" and is always in revision. The latest version includes worksheets for:

1. Mission/MET Clocks
2. STS-114 flight plan
3. STS-114 Quick-Look Facts and Figures
4. STS-114 Crew Thumbnail Background
5. Spacewalk Statistics
6. STS-114 Personnel Assignments
7. STS-114 Launch Windows
8. STS-114 Countdown Timeline
9. STS-114 Ascent Timeline (includes detailed trajectory data)
10. STS-114 Entry Timeline
11. Current/Projected Space Demographics
12. Shuttle Flight History
13. Shuttle Launch/Landing Details
14. Table of All Manned Space Flights (courtesy of Robert A. Braeunig)

Two versions of SpaceCalc are available, one formatted for Macintosh computers and the other for PCs. Both archives include two macros that can run various clocks and timers (Note: The user is assumed to be familiar with Excel).

In addition, the STS-114 edition of the CBS News Space Reporters Handbook is now available. The SRH (in pdf format) includes many of the charts featured in SpaceCalc, detailed background on STS-114 and appendices covering STS-51L, STS-107, abort scenarios, etc. The current version of the STS-114 supplement includes an 11,600-word CBS News mission preview written in December. An updated version of that story will be included after NASA's flight readiness review at the end of the month.

Revision histories are available below. As always, suggestions and corrections are welcome.

STS-114 SpaceCalc (Macintosh formatting; 1-megabyte Stuffit archive)

STS-114 SpaceCalc (Windows formatting; Zip archive)

SRH: STS-114 Supplement (70 pages; 5 megabytes)

SRH: Appendices 1-8 (109 pages; 3 megabytes)

SRH: STS-51L/107 Remembered (70 pages; 6 megabytes)

The space shuttle Discovery, bolted to an upgraded external tank and a fresh set of boosters, was hauled back out to pad 39B early today for work to ready the ship for blastoff July 13.

The 4.2-mile journey from NASA's cavernous Vehicle Assembly Building to the oceanside launch complex began at 1:58 a.m. and took more than 10 hours to complete. The trip took longer than expected because of an over heated bearing that prevented NASA's Apollo-era crawler-transporter from maintaining its normal 1 mph top speed.

While engineers are confident Discovery and its new tank will be ready to go when July 13 rolls around - the launch team has five full days of contingency time built into the processing schedule - final clearance to launch will hinge on the results of a critical meeting June 24 to assess the threat posed by ice impacts during ascent.

NASA had hoped to launch Discovery in mid May, but the long-awaited flight was delayed after a fueling test because of concern about ice buildups around a flexible liquid oxygen feedline bellows assembly. In addition, two of four hydrogen fuel sensors operated intermittently and a pressure relief valve in the hydrogen section of the tank cycled more often than expected.

NASA managers ultimately ordered engineers to move Discovery back to the VAB for attachment to an external tank and boosters originally slated for the second post-Columbia mission. The new tank is equipped with a heater to minimize ice buildups around the feedline bellows.

Before the rollback, however, a second fueling test was conducted May 20, confirming the hydrogen vent valve's unusual behavior. Engineers believe the valve cycling was associated with a jet-like device called a diffuser that injects helium into the tank to help maintain the proper temperature and to pressurize the tank for flight. A new dual-screen diffuser was used in Discovery's original tank and managers decided to switch back to the original single-screen design for the new tank.

As for the hydrogen sensors, which operated normally the second time around, engineers now believe whatever caused problems during the first tanking test was corrected during extensive post-test troubleshooting.

NASA managers decided June 6 to forego a third tanking test, saying they were confident the new tank would behave normally during Discovery's countdown. Even though the hydrogen pressurization system begins operating just two minutes before liftoff, the countdown can be safely stopped if any problems develop.

Wanting to avoid any such last-minute surprises, however, some engineers argued in favor of a third tanking test to make sure the analysis is correct. But senior managers decided to press ahead. Any serious problems with the tank almost certainly would delay the flight beyond the July 13-31 launch window regardless of when they were discovered.

Against this backdrop of engineering troubleshooting, an independent panel assessing NASA's implementation of post-Columbia safety upgrades is nearing the end of its work, waiting for the results of the June 24 debris verification review to close out a pair of lingering open items.

Even though the new tank is equipped with additional heaters to prevent ice buildups in critical areas, questions remain about the threat posed by ice shaking off the tank during launch from stand-off brackets that hold a pressurization line near the tip of the fuel tank.

While engineers believe the tank will not shed any foam insulation debris big enough to cause a catastrophic Columbia-class disaster, the data are less clear cut about the threat posed by ice.

The debris verification review will set the stage for a final meeting by the Return to Flight Task Group on June 27 and 28, followed by a two-day flight readiness review at the Kennedy Space Center June 29 and 30.

NASA managers today ruled out a third tanking test for the shuttle Discovery, keeping launch of the first post-Columbia mission on target for July 13. The launch window extends to July 31 and as of this writing, engineers have five days of contingency time in the launch processing schedule to handle unexpected problems between now and the opening of the window.

Still to come: final disposition of questions about the risk posed by ice debris during launch; a final report by the Stafford-Covey Return to Flight Task Group regarding NASA's implementation of post-Columbia safety upgrades; and a formal flight readiness review to clear Discovery for flight.

The debris verification review, which will quantify the threat posed by ice debris based on a flurry of recent tests and extensive analysis, is targeted for June 24, followed by the Stafford-Covey group's final meeting and the two-day flight readiness review, which will be held at the Kennedy Space Center.

Among the debris threats still on the table is the question of possible ice buildups around brackets near the top of the external tank.

Assuming no show stoppers emerge during the meetings later this month, commander Eileen Collins and her crew will fly to Kennedy July 9 for the start of the countdown to blastoff of the 114th shuttle mission. Liftoff July 13 is targeted for 3:51 p.m.

NASA had hoped to launch Discovery in May, but problems cropped up during a fueling test April 14 that contributed to a decision to delay the flight to mid July. During that test, two of four liquid hydrogen fuel depletion sensors (also known as engine cutoff sensors) failed to operate properly and a pressure relief valve that helps maintain hydrogen tank pressurization in the final two minutes of the countdown cycled more often than usual.

At the same time, the ongoing ice debris verification review raised questions about potentially dangerous buildups of ice around a liquid oxygen feedline bellows assembly that could shake off during launch and damage the shuttle's fragile heat shield tiles or wing leading edge panels.

NASA managers ultimately decided to delay launch and to move Discovery to a different set of boosters and a modified external tank equipped with a heater on the feedline bellows to prevent ice formation.

The tank also features an older-style single-screen diffuser, which injects a jet of helium gas into the hydrogen tank to help keep the supercold fuel circulating at the proper temperature. It also provides the pressurization needed after the tank is isolated from ground systems one minute 52 seconds prior to launch.

Engineers believe the unusual valve cycling during the April 14 tanking test was due to a newer style dual-screen diffuser. During the April 14 test and a second tanking test May 20, the valve cycled 13 times. Based on past experience, it was expected to cycle eight or nine times.

By switching to the single-screen diffuser, engineers are confident the problem will not recur during the launch countdown. But testing continues.

As for the engine cutoff sensors, NASA managers now believe the intermittent operation in April most likely was do to a wiring issue that was resolved during troubleshooting. The sensors worked properly during the May 20 tanking test and engineers believe the sensors in Discovery's new tank will behave normally as well.

Engineers pumped a half-million gallons of supercold rocket fuel into the shuttle Discovery's external tank early today, repeating a test that uncovered two unexpected problems in April. This time around, suspect fuel depletion sensors in the tank worked normally while a hydrogen pressure relief valve continued to cycle more often than normal.

Bill Parsons, shuttle program manager at the Johnson Space Center in Houston, said it will take engineers additional time to fully analyze the data. But it would appear whatever caused the fuel depletion sensor problem in April was corrected during earlier troubleshooting.

As for the hydrogen vent valve issue, today's test showed no difference in operation regardless of whether a new heater, installed as part of NASA's post-Columbia safety upgrade program, was turned on or off. That would appear to exonerate the heater.

Parsons said he was optimistic a thorough review of data from today's test will allow engineers to pin down the cause of the problem and clear the way for a launch attempt July 13 as currently planned.

"We had just a perfect test as far as I'm concerned," he said.

The three-hour fueling process began at 5:36 a.m. and was complete by around 8:30 a.m. At that point, just like in a real countdown, a small team of engineers headed to the launch pad for a detailed external ice inspection.

During the first tanking test April 14, humidity was relatively low and ice formation was minimal. Engineers were hopeful today's test would provide additional insights into where ice might build up on the tank. Contrary to initial predictions, however, the humidity was not much higher - 67 percent - and with a breeze to keep condensation down, ice and frost formation were only slightly higher.

Today's test was run in two stages. The first ended around 1:40 p.m. with a countdown cutoff at the T-minus 31-second mark. For that run, the bipod heaters at the bases of the two struts connecting the nose of the orbiter to the external tank were turned on to prevent ice buildups.

The shuttle Columbia's destruction was triggered by the loss of foam insulation around the left-side strut. In the wake of the accident, the foam in question was removed and the bipod heaters were installed to do the same thing: Prevent ice formation.

For the second part of today's test run, the countdown was recycled to the T-minus 20-minute mark and the heaters were turned off. The countdown resumed and continued to the T-minus 31-second mark a second time to find out if the bipod heaters played any role in how often the hydrogen pressure relief valve operated.

The pressure relief valve works during the final two minutes of the countdown when gas is being pumped into the tank to maintain the proper pressure levels for flight.

During the April 14 tanking test, the pressure relief valve cycled 13 times compared to a normal cycle rate of eight or nine. During today's test, with the bipod heaters were powered on, the valve cycled 13 times just like it did in April. During the second part of the test, when the bipod heater was off, it cycled 13 times once again.

Special instrumentation installed for today's test showed the valve was not leaking, focusing attention on a screen-like "diffuser" that engineers already planned to replace. The diffuser disperses the gas used to pressurize the tank just before launch. If that system was damaged or clogged, it could affect the operation of the vent valve.

Another problem seen in April involved two of four hydrogen depletion sensors in the tank that failed to operate properly. The sensors are used to make sure the shuttle's main engines shut down properly after reaching space and all four must be operational for a countdown to proceed.

During today's test, all four sensors operated normally. Engineers suspect cable harness replacements and other troubleshooting in the orbiter's engine compartment corrected whatever the problem was in April. But that remains to be confirmed.

And in any case, it's likely a moot point for Discovery. NASA managers decided earlier to move the shuttle to a different tank and boosters for its launch attempt in mid July. The new tank will feature an additional heater to prevent ice buildups around a flexible propellant line bellows, which prompted concern during Discovery's initial launch pad processing flow.

In addition, the new tank's engine cutoff sensors presumably will work properly and the tank features a diffuser with a known track record. But Parsons said engineers might ask for a third tanking test before Discovery's eventual launch just to make sure. As it now stands, he said, a third tanking test is not planned.

The shuttle will be hauled back to the Vehicle Assembly Building next Tuesday. If all goes well, Discovery will be moved back to pad 39B around June 14 for a launch attempt on July 13. If an additional tanking test is ordered, the flight could slip another week or so.

NASA's launch team readied the shuttle Discovery today for a second fueling test Friday to help find the cause of two vexing problems that cropped up during a similar test in April. Other shuttle engineers, meanwhile, are assessing a clearance issue between the shuttle's main Ku-band TV antenna and a new on-orbit inspection boom that could delay the transmission of post-launch images of the ship's redesigned external tank.

As part of NASA's post-Columbia safety upgrade program, a 50-foot-long boom equipped with a television camera and laser sensors was installed on the right, or starboard, side of Discovery's cargo bay. The boom, picked up by the shuttle's robot arm in orbit, will be used to inspect Discovery's wing leading edge panels and heat shield tiles for any signs of launch impact damage.

Just in front of the orbiter boom sensor system, or OBSS, on the starboard sill of the cargo bay is the shuttle's Ku-band dish antenna, which is used to send high-speed data and television signals from the orbiter to the ground via NASA communications satellites.

Once in orbit, the antenna is deployed by swinging it out over the side of the cargo bay sill so it can lock onto and track the appropriate satellite. The antenna normally is deployed on the first day of a shuttle mission and in Discovery's case, flight controllers were counting on a flight day 1 deploy to downlink photographs of the external tank to look for signs of foam shedding.

But an analysis of clearances between the antenna and the OBSS shows that as the dish swings out, it could come within 0.6 of an inch of the forward end of the boom. And that doesn't include the boom's insulation blankets. That tiny margin could shrink to just 0.3 inches in orbit due to thermal issues. The shuttle Atlantis has a similar problem.

No one wants to risk damage to the Ku-band antenna, which is critical to the conduct of a mission, and NASA managers plan to meet next week to reassess the clearance issue. But neither the boom nor the antenna can be easily repositioned. Barring a dramatic change, Discovery's crew could be forced to delay deployment of the Ku-band antenna until the second day of the mission, after the OBSS has been picked up by the robot arm.

While downlink of external tank video and photographs would be delayed by one day, officials said the overall impact would be relatively minor. And it would not affect transmission of data from new wing leading edge sensors using the shuttle's S-band radio system on flight day 1.

The clearance issue could have a more noticeable impact toward the end of the mission. Managers are debating two possible options: Stowing the antenna before Discovery undocks from the international space station or stowing the OBSS prior to undocking and then redeploying it the day before entry to re-stow the antenna. The first option would end normal television from the shuttle for the remainder of the flight while the second option would add complexity and additional failure scenarios.

Agency officials do not yet know why the clearance issue took so long to come to light. A contractor analysis earlier this year concluded there was more than an inch of clearance and as such, no problem. But measurements carried out by the astronaut office showed the actual clearance was half the earlier estimate.

The shuttle was designed so a robot arm could be installed on either side of the cargo bay. The OBSS was installed using those pre-existing mounting points. In that sense, the clearance problem has been there ever since the shuttle was built. But NASA never mounted anything on that side of the bay before the OBSS aboard Discovery.

The clearance issue is just one of several nagging problems facing the shuttle launch team as it readies Discovery for takeoff on the first post-Columbia mission. Launch currently is targeted for July 13, the opening of a window that extends through July 31.

Launch had been planned for May 22, but the flight slipped to the July window because of late-breaking concerns about ice formation on the external tank, and its potential for impact damage during ascent, and because of problems that were noticed during a tanking test April 14.

During that exercise, two of four fuel depletion sensors, also known as engine cutoff sensors, in the liquid hydrogen tank failed to operate properly. In addition, a hydrogen pressure relief valve cycled more often than normal.

The cutoff sensors are used to make sure the shuttle's three main engines do not run out of hydrogen while running, which would cause a potentially catastrophic oxygen-rich shut down. All four sensors are required to be operational for a countdown to proceed.

As for the relief valve, it operated within specification but its behavior was clearly unusual and engineers want to understand why. One possible explanation involves the operation of a new post-Columbia heater installed at the base of the bipod struts that hold the nose of the shuttle to the tank.

NASA managers earlier decided to move Discovery back to the Vehicle Assembly Building and attach it to a tank and booster set being prepared for the second post-Columbia flight. That tank will be equipped with a new heater to ease concerns about ice buildups on an oxygen feedline bellows assembly.

Still unresolved is what to do about potential ice buildups on brackets near the top of the tank. Engineers have suggested using infrared spotlights to prevent or minimize ice formation, but additional tests are required to determine the plan's feasibility.

In the meantime, NASA managers ordered a second tanking test Friday to collect additional data before Discovery is moved back to the Vehicle Assembly Building. The currently-installed bipod heaters will be on for part of the test and then turned off to find out whether the heaters influence the operation of the pressure relief valve.

Here is a tanking test timeline (all times in EDT):

   May 19
   08:00 p.m....Rotating service structure retraction
   11:30 p.m....Final loading preps begin
   
   May 20
   02:30 a.m....Pad cleared of personnel
   04:00 a.m....Mission management team meets for go/no-go decision
   04:30 a.m....Begin a 1-hour built-in hold
   05:30 a.m....Resume countdown
   05:30 a.m....Begin propellant line chilldown (fueling begins)
   08:30 a.m....Fueling complete; stable replenish mode begins
   08:30 a.m....Begin a 1-hour 45-minute built-in hold
   10:15 a.m....Resume countdown with bipod heater on
   01:35 p.m....Countdown is stopped at T-minus 31-second mark
   01:35 p.m....Recycle countdown to T-minus 20-minute mark
   01:45 p.m....Resume countdown with bipod heater off
   02:45 p.m....Final cutoff at T-minus 31-second mark
   02:45 p.m....Begin propellant drain back and boiloff
   
   May 21
   03:00 p.m....Rotating service structure moved back around shuttle

The launch team plans to test fire one of Discovery's three auxiliary power units, which generates the ship's hydraulic power, on May 23. The next day - May 24 - the orbiter will be moved back to the Vehicle Assembly Building for attachment to the new tank and boosters on June 7.

Discovery's space station cargo is scheduled to be moved to launch pad 39B on June 9 with Discovery following suit on June 14. If all goes well, the shuttle will be ready for launch July 13 as currently planned.

But engineers are still debating whether to stage a third tanking test to make sure the new tank operates normally when loaded with a half-million gallons of supercold propellants. If a separate, stand-alone test is required it likely would be staged around June 22, setting the stage for a launch attempt around July 19. But some engineers have suggested extending Discovery's countdown and inserting a tanking test a few days before the July 13 launch target.

A decision on whether to stage a third tanking test is expected early next month.

NASA managers today decided to stage a second tanking test next weekend or shortly after to troubleshoot problems with the shuttle Discovery's external fuel tank. The shuttle then will be hauled back to the Vehicle Assembly Building where engineers will attach the ship to a different set of boosters and a fresh external tank, officials said late today.

The shuttle will be returned to the launch pad in mid June for launch on the first post-Columbia mission around July 13, the opening of the next available space station launch window. But that date could slip if program managers order yet another tanking test after rollout to verify the performance of the new fuel tank.

NASA had hoped to launch Discovery on the first post-Columbia mission this month, but managers decided last week to delay the flight to the July window because of lingering concerns about the potential threat of ice on the external fuel tank and because of two problems that cropped up during a tanking test April 14.

The ice issue will be addressed by a heater around a liquid oxygen feedline bellows assembly similar to the one used to prevent ice buildups on the struts holding the nose of the shuttle to the external tank. The other two issues with the current tank are more subtle.

During the tanking test last month, two of four hydrogen sensors inside the tank, which are used to control the main engine shutdown sequence when the shuttle reaches space, failed to operate properly. Engineers have not yet pinned down what caused the problem, but all four must be operational for a launch to proceed.

In addition, a valve used to bleed off pressure in the hydrogen tank cycled more often than usual.

NASA managers spent the week debating a variety of processing options, including whether to stick with the current tank, whether to carry out one or two additional tanking tests and whether to swap out Discovery's tank and boosters for a set being assembled for the second post-Columbia flight (STS-121).

During a meeting late today, shuttle managers decided to implement a version of the latter scenario. Here is a timeline of major processing milestones (dates are approximate "no earlier than" targets and should be taken with a grin of salt):

• 05/15: Discovery's current tank is reloaded with a half-million gallons of rocket fuel to collect additional troubleshooting data on engine cutoff sensors, pressure relief system
• 05/28: Discovery is moved back to the Vehicle Assembly Building*
• 06/09: The orbiter is attached to the new booster/tank stack*
• 06/16: Discovery is moved back to pad 39B
• 07/13: Launch (assumes no additional tanking test)

  * Discovery's rollback depends on when the new tank/booster stack is completed. Engineers hope to finish the work ahead of schedule, allowing them to move rollback up by several days or more.

Before Discovery is returned to the pad, engineers must decide whether an additional tanking test is needed to verify the performance of the new tank. Such a test would add several days to the processing schedule, pushing launch to around July 18. But if the test is not required, July 13 remains a viable launch target.

Discovery's launch window opens July 13 and closes July 31. The next launch window opens Sept. 9 and closes Sept. 24. A three-day window is available Nov. 8-10, a four-day window opens Jan. 4 and a lengthy window opens March 3 and closes March 19.

The launch window is based on an internal NASA requirement to launch at least the first two post-Columbia missions in daylight. But the issue is complicated by a requirement to also ensure the external tank separates half a world away in enough light to allow documentary photography.

To reach the space station, the shuttle must launch within five minutes of the moment Earth's rotation carries the launch pad into the plane of the station's orbit. When all of those requirements are met, along with others involving temperature constraints on the station, only a limited number of launch windows is available.

The shuttle Discovery's crew strapped in today for a dress rehearsal countdown while program managers continued assessing what work needs to be done - and whether it can be completed in time - for a July 13 launch attempt.

In a traditional pre-flight milestone, commander Eileen Collins, pilot James Kelly, Stephen Robinson, Soichi Noguchi, Andrew Thomas, Wendy Lawrence and Charles Camarda donned their bright orange pressure suits and began climbing aboard Discovery shortly after 8 a.m.

The countdown proceeded to the T-minus four-second mark when it ended with a simulated abort and main engine shut down at 11 a.m. The test went smoothly and other than a minor communications glitch caused by a switch in the wrong position, no major problems were encountered.

"As I said in the debrief to our launch director, it felt to me like it was a real launch day the way people were talking and handling issues as they came up," Collins told reporters after the test. "The space shuttle is very close to being ready to go, but we want to make sure that we understand everything that we are studying and we don't want to rush into anything."

The terminal countdown demonstration test, or TCDT, was already scheduled when launch managers last week decided to delay the first post-Columbia mission from May 22 to no earlier than July 13, the opening of the next available space station launch window.

The launch delay was ordered because of concern about the possibility of ice buildups around a bellows assembly that allows the external tank's 17-inch-wide liquid oxygen line to flex slightly during launch. At issue is whether such ice could break off and damage the shuttle's heat-shield tiles or wing leading edge panels during ascent.

Wanting more time to assess the overall ice threat, and to install a heater on the liquid oxygen feedline bellows, program managers opted to delay launch. They also needed more time to troubleshoot two potentially serious problems that cropped up during a test April 14 when the external tank was loaded with a half-million gallons of supercold rocket fuel.

Two of four sensors in the liquid hydrogen tank that detect low levels and help control the engine shutdown sequence failed to operate properly. In addition, a pressure relief valve cycled more often than expected, indicating a problem of some sort in another system.

The feedline bellows heater assembly must be installed back at the Vehicle Assembly Building. But engineers have not yet decided when to move Discovery off its oceanside launch pad. They are considering another tanking test before rollback to gather additional data on the fuel depletion sensors and the relief valve.

They also are considering whether to move Discovery to a different set of boosters and a fresh external fuel tank currently being assembled for the second post-Columbia mission. The tank for that mission includes additional instrumentation not available in Discovery's current tank.

As of this writing, no such decisions have been made. But given the amount of work left to do, sources say it will be difficult to meet the July 13 launch target.

"The shuttle program as we speak, well this week, they are looking at what we are going to have to do to get our external tank ready to fly," Collins said. "There's some troubleshooting that needs to be done external to the tank and it will take, what I've been told, at least a week and I'm not sure maybe longer, maybe a little bit less.

"We're going to get the shuttle flying, we'll get the international space station built," she said. "Keep in mind this is a very short delay in the big picture of things and it's important that we do it right."

The shuttle Discovery's launch on the first post-Columbia mission is on ice until at least July 13, officials said today, primarily because of recent tests showing ice buildups pose more of a potential impact threat than previously thought.

Because of time needed to complete a thorough debris analysis, to implement possible fixes and to address a variety of other late-breaking problems, NASA Administrator Mike Griffin today announced that Discovery's launch would be delayed from May 22 to no earlier than July 13.

"No one thing but the sum of all those things together necessitates we move out six or seven weeks into the July window," Griffin said. "This is consistent with our overall approach to return to flight, which is we're going to return to flight, we're not going to rush to flight. We want it to be right, so we're doing what we need to do to ensure that."

The primary problem facing shuttle program managers is the potential threat of ice debris shaking off the external fuel tank during launch and causing impact damage to the shuttle's fragile heat-shield tiles or wing leading edge panels.

The shuttle Columbia was destroyed during re-entry Feb. 1, 2003, because of a hole in the left wing's leading edge that was caused by the impact of external tank foam insulation that broke off during launch 16 days earlier.

The foam responsible for Columbia's demise was intended to prevent ice from building up around the fittings that attach struts holding the nose of the shuttle to the tank. The so-called bipod foam has been eliminated in favor of small heaters.

Foam application techniques were changed to minimize the chances for foam shedding in general. Engineers believe the largest piece of foam that can come off the tank today is less than a half ounce. The piece that hit Columbia weighed some 1.67 pounds.

But recent testing shows ice buildups in two areas of the tank still pose a threat. One of those areas is where an oxygen feedline bellows is located. The bellows allows the propellant line to respond to the effects of supercold liquid oxygen and to flex slightly during launch.

The testing shows ice can build up on the bellows or on a bracket holding the line in place. Another ice problem area is near the tip of the tank around brackets that hold a pressurization line in place.

During a detailed review of past launches, engineers were able to identify signs of past ice damage on the belly of the shuttle. The review showed a clear bias for damage on the right side of the belly, which is consistent with ice falling from the feedline bellows area. Recent testing shows the impact energy of ice debris does not depend on whether it is rock solid or somewhat slushy.

The question is, when does the ice break off? And how does the airflow between the shuttle and the external tank affect the trajectory of any such debris?

Ice and foam behave very differently when they enter the airstream around a space shuttle. That airflow tends to stop a piece of lightweight foam in its tracks, which increases the relative velocity of an impact as the shuttle, in effect, runs into the debris at high speed. Ice does not decelerate as rapidly, which lowers the relative impact velocity and results in a very different transport mechanism.

"This is a very complex problem," said shuttle program manager Bill Parsons. "Ice can come off early in the flight and it doesn't have a transport mechanism to ever get to the vehicle. There's a small region in there when you're at a particular Mach number that you have a transport mechanism that gets this ice to the vehicle. That's what we've learned.

"We've learned that through a lot of research and through previous damage the vehicle received. We've had hits on (solid rocket booster) cork that we attribute to the LOX (liquid oxygen) feedline bellows ice, we've had some hits in the tile that we attribute at this point in time to the LOX feedline bellows ice.

"That was the information that said, hey wait a minute, maybe we've been a little lucky (in past flights), maybe we don't understand this problem as well as we should and therefore we need to go and understand this problem and understand if we've been lucky or the design of this vehicle, and the way this ice comes off, won't transport it towards the vehicle."

While testing showed a low probability of ice damage, there is "still a probability that pieces of ice can come off and hit the vehicle and cause damage," Parsons said. "Because of that, it became important to eliminate that."

NASA plans to install a heater on both of the external tanks currently at the Kennedy Space Center, including Discovery's, to eliminate the problem once and for all. After an already scheduled practice countdown next week with Discovery's crew, the shuttle eventually will be rolled back to the Vehicle Assembly Building where engineers will attach a feedline bellows heater.

The shuttle then will be hauled back out to launch complex 39B for work to ready the ship for takeoff July 13. Engineers believe they have about 20 days of contingency time in the current schedule to handle unexpected problems.

Managers might even opt to move Discovery to a tank and boosters being prepared for the second post-Columbia mission, although as of this writing, most insiders believe that's a remote possibility.

