Realtime coverage of U.S. EVA-18
08/30/2012 07:52 PM Filed in: Space News | International Space Station
By WILLIAM HARWOOD
CBS News
07:45 PM EDT, 08/30/12: NASA assesses spacewalk repair options to complete MBSU installation
The failure to install a replacement electrical distribution box during a marathon spacewalk Thursday has temporarily left the International Space Station unable to take in power from two of its eight solar array wings. But mission managers said jumpers installed before the excursion will allow near-normal operations while engineers study possible spacewalk repair options.
"We are down to six power channels," said Flight Director Ed Van Cise. "We have flight rules in place that give us a maximum loading of kilowatts per channel, and that's what we work to balance across the channels we have available.
"The space station, now that it's fully assembled and operational, has a lot of flexibility to route power around through different means so we can be creative with how we share power across the power channels to be able to provide as much balance as we can. But that said, there are some things that do require more power and are only connected to certain power feeds."
Until the installation issue is resolved and main bus switching unit No. 1 is re-connected to the station's electrical grid, equipment that is not considered crucial will be powered down or used when scheduling permits to avoid conflicts. A robotics work station in the U.S. lab module, for example, will be powered down until it is needed. Likewise automatic heaters may be operated manually to ease the load.
Major, on-going experiments like the Alpha Magnetic Spectrometer will continue to draw power, but others that operate on a more transient basis may require tighter scheduling.
"Really, on a bigger scale, we're just balancing the loads across the six channels we have without minimizing or reducing our operational redundancy we need to actually go fly the vehicle", Van Cise said. "Obviously, we want to keep doing that world-class science that we do, we just have to be smart on which facilities we have up at a given time so we don't have two facilities that use a lot of power on the same feed."
Power generated by the station's eight solar array wings, four on each end of the lab's main truss, is routed to four main bus switching units, all located in the central S0 truss segment. Two power channels feed into each 220-pound MBSU, which in turn deliver 160-volt array power to a pair of transformers known as DC-to-DC converter units. The DDCUs step the primary power down to the 124 volts used inside the station.
The MBSUs are critical to station operation, providing grounding and allowing flight controllers or station astronauts to crosstie power channels or to isolate them as needed when problems develop.
Several months ago, MBSU No. 1 stopped responding to commands, although it still delivered electricity from power channels 1A and 1B, generated by a right-side inboard solar array and one starboard outboard panel respectively.
Replacing MBSU No. 1 was the top priority of a spacewalk Thursday by astronauts Sunita Williams and Akihiko Hoshide. They successfully removed the malfunctioning unit but were unable to install a replacement when a bolt used to pull the 220-pound device down onto a gang of electrical connectors and cooling fins would not drive home.
When all was said and done, the astronauts spent eight hours and 17 minutes outside the lab -- the third longest EVA ever conducted by NASA -- but they were forced to leave the replacement MBSU partially torqued down and not connected to power channels 1A and 1B.
"The team installed some jumpers prior to powering down the MBSU in preparation for the EVA and those jumpers allowed us to get power to all the ISS systems and payloads," said Program Manager Mike Suffredini. "So really, what the team has to do, they can get power to all the systems but we only have three quarters of the power available that we normally have.
"Depending on the task at hand over the next several days, the team may have to manage power loads a little bit, but this is familiar territory and we'll be able to deal with that while we decide what our next plan is."
The problem comes at a busy time for the station crew, with the departure of two cargo ships next month and three of the station's six crew members.
"While we think about our plan and when we might do another EVA if we decide that's necessary, we'll have to take all of that into account and move things around in order to accommodate that," Suffredini said. "But if we do decide to do an EVA ... you'll hear us talking about doing that sooner rather than later in the program."
If engineers can figure out what went wrong with the bolt on the replacement MBSU and how to fix it or work around the problem, Williams and Hoshide possibly could stage another spacewalk as early as next week.
But as of this writing, that's far from certain. For the MBSU's electrical connections and cooling fins to engage their counterparts in the truss housing, the box must be firmly seated.
Engineers will use ground spares to study possible techniques to free and then drive a possibly galled bolt without breaking it off or stripping its threads. Another option may be to partially disassemble the attachment fittings to work around the bolt.
In the meantime, Van Cise thanked Williams and Hoshide for "an awesome job today."
"Everybody down here passes on their congratulations for going way above and beyond and they expressed their thanks, all the way up to the program manager. He wanted me to specifically thank you for all that you put into making this as successful as it could be given the circumstances", Van Cise radioed.
"So thank you very much, job well done. Get some rest tonight, and I'll make sure (Flight Director) Tony (Ceccacci) gives you plenty of time to sleep in. Good job."