Engineers also plan to either clean or replace 18 insulation blanks on Discovery's rear orbital maneuvering system rocket pods that were contaminated with hydraulic fluid during earlier pad processing. Engineers were initially concerned that normal ascent heating could cause the residual fluid to ignite, but Parsons said additional testing has shown the contamination is not as extensive as initially believed and that cleaning alone may be sufficient. Either way, the work can be done in parallel with other processing.

Of more concern to the launch team are two problems that cropped up April 14 during a key test in which the external tank was loaded with a half-million gallons of supercold liquid oxygen and hydrogen.

Two of four sensors in the hydrogen portion of the tank operated intermittently. The sensors detect low fuel levels when the shuttle's engines must be commanded to shut down. Running an engine out of hydrogen, resulting in an oxygen-rich shutdown, could be catastrophic and all four fuel depletion sensors must be operational for a countdown to proceed.

Engineers believe the sensors are not to blame but so far, they have been unable to find a wiring problem or any other explanation for their behavior during the tanking test.

Another vexing problem is the performance of a hydrogen pressurization relief valve. The regulator operates periodically when the tank is loaded to bleed off pressure that builds up as liquid hydrogen turns into a gas. During the tanking test, the valve operated, or cycled, much more than normal and engineers are not yet sure why.

Wayne Hale, deputy manager of the shuttle program, said he does not believe the new heater used to keep ice from forming on the forward attachment struts caused the problem, but engineers have not yet ruled out the possibility.

"I personally think that's low priority but ... every day intuition can mislead you," he said. "We've got to be rigorous and make sure that's not the problem. It's a little bit of a puzzle to us and we're going to have to do some troubleshooting.

"It is possible one of the changes we made to the tank contributed to this situation. There were other things that were done to the tank that had nothing to do with the Columbia accident. ... We're in the process of playing all those things out and we'll work through all of them."

Parsons said engineers are well aware of the "law of unintended consequences" and that as you make changes to this vehicle you better be very careful to understand what those changes do to the performance of this vehicle."

"One of the things we will be looking very closely at is when we put this LOX feedline bellows heater in that we do no harm. That's the first rule, do no harm."

As for why engineers just now realized ice formation around the feedline bellows posed a serious threat, Hale said "we concentrated on the foam because that was the cause of the (Columbia) accident."

"But we knew we had to do an exhaustive search through everything that could be a potential problem," he said. "After a great deal of testing and analysis, we've been able to take some 175 potential debris sources off our worry list. We believe we've mitigated those. And we have the engineering evidence to prove they're not a concern.

"We knew we had three or four more items to work on and we also knew there was this ice that forms in certain places on the external tank, which we thought was probably not a major concern but we needed to ensure that. So what you've seen here is the diligence and rigor of going through every piece of the process to ensure that we've eliminated at least to the best of our ability the hazard from ascent debris.

"We've come to the conclusion we really need to do something about this ice. We have several options to deal with it and it's going to take us just a few more weeks to deal with that problem. We certainly cannot fly until we've convinced ourselves that it's safe to fly."

The threat of ice forming around the liquid oxygen feedline bellows prompted NASA to modify the tank in the wake of Columbia to minimize its formation. Engineers thought that would be sufficient and indeed, they believe the so-called "drip lip" modification eliminates 70 percent of the ice that otherwise might form in that area."

But the recent testing suggests that is not enough.

"This is a 17-inch-diameter pipe that's got a small area that is exposed, cryogenic temperatures are exposed to the outside air and water can condense and form ice inside that cavity," Hale said. "In some of that testing, we liberated pieces as large as 5 inches long ... by probably a couple of inches of ice. That's a pretty sizeable piece of ice."

He said engineers plan to release ice in a wind tunnel capable of velocities three times the speed of sound "and see if ice will hold together" in a bid to better understand the threat.

"So there's testing going on," Hale said. "But clearly a piece of ice that big, going three times the speed of sound, can do some serious damage. So we need to go understand a little bit about the dynamics. But the bottom line is, if we can eliminate it that would be the best."

The proposed heater modification should do just that, assuming, of course, it doesn't cause any "unintended consequences." Other options include the use of infrared spotlights near the pad to warm the outer surface of the tank just enough to preclude ice formation.

In any case, Griffin said NASA will not launch until engineers reach agreement that it's safe to do so. That doesn't mean, however, that schedule is not important.

"We constantly hear, we tell ourselves, we try to tell others, that schedule doesn't matter, we'll do the right thing," Griffin said. "I appreciate the philosophical intent behind those words. But schedule does matter. There are no human activities in which it's just OK to perform them whenever you like. Schedule matters. It shouldn't matter to the point of causing people to do dumb things or to take ill-advised actions, and that is where we want to get to.

"But schedule is one factor in the equation and we want to launch Discovery when we can. Because the completion of the international space station depends upon an expeditious launch schedule."

As expected, the shuttle Discovery's launch on the first post-Columbia shuttle mission will be delayed from May to at least July 13 because of time needed to complete ice debris analysis, implement proposed fixes and to resolve a handful of nagging problems, NASA managers announced today.

Liftoff had been targeted for May 22 in a launch period that extended to June 3. But after a detailed assessment of the work remaining and the uncertain resolution of several key issues, senior managers decided to delay the flight to the next available launch period, which opens July 13 around 3:45 p.m.

A detailed story will be posted here after an 11:30 a.m. briefing by shuttle program manager Bill Parsons and deputy manager Wayne Hale.

NASA managers are debating whether to delay the shuttle Discovery's launch on the first post-Columbia mission to July because of time needed to complete ice debris analysis, implement proposed fixes and to resolve a handful of nagging problems, sources say.

Launch currently is targeted for May 22 in a launch period that extends to June 3. But senior managers are considering a delay to the next available launch period, which opens July 13 around 3;45 p.m. Engineers at the Kennedy Space Center have delayed loading maneuvering thruster rocket fuel pending the outcome of the discussion.

A teleconference is planned later today between shuttle program managers at the Johnson Space Center in Houston and at agency headquarters in Washington. A decision could be announced later today or tomorrow.

The primary problem facing program managers is the potential threat of ice debris shaking off the external fuel tank during launch and causing impact damage to the shuttle's heat-shield tiles or wing leading edge panels.

The shuttle Columbia was destroyed during re-entry Feb. 1, 2003, because of a hole in the left wing's leading edge that was caused by the impact of external tank foam insulation that broke off during launch 16 days earlier.

The foam responsible for Columbia's demise was intended to prevent ice from building up around the fittings that attach struts holding the nose of the shuttle to the tank. The so-called bipod foam has been eliminated in favor of small heaters.

Foam application techniques were changed to minimize the chances for foam shedding in general. Engineers believe the largest piece of foam that can come off the tank today is less than a half ounce. The piece that hit Columbia weighed some 1.67 pounds.

But recent testing shows ice buildups in two areas of the tank still pose a threat. One of those areas is in the so-called intertank region between the upper oxygen tank and the lower hydrogen tank where an oxygen feedline bellows is located. The bellows allows the line to flex slightly during launch.

The testing shows ice can build up on the bellows or on a bracket holding the line in place. Another ice problem area is near the tip of the tank around a bracket that holds a repressurization line.

NASA managers held a second debris verification review, or DVR, earlier this week and still were unable to conclusively demonstrate ice was not a threat, sources said.

NASA plans to install at least one heater on the next shuttle tank to eliminate the problem and they may opt to go ahead and do the same for Discovery's tank. Other options under discussion include the use of infrared spotlights near the pad to help prevent ice formation, but those plans are not yet mature.

If a heater must be installed, Discovery would have to be hauled back to the Vehicle Assembly Building, which would rule out a launch during the May-June window.

Other problems facing the launch team:

One of four hydrogen depletion sensors inside the external tank did not operate properly during a tanking test April 14. Engineers initially thought a wiring problem was to blame, but so far no such problems have been found. The sensor is critical and flight rules require all four to be operational for launch. To replace the sensor, if that is required, engineers would need to remove insulation, open an access hatch and enter the hydrogen tank.

A pressurization relief valve operated, or cycled, more often than expected during the tanking test. Engineers are not yet sure if this is an actual problem or not, but they are looking into it.

Hydraulic fluid was blown onto one of the shuttle's aft rocket pods during high winds, contaminating nearly 20 insulation blankets. Engineers want to replace the blankets because of concerns about normal ascent heating and the possibility some of the residual fluid could actually ignite.

Much of the work could be completed at the launch pad, but getting it all done in time to make the May-June launch window - and completing the debris impact analysis - no longer appears likely.

A launch delay to July would allow NASA to roll Discovery back to the Vehicle Assembly Building where engineers would have much better access and a controlled environment for any repairs or upgrades, including installation of a new external tank heater and replacement of the insulation blankets.

NASA managers have said for months they are not being driven by the launch schedule and a delay to July would appear to be in keeping with that post-Columbia philosophy. But some senior managers, sources say, argue against giving up the May-June launch window unless it's absolutely necessary.

As expected, NASA today set a May 22 target date for shuttle Discovery's liftoff on the first post-Columbia mission, saying time needed to close out a handful of open issues precluded an attempt at the May 15 opening of the actual launch period.

An official launch date will be set during a formal flight readiness review next month but as of today, NASA managers said they are confident Discovery will be ready to go by May 22, nearly two-and-a-half years after Columbia's destruction during re-entry Feb. 1, 2003.

"We've always felt that the processing was not the driving factor in this," Bill Parsons, shuttle program manager, told reporters today. "Things have gone very well (at the Kennedy Space Center). Most of this is because of the analysis we're doing and some of the things we want to go ahead and close out and give ourselves a little bit of dwell time to look over it and review it the way we always do. So that's what we did."

The goal of Discovery's mission is to deliver supplies and equipment to the international space station. During three spacewalks, shuttle astronauts Stephen Robinson and Soichi Noguchi will test heat-shield tile and wing leading edge repair procedures; install a replacement gyroscope; and attach a space parts storage platform needed for future assembly flights.

Discovery's launch period, based on lighting constraints imposed for at least the first two post-Columbia missions, opens May 15 and closes June 3.

NASA will not launch Discovery unless it can dock with the space station on the second full day of the mission, known as flight day 3. As it now stands, FD-3 opportunities are not available on May 23, 25 and 27. But NASA plans to ask Russian space program officials to adjust the international space station's orbit in the coming weeks to provide FD-3 docking opportunities for every day of the launch period.

Assuming the May 22 target holds up, Discovery' countdown would begin around 3:15 p.m. on May 19 for a liftoff at 1:03:49 p.m. on May 22. Here's a brief timeline overview of the mission (a detailed flight plan and countdown timeline are available below; all times EDT).

NOTE: These times likely will change slightly after the station's orbit is adjusted; updates will be posted here as they are made available. This quick-look timeline is posted for guidance and planning purposes only:

   05/22...01:04 PM......Launch (current T-0: 1:03:49 PM)
   05/23...07:09 AM......Orbiter thermal protection system survey begins
   05/24...09:34 AM......Discovery docks with space station
   05/25...04:34 AM......Supply module move to station begins
   05/26...06:29 AM......Spacewalk No. 1 begins
   05/26...01:04 PM......Spacewalk ends
   05/27.................Supply transfers
   05/28...06:39 AM......Spacewalk No. 2 begins
   05/28...01:09 PM......Spacewalk ends
   05/29...07:19 AM......Joint crew news conference
   05/30...06:39 AM......Spacewalk No. 3 begins
   05/30...12:39 PM......Spacewalk ends
   05/31...09:09 AM......Supply module moved back to shuttle
   06/01...06:51 AM......Shuttle undocks from space station
   06/02.................Re-entry preparations
   06/03...08:36 AM......Landing

Discovery's payload is scheduled to be delivered to the launch pad for installation in the shuttle's cargo bay on April 28. Commander Eileen Collins and her crew will fly to Kennedy May 1 to participate in emergency training and a dress-rehearsal countdown May 2-4.

A formal flight readiness review to close out any open items and to set an official launch date is now targeted for May 10 and 11. At some point soon after, NASA will present those results to an independent panel of experts charged with monitoring the agency's implementation of post-Columbia safety upgrades.

The panel, chaired by former Apollo astronaut Tom Stafford and former shuttle commander Dick Covey, has been following NASA's return-to-flight effort for a year. The group had hoped to present its final report to new NASA Administrator Mike Griffin a full month before launch, but that is no longer possible.

Hale said the timing was not a critical issue, however, because panel members have kept up to date with NASA's progress throughout the return-to-flight effort. He said several panel members sat in on a design certification review Tuesday that provided a thorough review of all open items.

"We've been in contact with them a great deal," Hale said. "As a matter of fact, we had a number of Stafford-Covey members at the design certification review and they have been through the entire process with us. In fact, they have raised many useful suggestions, which we have incorporated as we've gone through the last two years.

"So I think we have a great deal of information from them already and if they come in their final report with a significant concern ... then I think we will consider it. And if we need to deal with it, I'm sure we will.

"We are aiming for a launch date based on our best knowledge," Hale said. "But if somebody comes in tomorrow and says look, you forgot something, then I don't care if it's Stafford-Covey, one of our own engineers or anybody else, we will give them serious consideration."

The design certification review Tuesday marked a major milestone in the shuttle's return to flight. Hale said senior managers and engineers, including Griffin, reviewed 20 major post-Columbia modifications and improvements to the external fuel tank, main engines, solid-fuel boosters and the shuttle itself.

The only technical issues that remain open as of this writing involve the amount of launch stress a new inspection boom in the cargo bay will experiences during ascent - and the outcome of tests to prove it can, in fact, endure those stresses - and three ice debris issues involving the external fuel tank.

Hale said he is confident the inspection boom is ready for flight and he said tests were underway to make sure any ice that builds up in the areas of concern will safely shake off when the shuttle's engines and boosters roar to life.

All three areas of the tank are visible to launch pad cameras and Hale said regardless of what the testing shows, if ice actually forms in any of those areas during Discovery's countdown, NASA would simply delay launch.

Engineers are looking into techniques for melting any ice that might form on high-humidity launch days. At some point down the road, NASA plans to add additional heaters to the tank to preclude ice formation.

New NASA Administrator Michael Griffin said today he would consider pressing ahead with launch of shuttle Discovery next month even if an outside review panel found fault with NASA's implementation of post-Columbia safety upgrades.

Griffin said NASA's shuttle management team, not the review committee, has the ultimate responsibility for deciding whether or not to launch Discovery and if those managers recommend pressing ahead, he will consider it.

"In concept, yes I would, if (senior managers) recommend that we should consider launching despite not filling all the squares on Stafford-Covey, that is something I would consider," Griffin said.

A panel of outside experts led by former Apollo astronaut Tom Stafford and former shuttle commander Richard Covey was chartered by NASA to monitor the agency's implementation of recommendations by the Columbia Accident Investigation Board.

Fifteen of the CAIB's 29 recommendations were classified as "return to flight," meaning they were to be implemented before shuttle launches resumed. The Stafford-Covey panel had hoped to present its final report to the NASA administrator a full month before return to flight to give the agency time to respond.

But the panel's report has been held up to give NASA time to complete last-minute testing. With the May 15 opening of Discovery's launch window fast approaching, it now appears unlikely the Return to Flight Task Group can finish its report a full month in advance.

Complicating the picture, some members of the panel have raised questions about whether NASA has, in fact, met the intent of at least some return-to-flight recommendations, including the ability to repair damage to the shuttle's heat-shield tile wing leading edge systems in orbit.

Columbia was destroyed during re-entry Feb. 1, 2003, by a hole in its left wing that was caused by the impact of external tank foam debris during launch 16 days earlier. NASA managers believe the redesigned insulation scheme will preclude major form shedding but so far, engineers have not been able to come up with reliable techniques for repairing significant heat-shield damage.

During Discovery's mission, the astronauts will test a few rudimentary repair techniques that might prove effective fixing minor impact damage. But no techniques are available yet that could handle major damage to a wing leading edge or critical heat-shield tiles.

In a sense, NASA is faced with a catch-22. Repair techniques are needed before flights can resume, but any such repair techniques must be tested in the space environment to confirm they will work. Many agency managers believe it is safe to resume flights in the absence of certified repair procedures because:

• Improved launch imagery, radar, on-orbit imagery and new wing leading edge sensors will show, without question, whether Discovery suffered any damage during launch;

• If damage is seen, and if it's too extensive to be repaired, the crew can move into the international space station and await rescue by another shuttle crew. For at least the first two flights, NASA plans to process a second shuttle in parallel that could reach the station within a month or so if required.

Griffin did not discuss details of any such scenarios. But he expressed faith in the shuttle management team to make the right decision.

"I don't believe engineers make blanket decisions in advance and I don't believe the technical decisions are a voting matter," he said. "Stafford-Covey will have their criteria, the line managers in charge of the program will have theirs.

"Now, I cannot begin at this time to say under what specific conditions that NASA might elect to go ahead with the launch given a disparity of opinion between various interested parties as to whether we should or should not. That will depend on the technical details of the issue at hand. But that is precisely the point.

"We study those issues and we resolve them as they occur and then we make our decision and we hold ourselves responsible for it," Griffin said. "Advisory groups advise. The NASA line managers have the responsibility for executing the program. We need to take our advice very seriously and very carefully when it is given and we need never to be defensive about receiving advice from outside.

"But at the end of the day, the people wearing government and contractor badges charged with launching the vehicle will be the ones who are responsible and accountable for their actions."

Griffin said he had no illusions "about the fact that I am the person in the chain of command least knowledgeable about the full details of shuttle operation and its readiness for return to flight."

"I have a lot to learn and I have a lot to learn very quickly and there's no possible way I can learn it all," he said. "Nonetheless, I have enormous confidence in the shuttle team, both NASA and contractors, and what my focus will be on will be learning everything about the process that has gone into fixing the problems that led to the loss of (Columbia) and moving forward."

He said he would do everything possible to ensure good communications from "bottom to top" and "from side to side."

"Again, in the end someone must decide yes or no," he said. "The thing to do is take into account all of the knowledge that we have. I will be one person in that chain. I will probably know less about it than anyone else, but I will make certain that everyone has given me the most convincing technical arguments on why it's OK to launch, if it is OK to launch, before we commit to going ahead."

Immediately after leaving his first press conference at NASA headquarters in Washington, Griffin flew to the Kennedy Space Center to sit in on a long-awaited space shuttle design certification review Tuesday.

NASA hopes to launch Discovery sometime during a window that opens May 15 and closes June 3. It is widely believed NASA will announce a new target launch date later this week, after the design certification review. May 22 is one possible target, sources say, but no final decisions have been made.

In any case, Griffin said a go, no-go decision will not be based on the crew's ability to repair tile damage.

"It's not at all clear from a fundamental viewpoint of the hypersonic aerodynamics involved that tile repair is ever going to be a very easy thing," he said. "It would be very, very easy to make a fix, quote unquote, to a tile known to be damaged and make the problem worse by inserting material or other changes that would (affect the ship's aerodynamics). It would be very easy to make the problem worse rather than better.

"The whole idea of tile repair is a very good idea, but the implementation of it could well be beyond that which we know how to do. ... If it comes down to the fact that we simply don't know how to repair shuttle tile that suffers a certain amount of damage in orbit, then that will be the answer. I don't know that that will be the answer, but that's what the squabble is about."

If that is how the discussion plays out, he said, "then we now need to elevate it to a higher level of decision making."

"Do we think we have solved the problems with foam shedding on the external tank with a high enough degree of confidence, that we believe nothing will fall on the orbiter that will damage it? That's a different question.

"Are we willing to take the statistical risk to fly the orbiter again in the event that we don't have a tile repair capability? That's yet another question. I don't know what the answers to those are.

"But the clearance for return to flight cannot be simply a go or no-go decision based on can you repair a tile in orbit. Even if a tile repair mechanism is offered up as a good idea and even if the capability is on board the orbiter to implement that, there is not a certain way of knowing whether that repair will have worsened the situation or, in fact, improved it.

"We need to get these kinds of facts out on the table so that people at large understand that this is not a simple issue."

The space shuttle's redesigned fuel tank sailed through a critical tanking test today, giving NASA managers increased confidence about launching Discovery on the first post-Columbia shuttle mission next month.

"We had an outstanding day today," said Wayne Hale, deputy manager of the shuttle program and chairman of NASA's mission management team. "After a long period of preparation, we're beginning to see our operations come to fruition.

"Today, we saw the newly modified external tank perform in an outstanding manner, we saw Discovery, which has gone through about three years of modifications, perform in an outstanding manner. And I have to also say the launch pad, the facility, which has gone through a major modification and extensive re-work, performed almost without flaw of any consequence."

Engineers pumped more than 500,000 gallons of supercold rocket fuel into Discovery's external tank today and then monitored the tank and the shuttle's systems down to the T-minus 31-second mark in a mock countdown. It was the first such tanking operation since Columbia's launch Jan. 16, 2003.

"Today's test was a major milestone," said launch director Mike Leinbach. "The next one is launch, and I'm really looking forward to that one."

The goals of the exercise were to give the launch team a chance to rehearse launch-day procedures, to verify the operation of critical launch pad systems and to demonstrate the tank's foam insulation could stand up to the stress of ultra-low temperatures without any major ice buildups.

During a debris verification review late last week, engineers discussed all possible sources of launch debris and retired all but three. The remaining open items involve the possibility of ice buildups around liquid oxygen feed line fittings near the so-called intertank region and pressurization line support fittings at the tip of the tank.

Post-Columbia impact testing shows even small pieces of ice, shaking off during launch, could cause serious damage to the shuttle's wing leading edge panels or its nose cap. Columbia was destroyed during re-entry Feb. 1, 2003, because of damage done by the impact of foam insulation during launch 16 days earlier. The insulation has now been been modified to minimize such debris shedding.

During today's test, ice buildups in the areas of concern were minimal and Hale said engineers are hopeful additional testing will close the remaining open items on the debris impact list.

"Ice is a biggie," Hale said. "One of the things we were very interested to look at today was the performance of the tank and ice growth on the tank," Hale said. "It performed very well, that is heartening news. But that is not all the story.

"We're doing some testing at Redstone Arsenal and other places regarding the potential for ice to shed off the tank during the ascent and we think there are a number of ways we're going to come to a good conclusion on that. But there is work remaining before us on that."

Discovery's launch window opens May 15 and closes June 3. The next window opens July 13. Leinbach said now that the tanking test is over, the pad processing flow is fairly standard with no other unusual tests. Between now and May 15, he said, the pad processing flow includes three days with no work planned.

"That's good news for us," he said. "Even better news for us is the test we performed today was the last non-standard activity we have at the launch pad. From here on out, it's standard pad processing. ... From here on out the rest of the pad flow is well understood. The fact that we have 'just' three days, if you want to say it that way, is not that bad."

But the time needed to resolve the remaining debris concerns forced NASA, in part, to delay a planned design certification review from Friday to next Tuesday. Another debris review is planned late next week and that information must then flow into NASA's flight readiness review process and to an independent panel charged with monitoring NASA's implementation of post-Columbia safety upgrades.

The review panel, headed by former Apollo astronaut Thomas Stafford and former shuttle commander Richard Covey, had hoped to present its final report to NASA's administrator a full month before launch to give the agency time to respond. That no longer appears plausible, but no firm dates have been set.

Hale would not speculate on when the work might get finished, saying only that NASA will not launch Discovery until it is safe to do so. But he was clearly encouraged by the results of today's tanking test.

"Both the debris verification review, the upcoming design certification review and the tanking test that we did today are key items on the way to being ready to go fly," he said. "We're not done yet, but these are key things that we needed to get accomplished. We're whittling down the to-do list of things we must do before we can we can certify the vehicle for flight readiness."

The shuttle Discovery's external tank has been loaded with more than 500,000 gallons of liquid oxygen and liquid hydrogen rocket fuel in a key countdown rehearsal and test of the upgraded tank's insulation. Fuel loading was completed by 11:04 a.m. and an ice inspection team arrived at the launch pad at 11:18 a.m. for a close-up look at the tank and other critical systems.

Engineers now plan to continue the countdown test until a cutoff at the T-minus 31-second mark around 4:30 p.m. The tanking test got off to a late start this morning, prompting managers to initially consider a cutoff at 2:30 p.m.

So far, officials say, the tank appears to be performing normally and as of this writing, the test team is not working any technical issues. A news conference with Wayne Hale, deputy shuttle program manager, is planned for 5 p.m.

Engineers began the process of pumping a half-million gallons of supercold rocket fuel into the shuttle Discovery's external tank at 8:14 a.m. after NASA managers decided the threat of afternoon thunderstorms was low enough to proceed. The tanking test had been scheduled to begin at 5:30 a.m., but it was held up based on a forecast calling for a 30 percent chance of afternoon thunderstorms with a possibility for hail.

The external tank should be fully loaded with liquid oxygen and liquid hydrogen rocket fuel by around 11:30 a.m. Engineers had planned to maintain the tank in so-called "stable replenish" mode for five-and-a-half hours, but they now will shorten that to three hours because of the late start. Under the revised schedule, the test will end around 2:30 p.m. at the T-minus 31-second mark.

The major goals of today's exercise are to rehearse launch-day procedures, to make sure ground equipment upgrades work properly and to assess the performance of the tank's redesigned insulation. A major concern in recent weeks is the potential buildup of ice in critical areas that could pose a debris threat during launch.

An ice inspection team will begin a thorough examination of the tank beginning shortly after fueling is complete around 11:30 a.m. A news conference to discuss the test is planned for later in the afternoon (time TBD).

During a three-day debris verification review late last week, managers and engineers went over 173 potential sources of launch debris. At least three issues regarding ice formation required additional discussion. NASA managers are optimistic today's test and additional testing at the Marshall Space Flight Center in Huntsville, Ala., will provide the data necessary to resolve the open issues.

But as it now stands, an additional debris verification review will be required late next week. A broader design certification review, originally scheduled for Friday, has been delayed to next week as well.

The results of those meetings eventually will be folded into NASA's formal flight readiness review process and presented to an independent panel monitoring NASA's implementation of post-Columbia safety upgrades. That panel, in turn, will present its findings to incoming NASA Administrator Michael Griffin.

How long that process might take, and what impact it will have on Discovery's eventual launch date, is not yet clear. Discovery's launch window opens May 15 and closes June 3. The review panel, headed by former Apollo astronaut Thomas Stafford and former shuttle commander Richard Covey, had hoped to present its final report to the administrator a full month before launch. That no longer appears plausible.

Engineers have delayed loading the shuttle Discovery's external tank with rocket fuel this morning because of concerns about possibly severe weather later in the day. The forecast calls for a 30 percent chance of afternoon thunderstorms and a 60 percent chance of hail if the storms do, in fact, materialize.

NASA managers plan to meet at 7:30 a.m. to reassess the weather and to decide whether or not to proceed with the tanking test. If the forecast remains unchanged, they likely will opt to roll the launch pad service gantry back around Discovery to protect the orbiter. In that case, the tanking test would be delayed to Friday, weather permitting. Live commentary on NASA television will resume at 8 a.m.

Engineers are gearing up to pump a half-million gallons of rocket fuel into the shuttle Discovery's external tank early Thursday to rehearse launch-day procedures and to observe the effects of the supercold propellants on the redesigned tank.

Fueling is scheduled to begin around 5:30 a.m. and if all goes well, the test will end around 3:30 p.m. after a countdown cutoff at the T-minus 31-second mark. The shuttle will be unmanned during the test.

NASA plans to provide live television coverage of the test beginning at 5:30 a.m. with commentary continuing every hour on the half hour. Here is a timeline of events (all times in Eastern and subject to change):

EDT..........EVENT

Wednesday
08:00 p.m....Retract rotating service structure from shuttle
11:30 p.m....Begin final fueling preps

Thursday
02:30 a.m....Clear launch pad
04:30 a.m....Begin one-hour built-in hold at T-6 hours
05:30 a.m....Resume countdown at T-6 hours
05:30 a.m....Fuel loading
08:30 a.m....Fueling complete; stable replenish mode
08:30 a.m....Begin a 3-hour built-in hold at T-3 hours
08:30 a.m....Begin ice/debris inspection/evaluation
11:30 a.m....Resume countdown
03:20 p.m....T-31 seconds (estimate)
03:30 p.m....Test cutoff (estimate)
03:30 p.m....Begin propellant draining procedures
04:30 p.m....Post-test news briefing with:
             Wayne Hale, deputy shuttle program manager
             Mike Leinbach, shuttle launch director
             Neil Otte, external tank chief engineer
05:00 p.m....Liquid hydrogen boiloff begins
05:30 p.m....Begin pad safing and securing
07:30 p.m....Pad re-opened for limited access

While engineers do not expect any major problems with the tanking test, a major objective will be to monitor ice formation in critical areas that could pose a debris threat during launch. Engineers also want to make sure the tank's foam insulation performs as designed.

External tank (web page) and thermal protection system (pdf) fact sheets are available from the Marshall Space Flight Center.

The shuttle Discovery was hauled toward launch pad 39B today at the Kennedy Space Center, the first shuttle "rollout" since Columbia's ill-fated mission and a major milestone on the road to resuming shuttle flights in May.

The glacial, 1-mph trip to the launch pad began at 2:05 p.m. after a brief delay to discuss a hairline crack seen in the insulation covering Discovery's external fuel tank.

Engineers decided the two-inch-long crack did not require repair and a powerful Apollo-era crawler-transporter slowly hauled the shuttle and its mobile launch platform out of the cavernous Vehicle Assembly Building and into a brilliant spring sky.

Looking on were scores of space center employees, reporters, tourists aboard visitor's center buses and even the crew of the international space station, which passed almost directly overhead around 4:30 p.m. If all goes well, Discovery will dock with the station two days after launch on the 114th shuttle mission.

Rollout originally was planned for late March, but the 4.2-mile trip was held up because of minor processing snags and technical issues that used up most of the contingency time built into the schedule to handle unexpected problems at the pad.

By rolling to the pad today, NASA can still, in theory, launch Discovery as early as May 15. But any additional problems almost certainly will delay launch. NASA has until June 3 to get Discovery off the ground on the first post-Columbia mission or the flight will be delayed to at least July 13, the next available daylight launch opportunity to the international space station.

As engineers in Florida were preparing Discovery for its long-awaited trip to the pad, the shuttle's crew and flight controllers at the Johnson Space Center in Houston were practicing re-entry procedures in a series of morning simulations riddled with mock failures and problems.

Speaking to reporters in mission control, ascent-entry flight director LeRoy Cain said Discovery's rollout was "an important milestone because it's such a collaborative effort to get to this point."

"What I see here is the people and the machinery coming together," he said. "This is a point in time where we begin to converge all of the people with all of the machinery and we move toward a common goal of launching."

Discovery commander Eileen Collins said "it's a great sight to see Discovery rolling out to the launch pad. We know we're getting close."

While some outside observers have questioned whether NASA has done enough to fully implement safety recommendations made in the wake of the Columbia disaster, Cain said he has no doubts Discovery's crew and his flight control team are ready to go.

"I'm happy to be here, I'm excited to be here," he said. "We've worked very hard to get to this point. As I said earlier, this team is as ready as any team that I have seen when we're this many days from launch. I feel very confident in our abilities. I don't look at this as closure. I look at this as a new beginning. I consider ourselves ready to go to the next milestone for return to flight."

With the first post-Columbia shuttle flight a month and a half away, engineers are still assessing what sorts of damage might result from impacts of foam debris falling from the shuttle's redesigned external fuel tank. Engineers are confident the new tanks will be the safest ever launched and that expected debris will not harm the shuttle's sensitive wing leading edges, which experience the fiercest heat during re-entry.

But officials said today test data indicates expected debris, in a worst-case scenario, could cause potentially catastrophic damage if the debris in question fell off at the worst possible moment and hit a sensitive area of heat-shield tiles at the proper angle and impact velocity. The precise nature of that risk is not yet known and engineers say they cannot provide details until after a three-day debris verification review, which starts Thursday.

"In terms of the foam, we have our best estimates of what the foam debris loss is going to be," said John Muratore, manager of shuttle systems engineering and integration at the Johnson Space Center in Houston. "And with our best estimates of the foam debris loss and our best estimates of the (wing leading edge) impact tolerance, we don't believe that expected foam can cause any damage" to the reinforced carbon carbon - RCC - wing leading edge panels.

A hole in the leading edge of Columbia's left wing, caused by the launch day impact of a 1.67-pound chunk of foam debris, led to the orbiter's destruction during re-entry Feb. 1, 2003.

Since then, the tank has been redesigned to eliminate the foam in question and to improve the application of foam in other areas to minimize debris shedding.

The shuttle Discovery's external fuel tank is not expected to release any foam heavier than 0.023 pounds - 0.37 ounces - from the upper portions of the tank during launch. Debris from the upper part of the tank poses the greatest threat to the ship's nose cap and reinforced carbon carbon leading edge panels. The 0.023-pound limit is for foam covering the liquid oxygen tank at the very top of the structure.

Foam from the intertank area, where bipod struts attach the nose of the shuttle and where the foam originated that doomed Columbia, can be slightly more massive: 0.03 pounds, or 0.48 ounces. Pieces of foam from the bottom of the tank pose less of a threat and thus can be larger still: up to 0.75 pounds.

Here is a partial table of allowable debris as approved by shuttle program managers:

REGION...............................FOAM (pounds/cubic inches)

Oxygen tank..........................0.023/15.9
O2 tank to Intertank flange..........0.026/20.7
Intertank............................0.03/20.7
Hydrogen tank and intertank flange...0.03/20.7 to 0.075/51.8
LH2 tank.............................0.075/51.8

The question is, can the shuttle's heat-shield tiles withstand impacts from foam debris expected to be released from the upper regions of the tank during launch? While the numbers aren't yet available, Muratore said he expects worst-case scenarios can, in fact, cause potentially catastrophic damage.

The most sensitive areas to damage are the edges of the landing gear doors and smaller doors that cover external tank propellant line connections after the external tank is discarded in space.

"The sizes that we're expecting to be liberated from the tank in a worst-case condition can cause significant damage," Muratore said. "It all depends on the location and the depth (of the damage) that actually occurs and the underlying structure."

But such worst-case scenarios have never happened before - "that is significantly outside of our flight experience," Muratore said - so the question becomes one of how conservative NASA's assumptions might be and "we're struggling with that problem right now. It's a very difficult engineering problem."

Those issues will be discussed in depth during the upcoming debris verification review. Those results, in turn, will be presented to senior NASA managers later this month and ultimately to an independent panel monitoring NASA's implementation of post-Columbia safety upgrades.

That panel, led by former Apollo astronaut Thomas Stafford and former shuttle commander Dick Covey, has delayed a final public hearing and completion of its report in part because of the time NASA has needed to complete the impact test analysis.

While the numbers aren't yet available, Muratore cautioned that NASA has not eliminated all risk of foam impact damage.

"So there is some risk we're accepting that if the conditions are different than our best estimates, that we could take serious damage," he said. "And that's the risk we're going to have to take."

Asked if NASA could gain any additional safety margin by taking more time to improve the tank still more, Muratore said "we feel that the main way we can get smarter about this problem is go fly."

"We have come to the point, as it comes in every experimental vehicle, where the amount of analysis and modeling and ground tests has sort of saturated the problem and now the only way to really know what's going to happen in the real world is to go actually perform a flight test."

Launch currently is targeted for May 15, but getting Discovery to the pad has taken longer than expected and used up what spare time was built into the schedule to handle unexpected problems. NASA hopes to move the shuttle to the pad Wednesday and many observers expect the launch date ultimately will slip five days to a week when all is said and done. But shuttle program manager Bill Parsons has said he does not plan to consider possible changes until after a fueling test around April 14. As of this writing, Discovery can still, in theory, make May 15.

NASA finally has test data indicating any foam debris falling from the space shuttle's redesigned external fuel tank almost certainly will be below the threshold expected to cause "entry critical" damage to wing leading edge panels, officials said today.

But with less than two months to go before Discovery's launch on the first post-Columbia shuttle mission, engineers are still assessing whether worst-case foam debris impacts could cause entry critical damage to the orbiter's heat-shield tiles.

In some areas, like certain places around the landing gear doors or the area where external tank propellant lines enter the belly of the shuttle, worst-case impact damage could be potentially catastrophic.

The results of ongoing tests and analyses will be discussed during a debris verification review in early April and a broader shuttle-wide design certification review before flights resume in mid May.

But NASA managers are optimistic test data and the results of sophisticated computer modeling will show the tank's insulation does not pose a significant safety threat to the wing leading edges.

"If you think back to what we knew before Columbia and what we thought our vulnerabilities were and where we are now, it is clear that before we were guessing and now we know, within a few fractions of a percent, exactly how vulnerable we are," deputy shuttle program manager Wayne Hale said in an interview late today. "We've come a long way."

Columbia was destroyed during re-entry Feb. 1, 2003, because of a hole in the leading edge of the ship's left wing that was caused by a major foam debris impact during launch 16 days earlier.

NASA's primary rationale for returning the shuttle to flight this May is the elimination of the foam in question, along with other design changes expected to minimize the amount - and size - of any foam debris that might shake loose or pop off other areas during launch.

The shuttle's redesigned fuel tank has been modified to ensure no pieces of foam heavier than 0.023 pounds - 0.37 ounces - can break away from the upper portions of the tank during launch.

Debris from the upper part of the tank poses the greatest threat to the ship's nose cap and reinforced carbon carbon leading edge panels, which experience the most intense heat during re-entry. The 0.023-pound limit is for foam covering the liquid oxygen tank at the very top of the structure.

Foam from the intertank area, where bipod struts attach the nose of the shuttle and where the foam originated that doomed Columbia, can be slightly more massive: 0.03 pounds, or 0.48 ounces. Pieces of foam from the bottom of the tank pose less of a threat and thus can be larger still: up to 0.75 pounds.

A major question as the shuttle Discovery's May 15 launch target approaches has been whether foam in that range of sizes from the top of the tank could cause the kind of wing leading edge damage during ascent that could threaten a shuttle during re-entry.

To the surprise of many, recent testing showed major cracks or holes in leading edge panels are not required to bring down a space shuttle. Coating damage alone could do it, if the damage was in a particularly bad spot and if the impact that marred the coating also caused internal delamination. Small cracks and holes, obviously, also pose serious threats.

The problem was in the testing. Using a nitrogen gas-powered cannon and a wing leading edge mockup at the Southwest Research Institute in San Antonio, Texas, engineers were able to create entry critical damage from pieces of foam as small as 0.044 pounds - 0.7 ounces - depending on impact velocity and angle of incidence.

But they were not immediately able to test down to the range of sizes expected to be liberated from a redesigned fuel tank because smaller foam bullets tended to come apart before hitting the leading edge panels.

Steve Poulos, manager of the shuttle project office at the Johnson Space Center, said in an interview today that those issues appear to have been resolved and test data show the sort of debris that could be expected to fall off the redesigned tank would be below the critical damage threshold.

"We have done testing that exceeded the parameters to help us in the validation of our models: higher speeds, higher angles, different geometries," he said. "And in some of those instances we did do damage to the panels and quite honestly, we expected to do it because (the impact energy was) greater than 1,500 foot pounds.

"I'm not one to live in the world of assumptions," he said. "I'm a test person. I've got empirical data where we've impacted the panel and we haven't damaged it once below 1,250 foot pounds of kinetic energy."

That's a measure of the impact energies expected during an actual launch.

"Recently, they've done testing down to very small masses, I think the last number I heard, and I could be wrong, was about .02 pounds, .025, at velocities on the order of 2,500 feet per second. And zero damage (was seen)."

Poulos discussed the impact testing immediately after attending a meeting to review what level of damage a new laser sensor will be able to detect in orbit if, despite the test results, an impact causes a crack or mars the surface coating of a wing leading edge panel.

The sensor, or laser dynamic range imager - LDRI - and a television camera, both mounted on the end of a 50-boot boom, will be used during the second day of Discovery's mission to inspect the shuttle's nose cap and wing leading edge panels.

"I saw the latest certification level test results from the LDRI and it shows clearly the ability to detect down to as low as a 15 thousandths (of an inch) crack," Poulos said. "Coating loss is easily detectable with these sensors as is a crack. So there's no concern from the sensor perspective to being able to detect coating loss or a crack."

All in all, he said, "there is nothing today that I have seen from the external tank data that gives me any pause or any concern about having any damage at all on our RCC panels."

He said NASA now has "sufficient capability, based on all the testing that's been done at Southwest Research and the Glenn Research Center and the Kennedy Space Center, to verify in my mind that we have sufficient capability for what the tank is expected to liberate."

But Hale cautioned that impact testing alone is not enough. Sophisticated computer modeling also is required to predict how debris might behave after it comes off the tank and moves into the airflow around the shuttle. As such, there is still room for debate in the upcoming design reviews.

"The transport mechanism, which is a key element in this, is a model, a computer model of aerodynamics," he said. "We've done a lot of wind tunnel work to try to make sure it's right, but it's a model. So we do have an ongoing discussion about the validity of models and usefulness of models.

"If you state it in absolutely black-and-white terms, I think you will get some discussion whether this is proven to within the last decimal place. ... I'm almost hesitant to say anything in black and white until that review. But you've got a sense of where we're going."

The debris verification review is planned for the first week in April, just a few days after the shuttle Discovery's planned April 4 rollout to the launch pad at the Kennedy Space Center. Launch is targeted for May 15.

Rollout had been planned for late this month, but engineers and technicians needed more time to complete final processing, eating into the 12 days or so of on-pad contingency time managers had built into the schedule. An April 4 roll out will leave the team with just a few days of spare time to handle unexpected problems and still make the May 15 target.

NASA insiders say the launch date likely will slip up to a week or so when all is said and done, but shuttle program manager Bill Parsons said today no decisions will be made until after Discovery reaches the launch pad.

"We've slipped a few things (but) we feel very comfortable with where we're at in the processing and the milestones we still have in front of us," he said at a news briefing. "We probably need about 30 more days of processing before we can really say whether we should slip the May 15th launch date or not. But that is the opening of the window and currently, if everything was to go all our way, we feel we can make that."

In the wake of the Columbia disaster, an independent accident review board made 29 recommendations to improve safety. NASA tracks its progress meeting those recommendations in a formal document known as the shuttle return to flight implementation plan.

The latest version was released today and it included a new approach to shuttle re-entries based on the expected risk to the public below the orbiter's flight path if a shuttle has known damage or flight control issues.

When Columbia broke apart above Texas, debris rained down along its flight path stretching from just west of Fort Worth all the way into southwestern Louisiana. In what still ranks as a remarkable stroke of luck, no one on the ground was injured and property damage was minimal.

But new flight rules are being developed to minimize that risk. The crew of any shuttle with known damage that could affect structural integrity or any other problems that might affect flight control will be directed to land at White Sands, N.M., one of NASA's backup landing sites.

"If there is some kind of problem with the shuttle, a loss of its flight control redundancy, for example, or a threat to the structural integrity of the vehicle ... these flight rules will call for us to go to (White Sands)," said Bryan O'Connor, NASA's director of safety.

Descents to White Sands carry the shuttle over less densely populated areas than those to the Kennedy Space Center in Florida or Edwards Air Force Base in California's Mojave Desert.

"On average, entry opportunities into KSC are half the public risk level of entries into EAFB," according to the implementation plan. "On average, entry opportunities into WSMR (White Sands Missile Range) are one-seventh the public risk level of EAFB and one-third the public risk level of KSC."

But because White Sands does not have the infrastructure needed to support routine shuttle landings, the preferred landing site for undamaged orbiters will remain the Kennedy Space Center with Edwards as backup.

"The bottom line is that the orbiter will normally land at KSC," according to the implementation plan. "But if it is compromised in a way that poses a threat to the public, it will land at WSMR."

The shuttle Discovery docked with the international space station today in a dramatic mission simulation that included presumed foam debris impacts to the orbiter's right wing leading edge and an aft rocket pod during the make-believe climb to space.

The simulation, which began Wednesday with a mock launch from the Kennedy Space Center, is designed to put NASA's post-Columbia management system to the test while giving the shuttle crew and mission control team a chance to sharpen their skills.

Flight directors and members of NASA's mission management team even staged faux news briefings complete with questions from public affairs officers playing the roles of reporters.

"It's been great. Everybody's played really hard," MMT Chairman Wayne Hale said late Friday. "This is like a real flight, they are playing hard and we've had a lot of emotion, which is good, that means people are taking it seriously, but it hasn't gotten out of hand. People are being reasonable in their conclusions and I think that's very good."

Said John Shannon, a senior member of NASA's mission management team: "This is like a play. This is our dress rehearsal. I'm just blown away by the fidelity of this thing, how many people are participating."

For launch, the MMT gathered at the Kennedy Space Center earlier this week for a full-fledged dress rehearsal countdown led by launch director Mike Leinbach and NASA test director Jeff Spaulding.

Five reporters, including this writer, were allowed to sit in on an early morning MMT meeting Wednesday to clear the launch team for a simulated fueling and later, observed the terminal countdown from the firing room in Kennedy's Launch Control Center.

The countdown was relatively straight forward and while a few problems cropped up that required extensive troubleshooting, the simulated liftoff occurred on time at 5:55 p.m. But seconds after launch, the simulation team began throwing problems at Discovery's crew and flight controllers in Houston, including simulated impacts by foam debris from Discovery's external fuel tank.

Columbia was brought down Feb. 1, 2003, by wing leading edge damage caused by a chunk of external fuel tank foam insulation that broke away during ascent.

Discovery's tank has been modified to prevent large pieces of foam from breaking off in flight. While it's not possible to completely eliminate foam shedding, Hale said Wednesday he's optimistic the tank will be deemed safe enough to launch after two years of work to minimize debris.

"In a perfect world, you would say the biggest thing that can come off the tank can't possibly hurt the (wing leading edge)," he said. "In a worst-on-worst kind of case, you can't say that, even after what we've done. But what we can say is in a realistic sense ... from what is reasonable to happen, we have sufficient safety margin."

To make absolutely sure the tank performs as expected, at least the first two shuttle missions will be treated as test flights. New high definition television tracking cameras will photograph Discovery's real launch in May with unprecedented resolution and other cameras on the shuttle, the tank itself and the orbiter's twin solid-fuel boosters will provide extensive photographic coverage during the climb to space.

In addition, cameras aboard two modified jets will "shoot" the shuttle through booster separation, the astronauts will photograph the tank in space and military spy satellites will snap classified photographs of the shuttle to look for signs of damage.

Sensors mounted just behind the wing leading edge panels will record the force and location of any impacts, the shuttle's crew will use laser sensors mounted on the end of a long boom to scrutinize the wing leading edges from the outside and the crew of the international space station will photograph the heat-shield tiles on the belly of the orbiter during Discovery's final approach.

The severity of any actual damage in a real flight will dictate NASA's response. Minor damage might be deemed acceptable for re-entry as is. More serious damage could raise the prospect of a spacewalk repair job and in cases where the issue was not clear cut - or the damage is obviously too severe for a repair - the astronauts could move into the space station and await rescue by another shuttle.

While no one expects any significant foam shedding during Discovery's May 15 launching, this week's simulation has forced mission managers to exercise all of the post-Columbia inspection techniques in a real-world scenario.

During Wednesday's mock launch, commander Eileen Collins had to deal with a fuel cell glitch seconds after liftoff and was told the center main engine's nozzle was leaking hydrogen. The shuttle's main engines shut down slightly early, resulting in a 35 foot per second shortfall in velocity. To save fuel, the crew was told not to perform a maneuver that would have let them photograph the external tank from an overhead window.

More worrisome to mission managers, two debris shedding events were noticed in ascent photography and radar tracking. By Thursday, the second day of the simulated mission, photography and radar analysis had identified at least nine "debris events," including damage to tiles on the shuttle's right rear rocket pod and a possible impact around leading edge panel No. 6 on Discovery's right wing.

The Discovery astronauts, working in a flight simulator at the Johnson Space Center, spent flight day two inspecting the 44 reinforced carbon carbon - RCC - leading edge panels on both wings, along with the ship's RCC nose cap, using a new boom-mounted laser system attached to the end of the shuttle's robot arm.

Aside from the use of computer graphics and animation in place of actual video from space, this writer, observing from the mission control room at the Johnson Space Center, could not tell the difference between the simulation and an actual mission as lead flight director Paul Hill led his team through the complex inspection work.

But in what might be a surprise to at least some observers, mission managers overseeing Discovery's simulated docking with the international space station Friday afternoon - two full days after launch - still did not yet know for sure whether the RCC impact had caused any damage.

Data from the laser scanner used to inspect the leading edge Thursday was inconclusive, officials said today, and close-up, targeted inspections were to be simulated Saturday. In addition, photos of the shuttle's belly taken by the crew of the space station during Discovery's final approach earlier today, showed unusual white speckles across the black heat-shield tiles that now must be investigated.

Senior managers said the pace of the simulation gave a sense of how long it will take NASA and contractor engineers to collect the data they would need in an actual flight to determine whether or not the shuttle had been damaged during launch or if repairs might be needed.

"We know we have some warning signs, we know we had some different debris events, we know we had one of those events gave us an indication on a wing leading edge sensor," Shannon said late Thursday. "That doesn't tell you if you have damage or not, it tells you potentially you had a hit in that area.

"The thing is, we go from a lot of data that's pretty low fidelity - from the imagery, from all the ground stuff, the external tank camera and the radar stuff - it's not very good fidelity but it tells you hey, something was going on and it clues you in to the next stage, which is what we're doing now and you go look carefully.

"It's going to be a little frustrating because you say OK, maybe they've got something here, here and here but it's going to take a few days to collect the data to see what is there. ... This is extremely complex."

In a real flight, the mission management team would not expect to have an impact site fully characterized and understood until flight day six. That is when the MMT would have to decide whether a shuttle could return to Earth as is, whether repairs were needed or whether a crew might have to abandon ship and use the space station as a "safe haven."

"It's really going to take us until flight day six to get all the reports, all the data, do all the analysis and come to a conclusion," Hale said. "Other than being anxious to get the answer, that's OK."

This week's simulation is working through that analysis and decision-making timeline in exhaustive detail.

"I would characterize the process as extremely realistic," Shannon said. "The systems engineering and integration guys are tracking nine different debris cases, the data collection's going on, two of them could be fairly significant. ... It's such a great, real-time prep for us to get real problems coming down, crunching them through all the people, all new processes, all the debris assessment stuff, this is all new. ... Just going through this whole cycle is so valuable."

The simulation was scheduled to end Friday evening, but the mission management team planned to work through the weekend assessing the impact data and debating all of the options they could face in a real flight.

Asked about the deliberate pace of the post-launch analysis during a real flight, John Muratore, a senior shuttle manager at the Johnson Space Center, said "my impresseion is this is going to be high drama for 14 days."

NASA hopes to move Discovery to launch pad 39B at the Kennedy Space Center around March 25. The launch window opens May 15 and closes June 3. The next launch window opens July 12.

"I think we're in pretty good shape," Hale said Wednesday. "We're working a number of problems on the orbiter, but that is fairly typical. ... The real issues we've got, I don't think it's orbiter processing. I think we're within the box on orbiter processing on Discovery. The real issue we've got is getting closure to the engineering analysis.

"We have to have the engineering analysis complete that proves it's safe to fly before we can go fly. If that means go park Discovery at the beach for an extra month, then so be it. Now, I don't think it's going to come to that. But we have got to prove it's safe to fly before we light the fuse."

NASA managers today set May 15 as the target launch date for the first post-Columbia shuttle mission, saying they are confident remaining technical issues, an independent review and a mountain of paperwork can be closed out in time for flight.

Launch director Michael Leinbach said the processing schedule includes about 12 days of contingency time to handle unexpected problems between now and then and "we feel good about that date."

The current schedule calls for engineers to attach Discovery's redesigned external fuel tank to a pair of already assembled solid-fuel boosters around Feb. 25 and for Discovery to be bolted to the side of the tank March 18.

The assembled spacecraft then will be hauled to launch pad 39B on March 25 and the tank will be loaded with supercold liquid oxygen and liquid hydrogen rocket fuel April 7 in a test that will serve as a dress rehearsal for launch.

Commander Eileen Collins and her six crewmates plan to strap in aboard Discovery on April 29 for a practice countdown and if all goes well, the actual countdown will begin May 12 for a launch around 3:50 p.m. on May 15.

Columbia's launch window extends to June 3, based on the orbit of the shuttle's destination - the international space station - and because of a self-imposed requirement to not only launch the first two post-Columbia flights in daylight but also to ensure external tank separation in sunlight for photo documentation.

If NASA can't get Discovery off the ground by June 3 or thereabouts, the flight will slip to mid July. But Leinbach is optimistic it won't come to that.

"After the tanking test is done, the remainder of the pad flow is very standard to us," he said. "And so I'll just tell you, this date feels real good to me."

Fifteen of the 29 recommendations made by the Columbia Accident Investigation Board must be completed before Discovery's return to flight. As of today, only seven of those 15 have been fully addressed to the satisfaction of an independent panel charged with monitoring NASA's implementation of those recommendations.

But on Thursday, panel co-chairman Richard Covey, a Boeing executive and former shuttle commander, said he saw no major roadblocks to closing out the remaining items before the board ends its work March 31. William Readdy, NASA's associate administrator for space operations agreed and told reporters today "we have every expectation we are going to close all of them."

"We have a continuing dialogue with the three panels that they have," he said. "At this point, we really don't see any show stoppers, that's been their commentary all along, so we expect to close all of them."

Major technical questions remain, however, including work to determine how much damage the shuttle's wing leading edges and heat-shield tiles can withstand before repairs are needed.

The Spaceflight Leadership Council, co-chaired by Readdy, met today at the Kennedy Space Center and approved plans to test three rudimentary tile and leading edge repair techniques during Discovery's mission. But those techniques will not be certified before launch and as such would only be used in a true emergency.

Readdy said any repair techniques would have to be tested in space before certification and in any case, the kind of external tank foam debris blamed in the Columbia disaster has been eliminated. Other potential weak spots have been addressed as well.

"Given the depth of inspection that we've gone into in each and every last subsystem and element of the program, I'd be very, very surprised if we had any kind of damage as a result of debris shedding," he said. "We just heard ... a very thorough debrief on the analysis they have conducted. That analysis continues, but I think it should give everyone tremendous confidence that we have done what we needed to to eliminate critical debris from the external tank and other sources on the solid rocket motors."