"Thank you very much for those words," astronaut Joseph Acaba replied from the Quest airlock, where he was helping Williams and Hoshide out of their spacesuits. "As you can see, these guys are happy, we're glad they went out and got back safe, that's what's important.
"You guys worked just as hard as we did, so thank you very much. We'll get these guys out and they can relax a little bit. Have a great night and we'll touch bases with you guys tomorrow."
04:40 PM EDT, 08/30/12: Spacewalk ends
Astronauts Sunita Williams and Akihiko Hoshide began repressurizing the space station's Quest airlock at 4:33 p.m. EDT (GMT-4) to officially conclude an eight-hour 17-minute spacewalk, NASA's first since the last shuttle flight. Thanks to problems with bolts that prevented the installation of a power distribution unit, the spacewalk was the third longest in NASA history. The record holder, a station assembly flight in 2001, lasted eight hours and 56 minutes.
For readers scoring at home, this was the 164th spacewalk devoted to station construction and maintenance since assembly began in 1998, the third so far this year, the fifth for Williams and the first for Hoshide. With this EVA, 109 astronauts and cosmonauts have logged 1,035 hours and 55 minutes -- 43.2 days -- of station EVA time.
Williams has now logged 37 hours and 34 minutes of EVA time, moving her up to 27th on the list of most experienced spacewalkers. She is second only to Peggy Whitson in total EVA time for a female astronaut. Hoshide is the third Japanese astronaut to walk in space.
An updated story will be posted here following a news conference with NASA managers to discuss the outcome of today's spacewalk and the impact of deferring installation of a power distribution unit.
03:50 PM EDT, 08/30/12: Balky bolt blocks installation of replacement power box
After repeated attempts to drive home a balky bolt needed to lock a replacement power switching unit into place on the space station's solar array truss, flight controllers told Japanese astronaut Akihiko Hoshide and fellow spacewalker Sunita Williams to call it a day, deferring installation to a future spacewalk.
Thanks to internal jumper cables installed earlier, the station's three operational main bus switching units, or MBSUs, are able to distribute solar array power throughout the lab without the need to power down any equipment. But the replacement unit is needed to restore full redundancy and to protect against possible downstream failures.
Before giving up for the day, the astronauts made one final attempt to drive the bolt home, with Hoshide wielding a power tool at a higher torque setting while Williams attempted to hold the 220-pound box level over its attachment plate. But again, the bolt jammed and rather than risk stripping it with an even higher power setting, the astronauts were told to tie the box down and to begin collecting their tools and tethers.
"It torqued out, less than one rev," Hoshide reported.
"Copy that, buddy. All right, we're going to clean things up and tie this down," replied Jack Fischer from mission control. "We'll figure this out another day."
Power generated by the station's eight solar array wings, four on each end of the lab's main truss, is routed to four main bus switching units, all located in the central S0 truss segment. Two power channels feed into each 220-pound MBSU, which in turn deliver 160-volt array power to a pair of DC-to-DC converter units. The DDCUs step the primary power down to the 124 volts used inside the station.
The MBSUs are critical to station operation, providing grounding and allowing flight controllers or station astronauts to crosstie power channels or to isolate them as needed when problems develop. MBSU's 2, 3 and 4 are operating normally. But several months ago, MBSU No. 1, while providing power normally, stopped responding to commands because of an internal problem of some sort. Replacing the device was the top priority of today's spacewalk.
But Hoshide ran into problems right off the bat when he attempted to unbolt the faulty MBSU. After an inspection, Williams reported metal shavings and signs of cross threading in one of the bolt receptacles and on the bolt that was particularly difficult to loosen. But after a bit of a struggle, the box was removed and Williams was able to lock the unit down on a storage platform as planned.
The astronauts ran into more problems bolting the replacement box in place. The initial bolt refused to run in smoothly, forcing Hoshide to try different torque settings on his power tool while Williams attempted to hold the box level above its mounting plate. But the bolt could not be driven in far enough to pull the box down onto a gang of electrical connectors.
Mindful of possibly stripping the bolt, flight controllers told Hoshide to back the bolt all the way out and to pull the power unit away from its mounting plate. Williams then aimed a small can of compressed nitrogen at the bolt receptacle in a bid to blow out any shavings or other debris that might have been interfering with the bolt's progress.
"All right. Give her a puff," Fischer radioed from mission control.
"That looked like that cleaned out a lot," Williams reported a moment later. "It's pretty clean compared to what it just looked like."
She then retrieved a second canister of nitrogen and directed a jet at the bolt assembly itself.
"I puffed at it," she said. " I didn't see much come out, but it does look cleaner, actually."