Combined with efforts to fully characterize the strength of the thermal protection system and improvements in other systems, "we'll be flying much more safely than we ever have before."

Walter Cantrell, deputy chief engineer for NASA's new Independent Technical Authority and co-chairman of the agency's Spaceflight Leadership Council, said the Return to Flight Task Group chaired by Covey and Thomas Stafford has participated in the engineering discussions and understands the processing schedule.

Regarding the open items in the RTF recommendations, Cantrell said "we have received from the Stafford-Covey group their expectations that, if satisfied by us, they could be comfortable in saying that their assessment would be that we have complied with the intent of the CAIB recommendations. Some of them are obviously harder than others because of timing, given that the Stafford-Covey group wants to be able to give its recommendations to the administrator at least one month before return to flight.

"We're in careful contact with them and know what we think we need to do and we know what they think we need to do," Cantrell said. "A significant number of their members participated today, not only listened but actually provided comments during the Spaceflight Leadership Council (meeting), so we're all tracking the same things.

"Our sense of it, and the last sense that we have from them, is there are no show stoppers, we're in very tight agreement on the schedule for closure. Anything can happen, but we do not anticipate that being a problem."

He said NASA had set higher standards "in almost every case" than the CAIB recommendations required and "we're holding ourselves to that raised bar."

"Obviously, we're going to comply with what Stafford-Covey is looking for and what the CAIB is looking for," he said. "But we are the ones who accept the risk and we've set that standard where we think it should be."

The shuttle Discovery's astronauts will test three, not four, tile and wing leading edge repair techniques during the first post-Columbia shuttle flight in May, NASA officials clarified today.

Shuttle program manager William Parsons told CBS News that one proposed technique, the so-called overlay tile repair, will not be demonstrated by Discovery's crew because of safety issues.

The technique calls for fixing damaged heat-shield tile by filling in voids with insulation and then covering it with a heat-resistant sheet that would be held in place by drywall-type bolts screwed directly into the surrounding tile.

One option debated during a lengthy meeting Thursday at the Johnson Space Center was for the Discovery astronauts to demonstrate the overlay technique in the shuttle's crew cabin. Asked to clarify the results of the meeting, Parsons said today that concern about fine tile particles that could be released into the cabin air supply ruled out an overlay demonstration.

"So we have decided to carry the overlay and tools up but not pull them out and do a demonstration," he said. "The repair technique is so straight forward that we do not believe the demo is really necessary. We may pull them out and show them, who knows, but we won't demo."

The astronauts will, however, test a leading edge repair technique using a plug-like patch that would be inserted into a hole in a leading edge panel and held in place by an expansion bolt. They also will stage a spacewalk to test two other rudimetary repair techniques as reported Thursday.

In one, astronauts Stephen Robinson and Soichi Noguchi will test a leading edge crack repair technique and in the other, they will coat deliberately damaged tile with a material that should increase their ability to reject heat.

Engineers, meanwhile, are still assessing whether additional robot arm inspections will be required to look for signs of actual damage.

If significant additional inspections ultimately are required after Discovery docks with the international space station, the repair demonstration spacewalk could be elminated because of scheduling conflicts. But shuttle managers are hopeful that will not be required.

After weeks of internal debate, testing and analyses, NASA managers today selected four rudimentary tile and wing leading edge repair techniques to demonstrate during the first post-Columbia shuttle mission.

Two of the repair procedures will be carried out inside the shuttle Discovery's crew cabin, a so-called "overlay" technique that could prove useful for fixing damage to heat-shield tiles and a "plug" procedure for repairing larger holes in wing leading edge panels. A less sophisticated tile repair technique, one intended for minor damage, will be demonstrated during a spacewalk, along with a technique for repairing small cracks in leading edge panels.

A tile repair technique that had long been considered a front runner was ruled out, however, and while two large backpacks and high-tech caulk guns will be carried aloft aboard Discovery, they will not be tested in flight, according to NASA spokesman Kyle Herring.

Instead, engineers will continue work to perfect the technique and if all goes well it may be tested on the second post-Columbia mission.

Today's Program Requirements Control Board was chaired by shuttle program manager William Parsons. His decision will be presented to NASA's Spaceflight Leadership Council a week from Friday for formal approval.

Discovery is scheduled for launch on the first post-Columbia shuttle mission around May 15. Three spacewalks are planned, two devoted to space station repair and servicing and one to demonstrate repair procedures. But technical questions and concerns have held up a decision on which repair techniques to demonstrate and with time running out to complete crew training and equipment testing, Parsons, deputy program manager Wayne Hale and other senior managers heard final presentations today.

Three options were on the table.

The first option called for spacewalkers Stephen Robinson and Soichi Noguchi to use the cure in-place-ablator applicator - CIPAA - backpacks, loaded with a tile repair material known as STA-54, to fill in deliberately damaged tiles in Discovery's cargo bay.

Questions about the reliability of the procedure surfaced last year when engineers noticed the formation of air bubbles in the viscous STA-54 material as the two compounds that made it up were mixed together in the backpack. After extensive troubleshooting, engineers were able to reduce the bubbling but they have not yet eliminated it. The concern is that bubbles could migrate in weightlessness and form large voids as the material cures. Those voids could weaken the patch and its ability to shield against re-entry heating.

Chief astronaut Kent Rominger told CBS News his office opposed in-flight testing aboard Discovery's flight and sources said later that Discovery commander Eileen Collins agreed with that position.

A second option debated today called for eliminating a repair demonstration spacewalk altogether. Instead, the crew would demonstrate the overlay tile repair procedure in the shuttle's cabin, along with the plug technique for repairing small holes in leading edge panels. By eliminating the spacewalk, the crew would have more time for external tile inspections and logistics transfers to the international space station.

A third option, the one ultimately selected, was chosen because the techniques in question were the most technically mature and offered the best opportunity to collect useful in-flight data, Herring said.

Robinson and Noguchi now plan to test a tile repair technique known as "emittance wash" in Discovery's cargo bay. Using a demonstration kit with deliberately damaged tiles, the spacewalkers will paint exposed surfaces with a material that will replace damaged or eroded coating and improve heat rejection.

Columbia was destroyed two years ago by a hole in a reinforced carbon carbon wing leading edge panel that hot gas to burn its way into the wing's interior during re-entry. NASA still has no way to repair that level of damage, but Robinson and Noguchi will test a rudimentary technique in which a heat-resistant material known as NOAX will be smoothed over small cracks in RCC material.

NOAX, which stands for non-oxide adhesive experimental, will be squirted from a caulk gun-like device and then smoothed out with trowels. The original procedure required the RCC to be heated prior to NOAX application and for the patch itself to be heated for a half hour after that to cure the material. Whether a heater remains part of the new spacewalk demonstration is not yet known.

Two mechanical fixes that would not be affected by exposure to vacuum will be tested inside the shuttle's crew cabin, assuming a safety analysis determines the equipment poses no risk to the astronauts.

The so-called "overlay" technique for tile repair calls for the astronauts to cover a panel of damaged tiles with a thin, flexible sheet of heat-resistant carbon silicon-carbide that would be mounted atop a gasket and attached with fasteners similar to drywall bolts that would be screwed into surrounding tile.

The final repair procedure, aimed at fixing small holes in RCC panels, requires a flexible carbon silicon-carbide patch called a "plug." After fit checks and application of a sealant, a plug would be inserted into a hale and held in place from behind by expansion bolts.

Between 20 and 30 different plugs, each with slightly different geometries, would be needed in a real repair kit to ensure a good fit virtually anywhere in the curving leading edge.

See the Feb. 6 Status Report below for the first take of a detailed overview of all the shuttle in-flight repair options. The entire story is available in the CBS News STS-114 Mission Archive. See the Status Report immediately below for an interview with retired Adm. Harold Gehman, chairman of the Columbia Accident Investigation Board, on his reaction to NASA's plans to fly without certified tile and RCC repair techniques.

Editor's note...
See the Feb. 6 Status Report below for the first take of a detailed overview of shuttle in-flight repair options and the ongoing debate about which procedures to test during Discovery's flight in May. The entire story is available in the CBS News STS-114 Mission Archive.

The chairman of the Columbia Accident Investigation Board believes NASA has made a credible attempt to meet the spirit and intent of the board's return-to-flight recommendations, even though the agency will not have certified tile or wing leading edge repair techniques in place before Discovery blasts off in May.

NASA managers, engineers and astronauts at the Johnson Space Center in Houston, meanwhile, are meeting today to decide what, if any, repair techniques to actually test during Discovery's mission. Sources familiar with the ongoing debate say additional time is needed to perfect more ambitious repair procedures and that Discovery's crew may be limited to testing relatively simple techniques for fixing relatively minor damage.

Even so, Harold Gehman, the retired admiral who chaired the independent panel that investigated the Columbia disaster, said he has no objections to NASA's plan to resume shuttle flights in May while testing and analyses continue. But Gehman said that does not mean NASA can ever give up trying to perfect repair techniques, or worse, stop listening to the concerns of the agency's engineering community when problems arise.

In a telephone interview Wednesday with CBS Space Consultant William Harwood, Gehman discussed his impressions of NASA's recovery from the 2003 Columbia disaster and the intentions behind the CAIB's recommendations.

Q: Have you been following NASA return to flight efforts? And do you think NASA is meeting the intent of your recommendations?

Gehman: One of the things the CAIB said when we released our report, and we've said it consistently since, we were not going to be sitting on the sidelines doing what we called 'grading NASA's papers.' So we have been following, and I have been following, loosely and from a distance, the return to flight efforts and the function of the return to flight task group, the Stafford-Covey group.

[The Return to Flight Task Group, chaired by former Apollo astronaut Thomas Stafford and veteran shuttle commander Richard Covey, is providing an independent assessment of NASA's implementation of the CAIB's recommendations.] Gehman: So I've been following this business, but not in great detail because our policy was that unless they just gaff us off, we weren't going to critique them. And they certainly are not gaffing off our recommendations. They certainly are making a try.

Our intent was, and I think that we said this in the report and I could quote you the page, what we wanted to do was break what we considered a very rigid, or tightly coupled, chain of events that starts with debris and ends with losing astronauts. It's a chain of events and what you need to do to ensure, or maximize the safety of the astronauts, is to do everything you can to strengthen every link in the chain. Or, if you look at it the other way around, if it's a chain of events that leads to an untoward incident, then break the chain and break it in as many places as possible.

All you've got to do is break the chain of tightly coupled events, which leads to the loss of an astronaut. Where you break it in the chain doesn't make any difference. So what we said was, you've got to do essentially four things in order to break this closely coupled chain of events.

First of all, you've got to understand foam creation and the creation of the hazard in the first place and you've got to do everything you can to prevent the creation of foam (debris) in the second place. The second thing you've got to do is, you've got to have much better pictures on launch and ascent to know whether or not there's been a foam event, or a debris event. You've got to know that. The third thing you've got to do is, you've got to essentially re-certify the orbiter to be ready to come back into the Earth's atmosphere. That translates into some kind of an inspection in orbit. And we didn't specify what kind of an inspection, (just that) you've got to be able to inspect it to see if there's anything wrong with it.

[In an aside, Gehman said the board was "quite struck by the first law of thermodynamics. I mean all the energy that goes into putting this thing into orbit, every single BTU of energy has got to come back out of it again. It's much more dramatic taking off than it is coming back to Earth, but that's only because we're standing there listening to the thunder and watching the light. But the coming back is just as exciting. So they go to all (this trouble) to certify the thing to take off and then they kind of gaff off the coming back in part. We were kind of struck by that."]

Gehman: And then, if you do have something wrong with the orbiter, you have to have some minimal, practical kind of capability to do some kind or orbital repair, the best practicable kind of a repair. Knowing full well, depending on the size of the damage or what the nature of the damage was, there are some repairs that are beyond your capability to do in space. And it has been our unwritten policy (that) you must attack all four of these things.

Now, you can do some better than others. If you really think you've done a fabulous job of preventing the creation of debris in the first place, you've got some really good ways to take pictures to make sure your orbiter hasn't been struck or anything like that and you're really sure that it's in good condition, then you can do some of the other stuff to a lesser degree. But you do have to make an attempt at all four areas. Now, within those four areas, there are sometimes one, two, three or four things that you've got to do. But that was what our intent was.

Q: NASA managers clearly believe they have reduced foam shedding to a minimal, reasonable level. They believe nothing much larger than 0.03 pounds can come off a section of the tank high enough up the stack to possibly hit a wing leading edge or any other critical structure. Are you comfortable with that?

Gehman: Not only have they minimized the foam but as I said, there are two or three sub bullets under each one of those things. One is, at least the people we talked to (during the investigation), they didn't have an understanding of the true strength of RCC. So they've been testing RCC to see how strong it really is, how big a piece of debris is dangerous and all that kind of stuff. And of course, the big picture, the big problem with the debris assessment team in the case of Columbia, was they had these crummy photographs and couldn't tell whether or not the thing was really a hazard or not. Well, we want them to be able to judge whether they've got a hazard or not. So, they really have taken us to heart on all those things.

[NASA will use high-definition television cameras attached to large telescopes to augment high-speed film and movie cameras to fully document launch and the initial climb toward orbit. Cameras on the external tank, the solid-fuel boosters and in the belly of the shuttle itself will look for signs of foam shedding, as will two camera-equipped WB-57 jets flying offshore. Impact sensors behind the wing leading edge panels will record - and pinpoint - any real debris strikes. In addition, the astronauts will photograph the tank after it separates from Discovery and use cameras and laser sensors on the end of a long boom to inspect both wing leading edges and the shuttle's carbon composite nose cap on the second day of the mission. Finally, the crew of the international space station will photograph the belly of the shuttle during the ship's final approach on flight day three to look for signs of tile damage. Additional inspections are planned later in the flight.] Q: With improved imagery and wing leading edge sensors, engineers believe they will know if there's any sort of impact damage. They've minimized foam shedding and in a worst-case scenario, the crew could use the station as a safe haven while awaiting rescue. The real issue comes down to whether that's enough and whether it's safe to go fly without a certified repair technique. They will not have one by the time they fly.

Gehman: We understand that and it is our judgment that they're efforts have passed the criteria that we set up for them.

Q: So you think it's a reasonable thing to go fly without a certified repair technique given everything else they've done.

Gehman: That's correct. But that doesn't mean they're allowed to give up on the repair. In our view, they have to keep working at it. And of course, we're talking about our 15 return-to-flight things (recommendations). Remember, there are 14 other recommendations, all of which they've got to work on. And they are. They are. Now that's not to say that some NASA engineer who's working on some subsystem deep down inside may feel that his box is not checked. There probably are some engineers who say, 'wait a minute, we're still working on this camera thing or the boom thing and it's not ready.' And he's probably right. So don't take my words as necessarily being construed that NASA engineers with subsystem problems don't need to be listened to.

Q: The perception seems to be that the CAIB told NASA to develop repair techniques and that, if NASA doesn't have them, the agency is not meeting the intent of the CAIB.

Gehman: Right. And we had no idea we were being prescient when we did this, but we worded that recommendation as broadly and as loosely as we possibly could, just to make sure that we did not box either ourselves or NASA in.

Q: Are you surprised it's taken this long to develop repair procedures and return to flight?

"Yeah. Some of it is our fault in that something as simple as putting a camera on the belly (of the shuttle), when you really go through certifying it for spaceflight it turns out to be a pretty large step. It's not just a matter of buying a camera, getting some rivets and screwing it on. Turns out, the steps that they go through to make sure everything is done right - and now, of course, they're being really scrupulous about it - it does take longer. We probably underestimated it. But we didn't think each of our recommendations by themselves, except for maybe on-orbit repair, actually were not very difficult. We did say in our press conference, though, that when you add them all up it might be a pretty tall hill. So yes, I must admit I thought this would get done in a year. But that's just my layman's voice speaking.

Q: In a hearing after the CAIB report was released, you were asked about coming back and taking a look at NASA's progress. Has anyone asked you to actually do that?

Gehman: Let me make sure you and I are on the same page here. The question was culture. How do you know if the culture has really changed? It wasn't a question about certification for flight, return to flight or anything like that. The question was, how would you know the culture has really changed? And I foolishly replied that we would know, we could tell in a week or two because we know exactly where to go and exactly what questions to ask. And they said well, would you be interested in coming back in a year to see whether or not the culture has changed? Culture was the issue. As a matter of fact - I had already cleared this with the board - I said 'if directed, we will serve.' In other words, we aren't volunteering. But so far, the Congress has not asked.

Q: Does the difficulty of coming up with a repair imply anything about the overall worthiness or safety of the shuttle? Recent impact tests show even coating damage to a reinforced carbon carbon (RCC) wing leading edge panel can be entry critical.

Gehman: I'll tell you what it says to me. It says to me that the organizational changes that we prescribed are more needed than ever. In other words, this kind of testing and this kind of awareness should have been known 15 years ago, or 20 years ago. When we probed into what is RCC, how strong is it, what are the specs and all that kind of stuff, they did not demonstrate a lot of detailed knowledge and they had not been testing or examining. ... So what it indicates to me is that the independence of the engineering department, independently funded, to be able to continuously test and continuously examine the specifications and requirements to see that they're right and that they're adhered to is just very important.

I understand what their testing has found and things like that, but you've got to remember that shuttles have returned safely with small delaminations and things like that. So you know, I'm pleased that they're learning more about RCC and I'm pleased that they're more nervous about it. But it doesn't strike me as changing the air worthiness of the vehicle. That's not the way I take it.

Q: So to reiterate, given your knowledge of all this, you think it's reasonable to go fly in May?

Gehman: Yes. As far as the CAIB is concerned, we think they have fulfilled their requirements and that they are diligently working on all the things we asked them to, to the degree that once it passes their internal return to flight certification, we think that they've done their job.

Q: Do you and the other board members still keep in touch?

Gehman: Yes, all the time. We're very active emailers to each other, it became a very close group. We speak at each other's seminars and things like that, see each other on a regular basis.

Q: Do you think your report will stand the test of time?

Gehman: So far, I've been invited on speaking tours at many universities and colleges and places like the nuclear power industry and other risky kinds of things, and they all say 'you're report is textbook, that's what we use for teaching and risk assessment.'

Q: One final question. Can the culture change the CAIB kicked off be too much of a good thing? Giving everyone a voice means it takes a long time to reach a consensus.

Gehman: Right. I know. They're going to have to sort they're way through that. As we indicated in our report, we went around and looked for best practices in other enterprises which are highly risky, like the Navy submarine program. And they have the same issue and they've learned mechanisms for how to deal with that. What NASA needs to do is they need to learn how to deal with those things. But suppressing the engineers isn't one of them.

Q: Are you coming to Discovery's launch in May?

Gehman: If I get invited.

By WILLIAM HARWOOD
CBS News

KENNEDY SPACE CENTER, Fla. - With the first post-Columbia shuttle flight just three-and-a-half months away, astronauts and engineers are engaged in a sometimes heated, down-to-the-wire debate over which tile and wing leading edge repair techniques to test in orbit.

The astronaut office at the Johnson Space Center in Houston now opposes one tile repair technique once considered a front runner, arguing it's too inconsistent, and thus unreliable, to warrant tests aboard the shuttle Discovery during mission STS-114 in May.

"It is a really challenging problem," chief astronaut Kent Rominger said in a telephone interview Friday. "But we're at the point, from our insight with looking at where it is and where we are, we do think we're better off without it on STS-114."

Many engineers disagree, saying tests in the airless, weightless environment of space are precisely what are needed to work the bugs out and learn whether the technique can ever be formally certified for use.

"For many of the repair techniques, not all but many of them, one of the key elements is to do it in vacuum and zero G and let it cure, or set up, and then bring it back and put it in our re-entry simulator, the arcjet," said Wayne Hale, deputy manager of the space shuttle program. "To most of us, it makes sense that that is the last step in certification. Do it on orbit, bring it back and put it in the highest fidelity re-entry simulator."

The procedure requires a spacewalking astronaut, wearing a large backpack with a sort of high-tech caulk gun, to fill or patch areas of damaged tile with a thick heat-resistant material known as STA-54 that would then cure in place, providing a barrier against re-entry heat.

Recent tests, however, show air bubbles can form in the material as it is mixed in the backpack, possibly reducing its effectiveness as a barrier against re-entry heating. Other questions involve the crew's ability to apply the material properly and its ability to adhere to the underlying surface.

"The office has taken a position that the systems are really just not developed along enough to do an effective demonstration," said Andrew Thomas, a member of Discovery's crew.

In the midst of that debate, a new tile repair technique, developed by a small group of engineers working on their own, has emerged in recent weeks that could provide a way out of the tile repair quandary if additional testing and analyses confirm initial results.

The so-called "overlay" technique calls for spacewalking astronauts to cover damaged tiles with thin, flexible sheets of heat-resistant carbon silicon-carbide that would be mounted atop a gasket and attached with fasteners similar to drywall bolts that would be screwed into surrounding tile.

The procedure, which appears relatively easy to carry out in weightlessness, is essentially a mechanical fix and as such, not susceptible to the sort of problems seen in STA-54 testing. But the overlay technique has not been subjected to the same level of scrutiny as STA-54 and the jury is still out on whether it might ultimately make its way on board Discovery.

"It would have some pluses and minuses," Thomas said in an interview. "You don't have the problem of the material flowing, the adhesion problem, which is really a very tricky problem. But you've got other issues, namely you've got a material edge which you wouldn't want air flow to get under so you've got to somehow button that down.

"The fasteners I've seen are actually pretty slick, though," he said. "They're low profile, very smooth and flush mounted, they're made of RCC-type materials. And they work, by all accounts, they seem to work very well. But there's a lot more development that's needed on that, too. It's not ready for prime time."

Not yet, maybe. But supporters believe the technique has great promise and Rominger said he supports in-flight tests by Discovery's crew.

"Absolutely. As a matter of fact, that looks really encouraging to me," he said.

The shuttle Columbia, of course, was not brought down by tile damage. It was the victim of a hole in a reinforced carbon carbon wing leading edge panel that was caused by the impact of foam debris falling from the ship's external fuel tank during launch.

Shuttle managers initially downplayed any realistic chance of developing techniques for repairing the shuttle's RCC panels in time for return to flight, citing the technical challenge of coming up with fixes capable of withstanding 3,000 degrees during re-entry.

Instead, the approach was to upgrade the external fuel tank's insulation to eliminate the possibility of large debris strikes.

The 1.7-pound chunk of foam that triggered Columbia's destruction came from an area known as a "bipod ramp," which insulated one of the fittings used to attach the shuttle's nose to the tank. Acting on a recommendation from the Columbia Accident Investigation Board, NASA has eliminate the bipod foam and implemented other changes to ensure no debris larger than about 0.03 pounds - half an ounce - can come off the tank during launch.

But engineers never stopped working on RCC repair options and recent testing shows those techniques may be more mature than any of the proposed procedures needed to repair the tiles on the belly of the shuttle.

"I was of the opinion early on that it was more important for us to demonstrate the RCC repair because that was the immediate cause of the Columbia accident," Hale said. "Several months ago, folks in the technical community were of the opinion that tile repair was more mature, RCC repair was less mature, we ought to do tile repair (aboard Discovery).

"Now it appears that RCC repair may be more mature and tile repair less. So we may be doing the first flight demonstration test on RCC repair rather than tile repair because it's readier."

Rominger agreed, saying "I've been really impressed with the kind of progress we have made in RCC repair."

"For some reason, from the start, we really tied ourselves to tile repair when in fact that's not what killed us on Columbia," Rominger said in an interview. "On every shuttle flight we've flown, we've damaged the tile and if anything, we've proven how robust it is."

Luck, however, played a role in previous tile damage - no critical systems were affected before Columbia's flight - and Rominger said NASA clearly needs to develop a reliable tile repair procedure.

"But I am really excited seeing all the progress we've made in RCC repair," he said. "I'm cautiously optimistic that with the number of different repairs we have out there (that), maybe not on 114 but not too far down the road, we will have pretty decent RCC repair capability."

Hale said Friday that meetings are planned in the next week or so to discuss the technical feasibility of the proposed RCC and tile repair procedures and to select those that will be tested by the Discovery astronauts in May.

"The question we're about to come to is, what is ready to fly in May?" Hale explained. "We have got to decide what we're going to fly and what we're going to train the crew for."

Whatever is decided, Rominger said the astronaut office will accept the outcome.

"In today's environment, they go around and ask everybody at the table, what do you think?" he said. "All along we've said, here's our input, take it for what it's worth, and when you come back with a decision we will salute sharply and do our best with whatever that is."

REPAIR OPTIONS AND 'THE INTENT OF THE CAIB'

In a very real sense, the need to test tile and RCC repair procedures aboard Discovery is as much a matter of perception as engineering.

None of the repair procedures ultimately flown aboard the shuttle will be "certified" by the time Discovery flies. Barring a worst-case scenario that left no choice, a crew would not be allowed to attempt a re-entry with a damaged shuttle until the repair techniques in question have gone through much more testing and analyses.

And that includes in-orbit testing.

"That clearly is part of the necessary certification," Hale said. "So that said, by definition almost, you can't fly the first flight with a certified repair technique. The only way to certify it is to take it to orbit, bring it back and see how it works. So we've recognized that for a long time.

"The goal, however, has been, is, will be, to get the best capability we can as soon as we can and fly with it. And if you get caught in a tight spot, you do the best you can with what you've got. The story's still in play. Tests are continuing as we speak, they're going to continue almost right up to launch day."

During at least the first few post-Columbia missions, with the space station available as a "safe haven," the crew of a shuttle with major tile or RCC damage more likely would be ordered to remain aboard the lab complex until a rescue flight could be launched.

"If we had damage that everyone concluded would be a threat to the vehicle, we would not, I'm sure, use an untried, untested repair technique to bring the vehicle home," Thomas said. "We would go into safe haven mode and await a rescue mission."

It's a tricky issue for NASA because the Columbia Accident Investigation Board concluded that before shuttle flights resumed, NASA should "develop a practicable capability to inspect and effect emergency repairs to the widest possible range of damage" to the shuttle's protective tiles and leading edge reinforced carbon carbon panels.

The key words, of course, are "practicable" and "widest possible range of damage." That gives NASA some leeway in determining what is required before flights can resume.

To meet the intent of the CAIB, the space agency is relying on safe haven and a suite CAIB-recommended post-Columbia fixes to resume flights without having certified tile and RCC repair techniques in place.

"We think launching in May on 114 looks like a very reasonable and safe thing to do," Rominger said. "Our office has never said, given we are going to station, that we have to have repair capability."

On paper, the safe haven concept calls for the station to support a combined shuttle-station crew of nine to 10 astronauts for up to six months. But for at least the first two flights, NASA plans to be prepared to launch a rescue shuttle within 45 days or so of the declaration of an emergency.

Even that's not soon enough for Sergei Krikalev, the Russian commander who will be on board the international space station when Discovery arrives. He said last week the lab cannot safely support a combined crew for much longer than that.

"First, everything possible needs to be done to prevent a situation that may require people to stay for such a long time," he said. "Second thing, if it happened, it should be a relatively short period of time ... because station would not be able to fly for many months in this configuration. For several weeks, we can deal with it, but we need to prepare kind of a backup plan for this backup scenario."

The major concern, he said, "would be supplies and also to maintain the crew in good health and good spirits. Exercise equipment we have on orbit is limited. ... I don't think we would be able to support all nine crew members with full-scale exercise."

Hale agreed, in general, that "the rescue scenario has got a number of its own risks."

"He's representative of a lot of folks who have pointed out that when you've got a lot of people at the station, their margins are kind of thin and then trying to get the next shuttle off, even though we've put our plan together, that plan doesn't allow for things to go wrong," Hale said.

Despite the unknowns still facing return to flight, Scott Hubbard, director of NASA's Ames Research center and a member of the Columbia Accident Investigation Board, said in his view, NASA appears to be meeting the intent of the CAIB recommendations.

"During all the deliberation about whether this should be written as you would write an FAA or a National Transportation Safety Board report or whether it's to be something that's more general, we came down on the side of being more specific," Hubbard said in an interview. "However, if you look at some of the preamble language and if you talk to the individuals who were there, there was always this sense that 'we know it's going to be hard, we know this is a high-risk machine and it will remain so.'

"The sense (CAIB Chairman Harold) Gehman captured that represented that was, 'do the best you can,'" Hubbard said. "What we meant by that was, it may not be that you can meet exactly every single recommendation to the 100.0 percent level. The thing to do is to take a systems look at it and say all right, if we can't get three perfect views with a perfect on orbit inspection with a perfect safe haven, with a perfect repair capability, what can you do in combination of those things that make this as safe as you can get it?"

The CAIB made 15 recommendations for changes needed before shuttles could safely resume flights. An independent panel of experts chaired by former Apollo astronaut Tom Stafford and former shuttle commander Richard Covey is monitoring NASA's implementation of those recommendations.

"Each of those 15 recommendations, each comma, each word was debated endlessly within the board as we were writing the document," Hubbard said. "We've got to be sure that we didn't tell people to go do something that was impossible and that we left it up to the agency and the Stafford-Covey review as to what the ultimate sense would be and what would be, in effect, as good as you can do to return to flight."

Following CAIB recommendations, the space agency has beefed up ground-based camera coverage to improve detection of foam strikes or other problems during launch and sensors have been mounted in the wing leading edges to detect impact forces. The sensors will not only record the force of any strikes, they also will pinpoint their locations.

Following another CAIB recommendation, Thomas will use Discovery's robot arm, and a long boom equipped with lasers and a TV camera, to inspect the shuttle's nose and wing leading edges for signs of impact damage the day after launch. The tile system will be inspected, with one-inch resolution, by the space station astronauts using telephoto lenses prior to docking and later, by more robot arm work.

Given NASA's ability to detect damage in the first place, the elimination of the external tank foam that caused Columbia's destruction and the ability of a shuttle crew to use the station as a safe haven, agency managers say certified tile and RCC repair techniques are not required to resume flights. Some engineers question the need to rush a repair technique into orbit aboard Discovery in the first place.

"You don't need it for the flight for that very reason, because we've got all of these other things," Thomas agreed. "If we didn't have it, we could still fly. I think that's an important point."

But, he added, "I think it would be nice for the program to have a capability that we could contribute to, to demonstrate and help those engineers work all those tricky problems. If one of these techniques was shown to be viable, we could put it on the flight, although time is really getting close for us. The train is leaving the station."

Asked if that implied NASA was bowing to the sort of schedule pressure blamed in part for the Columbia disaster, Thomas said "we do believe it's important to go fly. I think it's important not just for the agency, but for the country."

"We've got a space station that needs to be supported, the mission of the shuttle is to support the space station and we need to go do that and continue to work these issues so that any techniques that are developed really are themselves not driven by an arbitrary launch schedule, but can evolve in the natural engineering development cycle that they deserve," he said.

REPAIR OPTIONS POSE UNIQUE CHALLENGES

Discovery is targeted for launch on the first post-Columbia mission around May 15. Three spacewalks are planned, two of them devoted to space station repairs and outfitting and one to testing new tile and wing leading edge repair materials and procedures.

NASA calls that spacewalk a "detailed test objective," or DTO, and until recently, it was not certain such tests would even be attempted. But Hale said it now appears repair techniques of some sort will, in fact, be tested by spacewalkers Stephen Robinson and Japanese astronaut Soichi Noguchi.

"I think it is a done deal we are going to have a DTO and test whatever we get to carry," Hale said.

But with just three-and-a-half months to go before launch, engineers, astronauts and managers are still assessing which techniques to test. At least part of the reason it's taken so long to reach a consensus is NASA's post-Columbia effort to improve communications between managers and engineers.

"We have spent the last two years in this culture change, where we say that everybody needs to be involved, everybody's opinion needs to be heard and I think we've been successful, because we're hearing from everybody," Hale said. "And I've got to tell you, some of the gray beards, the retired NASA alumni league, former managers, have told me face to face that we are absolutely nuts to try to manage the program this way, that we will never reach a conclusion, fly the vehicle, you name it, because you get so much (feedback).

"I don't know, maybe they're right, but I've got to tell you our goal is to try to hear folks out and apply some logic and where possible, test and analysis, to come to a conclusion. But if you don't hear people out, you may miss something. That's the lesson I think we've heard."

In the meantime, he said, "what you're seeing is the sausage making."

In the RCC arena, the engineering community is focused on developing repairs to fix everything from coating damage, which in some circumstances could be "entry critical," to holes up to six inches across.

To fix cracks, engineers are testing a technique that calls for the astronauts to first heat a damaged RCC panel and then to apply an adhesive ceramic material that would be squirted from a gun-like applicator and smoothed out with trowels. A heated frame then would be used to cure it in place for half an hour at 300 degrees.

A team of Boeing engineers is working on a patch repair concept that potentially could be used to fix everything from coating damage to holes up to four inches across.

For this technique, the astronauts would first sand and smooth the damaged RCC panel. A two- to three-layer patch made up of silicon carbide and zirconia fabric impregnated with a "preceramic polymer matrix" then would be applied with a hand roller or paddle and cured in place for 30 minutes at 450 degrees.

Yet another technique, one that is perhaps the most technically mature, calls for using a flexible carbon silicon-carbide patch seven inches across to plug holes up to six inches in diameter. After fit checks and application of a sealant, the plugs would be held in place from behind by expansion bolts inserted into the hole.

Between 20 and 30 different plugs, each with slightly different geometries, would be needed in a repair kit to ensure a good fit virtually anywhere in the curving leading edge.

"The plug is very mature, the crack repair, it's very mature if we can develop a heater because this stuff has got to be heated after it's applied," Hale said. "We're going through some difficulty in building that heater. They're off doing a whole bunch of engineering tests to see if this crack repair would work in an unheated state so that it actually cures during the early part of re-entry. Sounds kind of dicey, but they're off looking at that. So we may fly that and bring it back."

The heater requires a battery pack and must be mounted to the leading edge to do its work. "It looks like a Rube Goldberg kind of device," Hale said. "It's not as simple as you would like EVA equipment to be. But there is this group of experts who say (the repair material) could work in a non-cured state, the early re-entry actually allows it to cure before you get to the maximum heating. Of course, there are a lot of folks who have a lot of concerns that that may not work. So here we are."

Engineers currently are studying ways to combine components of different RCC repair techniques to come up with procedures that might be easier to implement address a wider range of damage scenarios.

As for tile damage, engineers have been focusing on the caulk-gun/STA-54 technique to fill major cracks or voids in a shuttle's protective tiles. Minor damage may be addressed with yet another technique, known as "emittance wash," in which a coating is applied to damaged tile to help reject heat.

The STA-54 backpack applicator is known by the acronym CIPAA, which stands for "cure in-place ablator applicator." The constituents of STA-54 are carried inside two tanks and, like epoxy, are mixed together as they are pumped out. The astronauts then smooth and shape the viscous mixture before letting it harden and cure in the vacuum of space.

In ground tests last summer, however, engineers discovered air bubbles can form in the material as it is mixed together in the applicator. In the weightlessness of space, such bubbles could form large voids, weakening the material's strength and insulating ability.

Engineers do not yet understand the mechanism responsible for the air bubbles and while they've made changes that have reduced the amount of bubbling, they want to test the technique in orbit to find out exactly how the space environment affects the material.

"The problem the office has is not actually the CIPAA itself, by which I mean the idea of squirting the material out," Thomas said. "It's the material itself. It's not performing the way you would like ... because of the bubbling, it's outgassing and so on. So I think a lot of work needs to be done on the material itself."

Even so, he agreed with Hale that a CIPAA demonstration during a Discovery spacewalk "has some merit in so far as you could address some of the questions of how you deploy a goo like this in zero gravity, that's a tough problem. But the office has taken a position that the systems are really just not developed along enough to do an effective demonstration."

For Rominger, the backpack and equipment required for the CIPAA/STA-54 repair technique pose problems in their own right.

"For the repair with the STA-54, you have this backpack, for the EVA guys it's very technique sensitive," he said. "If you've ever caulked around your bathtub or other areas, you (know you) need to have a little bit of the artist in you. So to do that kind of work, we need a pretty secure work platform."

STA-54, he said, "is still in the research-and-development stage and it really is still a science project."

By not flying STA-54 aboard Discovery, Robinson and Noguchi would have more time during the repair DTO to test other procedures more thoroughly. Like the potentially more reliable overlay technique.

Instead of filling a void in tiles with a material that must cure in space, the idea with the overlay is to fill a void with packets of insulation and then to cover it with a thin, flexible sheet of high-temperature carbon silicon-carbide material. The sheet would be held in place by screws driven directly into the surrounding tiles like drywall bolts. A thin temperature-resistant fabric would serve as a gasket between surrounding tile and the cover sheet.

In recent tests, a section of tile repaired with this "overlay" sheet endured back-to-back sessions in NASA's arcjet facility getting blasted by temperatures similar to those that would be experienced during re-entry. The test sample survived both sessions in good shape.

Supporters argue the overlay technique is inherently superior to STA-54 because it is a mechanical fix and as such, immune to the effects of the space environment. And unlike STA-54, which has a limited shelf life, the overlay sheets could be stored indefinitely for use as required.

"It's actually pretty interesting what they've come up with," Thomas said. "It's a material that you would basically screw on and they've come up with methods of making fasteners that can take all the heat. ... The material is impregnated with a glass so it kind of liquefies a bit and fills the voids and potentially has applications for RCC (the shuttle's leading edge).

"On tile, you could use it in conjunction with something underneath it. If you had a big gouge in a tile, you'd put something in there to fill it in and then put this wrap over it. ... You don't have the problem of the material flowing, the adhesion problem, which is really a tricky problem. It kind of circumvents that."

Despite its initial promise, the overlay technique, like STA-54, cannot be certified before Discovery's launch. But testing the procedure in orbit "should tell us whether we should have more or less confidence in this as a repair," said one manager.

It may even be possible to combine the two repair techniques by using STA-54 to fill in voids and then covering them with the carbon silicon-carbide sheets. But much more testing is needed.

All in all, however, NASA has made solid progress toward meeting the intent of the Columbia Accident Investigation Board's recommendations, Covey told reporters in a recent teleconference.

"If we look at it in the total context of all the things that are being done, and if you look at the intent of the CAIB recommendation, which is one that I will admit we continue to have considerable debate over, then you could say well, OK, the real control for the hazards that come from the external tank is to reduce the debris that comes off the external tank to acceptable levels," he said. "That then reduces the requirement to be able to repair and if you look at the total context, even if we can't repair something, then for the first two flights while we gain confidence in the changes made to the external tank, the agency is prepared to have a safe haven capability on the space station and be able to launch a rescue mission.

"If you look at all those things together, then whether you have a repair capability that can be tested on STS-114 or not really probably isn't as significant as the overall control (of debris and the ability to inspect the thermal protection system during launch and in orbit).

"So, whereas that's probably still in my mind the remaining most difficult technical challenge, I still believe that the agency has gone a long ways toward meeting the intent of the CAIB recommendation, which was to develop and implement a program to develop those capabilities to the greatest range possible."

Ongoing space shuttle wing leading edge impact tests show tiny cracks or even damaged surface coating, coupled with internal delamination, can lead to catastrophic failure.

What is not yet known is how small a piece of debris can be and still cause the kind of damage that, if unrepaired, could bring down a shuttle; and whether chunks of foam insulation from the shuttle's upgraded external fuel tank will be above or below that critical size, whatever it turns out to be.

"You're asking the $64,000 question," said Don Curry, a senior engineer at the Johnson Space Center in Houston. "We're trying to establish that right now."

The issue is important because NASA's new external fuel tank, now at the Kennedy Space Center undergoing launch processing, was built to ensure no pieces of foam heavier than 0.023 pounds - 0.37 ounces - can break away from the upper portions of the tank during launch.

Debris from the upper part of the tank poses the greatest threat to the ship's nose cap and reinforced carbon carbon leading edge panels, which experience the highest heat loads during re-entry. The 0.023-pound limit is for foam covering the liquid oxygen tank at the very top of the structure.

Foam from the intertank area, where bipod struts attach the nose of the shuttle and where the foam originated that doomed Columbia, can be slightly more massive: 0.03 pounds, or 0.48 ounces. Pieces of foam from the bottom of the tank pose less of a threat and thus can be larger still: up to 0.75 pounds.

Here is a partial table of allowable debris as approved by shuttle program managers:

REGION...............................FOAM (pounds/cubic inches)

Oxygen tank..........................0.023/15.9
O2 tank to Intertank flange..........0.026/20.7
Intertank............................0.03/20.7
Hydrogen tank and intertank flange...0.03/20.7 to 0.075/51.8
LH2 tank.............................0.075/51.8

Engineers using a nitrogen gas-powered cannon and a wing leading edge mockup at the Southwest Research Institute in San Antonio, Texas, have been able to create "entry critical" damage from pieces of foam as small as 0.044 pounds - 0.7 ounces - depending on impact velocity and angle.

But they have not yet been unable to determine the damage threat posed by 0.03-pound pieces of foam because the small, low-density foam test "bullets" are deformed too much when fired from the cannon.

NASA hopes to launch the shuttle Discovery on the first post-Columbia mission in mid May. Curry said the engineering community is working to refine test methods and computer models to verify that pieces of foam debris weighing 0.03 pounds or less pose no threat to the shuttle.

But the testing already has revealed that entry critical damage can be caused by much smaller impacts than previously thought.

Columbia's left wing was damaged 81 seconds after liftoff Jan. 16, 2003, when a 1.67-pound chunk of foam broke away from an aerodynamically shaped ramp insulating one of two bipod fittings used to attach the nose of the orbiter to the tank.

The briefcase-size piece of foam slammed into the left wing at a relative velocity of more than 500 mph. Enhanced video from the one camera that viewed the impact point indicated the foam struck the leading edge at or very near the lower side of reinforced carbon carbon panel No. 8, one of 22 such panels making up the leading edge of the left wing.

But the grainy video, unable to resolve anything smaller than two square feet, provided no direct evidence of actual damage. In the wake of the mishap, the Columbia Accident Investigation Board, working with NASA, decided to conduct a complex series of tests to find out whether impacts by low-density foam could, in fact, cause the kind of damage needed to bring down the shuttle.

Initial tests at SRI showed foam impacts could cause damage, but the results were not clear cut. Then, on July 7, 2003, a properly sized foam bullet was fired at the lower side of RCC panel 8 using an impact point, angle and velocity thought to more closely mimic conditions possible during Columbia's launch. The result was a gaping hole 16 inches across.

Few engineers believe Columbia's wing had a hole that size and, based on more recent test data, it likely was on the small end of the 6-to-10 inches cited by CAIB investigators.

The question today is, what is the threshold for entry critical damage, that is, what is the smallest size piece of foam that could cause the sort of damage that would have to be repaired before a crew could safely return to Earth?

To find out, engineers are running exhaustive tests and analyses to define, for each of the 22 RCC panels making up a leading edge, what sort of impacts cause entry critical damage. Anything at or above that level must be repaired.

Because the RCC panels are custom built to match the sweeping curvature of the wing, each panel is divided into six zones and each zone is assigned its own number to more accurately model real-world conditions.

To find out what level of damage leads to oxidation, or burning, Curry said two approaches are in use. In one, 2.8-inch disks of RCC material are deliberately damaged and then subjected to re-entry heating with an arcjet plasma. The arcjet can produce temperatures of 2,960 degrees, closely matching the maximum experienced during re-entry.

Initial tests showed even small cracks could produce severe damage if the cracks penetrated a surface coating down into the underlying material, or substrate. Intrigued, engineers ran tests using a piston mechanism to push on a disk to impart loads, or forces, that could cause interior layers of RCC material to delaminate, or separate.

The results showed that as long as the surface coating remained intact, the test disks survived the arcjet without any major damage, even if delamination was present. But if the surface coating was damaged or removed, and if delamination was present, the reinforced carbon carbon burned up.

And in all cases, Curry said, "if we break the coating, we will have a delamination."

As a result, coating damage alone may be classified as entry critical. A shuttle could, in theory, be brought down by an impact that did not cause a hole in an RCC panel, just a visible crack in the coating, along with internal delamination. But engineers have not yet identified the debris threshold for causing such damage.

It is a complex problem because the force imparted to a leading edge panel depends not only on the size of the debris, but also the impact velocity, the angle of the impact relative to the surface of the RCC panel, the precise location of the strike and the panel's location on the leading edge.

To improve accuracy, engineers now are running so-called "wedge" tests in which the RCC is mounted at an angle to the flow of the arcjet to more realistically mimic real world conditions.

In disk tests, where the arcjet hits head on at a 90-degree angle, damage grows in a circular fashion as one might expect. In wedge tests, which more accurately reflect reality, the damage is more elliptical in shape with the worst heating occurring toward the downstream area of the ellipse.

Engineers currently are working to model damage patterns in each of the six zones in each RCC panel of a leading edge. Computer models are being refined to predict damage based on multiple variables and so far, the models show good correlation with the disk tests. Wedge models are more complex and are not yet fully developed.

In any case, Curry said Discovery's astronauts, using a boom-mounted camera and laser sensor package to inspect the shuttle's wing leading edge panels in orbit, will be able to spot the sort of surface coating damage that might pose a threat.

The upgraded external fuel tank that will launch as part of the first post-Columbia shuttle mission in May arrived at the Kennedy Space Center today, setting the stage for attachment to a set of solid-fuel boosters.

A barge carrying external tank 120 was tied to a dock near the Vehicle Assembly Building around 2 p.m. after a five-day voyage from NASA's Michoud Assembly Plant near New Orleans.

"This is a very important day for the nation and for its space program," said external tank project manager Sandy Coleman. "This is a big boost (to morale). We're all excited we have all of the vehicle here at the Kennedy Space Center."

The shuttle Columbia was destroyed during re-entry Feb. 1, 2003, because of a crack or hole in its left wing that was caused by the impact of a piece of insulating foam that broke free from the orbiter's external tank during launch.

The foam in question has been eliminated from the new tanks and other improvements have been implemented that, taken together, should make ET-120 "the safest and most reliable tank that's ever been produced," Coleman said.

The tank arrived at KSC a day early. It will be off loaded and towed to the Vehicle Assembly Building early Thursday. NASA plans a media photo opportunity and news conference later in the morning.

Shuttle commander Eileen Collins and the crew of the STS-114 mission are scheduled to visit the Kennedy Space Center Friday to view the tank and meet with spaceport workers. They also plan to hold their first question-and-answer session with reporters before heading back to Houston.

Editor's note (12/15/04):
With the shuttle Discovery's launch on the first post-Columbia shuttle flight fast approaching, I have updated the Current Mission page to include the latest flight plan, a global mission timeline, mission control personnel assignments, crew assigtnments and seating, links to bios, hardware assignments and other useful information.

The 11,600-word story below is focused on details about Discovery's mission and safety upgrades to improve ascent damage detection and potential repair options. It was current as of Dec. 17. It does not address management changes or other related topics ordered in the wake of the Columbia mishap. Those issues will be covered in subsequent stories.

NASA Sets Sights on Next Shuttle Flight

By WILLIAM HARWOOD
CBS News Space Consultant

KENNEDY SPACE CENTER, FL (CBS) - NASA is gearing up to resume shuttle flights this spring with a three-spacewalk mission to repair the international space station's stabilization system, to deliver critical supplies and equipment and to prove the design defects that contributed to the Columbia disaster have been corrected.

Implementing an overlapping suite of approaches, NASA managers believe the chances of Columbia-like damage to Discovery during launch are minimal. But if damage does, in fact, occur, they are equally confident they will be able to detect it, determine if it is entry critical and, if it is, carry out at least rudimentary spacewalk repairs.

Those repair procedures are still evolving and may not be fully tested and certified by the time Discovery's May/June launch window rolls around. But NASA managers say certified repair procedures are not required for flight because of the elimination of major debris, improved damage detection and the crew's worst-case ability to use the space station as a "safe haven" until another shuttle, already prepped for flight, could be launched on a rescue mission.

"I believe in our flight rationale, which says we are fixing the vehicle," said LeRoy Cain, the ascent-entry flight director for Columbia's final mission and now, for return to flight. "We are eliminating critical debris from being liberated from the tank and the boosters, the launch pad, every source that we can think of, we think we're eliminating critical debris. That's number one for me.

"I really feel like the chances of us having something come off of the stack and create a problem for us in our flight are exceedingly low."

Even so, engineers are working around to clock to perfect techniques for repairing damage to the ship's heat-shield tiles and the reinforced carbon carbon panels making up the wing leading edges and the shuttle's nose cap.

"The objectives of what's intended in the Columbia Accident Investigation Board report is to have the capacity to repair damage to the thermal protection system," said NASA Administrator Sean O'Keefe. "The only way you're going to be able to determine whether or not you can do any level of repair is after the first two flights.

"So the answer to that probing question in this semantical debate will be settled as a reality of what's possible after the first two flights. One test is worth worlds of opinions on this kind of thing and it settles all kinds of arguments."

Astronauts Stephen Robinson and Soichi Noguchi will provide at least some of the answers by testing tile repair materials and procedures in orbit. They may also test rudimentary techniques for repairing RCC panels, although that is problematic. While repair procedures may not be certified, tests in the space environment are considered critical to validating whatever techniques might ultimately be adopted.

"When we started this, we knew it would be extremely difficult to do a repair technique both on tile and on RCC," said shuttle program manager William Parsons. "The CAIB (Columbia Accident Investigation Board) said to do the best that we could to come up with some technique to repair RCC and repair tile. We have done that. We have continued to put the best and brightest this agency has to offer on this, we've used every resource this agency and this nation have to work on this, we've made a lot of progress.

"We think there are some issues that we still need to resolve, but we still have some time to go resolve that. And then we're going to go fly a test mission. We're going to have a detailed test objective in the payload bay of the orbiter, we're going to go out there and we're going to test some of the techniques, we're going to bring them back and (test them).

"We can do a lot of things on the ground, but we have to take them on orbit, put them in work on orbit, bring them back and then see if they work as well as we think they'll work. In a state of emergency, we would have a technique that we would be ready to perform. But right at this moment, we're still working through some of the technical details on how to do good tile repair and RCC repair."

Discovery currently is targeted for launch on the 114th shuttle mission around 4:11 p.m. on May 14. That date is little more than a target, however, and launch easily could slip a few weeks depending on the progress of work to close out open issues.

But engineers believe Discovery has a good chance of getting off the ground before its launch period closes June 3. The next available window, which depends on the space station's orbit, the shuttle's ability to reach it and a post-Columbia requirement to launch in daylight for photo documentation, opens July 13 and runs through Aug. 1.

STS-114/International Space Station: Upcoming Events
Editor's note:
For the purposes of ground processing, NASA is tracking May 12 for Discovery's launch even though lighting constraints are not met until May 14.

December
12/22/04: Progress 15 supply ship undocks from international space station
12/23/04: Progress 16 supply ship launches from Kazakhstan (5:19 p.m. EST)
12/25/04: Progress 16 docks with station (7:05 p.m. EST)
12/31/04: STS-114: External tank departs Michoud, La.

January
01/06/05: STS-114: External tank arrives at Kennedy Space Center
01/26/05: ISS Expedition 10 crew: Spacewalk No. 1
01/20/05: STS-114: OBSS thermal protection system survey boom installation in shuttle cargo bay

February
02/09/05: STS-114: External tank mated to solid-fuel boosters
02/11/05: STS-114: Crew equipment and interface checks
02/27/05: Progress 16 supply ship undocks from space station
02/28/05: Progress 17 supply ship launches

March
03/02/05: Progress 17 supply ship docks with space station
03/02/05: STS-114: 48-hour full-up mission simulation at Johnson Space Center
03/07/05: STS-114: Shuttle rollover to Vehicle Assembly Building
03/16/05: STS-114: Shuttle stack moved to launch pad 39B
03/25/05: ISS Expedition 10 crew: Spacewalk No. 2
03/30/05: STS-114: External tank is loaded with fuel for a test

April
04/15/05: ISS Expedition 11/ESA crew launches aboard a Soyuz
04/17/05: ISS Expedition 11/ESA crew docks with space station
04/25/05: ISS Expedition 10/ESA crew lands
04/26/05: STS-114: Flight readiness review
04/29/05: STS-114: Terminal countdown demonstration test

May
05/08/05: STS-114: Crew flies to Kennedy Space Center
05/09/05: STS-114: Countdown begins
05/12/05: STS-114: Launch

At the controls on Discovery's flight deck will be veteran commander Eileen Collins, pilot James Kelly and flight engineer Robinson. Seated to Robinson's right will be Japanese native son Noguchi.

Strapped in below on the shuttle's middeck will be Mir-veteran Andrew Thomas, Wendy Lawrence and Charles Camarda. All but Camarda and Noguchi are shuttle veterans.

The shuttle's primary cargo includes a refurbished control moment gyroscope to replace one that failed earlier aboard the space station; a tool kit and spare parts module that will be mounted on the station's airlock to enable future assembly work; and a pressurized logistics module loaded with space station equipment and supplies.

In an interview by the author for the book "Comm Check: The Final Flight of Shuttle Columbia," Collins was asked if she had any second thoughts about commanding mission STS-114.

"Absolutely not," she said. "In fact, I am more committed to flying this mission than I ever would have been. ... I am excited, I am going to be extremely confident because look at all this work that is being done, not just done because of (Columbia), but other things that we think are risky. I am so confident, I am so excited, I want to get our country back flying in space again, so I am not one blink of an eye worried about safety."

And her crewmates?

"I have talked to everybody on my crew several times, 'Hey, I want to make sure that all of you are comfortable, that you feel safe, if you don't want to fly this mission you don't have to. No requirement.' They are all feeling the same way I am. They are onboard, they can't wait to get back and fly."

Robinson, a private pilot who owns three airplanes, lives on an airstrip and doesn't own a television, said spaceflight is inherently risky and that the Columbia disaster "does not reset that calculation."

"We feel that there is a risk and it is worth it," he said in an interview for "Comm Check." "I feel that it is an important enough thing to do, that the risk - and we have a very realistic view of risk, especially now - the risk is worth it. It doesn't change my mind at all. It is that important. I am totally dedicated to do it."

LIMITED LAUNCH PERIODS

During Columbia's launching Jan. 16, 2003, a suitcase-size piece of foam insulation broke away from an aerodynamically shaped ramp used to keep ice from forming on a strut connecting the shuttle's nose to its external fuel tank. The foam came off 81.7 seconds after liftoff and struck the underside of Columbia's left wing 0.2 seconds later, smashing into the lower side of a leading-edge RCC panel.