"You think that cleaned it out?" Fischer asked.
"Yes, I do, I saw a couple of little things come out of there."
"All right," Fischer said, "if this thing works we're going to name that tool 'Puff the Magic Dragon.'"
With Williams helping align the MBSU, Hoshide tried again to lock it in place. But again, the bolt refused to drive in.
"You know, at this point we're coming up with new ideas," Fischer said. "We've tried almost every backup we have on this stupid bolt."
While Hoshide was wrestling with the MBSU, Williams completed the second priority of the spacewalk, routing two long cables from a power distribution panel inside the solar array truss to the interface between the U.S. and Russian segments of the station. The cables will be needed next year when a new Russian lab module is attached to the Zvezda command module.
11:40 AM EDT, 08/30/12: Balky power unit pulled from truss
Japanese astronaut Akihiko Hoshide, anchored to the end of the space station's robot arm, ran into problems removing a 220-pound power switching unit out of its housing on the forward face of the lab's solar array truss. But he finally succeeded in freeing two tight bolts holding main bus switching unit No. 1 in place, clearing the way for its removal and replacement.
Astronaut Joseph Acaba, overseeing arm operations from inside the station, then pulled Hoshide and the power unit away from its housing, kicking of a slow 14-minute maneuver to move the astronaut down to external stowage platform No. 2 where a spare power unit was mounted.
After temporarily stowing the malfunctioning MBSU, Hoshide planned to remove the spare and move it back up to the solar array truss for installation. Fellow spacewalker Sunita Williams, meanwhile, was well ahead of schedule routing two long electrical cables that eventually will provide U.S. solar array power to a Russian laboratory module scheduled for launch late next year.
Tight bolts are not unusual on the space station, which undergoes extreme temperature swings as the lab repeatedly moves from sunlight to darkness and back again. Only two bolts held the MBSU in place, but Hoshide had problems getting both of them loose using an astronaut power tool. After multiple attempts at different torque settings, and making sure the box was properly aligned in its housing, Hoshide managed to get both bolts out.
After the power unit was pulled out of its mounting plate, Williams inspected the bolt housings and reported what appeared to be fine metal shavings in one of them. Williams was asked to inspect the bolts on the backside of MBSU 1 to see if there were any signs of cross threading.
"Yeah, I can see a similar type of pattern on this H2 bolt," she said. "Some galling and some metal shavings around the bolt. The threads are all still there, there are just small, hair-like metal shavings."
The spacewalkers then pressed ahead, temporarily mounting the power unit before retrieving the spare.
08:20 AM EDT, 08/30/12: Spacewalk begins
Floating in the Quest airlock, space station astronauts Sunita Williams and Akihiko Hoshide switched their spacesuits to battery power at 8:16 a.m. EDT (GMT-4) Thursday to begin a planned six-and-a-half-hour spacewalk.
The primary goals of the excursion -- the first for NASA since the space shuttle's retirement last year -- are to replace a balky power switching unit on the station's solar array truss and to install cables needed by a Russian lab module scheduled for launch next year. If time is available, the astronauts will carry out a variety of "get-ahead" tasks, servicing external cameras and installing a protective shield over the lab's forward docking port.
For identification Williams, call sign EV-1, is wearing a spacesuit with red stripes while Hoshide, call sign EV-2, is wearing an unmarked suit.
This is the 164th spacewalk devoted to station construction and and maintenance since assembly began in 1998, the third so far this year, the fifth for Williams and the first for Hoshide. Going into the EVA, 108 astronauts and cosmonauts have logged 1,027 hours and 38 minutes -- 42.8 days -- of station EVA time.
"There's a couple of things that are really critical on the spacewalk," Williams said in a NASA interview. "One of them is replacing an MBSU, main bus switching unit. We have four of them on the space station, one of them hasn't been working quite a hundred percent for probably the last eight or nine months.
"We've been talking about trying to get this guy replaced. It's nothing critical at the moment, it just decreases some of our redundancy, and, of course, with a humungous space station that we have and all the laboratories that are running and all the power that's coming from the solar arrays, we like to have as much flexibility as possible. So we'd like to replace that MBSU."
Power generated by the station's eight solar array wings, four on each end of the lab's main truss, is routed to four main bus switching units, all located in the central S0 truss segment. Two power channels feed into each 220-pound MBSU, which in turn deliver 160-volt array power to a pair of DC-to-DC converter units. The DDCUs step the primary power down to the 124 volts used inside the station.
The MBSUs are critical to station operation, providing grounding and allowing flight controllers or station astronauts to crosstie power channels or to isolate them as needed when problems develop. MBSU No. 1 is delivering power, but it no longer responds to commands or provides detailed diagnostic information.