Ground cameras were unable to see the point of impact. One long-range tracker that might have shown the impact site was out of focus. And given the resolution of the cameras in place at the time, it's not clear obvious signs of damage would have been detected.

In any case, without the benefit of high-resolution video of the impact, mission managers were forced to rely on computer modeling and other indirect techniques for determining whether the foam strike could have caused any entry-critical damage. In the end, they wrongly concluded Columbia could safely re-enter as is.

Commander Rick Husband, pilot William McCool, Kalpana Chawla, Laurel Clark, Michael Anderson and Israeli astronaut Ilan Ramon were killed Feb. 1, 2003, when hot gas burned its way into the interior of the left wing through a presumed breach on or near the underside of RCC panel No. 9. The wing failed and the shuttle broke up 37 miles above Texas.

In the wake of the disaster, the Columbia Accident Investigation Board ordered sweeping changes, including "an aggressive program to eliminate all external tank thermal protection system debris shedding at the source with particular emphasis on the region where the bipod struts attack to the external tank."

The bipod ramp was intended to keep ice from forming around the struts due to the ultra-low temperatures of the shuttle's liquid oxygen and hydrogen propellants. NASA's solution was to simply eliminate the ramps and to install heaters on the strut attachment fittings to prevent ice buildups.

Tank engineers also implemented a variety of other changes to minimize foam shedding and while additional work remains to be done, NASA managers plan to ship the first upgraded tank to the Kennedy Space Center in the first week of January.

"Through all our testing, we believe the amount of foam that can come off the tank and not cause serious damage is .03 pounds," said Wayne Hale, deputy director of the shuttle program at the Johnson Space Center in Houston. "If you think about that, that is three one hundredths of a pound. That is something like six tenths of an ounce. So that's a pretty small piece of foam.

"All our investigations of the foam indicate we will not get a piece of foam coming off bigger than .008, eight one thousandths, or almost an order of magnitude smaller than the requirement."

It is not possible to eliminate all foam shedding, but "we're clearly moving toward an era where we expect to see much less damage in the tile and no critical damage that will require a repair," Hale said. "So that's our goal in this and it's beginning to look very positive that we'll be able to accomplish that level of control on the ET foam."

But to make absolutely sure, NASA must be able to inspect the tank and the space shuttle after launch with much greater precision than before. The CAIB recommended that NASA "upgrade the imaging system to be capable of providing a minimum of three useful views of the space shuttle from liftoff to at least solid rocket booster separation."

The panel also told NASA to look into putting cameras aboard ships and/or aircraft to provide additional coverage, to develop a capability to obtain high-resolution images of the tank after separation from the shuttle, to develop techniques for high-resolution imaging of the ship's underside and wing leading edges; and to make arrangements to obtain imagery from spy satellites if needed.

On its own, NASA managers decided to launch the first two post-Columbia shuttle flights in daylight to improve photo coverage and to time the launchings so the external tank, separating from the ship half a world away, also would be lighted by the sun.

To reach the space station, the shuttle must launch within a few minutes of when Earth's rotation carries the launch pad into the plane of the lab's orbit. And as a final complication, the shuttle can only visit the station when the "beta angle" - the angle between the plane of the station's orbit and the sun - ensures the shuttle-station stack will not get too hot.

Throwing all of those requirements together, NASA can only launch a shuttle to the station during relatively short windows. As of this writing, the next six allowable launch periods are:

• May 14 through June 3
• July 13 through Aug. 1
• Sept. 9 to Sept. 25
• October: None
• Nov. 7 through Nov. 10 (not thought to be viable)
• December: None
• Jan. 4, 2006, through Jan. 6 (bad beta angle)
• February: None
• March 4 through March 20

Because engineers do not expect any useable launch periods from Sept. 25 through the end of February, the space agency needs to get Discovery off the ground during the May-June period to have any chance of launching more than two flights next year. And that has implications for space station resupply options and when NASA can add a third person to the station's full-time crew.

UNPRECEDENTED VIDEO COVERAGE

From a mission control standpoint, Cain said Discovery's flight will use the same rules and procedures in place for Columbia's launching with one major exception. NASA is finalizing activation of a new trans-Atlantic abort site in France that will replace Ben Guerir in Morocco.

While NASA will leave equipment at Ben Guerir for possible use in the future, security issues in the wake of the 9/11 terrorist attacks could not be ignored. The new landing site, known as Istres-le-Tube, is located northwest of Marseilles and is one of the largest military air bases in Europe.

"We're real excited about it," Cain said. "It's a great opportunity for us and the French air force and the air traffic control folks in southeastern France have been very helpful."

But barring an engine failure or some other malfunction that might trigger a TAL abort, Collins and Kelly will carry out the same ascent procedures as Rick Husband's crew aboard Columbia. But major changes have been implemented on the ground to document foam shedding from Discovery's external tank and any possible damage to the orbiter.

Bob Page, the engineer at the Kennedy Space Center who leads NASA's launch photo team, said Discovery's launching will be documented in unprecedented detail using a mixture of high-speed film cameras, high-definition TV cameras and even airborne sensors using two NASA WB-57 jets.

The high-definition video cameras are a major improvement over the standard television cameras in place when Columbia took off. Those operated at just under 30 frames per second, limiting how precisely engineers could track the debris from the fuel tank. The new HDTV cameras will operate at 60 frames per second and provide four times the resolution, or clarity.

But because the shuttle rolls into a "heads down" orientation shortly after launch, putting the belly on the other side facing upward, ground cameras, no matter how good, still will not be able to see certain areas of the shuttle's underside. But any major debris strikes should be readily apparent.

"For tracking debris, for determining debris size, for determining debris velocity, for location of impact, for source location, for a lot of those things, I have much better data," Page said in a recent interview. "But what so far has been determined as a damage size that would be a threat to the vehicle, it is smaller than the resolution that I can work with at the time frames from 70 seconds out to SRB separation. The damage size that's a threat to the vehicle is a whole lot smaller than anything I can see."

That's where cameras aboard the shuttle will come into play. But if anything large breaks away, and certainly anything the size of Columbia's bipod ramp, Page's cameras will spot it, track it and help managers determine what damage it may have caused.

More than 50 high-speed film cameras mounted on the launch pad, running at 400 frames per second, will capture the initial seconds of flight from extreme close range. Three short-range tracking platforms around the perimeter of the pad, each one equipped with two film cameras and an HDTV camera, will capture imagery through the first 57 seconds of flight.

Six medium-range tracking platforms, each with film and HDTV cameras, will capture the view through the first 100 seconds and 10 long-range trackers, five of them north and five south of the shuttle's ground track, will cover the flight through the first 165 seconds, well after booster separation.

Because of the high frame rate, imagery from the HDTV cameras will be stored on hard drives. The engineering team that will analyze the footage will be able to access it across the agency from internal NASA web sites. The public will see normal-resolution television views from those tracking in realtime over NASA's satellite network. Within six to eight hours, Page hopes to deliver processed HDTV imagery to public affairs for broadcast.

Along with switching to HDTV, Page also bumped up the frame rate of the film cameras to provide better temporal resolution.

"You've got to look at both the spatial and the temporal. Let's start with the film," he said. "We ran 35-millimeter film before and we're running it now. So the pixel, or grain, count is the same. But we've increased the frame rate from 64 frames per second to 100. So we have 50 percent more data points to track a piece of debris. As you're tracking a piece of debris as it moves down the stack during ascent, we have 50 percent more number of points, number of frames, to pinpoint where it is in three dimensional space. So that's important.

"From the video side, we're going from 30 frames per second interlaced, where every other line is scanned, and we're now going to 60 frames per second progressive scan, and that means every single line is scanned. So we have a hundred percent more data points, plus we have four times the resolution. We can measure the size better, we can see it better."

The smallest piece of debris or damage detectable by the ground cameras at the moment of solid-fuel booster separation is 15 to 16 inches across. To improve those numbers, NASA has equipped its two WB-57 jets with nose-mounted HDTV cameras and infrared sensors. The planes will fly 15 miles to the north and south of the shuttle's ground track at an altitude of 60,000 feet some 40 miles off shore. At booster separation, they will be 15 miles below the shuttle on a line perpendicular to the orbiter's ground track.

Each HDTV camera in the nose of each jet will be hooked up to a telescope with a 4.2-meter focal length. The telescopes should be capable of detecting debris or damage sites as small as six inches across at booster separation.

Page said the planes have two additional benefits for NASA: They can provide full video coverage of a return-to-launch-site abort and they will be used during re-entry to image a shuttle's plasma trail during the region of peak re-entry heating using both visible light and infrared. But footage cannot be downlinked from the aircraft. It must be delivered to analysts after landing.

For Discovery's re-entry, one of the WB-57s will be stationed over the Pacific Ocean, below and west of the point where peak heating begins, while the other will be stationed over Mexico, east of the point where peak heating ends. A third aircraft will be used lower, near the landing site, to provide additional photo documentation.

The jets normally are based at Ellington Field near Houston. For launch, they will be stationed at Patrick Air Force Base just south of the Kennedy Space Center. Page says it will take about a day to process the on-board imagery.

The CAIB recommended that NASA include the operational status of its tracking cameras in the agency's launch commit criteria, the set of rules used to determine whether a countdown can proceed or not. Page said the status of long-range trackers will be included in a management poll during a 10-minute hold at the T-minus 20-minute mark. A final poll of short-range camera status will be conducted during a hold at T-minus nine minutes. But the cameras are not part of the formal launch constraint criteria because they do not directly affect flight safety.

No additional polling will be conducted after the T-minus nine-minute hold and any subsequent failures in the camera system will not stop a countdown. The high-speed launch pad cameras fire up at T-minus 10 seconds and Page said it would be more dangerous to abort a countdown at that point than it would be to continue without complete camera coverage.

"The LCC comes into play when you look at these cameras on the pad," he said. "Now I have a computer system that is controlling every single one of these. If I lose that computer system, I lose all of these views. So what I do is, I have an LCC down to T-minus nine minutes for the control system for these cameras, to make sure it is operating and it is stable. And then I can continue on from T-minus nine."

NASA is spending $40 million to upgrade the imaging system at the Kennedy Space Center and $9 million to modify the WB-57s. Through the end of the shuttle program, the agency expects to spend another $40 million on operations, pushing the total cost of the new system to nearly $90 million.

ON-BOARD CAMERAS PROMISE DRAMATIC VIEWS

Ground- and air-based shuttle imagery mark a clear improvement over what was in place for Columbia's launch. But it still isn't good enough to spot small areas of potential entry critical damage or damage to areas of the underside of the shuttle that cannot be seen from the ground.

To close that gap, NASA is installing cameras on the external tank of the shuttle, on the two solid-fuel boosters and in the recessed cavity where 17-inch propellant feed lines enter the belly of the orbiter. In addition, as soon as the shuttle reaches space, the astronauts will use a hand-held video camera to "shoot" the tank from close range before it drifts away.

"We have added some cameras on the tank and on the solids that are primarily looking at tank and at the bottom of the vehicle for the higher impact concern areas," said Paul Hill, the lead flight director for STS-114. "From a crew perspective, they're all hands off, almost passive operations. Once we separate from the ET, we have modified the ET separation maneuver, which will pitch us around so the crew can take pictures at about half the range we used to take pictures at.

"What we're more excited about from an ET photography perspective is the umbilical well camera. Because that camera is going to give us such a good shot of the ET foam in particular that's on the orbiter side and we're going to see that at a really close range. That will give us a really good idea of how the ET insulation performed during ascent."

Within a minute or so of separation, the crew will get a full view of the tank from Columbia's flight deck.

"We would definitely be able to see if we had large pieces of foam come off," Hill said. "My expectation is, we will have really good resolution because it is a still camera instead of video, and because of the close range. Because it's a digital still camera, we'll also be able to downlink that day instead of waiting until post-flight like we would a film camera."

On board imagery will be stored and downlinked to mission control as soon as the crew sets up the shuttle's laptop computer system, along with data from new wing leading edge sensors that were added to the shuttle's wings as a post-Columbia upgrade.

Located on each wing's forward spar behind every RCC panel, the impact sensors will tell flight controllers whether anything struck the leading edges during launch. In fact, they may show engineers aspects of the shuttle they've never seen before.

The sensor system generates two types of data: Peak and detailed.

"Think about what a stereo equalizer looks like," Hill said. "You've seen these ones that, across the frequency band, as the signal bounces up and down, it leaves a hash mark. The system works kind of like that. It's recording very high rate frequency response data across the wing leading edge from all these accelerometers that are on the wing spar for every RCC panel. And it registers the peaks, the software pulls out where those little peaks are from T-0 all the way to after we've made it into orbit.

"The first thing we downlink is just the file that has all the peaks in it," Hill said. "That then tells us that we have a suspected impact somewhere and after we see that, then within an hour after the guys in the MER (mission evaluation room) see that and pick out the ones they think are potential impacts, then we put commands on board to downlink the detailed data around each one of those peaks."

Hill acknowledged that engineers worry "we're going to get data down that we don't understand, or because of the shake rattle and roll we'll get going up hill, we won't be able to interpret the data."

"What's in our favor on this is we've been flying accelerometers like this and the same data collection system in the aft compartment of the orbiter for years," he said. "Now we haven't had it on the wing leading edge and we weren't using it to detect impacts, but the hardware has been flying for some time and we have characterized a similar ascent vibe environment in the aft compartment."

In addition, the sensors have been used during impact tests at Southwest Research Institute in San Antonio, Texas, to collect actual data.

"So we have a certain amount of data on how the system will respond going up hill, we have other data to tell us what impact ought to look like," Hill said. "One thing's for sure, by the end of flight day one we'll have data on the ground and we'll know the answer to that question."

Flight Day 1 highlights (all times in Eastern; a detailed flight plan is available on the Current Mission page):

   DAY..EDT........DD...HH...MM...EVENT
   
   05/14/05
   Sat  04:11 PM...00...00...00...STS-114 Launch
   Sat  04:20 PM...00...00...09...Main engine cutoff
   Sat  05:01 PM...00...00...50...Post-insertion timeline
   Sat  06:41 PM...00...02...30...PGSC laptop computer setup
   Sat  07:06 PM...00...02...55...Remote manipulator system (RMS) powerup
   Sat  07:21 PM...00...03...10...RMS checkout
   Sat  07:46 PM...00...03...35...Elevon park
   Sat  08:06 PM...00...03...55...RMS powerdown
   Sat  09:16 PM...00...05...05...NC1 rendezvous rocket firing
   Sat  09:36 PM...00...05...25...Group B powerdown
   Sat  10:11 PM...00...06...00...Crew sleep begins

The astronauts will downlink hand-held video of the external tank shortly after reaching orbit. Hill said he expects initial, non-HDTV ground-camera ascent imagery to be available before the crew goes to sleep, as will most, if not all, of the digital still photos on board Discovery. Wing leading edge data will be downlinked late in the crew's day or during their sleep period. An imaging report based on a more detailed analysis of ground and air-based cameras should be available within 24 hours.

TAKING A CLOSE-UP VIEW OF THE WING LEADING EDGES

Even if no obvious signs of foam shedding or damage show up in the initial imagery or wing leading edge data, CAIB recommendation 3.4-3 calls for NASA to "provide a capability to obtain and downlink high-resolution images of the underside of the orbiter wing leading edge and forward section of both wings' thermal protection system." The Discovery astronauts are going to spend their second day in space carefully inspecting the leading edges of both wings, along with the RCC nose cap of the shuttle, using a new 50-foot-long boom on the starboard side of the payload bay known as the orbiter boom sensor system, or OBSS.

Thomas will unlimber the shuttle's 50-foot-long robot arm and lock it onto the OBSS. Thomas, Kelly and Camarda then will spend the entire day maneuvering the boom back and forth along the leading edges of both wings, using an OBSS television camera and one or more laser sensors to inspect every square inch of the RCC panels. The astronauts also will use the boom to inspect the nose cap.

Maximum speed of the boom survey: 2 inches per second.

"If you think about the laser, the way we're recording the data is similar to recording video," Hill said. "Imagine standing on the side of a soccer field watching one of your kids play soccer with a camcorder. And you're panning that camera real fast so you can watch him run down the field. But when you play it back at home, you can't make out hide nor hair because everything is blurred. That's the problem we've got. We've got a translation constraint. If we move too fast, we blur the image, which directly affects the resolution and we can't see the small stuff we're looking for."

As of this writing, it's not yet clear whether the boom will be equipped with one or two laser sensors. That's because engineers are still debating what constitutes entry critical damage to the wing leading edges.

"A year ago, we thought - actually until about six or seven months ago - we thought entry critical damage required a penetration of the RCC, not just coating damage or even small damage to the substrate on the outside," Hill said. "More recent arc jet testing has us worried that coating damage alone, if it's large enough and if we had internal damage - delamination - between the layers, that the combination of those two could be entry critical."

In the early years of the shuttle program, Hill said, tests indicated the leading edge RCC panels could tolerate penetrations a quarter of an inch across. But that testing was with a clean hole punched in the panel, which is what one would expect with a hypervelocity impact in space. But during launch, impact velocities would be much lower and any resulting penetrations would be more ragged.

Engineers then began wondering if lower-velocity impacts might be entry critical.

"What we found out in more recent arc jet tests, in the last couple or three months - this is the thing that's key - if (RCC coating is intact), our conventional wisdom is good, we don't care, internal delamination's not an issue.

"But if the coating is gone and underneath that coating you're delaminated, then picture the RCC itself from a side view like a cross section. Now you've got this bubble or this void in between layers. What you've done is, you've significantly reduced the density of this RCC that's exposed to the heat load. So it burns faster.

"So now instead of being this more solid material that's hard to light, kind of like if you take a piece of hard wood like oak and you hold a match to that oak, it won't light typically. But if you shave off some splinters of that oak, you can get them to flash. Damned if that's not what we found in a handful of RCC runs for uncoated RCC."

For the tests, engineers deliberately damaged an RCC panel by pushing on it with a metal cylinder. After confirming the panel developed delamination as a result, "they put that bad boy under the arc jet and it burned like there was no tomorrow. The whole area that covered the delamination burned off like a fuse."

Engineers are debating whether coating damage, coupled with delamination, represents a credible damage scenario.

"If it's credible for us to take an impact that has enough energy to cause a delamination and takes off the coating, then that does not have to be very big to be catastrophic," Hill said. "From an RCC damage perspective, that looks like a penetration. So now the question is, do we believe that testing? Have we done enough of those tests to be sure that is an entry critical damage form? And then, is it credible for us to take an impact that could cause that kind of damage? Those are the questions before the orbiter project office."

How the debate plays out could play a role in Discovery's eventual launch date. One OBSS laser sensor, known as the laser dynamic range imager, or LDRI, can support a May 14 launch date. But it does not have the resolution to detect the sort of coating damage engineers are assessing.

Another sensor under development, known as the laser camera system, or LCS, can detect such damage but more time is needed to ready it for flight. If the LCS is required, launch could slip a week or two.

"I'm holding out hope that what we will conclude is yes, if we take that kind of damage it's something we're worried about," Hill said. "But with our increased knowledge of the transport model, there's only very few parts of the leading edge where we're really at risk of taking an impact that can do that."

Flight Day 2 highlights:

   DAY..EDT........DD...HH...MM...EVENT
   
   05/15/05
   Sun  06:11 AM...00...14...00...STS crew wakeup
   Sun  07:41 AM...00...15...30...NC2 rendezvous rocket firing
   Sun  08:41 AM...00...16...30...Robot arm checkout (if necessary)
   Sun  09:26 AM...00...17...15...Thomas unberths the OBSS
   Sun  10:16 AM...00...18...05...OBSS RCC survey begins
   Sun  12:36 PM...00...20...25...Docking ring extension
   Sun  01:01 PM...00...20...50...Crew meals begin
   Sun  02:06 PM...00...21...55...OBSS survey resumes
   Sun  02:06 PM...00...21...55...Middeck equipment prepped for transfer
   Sun  02:41 PM...00...22...30...Robinson and Noguchi checkout their spacesuits
   Sun  04:36 PM...01...00...25...The OBSS is reberthed
   Sun  04:41 PM...01...00...30...Public Affairs event (audio only)
   Sun  05:21 PM...01...01...10...NPC rendezvous rocket firing
   Sun  06:51 PM...01...02...40...NC3 rendezvous rocket firing
   Sun  08:11 PM...01...04...00...Crew sleep begins

RETURN TO THE SPACE STATION

The OBSS will survey RCC panels only. To look for signs of tile damage on the underside of the shuttle, including possible damage to critical seals around landing gear doors, Discovery's crew will rely on help from the crew of the space station.

Docking is targeted for flight day three. Collins will guide Discovery through a standard rendezvous profile, approaching the lab complex from behind and below.

On final approach, at a distance of about 600 feet directly below the station, Collins will carry out a slow 360-degree rotational pitch maneuver, or RPM, that will point the belly of the shuttle at the station.

"I have been flying that since it was first conceived," Collins said in the "Comm Check" interview. "I am thoroughly convinced that we can do this very safely. ... It is not an easy maneuver when you start learning it, but once you have done it a few times it is relatively simple."

As the shuttle's underside rotates into view, Expedition 11 commander Sergei Krikalev and flight engineer John Phillips, shooting through windows at opposite ends of the station, will photograph Discovery's belly with handheld digital cameras equipped with 400- and 800-millimeter lenses. During an earlier expedition, science officer Donald Pettit took test photographs of approaching Russian spacecraft to determine what the station's cameras could actually see.

"The 800 millimeter gives them one-inch resolution, which is what we're looking for around (landing gear) door seals," Hill said. "400 millimeters gives three inches, which is what they're looking for everywhere else."

Again, imagery from the station will be downlinked that day for detailed analysis.

After completing the RPM maneuver, Collins will position Discovery directly ahead of the space station with the shuttle's nose facing deep space and its cargo bay facing the lab complex. She then will guide the spacecraft to a docking with a pressurized mating adaptor attached to the Destiny lab module, the first shuttle linkup with the outpost since Nov. 25, 2002.

After leak checks, Krikalev and Phillips will welcome the shuttle crew aboard and provide a brief safety briefing before all nine astronauts get down to work.

Because of clearance issues after the shuttle is docked, Discovery's robot arm cannot unberth the OBSS for additional tile inspections. Instead, the space station's arm - the SSRMS - will pluck the sensor boom from the shuttle's cargo bay and hand it off to Discovery's arm about five hours after docking.

Flight Day 3 highlights:

   DAY..EDT........DD...HH...MM...EVENT
   
   05/16/05
   Mon  04:11 AM...01...12...00...STS crew wakeup
   Mon  04:41 AM...01...12...30...ISS crew wakeup
   Mon  05:41 AM...01...13...30...Upper surface tile inspection
   Mon  06:31 AM...01...14...20...Rendezvous timeline begins
   Mon  07:11 AM...01...15...00...Rendezvous tools checkout
   Mon  07:11 AM...01...15...00...NH rendezvous rocket firing
   Mon  07:56 AM...01...15...45...NC4 rendezvous rocket firing
   Mon  08:11 AM...01...16...00...Spacesuits removed from shuttle airlock
   Mon  09:31 AM...01...17...20...TI rendezvous rocket firing
   Mon  10:51 AM...01...18...40...Begin final approach
   Mon  12:09 PM...01...19...58...DOCKING
   Mon  01:36 PM...01...21...25...Hatch opening
   Mon  02:21 PM...01...22...10...Handshake/Welcome
   Mon  02:31 PM...01...22...20...Safety briefing
   Mon  02:56 PM...01...22...45...Station arm (SSRMS) grapples OBSS
   Mon  02:56 PM...01...22...45...Middeck equipment transfer begins
   Mon  03:21 PM...01...23...10...OBSS is unberthed by SSRMS
   Mon  03:26 PM...01...23...15...Spacewalk tool transfer
   Mon  04:46 PM...02...00...35...SSRMMS hands off OBSS to RMS
   Mon  05:31 PM...02...01...20...SSRMS inspects Unity for MPLM install
   Mon  08:11 PM...02...04...00...Crew sleep begins

The astronauts also will begin moving more, than 1,000 pounds of station equipment stowed in the shuttle's middeck area over to the space station, along with tools that will be used in the upcoming spacewalks. The bulk of the supplies carried aloft aboard Discovery will be housed in the Italian-built multi-purpose logistics module mounted in the cargo bay. The 21,000-pound MPLM will be unberthed on Flight Day 4, using the station's robot arm, and attached, or mated, to the downward-facing port on the U.S. Unity module.

Once the MPLM is in place, the SSRMS will lock onto a mobile base system on the front side of the station's unfinished solar array truss to assist with additional tile inspections. Later that day, the station's arm will be moved back to its normal perch atop the Destiny module. The astronauts, meanwhile, will perform leak checks to make sure the MPLM is firmly mated, they will pressurize the vestibule between Unity and the supply module, activate critical system and then float inside to begin the process of moving supplies into the space station.

At roughly the same time, yet another shuttle tile survey will begin using the RMS-OBSS boom, with additional TV views provided by the SSRMS.

In addition, the tools that will be used for the upcoming spacewalks will be configured for use, two emergency jet backpacks will be moved aboard the station and the crew will spend an hour reviewing the procedures that will be used in the first spacewalk.

Flight Day 4 highlights:

   DAY..EDT........DD...HH...MM...EVENT
   
   05/17/05
   Tue  04:11 AM...02...12...00...STS crew wakeup
   Tue  04:41 AM...02...12...30...ISS crew wakeup
   Tue  06:16 AM...02...14...05...MPLM grappled with SSRMS
   Tue  06:46 AM...02...14...35...MPLM unberthed
   Tue  06:51 AM...02...14...40...Spacesuit transfer to station
   Tue  07:31 AM...02...15...20...RMS inspects MPLM hatch mechanism
   Tue  07:41 AM...02...15...30...MPLM installation begins
   Tue  09:06 AM...02...16...55...MPLM berthing mechanism bolts tightened
   Tue  09:56 AM...02...17...45...SSRMS ungrapples MPLM
   Tue  10:11 AM...02...18...00...Mobile base system grapple by SSRMS
   Tue  10:51 AM...02...18...40...Public Affairs event (audio only)
   Tue  10:51 AM...02...18...40...MPLM vestibule pressurization
   Tue  10:56 AM...02...18...45...SSRMS ungrapples lab
   Tue  11:26 AM...02...19...15...RMS to survey point
   Tue  11:41 AM...02...19...30...OBSS survey (SSRMS assists with TV)
   Tue  01:16 PM...02...21...05...MPLM leak checks
   Tue  01:21 PM...02...21...10...MPLM activation
   Tue  02:01 PM...02...21...50...Spacewalk tools prepped for use
   Tue  03:21 PM...02...23...10...The astronauts enter the MPLM
   Tue  03:36 PM...02...23...25...SAFER jet packs transferred
   Tue  03:51 PM...02...23...40...Spacewalk procedures review
   Tue  05:11 PM...03...01...00...SAFER checkout
   Tue  08:11 PM...03...04...00...Crew sleep begins

A LONG-AWAITED SPACEWALK TO TEST TILE REPAIR TECHNIQUES

As of this writing, NASA managers continue to debate the sequence of the three planned spacewalks and the relative priorities of the tasks. The current plan calls for Robinson and Noguchi to test tile repair procedures during the first spacewalk on Flight Day 5, even if those procedures are not formally certified and even if questions remain about whether the techniques will work as planned. The plan calls for the spacewalkers to install the new control moment gyroscope during the second spacewalk and to attach a space station tool kit known as ESP-2 during the third excursion.

"Our current requirement puts (the tile repair demonstration) as one of the highest priority objectives for this mission," Hill said. "Because there's so much focus on the ET foam and the health of the TPS (thermal protection system) after we've made it into orbit, one of the things we've talked about is, if we really thought there was a risk we'd have to repair tile, our interest in getting the (tile repair tests) done goes up even more because we know that even when we resolve the technical issues with tile repair, it's still going to be a difficult job.

"Since we haven't repaired tile in orbit and we don't have the ability to do all of it in a full up orbit-like environment, except when we're in orbit, if we thought there was a significant risk of needing a repair, we absolutely would want to be out on EVA-1 with this crew practicing and confirming to us the tools will work and we really can put this material into tile and it really will cure in orbit. In this case, the test would be important as practice for an actual repair.

"But that aside, from a nominal perspective our mission priorities have this DTO as one of the highest objectives of the mission."

But those priorities could change if the shuttle encounters a problem with some other system that would require the crew to make an early landing and NASA managers are still discussing what to do if the crew only has time to carry out a single spacewalk.

Flight Day 5 highlights:

   DAY..EDT........DD...HH...MM...EVENT
   
   05/18/05
   Wed  04:11 AM...03...12...00...STS crew wakeup
   Wed  04:41 AM...03...12...30...ISS crew wakeup
   Wed  05:26 AM...03...13...15...EVA-1: EVA preparations begin
   Wed  07:41 AM...03...15...30...Equipment transfer operations resume
   Wed  08:11 AM...03...16...00...SSRMS re-grapples a fixture on the lab module
   Wed  08:56 AM...03...16...45...SSRMS ungrapples the mobile base system
   Wed  09:31 AM...03...17...20...EVA-1: Airlock depressurization
   Wed  10:16 AM...03...18...05...EVA-1: Airlock egress
   Wed  11:21 AM...03...19...10...EVA-1: Payload bay equipment/tools setup
   Wed  12:16 PM...03...20...05...EVA-1: Tile sample repair demo
   Wed  02:46 PM...03...22...35...EVA-1: Payload bay cleanup
   Wed  03:46 PM...03...23...35...EVA-1: Airlock ingress
   Wed  04:46 PM...04...00...35...EVA-1: Airlock repressurization
   Wed  08:11 PM...04...04...00...STS/ISS crew sleep begins

After leaving the station's Quest airlock module, Robinson and Noguchi will float toward the rear of the cargo bay where they'll open up the tile repair demonstration kit. Using a sort of high-tech caulk gun, the astronauts will take turns pumping thick, viscous silicon-based tile repair material into the cracks and blemishes to form a relatively smooth surface. The material is designed to cure, or harden, in the vacuum of space to restore the ability of damaged tiles to withstand the heat of re-entry.

The astronauts may also perform rudimentary RCC repairs, although those procedures are not nearly as mature.

In recent weeks, tile repair tests have raised questions about the curing process and whether the material will, in fact, form the shield a damaged shuttle might need.

"The biggest concern we have right now ... is we have started seeing bubbling in the material in tests in vacuum," Hill said. "This is a problem we thought we had solved in May of last year. There are a couple of things that have changed and we're not sure where the bubbling is creeping in."

One theory is that air is somehow leaking into the supply canisters, which are designed to keep the repair material at vacuum. Another theory involves the applicator. In previous ground tests, the material was not subjected to the sort of pressures it will see in the gun.

"This material has little micro balloons of silica, little glass balls, in it," Hill said. "The little glass balls have air in them. One of the concerns was that as we put this material in the flight gun, the pressures we're operating at as we push it through the static mixer ... causes those little micro balloons to break and the very small quantity of air in those micro balloons, we've shown on paper would be enough to show the bubbles we're seeing."

Based on testing, engineers decided to reduce the pressure in the gun. But that effectively reduces the rate at which the material leaves the nozzle and that, in turn, could affect how the material bonds with the damaged tile. Testing continues.

On the ground, bubbling is not a serious issue because bubbles tend to rise to the surface, pop and disappear. But in the weightlessness of space, the bubbles would remain suspended in place.

"In orbit, there is some concern amongst the materials guys that as these bubbles form inside the material, they're going to coalesce together and they may form a big pocket," Hill explained. "Then during deorbit, instead of getting a nice, glassy external surface, instead we'll burn down a little bit and hit that pocket and then the plasma will be channeled down into that damage cavity and will dig that patch and then the damaged tile right out. That's the concern.

"The two main technical questions about this repair technique are one, if we can't make this bubbling go away like we thought we had, can we tolerate it? Or will it cause large voids that will end up burning through? We can't test that anywhere but in orbit. So that's question number one.

"Then the second one would be, when we extrude this into tile in zero G and in vacuum and in the orbit thermal environment where we could be swinging significantly in temperature, are we confident we're going to get a good enough bond to that powdery tile surface that the patch is not going to break loose? And the only place we can fully test both of those is in orbit."

If engineers are unable to resolve the bubbling issue, or otherwise demonstrate the techniques will work as required, "it means we go into 114 having less confidence than we had hoped we would have in this repair technique being effective," Hill said. "And it means we really need to get the test samples down from 114 ... and cut them open and look at them under the microscope and stick them in the arc jet before we can really have confidence in this tile repair technique."

In the end, NASA may not have a "certified" procedure for repairing damaged tiles before Discovery's launch period opens. Agency managers decided earlier that STS-114 could be safely launched without a certified RCC repair procedure "because we absolutely are going to have the capability to detect entry critical damage and protect the crew from entry critical damage," Hill said. "Whether we can repair or not, we'll know we have it and if necessary, we'll invoke CSCS (contingency shuttle crew support, or safe haven), launch on need and we'll rescue the crew."

A similar argument could be applied to tile repairs as well. But Parsons, Hale and other senior managers say a tile repair kit of some sort - and possibly a rudimentary RCC kit - almost certainly will be on board Discovery whether the procedures are formally certified or not.

The question is, will those repair kits meet the intent of the Columbia Accident Investigation Board? The recommendation in question calls for development of "a practicable capability to inspect and effect emergency repairs to the widest possible range of damage to the thermal protection system, including both tile and reinforced carbon carbon."

The Stafford-Covey Return to Flight Task Group, an independent panel of experts set up by outgoing NASA Administrator O'Keefe, is monitoring the agency's compliance with the CAIB's recommendations.

"The way I interpret the CAIB, I think a practicable repair technique is a requirement," said James Adamson, a member of the Stafford-Covey task force. "I don't believe it needs to be certified. It's an emergency technique for an emergency situation. I don't think it necessarily has to have completed all its testing. It has to be reasonable, doable and practicable. And I think NASA's going to have that."

A former shuttle astronaut, Adamson said it's possible "we might disagree that they have met the full intent, or goal, of the CAIB recommendation and still be OK with them deciding that it's OK to fly because of this over-arching reduction of risk.

"But it's really not our call to say the shuttle's safe to fly," he said. "We're looking at a very tiny subset of all the things NASA has to consider to fly again so we really can't be in a position of declaring the shuttle safe to fly. That's their call."

FIXING THE STATION'S GYRO SYSTEM

The Discovery astronauts will spend the day after the first spacewalk transferring more supplies and equipment to the station from the logistics module. Robinson and Noguchi will service their spacesuits and prepare the tools that will be needed for the second spacewalk to install the replacement control moment gyroscope. Two televised crew interviews are planned as the astronauts gear up for the critical station repair work.

Flight Day 6 highlights:

   DAY..EDT........DD...HH...MM...EVENT
   
   05/19/05
   Thu  04:11 AM...04...12...00...STS crew wakeup
   Thu  04:41 AM...04...12...30...ISS crew wakeup
   Thu  07:11 AM...04...15...00...Spacesuit servicing
   Thu  07:11 AM...04...15...00...Transfer operations resume
   Thu  07:41 AM...04...15...30...TPS/EVA tool transfer
   Thu  10:41 AM...04...18...30...EVA tools configured for use
   Thu  10:51 AM...04...18...40...Public Affairs event with crew
   Thu  12:11 PM...04...20...00...Joint ISS/STS  meal
   Thu  03:36 PM...04...23...25...Public Affairs event with crew
   Thu  03:56 PM...04...23...45...EVA-2: Procedures review
   Thu  08:11 PM...05...04...00...Crew sleep begins

The space station uses four massive control moment gyroscopes to maintain the lab's orientation in space without having to tap into limited supplies of on-board rocket fuel. They are housed in the Z1 truss, which was attached to the Unity module's upward-facing, or zenith hatch - hence the name - during shuttle mission STS-92 in October 2000.

Along with saving fuel, the 800-pound gyros, spinning at 6,600 rpm, allow station crews and flight controllers to reorient the outpost and keep it stable without using rocket firings that would jar sensitive microgravity experiments.

But on June 8, 2002, CMG-1 suffered a malfunction and shut down. Station astronaut Carl Walz reported hearing an unusual noise inside the Unity module. He said the noise appeared to be coming from the module's zenith area. Mission control then told Walz engineers were working an issue with a spin bearing in CMG No. 1. Walz said the noise was quite noticeable inside the module.

"We're hearing a pretty loud, audible noise, kind of a growling noise, from inside the node," Walz reported.

"It looks like we have a mechanical failure of the spin bearings on CMG-1," an astronaut in mission control replied. "It's currently spinning down right now. The growling noise is undoubtedly due to vibration."

The station's orientation, or attitude, can be controlled by just two CMGs in a worst-case scenario. And indeed, a second gyro was knocked off line last year because of trouble with a circuit breaker. But that problem was fixed during a station-based spacewalk and the gyro was returned to service.

While the overall system remains fully operational with three working gyros, NASA wants to replace CMG-1 as soon as possible to provide additional redundancy in case of subsequent failures down the road.

Flight Day 7 highlights:

   DAY..EDT........DD...HH...MM...EVENT
   
   05/20/05
   Fri  04:11 AM...05...12...00...STS crew wakeup
   Fri  04:41 AM...05...12...30...ISS crew wakeup
   Fri  05:26 AM...05...13...15...EVA-2: Preparations begin
   Fri  07:11 AM...05...15...00...Transfer operations resume
   Fri  09:31 AM...05...17...20...EVA-2: Airlock depressurization
   Fri  10:16 AM...05...18...05...EVA-2: Airlock egress
   Fri  11:26 AM...05...19...15...EVA-2: CMG removal and replacement (4:15)
   Fri  03:41 PM...05...23...30...EVA-2: Cleanup and ingress
   Fri  04:36 PM...06...00...25...EVA-2: Airlock repress
   Fri  08:11 PM...06...04...00...Crew sleep begins

The replacement CMG will be mounted on a carrier truss at the back of Discovery's cargo bay. Robinson and Noguchi first will float up to the Z1 truss, unfasten thermal blankets, disconnect electrical cables and remove CMG-1. They will maneuver it to a temporary stowage location and lock it in place. Then, using the SSRMS, the replacement CMG will be removed from the cargo bay truss and moved up to the Z1 truss for installation.

After Robinson and Noguchi complete electrical connections and re-fasten the thermal blankets, engineers in mission control will begin preparations for spinning up the new gyro. The spacewalkers, meanwhile, will move the old gyro back to the cargo bay truss and lock it down for return to Earth. If all goes well, the new unit will be spun up while they are still in the cargo bay.

The gyroscopes are critical to station operation. Here's a description from a NASA press kit:

The motion control subsystem (MCS) hardware launched as part of the Z1 element includes the CMGs and the CMG assemblies.

The CMG assembly consists of four CMGs and a micrometeorite/orbital debris shield. The four CMGs, which will control the attitude of the ISS, have a spherical momentum storage capability of 14,000 ft-lb/sec, the scalar sum of the individual CMG wheel moments. The momentum stored in the CMG system at any given time equals the vector sum of the individual CMG momentum vectors.

To maintain the ISS in the desired attitude, the CMG system must cancel, or absorb, the momentum generated by the disturbance torques acting on the station. If the average disturbance torque is nonzero, the resulting CMG output torque is also nonzero, and momentum builds up in the CMG system. When the CMG system saturates, it is unable to generate the torque required to cancel the disturbance torque, which results in the loss of attitude control.

The CMG system saturates when momentum vectors have become parallel and only momentum vectors change. When this happens, control torques perpendicular to this parallel line are possible, and controllability about the parallel line is lost.

Russian segment thrusters are used to desaturate the CMGs.

An ISS CMG consists of a large flat wheel that rotates at a constant speed (6,600 rpm) and develops an angular momentum of 3,500 ft-lb/sec about its spin axis. This rotating wheel is mounted in a two-degree-of-freedom gimbal system that can point the spin axis (momentum vector) of the wheel in any direction.

At least two CMGs are needed to provide attitude control. The CMG generates an output reaction torque that is applied to the ISS by inertially changing the direction of its wheel momentum. The CMG's output torque has two components, one proportional to the rate of change of the CMG gimbals and a second proportional to the inertial body rate of the ISS as sensed at the CMG base. Because the momentum along the direction of the spin axis is fixed, the output torque is constrained to lie in the plane of the wheel. That is why one CMG cannot provide the three-axis torque needed to control the attitude of the ISS.

Each CMG has a thermostatically controlled survival heater to keep it within thermal limits before the CMGs are activated on Mission 5A. The heaters are rated at 120 watts and have an operating temperature range of -42 to -35ˇF.

READYING THE STATION FOR CONTINUED ASSEMBLY

The day after installing the new gyroscope, the astronauts will enjoy a half-day off, share a joint crew meal and hold a traditional in-flight news conference. Krikalev and Noguchi also will participate in separate news conferences with reporters from their own countries. Supply and equipment transfers to and from the logistics module will continue and Robinson and Noguchi will configure their tools and service their spacesuits for their third and final excursion the following day.

Flight Day 8 highlights:

   DAY..EDT........DD...HH...MM...EVENT
   
   05/21/05
   Sat  04:11 AM...06...12...00...STS crew wakeup
   Sat  04:41 AM...06...12...30...ISS crew wakeup
   Sat  07:11 AM...06...15...00...Transfer operations resume
   Sat  08:46 AM...06...16...35...EVA tools configured for use
   Sat  11:21 AM...06...19...10...Crew photo
   Sat  11:36 AM...06...19...25...Crew news conference
   Sat  12:11 PM...06...20...00...Joint ISS/STS meal
   Sat  01:11 PM...06...21...00...Crew off duty time begins
   Sat  01:16 PM...06...21...05...Japanese Public Affairs event
   Sat  02:51 PM...06...22...40...Russian Public Affairs event
   Sat  04:11 PM...07...00...00...EVA-3: Procedures review
   Sat  08:11 PM...07...04...00...Crew sleep begins

The primary objective of the third spacewalk is installation of the external stowage platform, or ESP-2. Tipping the scales at 6,300 pounds, ESP-2 will be pre-packed with critical equipment needed for extensive station re-wiring during upcoming assembly missions when the lab's huge solar arrays will be attached to the currently unfinished boom. Robinson and Noguchi also will retrieve two materials science experiment packages used to expose various materials to the space environment and install a third.

The station's robot arm will be used to pull ESP-2 from its mounting in Discovery's cargo bay. The box then will be maneuvered up to the Quest airlock module on the starboard side of the Unity node for attachment by the spacewalkers. The SSRMS, meanwhile, will lock onto the logistics module attached to Unity's nadir port to set the stage for its detachment the next day.

Flight Day 9 highlights:

   DAY..EDT........DD...HH...MM...EVENT
   
   05/22/05
   Sun  04:11 AM...07...12...00...STS crew wakeup
   Sun  04:41 AM...07...12...30...ISS crew wakeup
   Sun  05:26 AM...07...13...15...EVA-3: EVA preparations begin
   Sun  07:41 AM...07...15...30...Transfer operations resume
   Sun  09:31 AM...07...17...20...EVA-3: Airlock depressurization
   Sun  10:16 AM...07...18...05...EVA-3: Airlock egress
   Sun  11:01 AM...07...18...50...EVA-3: Primary cable routing
   Sun  11:46 AM...07...19...35...EVA-3: MISSE 1 & 2 retrieval   
   Sun  12:11 PM...07...20...00...SSRMS ESP-2 grapple
   Sun  12:46 PM...07...20...35...EVA-3: MISSE 6 installation
   Sun  12:56 PM...07...20...45...SSRMS ESP-2 unberth
   Sun  01:46 PM...07...21...35...EVA-3: Secondary cable routing
   Sun  02:46 PM...07...22...35...EVA-3: ESP-2 installation
   Sun  03:11 PM...07...23...00...SSRMS ESP-2 ungrapple
   Sun  03:26 PM...07...23...15...SSRMS maneuver to MPLM
   Sun  03:56 PM...07...23...45...SSRMS grapples MPLM
   Sun  04:11 PM...08...00...00...EVA-3: Airlock ingress
   Sun  04:36 PM...08...00...25...EVA-3: Airlock repressurization
   Sun  08:11 PM...08...04...00...Crew sleep begins

The next day, the astronauts will complete their final logistics module equipment transfers, deactivate the module's systems and remove it from Unity using the station's robot arm. After re-berthing the module in the shuttle's cargo bay, the SSRMS will lock onto the end of the OBSS boom, take it from the shuttle's arm and put it back in the payload bay for return to Earth. Engineers hope to eventually mount the OBSS boom permanently on the space station, after assembly reaches the point where clearance issues become a problem.

Flight Day 10 highlights:

   DAY..EDT........DD...HH...MM...EVENT
      
   05/23/05
   Mon  04:11 AM...08...12...00...STS crew wakeup
   Mon  04:41 AM...08...12...30...ISS crew wakeup
   Mon  07:11 AM...08...15...00...MPLM deactivation
   Mon  09:01 AM...08...16...50...MPLM vestibule depressurized
   Mon  09:26 AM...08...17...15...Public Affairs event
   Mon  10:46 AM...08...18...35...EVA tools stowed
   Mon  12:16 PM...08...20...05...MPLM removed from Unity
   Mon  12:56 PM...08...20...45...MPLM mounted in shuttle cargo bay
   Mon  01:46 PM...08...21...35...Rendezvous tools checkout
   Mon  02:01 PM...08...21...50...SSRMS ungrapples MPLM
   Mon  03:31 PM...08...23...20...SSRMS takes OBSS from RMS
   Mon  04:26 PM...09...00...15...SSRMS berths OBSS
   Mon  05:06 PM...09...00...55...SSRMS ungrapples OBSS
   Mon  08:11 PM...09...04...00...Crew sleep begins

DISCOVERY RETURNS TO EARTH

In what promises to be an emotional day for the shuttle-station teams in orbit and on the ground, Collins and her crewmates will bid farewell to Krikalev and Phillips during a departure ceremony shortly before Discovery undocks from the lab complex on the 11th day of the mission.

Flight Day 11 highlights:

   DAY..EDT........DD...HH...MM...EVENT
      
   05/24/05
   Tue  04:11 AM...09...12...00...STS crew wakeup
   Tue  04:41 AM...09...12...30...ISS crew wakeup
   Tue  06:26 AM...09...14...15...Farewell ceremony
   Tue  06:41 AM...09...14...30...Egress and hatch closure
   Tue  08:41 AM...09...16...30...Undocking timeline begins
   Tue  09:26 AM...09...17...15...UNDOCKING
   Tue  11:56 AM...09...19...45...STS off-duty time begins
   Tue  08:11 PM...10...04...00...STS crew sleep begins

Collins and company will spend their final full day in space testing Discovery's re-entry systems and stowing loose gear for the return to Earth. Collins and Kelly also will practice landing procedures using a laptop-based shuttle flight simulator. Discovery's KU-band antenna will be stowed in the afternoon, ending normal television views from the orbiter.

Flight Day 12 highlights:

   DAY..EDT........DD...HH...MM...EVENT
      
   05/25/05
   Wed  04:11 AM...10...12...00...Crew wakeup
   Wed  06:41 AM...10...14...30...Flight control system checkout
   Wed  06:41 AM...10...14...30...Cabin stowage begins
   Wed  07:51 AM...10...15...40...RCS hot-fire tests
   Wed  08:36 AM...10...16...25...Collins and Kelly rehearse landing procedures
   Wed  11:36 AM...10...19...25...Deorbit review
   Wed  04:11 PM...11...00...00...KU-band antenna stow
   Wed  07:11 PM...11...03...00...Crew sleep begins

And then the stage will be set for the first shuttle re-entry since Columbia's fatal fall to Earth two-and-a-half years earlier.

"We're not changing anything as far as our trajectory planning or designing," Cain said. "We know we're right down the middle where we want to be. Of course, we've looked at all of that again. (But) we're going to continue to fly the way we've flown and what we consider to be the most benign entry profile we can do."

The shuttle's re-entry trajectory will carry the ship over the south Pacific Ocean, Central America, the Gulf of Mexico and then across Florida. The WB-57 jets will be in place west and east of the region of peak heating to document the shuttle's return. Engineers are hopeful the infrared sensors carried by the jets will help characterize the super-hot plasma around the spacecraft and perhaps improve understanding of at least some of the phenomena seen in amateur video of Columbia's descent.

But video from the modified WB-57 bombers will not be available until the day after landing.

In the wake of the Columbia mishap, NASA conducted studies to determine what risk a returning shuttle posed to the public in the event of another Columbia-class breakup at high altitude. Columbia's debris "footprint" was enormous, stretching almost all the way across Texas and into Louisiana. No one was injured by falling debris, but there were numerous close calls.

Based on population centers and the shuttle's ground track when returning from the space station, agency planners concluded the safest landing site, from a public risk perspective, was the Kennedy Space Center in Florida.

"We're going to plan to land at KSC, that's our prime landing site," Cain said. "Of course, our first line of defense and our prime rationale for flying to begin with is fixing the tank and the orbiter mods and the inspection and repair capability. We think that that rationale bolsters our ability to get back to KSC from a public risk standpoint.

If the weather or some other issue prevents a Florida landing, NASA will fall back on Edwards Air Force Base, Calif., and a backup site - Northrup Strip - at White Sands, N.M.

"The only difference is that if you look at the expectation of casualty information, there are some areas for Edwards and one specific area for Northrup where the public risk assessment is a little bit higher than the highest KSC entry approach. In other words, what was said fundamentally is any and all approaches into KSC are at a risk level that's equitable and acceptable from an agency policy standpoint. If you look at all the opportunities for all cross ranges to KSC and you look at all the areas that you're overflying during the entry ... it's all acceptable.

"When you go and plot that out for Edwards and Northrup, there are some areas that poke up above that line that defines the highest risk into KSC," Cain said. "You're going to have some cases where you're flying over the LA basin. And then even for Northrup, you've got a couple of cases where you fly over Mexico City or even the LA basin.

"So what we've said is, if I can't get into KSC for weather or whatever and I'm going to land at Edwards or Northrup, we're going to give consideration to not utilizing those approaches that have those higher public risk estimates that poke out above that line."

Flight Day 13 highlights:

   DAY..EDT........DD...HH...MM...EVENT
      
   05/26/05
   Thu  03:11 AM...11...11...00...Crew wakeup
   Thu  05:06 AM...11...12...55...Group B computer powerup
   Thu  05:21 AM...11...13...10...Inertial measurement unit alignment
   Thu  06:04 AM...11...13...53...Deorbit timeline begins
   Thu  10:04 AM...11...17...53...Deorbit ignition
   Thu  11:07 AM...11...18...56...Landing

The STS-114 timeline posted in this story was current as of the end of November. It inevitably will change in the weeks and months ahead as engineers tweak their planning and press ahead with work to perfect tile repair procedures. But mission managers are optimistic NASA has finally turned the corner and that Discovery will, in fact, get off the ground during the first or second launch window in 2005.

"We've made a lot of progress in the last several months," Hale said. "We've been doing major work in a lot of different areas for return to flight. ... We are beginning, I think, to really converge on how to operate as a team and make effective decisions so that we can ensure we have a safe space flight."

Even so, Hale stressed that NASA will not catch "go fever" and launch Discovery before it's safe to fly.

"We are going to fly when we have determined that the vehicle is ready to fly, when it is safe to fly," he said. "We're not being driven by a calendar date, we're being driven by our readiness to go fly. So when we are convinced the external tank is in a good situation, when we are convinced we have the warning devices, the OBSS and all those other things, wing leading edge sensors all installed, checked out and ready to go, when we are convinced we have an adequate repair capability, then we'll go fly."

07:00 p.m., 12/06/04, Update: NASA optimistic about resumption of shuttle flights next spring; certified tile, RCC repair techniques may not be in place

KENNEDY SPACE CENTER, Fla. (CBS) - NASA remains on track for launching the shuttle Discovery on the first post-Columbia mission next May or June, senior agency managers said today. While certified techniques for on-orbit repair of tile or wing leading edge damage may not be available by then, efforts to minimize foam debris impacts, coupled with a variety of other safety upgrades and the "safe haven" provided by the space station, give managers confidence the shuttle can safely return to flight sometime next spring.

The next available launch window for mission STS-114 opens May 12 and closes June 3. Engineers at the Kennedy Space Center currently are in the process of stacking the ship's twin solid-fuel boosters and installing three hydrogen-fueled main engines. Discovery's external fuel tank, the subject of extensive insulation design changes in the wake of the 2003 Columbia disaster, is scheduled to arrive at the spaceport around Jan. 5.

Columbia was destroyed because of wing leading edge damage caused by a chunk of insulating foam that broke off the ship's external fuel tank during launch. During entry, hot gas ate its way into the left wing's interior, triggering structural failure 37 miles above Texas.

The foam in question, used to insulate the fittings that help attach a shuttle's nose to the tank, has been removed and replaced with heaters to prevent dangerous ice buildups. Other changes have been implemented to minimize roam shedding from other areas of the huge tank.

"I think we have done a very thorough job of working our way through the understanding of how debris is liberated, understanding what sizes will come off, how that debris is transported through the air stream and how it might impact the orbiter," William Parsons, shuttle program manager at the Johnson Space Center in Houston, told reporters today. "So I'm feeling very confident we have our arms around this.

"But what we have to do is, we have to verify that. And the best way to verify that is to fly a mission. So we're going to go fly a couple of test missions and we're going to take a lot of imagery and we're going to do a lot of inspections and we're going to have sensors in place so that we can verify what we feel very confident that we've accomplished."

Wayne Hale, a former flight director who now serves as Parsons' deputy and chairman of the mission management team, said today the requirement is for nothing to come off the tank that weighs more than 0.03 pounds.

"All our investigations of the foam indicate we will not get a piece of foam coming off bigger than .008, eight one thousandths (of a pound), or almost an order of magnitude smaller than the requirement," he said. "Now, does that mean we won't see a chip in a tile here or there? I would say we'd be optimistic if we said that.

"But we're clearly moving toward an area where we expect to see much less damage in the tile and no critical damage that will require a repair. So that's our goal in this and it's beginning to look very positive that we'll be able to accomplish that level of control on the ET foam."

Parsons said engineers are still wrestling with one aspect of the tank's redesigned insulation - an area around a bellows in a liquid oxygen feed line - but he is optimistic the tank will ship on time. He said any additional work in that area can be performed at Kennedy.