The second major objective of Thursday's spacewalk is to route cables from the U.S. segment of the station to the interface with Russia's modules that will provide power from the station's U.S. solar arrays to a Russian laboratory module scheduled for launch late next year.
The multi-purpose laboratory module will be attached to the Earth-facing port of the Zvezda command module after the Pirs docking compartment currently attached is jettisoned next year.
CBS News
07:45 PM EDT, 08/30/12: NASA assesses spacewalk repair options to complete MBSU installation
The failure to install a replacement electrical distribution box during a marathon spacewalk Thursday has temporarily left the International Space Station unable to take in power from two of its eight solar array wings. But mission managers said jumpers installed before the excursion will allow near-normal operations while engineers study possible spacewalk repair options.
"We are down to six power channels," said Flight Director Ed Van Cise. "We have flight rules in place that give us a maximum loading of kilowatts per channel, and that's what we work to balance across the channels we have available.
"The space station, now that it's fully assembled and operational, has a lot of flexibility to route power around through different means so we can be creative with how we share power across the power channels to be able to provide as much balance as we can. But that said, there are some things that do require more power and are only connected to certain power feeds."
Until the installation issue is resolved and main bus switching unit No. 1 is re-connected to the station's electrical grid, equipment that is not considered crucial will be powered down or used when scheduling permits to avoid conflicts. A robotics work station in the U.S. lab module, for example, will be powered down until it is needed. Likewise automatic heaters may be operated manually to ease the load.
Major, on-going experiments like the Alpha Magnetic Spectrometer will continue to draw power, but others that operate on a more transient basis may require tighter scheduling.
"Really, on a bigger scale, we're just balancing the loads across the six channels we have without minimizing or reducing our operational redundancy we need to actually go fly the vehicle", Van Cise said. "Obviously, we want to keep doing that world-class science that we do, we just have to be smart on which facilities we have up at a given time so we don't have two facilities that use a lot of power on the same feed."
Power generated by the station's eight solar array wings, four on each end of the lab's main truss, is routed to four main bus switching units, all located in the central S0 truss segment. Two power channels feed into each 220-pound MBSU, which in turn deliver 160-volt array power to a pair of transformers known as DC-to-DC converter units. The DDCUs step the primary power down to the 124 volts used inside the station.
The MBSUs are critical to station operation, providing grounding and allowing flight controllers or station astronauts to crosstie power channels or to isolate them as needed when problems develop.
Several months ago, MBSU No. 1 stopped responding to commands, although it still delivered electricity from power channels 1A and 1B, generated by a right-side inboard solar array and one starboard outboard panel respectively.
Replacing MBSU No. 1 was the top priority of a spacewalk Thursday by astronauts Sunita Williams and Akihiko Hoshide. They successfully removed the malfunctioning unit but were unable to install a replacement when a bolt used to pull the 220-pound device down onto a gang of electrical connectors and cooling fins would not drive home.
When all was said and done, the astronauts spent eight hours and 17 minutes outside the lab -- the third longest EVA ever conducted by NASA -- but they were forced to leave the replacement MBSU partially torqued down and not connected to power channels 1A and 1B.
"The team installed some jumpers prior to powering down the MBSU in preparation for the EVA and those jumpers allowed us to get power to all the ISS systems and payloads," said Program Manager Mike Suffredini. "So really, what the team has to do, they can get power to all the systems but we only have three quarters of the power available that we normally have.
"Depending on the task at hand over the next several days, the team may have to manage power loads a little bit, but this is familiar territory and we'll be able to deal with that while we decide what our next plan is."
The problem comes at a busy time for the station crew, with the departure of two cargo ships next month and three of the station's six crew members.
"While we think about our plan and when we might do another EVA if we decide that's necessary, we'll have to take all of that into account and move things around in order to accommodate that," Suffredini said. "But if we do decide to do an EVA ... you'll hear us talking about doing that sooner rather than later in the program."
If engineers can figure out what went wrong with the bolt on the replacement MBSU and how to fix it or work around the problem, Williams and Hoshide possibly could stage another spacewalk as early as next week.
But as of this writing, that's far from certain. For the MBSU's electrical connections and cooling fins to engage their counterparts in the truss housing, the box must be firmly seated.
Engineers will use ground spares to study possible techniques to free and then drive a possibly galled bolt without breaking it off or stripping its threads. Another option may be to partially disassemble the attachment fittings to work around the bolt.
In the meantime, Van Cise thanked Williams and Hoshide for "an awesome job today."