Along with minimizing potential debris, NASA has installed sensors behind the shuttle's wing leading edge panels that will detect any significant impacts. Upgraded cameras on the ground and aboard the shuttle will document whatever foam shedding does occur and the astronauts will carry out an exhaustive leading edge inspection in space using cameras and laser sensors mounted on a long boom.

But NASA is struggling to perfect techniques for spacewalking astronauts to repair tile or leading edge damage in space in the event such damage actually occurs.

Agency managers earlier said certified repair techniques for reinforced carbon carbon leading edge panels likely would not be available in time for STS-114. Such repair techniques were not considered a requirement for flight. That was due in large part to a new capability for the crew of a stricken shuttle to use the space station as a "safe haven" until a rescue flight could be launched.

Today, Parsons and Hale said the same philosophy will apply to tile repair. While engineers are continuing efforts to perfect repair techniques, and while two astronauts will carry out tests of those procedures, a certified methodology is not a pre-launch requirement for STS-114.

"When we started this, we knew it would be extremely difficult to do a repair technique both on tile and on RCC," Parsons said. "I think we said that from the very beginning. The CAIB (Columbia Accident Investigation Board) said to do the best that we could to come up with some technique to repair RCC and repair tile. We have done that. We have continued to put the best and brightest this agency has to offer on this, we've used every resource this agency and this nation has to work on this, we've made a lot of progress."

In recent weeks, engineers have raised questions about the possibility of air bubbles forming in the tile repair material that could degrade its performance during entry.

"We think there are some issues that we still need to resolve, but we still have some time to go resolve that," Parsons said. "And then we're going to go fly a test mission. We're going to have a detailed (repair demonstration spacewalk) in the payload bay of the orbiter. ... We can do a lot of things on the ground, but we have to take them on orbit, put them in work on orbit, bring them back and then see if they work as well as we think they'll work.

"In a state of emergency, we would have a technique that we would be ready to perform (during STS-114)," he said. "But right at this moment, we're still working through some of the technical details on how to do good tile repair and RCC repair."

Pressed to say whether on-orbit repair capability of both RCC and tile were, or were not, launch constraints, Hale said "The Columbia Accident Investigation Board did not say it was a requirement."

Parsons added, "but they did say to work on it and they did say to do the best that we could and I think what Wayne's telling you is we have done the best that we could. We have made a lot of progress and we think we'll have something there that we can use and test on STS-114."

For the record, the Columbia Accident Investigation Board said that for missions to the international space station, NASA should "develop a practicable capability to inspect and effect emergency repairs to the widest possible range of damage to the thermal protection system, including both tile and reinforced carbon carbon, taking advantage of the additional capabilities available when near to or docked at the international space station."

Hale stressed that NASA will not be driven by target launch dates and that Discovery will not fly until the agency's management team is convinced it is safe to do so.

"We are going to fly when we have determined that the vehicle is ready to fly, when it is safe to fly," he stressed. "We're not being driven by a calendar date, we're being driven by our readiness to go fly. So when we are convinced the external tank is in a good situation, when we are convinced we have the warning devices, the OBSS (inspection boom) and all those other things, wing leading edge sensors, all installed, checked out and ready to go, when we are convinced we have an adequate repair capability, then we'll go fly.

"The date will kind of be what the date is," he added. "We're not going to succumb to some kind of emotional schedule."

08:45 a.m., 12/03/04, Update: NASA lifts post-9/11 launch time security restrictions

NASA has quietly lifted post-9/11 security restrictions that included keeping shuttle launch times secret until the day before liftoff. NASA spokesman Michael Braukus said today the agency's revised policy governing what NASA tells the news media and he public in advance of shuttle processing milestones mirrors procedures in effect prior to the Sept. 11, 2001, terrorists attacks in New York and Washington.

"It's because our security level is lower and our security posture is better than it was prior to 9/11," he said by telephone from NASA headquarters in Washington.

For the record, and assuming NASA can complete post-Columbia safety upgrades in time, the target launch time for the next shuttle mission, STS-114, is approximately 4:11 p.m. on May 14, based on the projected orbit of the international space station. The actual launch period opens May 12, but post-Columbia lighting constraints for photo documentation are not met until two days later.

In the wake of the 9/11 terrorists attacks, NASA managers imposed unprecedented security procedures similar in many respects to those once employed for classified military flights.

Starting with mission STS-110 in April 2002, NASA stopped announcing when a shuttle countdown would begin and when a crew would arrive at the Kennedy Space Center for final preparations. Broad shuttle launch windows were announced in advance, but the exact launch times were not revealed until the day before liftoff. News organizations using widely available satellite tracking software capable of determining launch times with fair accuracy agreed not to publish the information until the agency made its formal announcement.

In addition, shuttle processing milestones that normally were open to the media - the terminal countdown demonstration test at the launch pad, for example - were closed and crew itineraries were no longer announced in advance.

Braukus said today procedures in effect for news coverage of STS-114 will be virtually identical to those in place prior to 9/11. But agency officials reserved the right to impose stricter procedures on a case-by-case basis and it remains to be seen what, if any, policy changes may be in the works for international journalists, non-NASA VIPs and other "special guests" of the agency.

06:40 p.m., 04/30/04, Update: Hale defends Hubble decision; says NASA on track for March return to flight

NASA's latest return-to-flight implementation plan for the first time codifies an earlier decision by NASA Administrator Sean O'Keefe ruling out any non-space station flights, like one to save the Hubble Space Telescope, after shuttle flights resume next spring.

Such a flight would require development of an autonomous on-orbit shuttle repair capability, one that would not require the support or safe haven provided by the international space station. While the board that investigated the Columbia disaster said NASA's "ultimate objective" should be development of such a capability in case of problems that prevent a station docking, for example, deputy shuttle program manager Wayne Hale said today it will be quite a while before any such stand-alone repair options are available.

"Right now, we are really concentrating on return to flight," Hale said in an afternoon teleconference with reporters. "Clearly, if you want to repair something on the underside of the orbiter, you have got to have a boom or a flight system (to reach the damage site). We're investigating both of those."

But an ability to carry out such repairs in the absence of the space station "is farther downstream than the first two flights, which we've deemed test flights," he said. "So it's in the future ahead of us. We don't believe we're going to have a full-up capability for autonomous repair (by return to flight). That's got a lot of challenges ahead of it."

The Columbia Accident Investigation Board, in recommendation 6.4-1, said NASA must, for non-station missions, "develop a comprehensive autonomous (independent of station) inspection and repair capability to cover the widest possible range of damage scenarios."

Recommendation 6.4-1 concludes by saying "the ultimate objective should be a fully autonomous capability for all missions to address the possibility that an international space station mission fails to achieve the correct orbit, fails to dock successfully or is damaged during or after undocking."

The recommendation was one of more than two dozen that must be implemented before shuttles can return to flight.

Only one non-station flight remained on NASA's books in the aftermath of Columbia: A long-planned, final flight to overhaul and upgrade the Hubble Space Telescope. But in January, two days after President Bush unveiled his moon-Mars initiative, O'Keefe canceled the Hubble servicing mission.

O'Keefe said a Hubble visit would be inherently riskier than a flight to the station, where a crew could more easily make repairs or, in a worst-case scenario, camp out until another shuttle could be launched on a rescue mission. The administrator said in that context, it didn't make sense to devote precious resources to the development of complex repair techniques and equipment that would only be used once.

The scientific community, of course, disagreed. Recommendation 6.4-1, after all, required development of just such capabilities to cover scenarios in which an engine failure during launch or some other problem prevented a docking with the space station.

But O'Keefe has stood firm and today, the agency wrote the decision into its formal return-to-flight implementation plan, saying "there are additional risks associated with creating and deploying a fully autonomous inspection capability without ISS (international space station) resources."

"Therefore, NASA has decided to focus its development of TPS (thermal protection system) inspection and repair on those capabilities that enhance the shuttle's suite of assessment and repair tools while taking full advantage of ISS resources."

The implementation plan goes on to say NASA will focus its efforts on "mitigating the risk of multiple failures" that might result in a failed docking or damage after undocking. Those efforts will include "maximizing the shuttle's ascent performance margins to achieve ISS orbit, using the docked configuration to maximize inspection and repair capabilities and flying protective attitudes following undocking from the ISS."

How NASA might maximize "ascent performance," i.e., how a shuttle carrying a heavy space station component could reach the lab's altitude after an early engine failure, for example, was left unsaid. But NASA management believes the odds of multiple failures that would both prevent a station docking and require emergency repairs in orbit are sufficiently remote.

While the implementation plan says the agency "will continue to analyze the relative merit of different approaches," there appears to be little chance NASA will reconsider autonomous shuttle flights without direct orders from Congress. A congressionally mandated study assessing the relative risks associated with station and non-station flights is currently underway. In the meantime, NASA laid out its argument, listing five reasons why a non-station flight should not be considered:

1. Lack of Significant Safe Haven
   
   
2. Unprecedented Double Workload for Ground Launch and Processing Teams. Without a safe haven, the report said, shuttle teams would have to process two shuttles at once in case an emergency rescue was required.
3. No Changes to Cargo or Vehicle Feasible. Engineers would not have time to change out the second shuttle's cargo, the report said, or even modify it significantly in time to support a rescue mission. "The whole process would be under acute schedule pressure and undoubtedly many safety and operations waivers would be required."
4. Rescue Mission. Developing safe, credible ship-to-ship rescue techniques would be difficult and require extensive analysis, the report said.
5. Tile Survey (expanded inspection requirements) and Thermal Protection System Repair. Without the station, NASA says, inspecting and repairing critical areas using the shuttle's limited-reach robot arm would be difficult. "Such a concept represents a challenging undertaking, which could take months or years to develop to meet safety and mission assurance standards and requirements."

Even Hale, a widely respected flight director and no stranger to high-risk decisions, said today he agreed with O'Keefe's reasoning.

"It is clear, if you study on it, that autonomous flight, that is to say, not having the station available with its resources to help you repair or give you options for safe haven or extending a flight on orbit to allow people to think about how to fix a problem, any autonomous flight you take is clearly riskier than a flight that you take to the station where you have friends that have air and electricity and food and water and all the necessary means to hang out and give you options to fix the problem," he said. "That's just common sense.

"You have to ask yourself, is the risk worth the gain? I've been thinking an awful lot about Hubble. Hubble is clearly the premier scientific instrument this agency has ever launched. But you know, we've got another telescope in the pipe coming down in 2011. ... So the question you have to ask yourself is, is the risk worth the gain? I have to make tough risk calls every day at my level. I would not want to take this one away from (O'Keefe). I certainly don't disagree with his assessment. He's made an informed decision."

Hale said NASA is well on the way to implementing the CAIB's return-to-flight recommendations and that as of today, the program remains on track for meeting the current March 2005 launch target for mission STS-114. He said engineers have made good progress developing an instrumented boom that will allow astronauts to inspect the shuttle for damage and in developing inspection and repair techniques utilizing the space station.

He said a tile repair capability should be in place before the current launch date and that engineers have made solid progress developing techniques for repairing damage to the reinforced carbon carbon panels protecting the shuttle's wing leading edges. It was a breach in the leading edge of Columbia's left wing that destroyed the orbiter during re-entry last year.

"I've said it before and I'll repeat it, there were many people who told us when we started out to look at RCC repair on orbit that it was impossible task," Hale said. "We have identified a caulk-like material ... that we believe would repair a crack or a small hole, which we think would be the most likely kind of damage to occur. ... We have a plug concept that is being refined that we think will also cover us for holes up to 4 inches in diameter. That's not quite as mature, but we feel pretty confident that we'll have that capability for the first flight as well. So I think we're making progress that far exceeds what anybody thought we could do when we started this repair of reinforced carbon carbon last year."

In recent weeks, engineers have run into problems with shuttle wiring, trouble with cracks, corrosion and improperly installed gears in the ship's rudder-speedbrake and concern about cracks in flexible hoses used throughout the orbiter. But Hale downplayed those issues, saying they were not unusual and that engineers had time to carry out inspections and, where necessary, repairs.

"The orbiters are approaching 20 years old, the oldest one, and we have been paying a tremendous amount of attention to the aviation industry and what they've been dealing with, what I would call 'aging aircraft' issues," he said. "We have talked before about wiring and the issues that come along with wiring. We have talked to you about ... some of the interesting things we've found out in the gearboxes we use to control the rudder of the space shuttle. Those are the kinds of things that we have been facing for the last 10 years on the shuttle and we expect to continue to face as we continue to fly to the end of the program. They are not extraordinary and they're not going to hold us up. We have a plan in place for every one of these items that will get us to a safe place to fly.

"The real constraint to return to flight is clearly, No. 1, fixing the external tank," he said. "That has been the pacing item from the very beginning. We have learned an awful lot about foam, the material properties of foam insulation, how it adheres to complex geometries, what happens to it during aerodynamic heating and the aerodynamic loads that you can encounter at supersonic flight. And our focus has been on fixing the tank so that no size, no critical size foam can come off of it. That's the pacing item.

"All of these other items, and I would even include the tile and RCC repair, are things that are falling under the umbrella of fixing the tank. Right now, our launch date is based on fixing the tank and having some management reserve in that schedule for any surprises that we may encounter as we go down that path to make a launch in the spring."

Hale dismissed talk suggesting the shuttle might never fly again because of mounting technical problems. Trouble with flex hoses, wiring, etc., "are all kind of normal-course-of-affairs things we would deal with if we're flying or we're not flying," Hale said. "The big question is can we fix the problem that occurred to us on (Columbia), can we provide a risk mitigation to that fix in the way of inspection and repair and so forth that allow us to feel safe to go fly? We're going to work what I would call normal daily business until the last day we fly the last shuttle. And that's not going to hold us up."

01:00 p.m., 02/20/04, Update: External tank modifications more complex than expected; some launch constraints may be relaxed later

After months of testing and computer analyses, engineers believe they understand the phenomena that causes foam insulation to separate from the space shuttle's external fuel tank during launch. But preventing such foam shedding has turned out to be more difficult than originally thought, a top NASA manager said today, and tank modifications remain a major challenge.

Bill Readdy, NASA's associate administrator for spaceflight, said recertifying the external tank was at the root of a decision Thursday by senior managers to delay the first post-Columbia mission from September to March 2005.

"Return to flight has always been driven by fixing the tank because the tank was clearly the cause of the (Columbia) accident," Readdy told reporters. "An awful lot of what has been going on has been analysis of debris transport, how things that come off the tank (and are) transported from some area of the tank and then impinge on some other portion of the space shuttle stack.

"We're most concerned about the orbiter and aboard the orbiter, we're most concerned about the leading edge (of the wing), although there are more delicate areas of the thermal protection system around the mail landing gear wells and the body flap area."

A large piece of foam insulation broke away from the shuttle Columbia's external tank during launch last year and struck the leading edge of the ship's left wing. The impact blasted a large hole in the reinforced carbon carbon material used to protect the leading edge from entry temperatures of 3,000 degrees. During the descent to Earth 16 days later, hot gas burned its way into the left wing and triggered the shuttle's destruction.

At the time of Columbia's launch, engineers did not believe large pieces of foam from the external tank could strike the orbiter, relying on a debris transport model that indicated such debris would peel away and fall down along the tank without entering the complex airflow between the shuttle and the tank.

They now believe the mechanism responsible for foam shedding causes pieces of insulation to be blown away from the surface of the tank, out into the main airflow where it quickly decelerates. If the debris comes from an area toward the top of the tank, the shuttle can ram into it at a high relative velocity.

Prior to and during Columbia's flight, "we were able to run 400 or so detailed runs of simulations using computational fluid dynamics models," Readdy said. "Since that time, with the improved models and improved computing power we now have, we have been able to run millions of runs, literally, and characterized the debris transport mechanism.

"And what that has done has caused us to look at how the insulation is applied to the tank, where it's applied, where it had been coming off. We were able to review an awful lot of the photography we had of the external tank previously and as a result, levied new requirements on return to flight for the external tank and recertification of the tank for flight. So the external tank has certainly been a pacing item."

At the time of Columbia's destruction, NASA engineers believed a phenomenon known as cryopumping was responsible for foam shedding. When the external tank is fueled for launch, air trapped in voids in the insulation or near the skin of the tank can turn into a liquid. As the shuttle rockets away, aerodynamic heating can cause that trapped liquid to turn back into a gas. The pressure generated by that phase transition, it was believed, could blow overlying pieces of foam away from the tank.

The Columbia Accident Investigation Board concluded such cryopumping alone could not explain the separation of the suitcase-size chunk of debris that doomed Columbia. But Readdy said today additional testing shows a different type of cryopumping can, in fact, cause such shedding.

"We've found out that the bolts and the nuts being applied to actually construct the different areas of the tank ... before you put the insulation on, that any kind of gap in there might be an opportunity for liquid nitrogen or liquid air to form," he said. "And what happens is, during the ascent environment, when the shock waves form on the external tank, aerodynamic heating and friction occurs and as a result, even trapped air kind of expands."

The expansion of that trapped air "imparts a velocity to that particular piece that causes large pieces to come off and instead of (peeling) away from the tank, actually being pushed away from the tank due to that gas pressure behind it," Readdy said. "That is really the root cause we've been able to discover here.

"And part of the new design is to change the bolt configuration, to actually close those areas out so that there is no opportunity for the liquid nitrogen or liquid air to form and close out a whole number of other areas. The other thing is characterizing the condition of the foam application more carefully so we have a much more controlled environment, not only in terms of the humidity that we're able to apply this foam, but the rate at which the foam must be applied, the surfaces near it, a whole number of other factors."

The foam that doomed Columbia tore away from the so-called left bipod ramp, an aerodynamic wedge of insulation covering the fitting used to attach one of the shuttle's two forward attachment struts. The ramps, in place to prevent ice buildups on the attachment fittings, have been eliminated in favor of electric heaters.

Foam shedding is the major driver behind virtually all of the constraints facing the first post-Columbia flight.

Launch windows are severely constrained by requirements for launch and external tank separation to occur in daylight so engineers can visually inspect the tank and the orbiter. A boom-mounted sensor package is being developed so the astronauts can inspect the underside of the shuttle for signs of damage. Repair techniques are being developed to fix any such damage that might occur.

Based on testing in which foam "bullets" are fired at tile and RCC panels, engineers have concluded the largest allowable debris is .04 pounds. Nothing larger will be permitted from a region 75 degrees to either side of the tank's centerline on the side facing the shuttle.

"It does vary somewhat by location," Readdy said. "Obviously, the further toward the nose of the external tank it is, the smaller the allowable it is. ... It also depends on how far around it is on the external tank. But I think the number is .04 pounds. In terms of foam, though, that's still a fairly large piece."

Playing it safe, NASA managers recently decided to process a second shuttle in parallel to carry out a rescue mission in the event of damage that can't be repaired (see the Feb. 19 update for complete details).

Readdy said the first two post-Columbia missions should be viewed as test flights. If foam shedding is, in fact, reduced to the desired level, it's possible at least some of the current constraints, including the requirement for daylight launches, could be relaxed.

"It's a constraint we hope we won't have to live with for the life of the program, that being daylight launch, daylight external tank separation," Readdy said. "Because we're also developing radar techniques that may allow us to detect any kind of debris that might be separated during ascent instead of using visual cameras. ... We're also looking at other sensors we might use to document the condition of the external tank. So perhaps the daylight launch, daylight external tank separation may not be permanent constraints. ... But clearly they are here for the next two flights."

08:00 p.m., 02/19/04, Update: Shuttle launch slips to March 2005; flight assigned to Discovery; rescue mission scenario matures

The first post-Columbia shuttle mission will slip from September to March 2005 to give engineers more time to develop in-flight repair procedures and to take advantage of more favorable launch windows, officials said today. The slip also will give NASA and its contractors time to resolve problems with actuators in the shuttle's rudder and speedbrake assembly in the ship's vertical tail fin.

NASA's Spaceflight Leadership Council, a panel made up of the agency's top managers, approved the change during a meeting today at the Johnson Space Center in Houston. The panel also approved plans to assign the flight to the shuttle Discovery, targeting launch for no earlier than March 6, 2005.

At the same time, engineers continue to refine plans to have a second shuttle ready for launch on an emergency rescue mission in case something goes wrong during Discovery's flight. In that case, commander Eileen Collins and her crew could be forced to seek "safe haven" aboard the international space station until a rescue flight could be attempted.

NASA plans to process a second shuttle that could be rolled to the pad and launched within 70 days of notification, assuming a three-shift around-the-clock work flow and no waived requirements, sources said. The flow possibly could be shortened to 35 days in a crisis.

The rescue flight, known as STS-300, would be crewed by four of the six astronauts already assigned to mission STS-115, the third flight in NASA's post-Columbia launch sequence. The STS-115 crew is made up of commander Brent Jett, pilot Chris Ferguson, Joe Tanner, Dan Burbank, Steve MacLean and Heidemarie Stefanyshyn-Piper.

STS-300 would provide backup for Collins' flight, known as STS-114, and the second post-Columbia mission, STS-121. Jett, Ferguson and two other STS-115 crew members would be trained to rescue either crew.

The mission would feature a normal rendezvous and docking with the space station. The stranded crew would return to Earth strapped into recumbent seats bolted to the floor of the rescue shuttle's lower deck.

Recumbent seats are needed to prevent balance problems for astronauts returning to Earth after being weightless for more than a month. Seven such seats would be required for Collins' crew, all of them mounted in the middeck area. Studies show all 11 returning astronauts would have time to bail out, if necessary, despite the crowded conditions on the lower deck.

"It is our plan to put ourselves in a posture for return to flight, first launch, that the second vehicle will be ready to launch and go to the international space station and pick up the first crew if we had a problem with the vehicle and could not bring it down," said Michael Kostelnik, deputy associate administrator for the shuttle and station programs.

He told reporters today the space station could support a stranded shuttle crew for up to 90 days and possibly longer if required.

"For the first flight, we are going to have the capability to do this and we will have the proper software load and training for that mission such that we'd be in a position to have a crew capable of launching within the time period the station could support an expanded crew and bring down the first crew if we had a problem with the first vehicle," Kostelnik said.

"There's still a lot of planning to be done on this (but) this is the path we're going down for the first flight. Our experience for subsequent flights will be determined by our success and our problems associated with the first flight."

With Discovery now slated for the first post-Columbia mission, the shuttle Atlantis will serve as the STS-300 "launch-on-need" vehicle.

"I don't believe there's an awful lot of extra training or extra things we have to do for a rescue mission," said Bill Parsons, shuttle program manager. "Overall, it would be very similar to an STS-121 mission. It would be going to the international space station, docking, picking up crew, making sure we had the appropriate hardware and things we needed to bring that crew on board and then returning safely."

NASA managers originally said they hoped to launch the first post-Columbia flight this March or April. But last October, the target was pushed back to a launch window that opened around Sept. 12 and closed Oct. 10.

A variety of factors conspired to push the flight into 2005, including work to minimize foam shedding from the shuttle's external fuel tank and development of tile and wing leading edge repair materials and techniques. Other "long poles" include development of a camera and laser sensor package needed for in-flight inspection of the ship's thermal protection system.

Tile repair development is "going as well or better than expected," Parsons said today. Wing leading edge repair is "not as mature as the tile repair, but it is coming along."

Another complicating factor is a photo documentation requirement to have daylight lighting for both launch and external fuel tank separation to determine whether any large pieces of foam separated from the tank and, if any did, whether the shuttle suffered any damage. The lighting requirment, combined with thermal constraints at the space station, translated into very limited launch windows through early 2005. After the September window, NASA had just three available days in November, none in December and three in January.

NASA now plans to target a launch window that would open around March 6, 2005, and extend into early April. But a Russian Soyuz crew transfer flight is scheduled for launch around April 10 and flight rules forbid launching a shuttle within 13 days of that date. As a result, NASA will have until roughly the end of March to get Discovery off the ground during that window.

The STS-121 mission is tentatively targeted for launch in May 2005.

As Parsons said, developing repair techniques for the reinforced carbon carbon panels that protect the leading edges of the shuttle's wings remains a major concern. But sources say recently discovered problems with actuators in the rudder/speedbrake assembly on the shuttle could prove equally time consuming to resolve.

01:30 p.m., 12/17/03, Update: Posting initial STS-114 page

The STS-107 page has been archived and all status reports are available below in HTML format. From this point forward, the Current Mission page will focus on the next shuttle flight, STS-114. A preliminary flight plan, links to crew bios and other mission-specific data are posted below. As always, comments, suggestions and corrections are welcomed.

07:20 p.m., 10/03/03, Update: New launch target date; additional mission added

As expected, NASA today set a new, more realistic target date for launch of the next shuttle mission - between Sept. 12 and Oct. 10, 2004 - and announced a new flight that will be inserted into the launch manifest to complete tasks originally planned for the first mission.

Given post-accident requirements that sharply restrict when a shuttle can actually take off, however - launch and external tank separation in daylight and thermal constraints at the international space station - the shuttle's return to flight easily could slip into 2005 when all is said and done.

Those requirements currently translate into 26 days of launch opportunities between Sept. 16 and Oct. 11, 2004; Nov. 19-21, 2004; and Jan. 17 through 19, 2005. The next available launch window opens in March 2005.

Current estimates show photo documentation of external tank separation in daylight will not be possible until Sept. 16. But NASA managers say those estimates might change and so they added a bit of cushion to the front end of the target launch window.

Bill Readdy, NASA's associate administrator for space flight, stressed the new launch window, like the original March/April target window, remains a target only and that it may change again as planners get a better idea of how much work remains to be done.

Mission STS-114, currently assigned to the shuttle Atlantis, will include a robot arm extension and sensors to look for damage to the ship's heat-shield tiles and leading edge panels; a spacewalk to test tile and leading edge repair techniques; another spacewalk to install a new gyroscope on the space station; and supply and equipment transfers to and from the lab complex.

"We're going to be very much driven by the milestones and by the content we have to accomplish here in terms of the testing of the robotic arm, survey techniques, tile repair, modifications to the external tank, all the testing that's required," Readdy said in an evening teleconference with reporters. "So It comes down to being able to satisfactorily accomplish all of those.

"I can't tell you whether or not we're going to have more content creep in over time, whether we're going to come up on some technical hurdles. I can almost guarantee that this is going to be a long, uphill climb back to return to flight. But I also would tell you that we're getting an awful lot smarter about this and we're going to come back stronger and safer as a result."

To accommodate the tile/leading edge inspections and the repair demonstration spacewalk, NASA decided to split the original STS-114 mission into two separate flights. The new mission, known as STS-121, currently is assigned to the shuttle Discovery. It is targeted for launch Nov. 15. Again, that's a few days in advance of when a daylight tank separation because available.

Making the September launch date for the first post-Columbia mission will not be easy. Long poles include development of the robot arm extension boom and the sensors needed for in-orbit tile and leading edge inspections and development of viable techniques for repairing leading edge damage. A wild card is the impact of work to remove Atlantis' carbon composite nose cap for an extensive corrosion inspection.

But shuttle program manager Bill Parsons said it's too soon to predict how all of that might play out. As for the shuttle's sharply restricted launch windows, "I think we have some opportunities, some small opportunities in November and possibly January and some other places."

"But you know, again, we're still refining those requirements, we're still refining exactly what we need to do to meet the lit launch requirement," he said. "But I think there are some opportunities in November (2004) for us to go launch."