"Everybody down here passes on their congratulations for going way above and beyond and they expressed their thanks, all the way up to the program manager. He wanted me to specifically thank you for all that you put into making this as successful as it could be given the circumstances", Van Cise radioed.
"So thank you very much, job well done. Get some rest tonight, and I'll make sure (Flight Director) Tony (Ceccacci) gives you plenty of time to sleep in. Good job."
"Thank you very much for those words," astronaut Joseph Acaba replied from the Quest airlock, where he was helping Williams and Hoshide out of their spacesuits. "As you can see, these guys are happy, we're glad they went out and got back safe, that's what's important.
"You guys worked just as hard as we did, so thank you very much. We'll get these guys out and they can relax a little bit. Have a great night and we'll touch bases with you guys tomorrow."
04:40 PM EDT, 08/30/12: Spacewalk ends
Astronauts Sunita Williams and Akihiko Hoshide began repressurizing the space station's Quest airlock at 4:33 p.m. EDT (GMT-4) to officially conclude an eight-hour 17-minute spacewalk, NASA's first since the last shuttle flight. Thanks to problems with bolts that prevented the installation of a power distribution unit, the spacewalk was the third longest in NASA history. The record holder, a station assembly flight in 2001, lasted eight hours and 56 minutes.
For readers scoring at home, this was the 164th spacewalk devoted to station construction and maintenance since assembly began in 1998, the third so far this year, the fifth for Williams and the first for Hoshide. With this EVA, 109 astronauts and cosmonauts have logged 1,035 hours and 55 minutes -- 43.2 days -- of station EVA time.
Williams has now logged 37 hours and 34 minutes of EVA time, moving her up to 27th on the list of most experienced spacewalkers. She is second only to Peggy Whitson in total EVA time for a female astronaut. Hoshide is the third Japanese astronaut to walk in space.
An updated story will be posted here following a news conference with NASA managers to discuss the outcome of today's spacewalk and the impact of deferring installation of a power distribution unit.
03:50 PM EDT, 08/30/12: Balky bolt blocks installation of replacement power box
After repeated attempts to drive home a balky bolt needed to lock a replacement power switching unit into place on the space station's solar array truss, flight controllers told Japanese astronaut Akihiko Hoshide and fellow spacewalker Sunita Williams to call it a day, deferring installation to a future spacewalk.
Thanks to internal jumper cables installed earlier, the station's three operational main bus switching units, or MBSUs, are able to distribute solar array power throughout the lab without the need to power down any equipment. But the replacement unit is needed to restore full redundancy and to protect against possible downstream failures.
Before giving up for the day, the astronauts made one final attempt to drive the bolt home, with Hoshide wielding a power tool at a higher torque setting while Williams attempted to hold the 220-pound box level over its attachment plate. But again, the bolt jammed and rather than risk stripping it with an even higher power setting, the astronauts were told to tie the box down and to begin collecting their tools and tethers.
"It torqued out, less than one rev," Hoshide reported.
"Copy that, buddy. All right, we're going to clean things up and tie this down," replied Jack Fischer from mission control. "We'll figure this out another day."
Power generated by the station's eight solar array wings, four on each end of the lab's main truss, is routed to four main bus switching units, all located in the central S0 truss segment. Two power channels feed into each 220-pound MBSU, which in turn deliver 160-volt array power to a pair of DC-to-DC converter units. The DDCUs step the primary power down to the 124 volts used inside the station.
The MBSUs are critical to station operation, providing grounding and allowing flight controllers or station astronauts to crosstie power channels or to isolate them as needed when problems develop. MBSU's 2, 3 and 4 are operating normally. But several months ago, MBSU No. 1, while providing power normally, stopped responding to commands because of an internal problem of some sort. Replacing the device was the top priority of today's spacewalk.
But Hoshide ran into problems right off the bat when he attempted to unbolt the faulty MBSU. After an inspection, Williams reported metal shavings and signs of cross threading in one of the bolt receptacles and on the bolt that was particularly difficult to loosen. But after a bit of a struggle, the box was removed and Williams was able to lock the unit down on a storage platform as planned.
The astronauts ran into more problems bolting the replacement box in place. The initial bolt refused to run in smoothly, forcing Hoshide to try different torque settings on his power tool while Williams attempted to hold the box level above its mounting plate. But the bolt could not be driven in far enough to pull the box down onto a gang of electrical connectors.
Mindful of possibly stripping the bolt, flight controllers told Hoshide to back the bolt all the way out and to pull the power unit away from its mounting plate. Williams then aimed a small can of compressed nitrogen at the bolt receptacle in a bid to blow out any shavings or other debris that might have been interfering with the bolt's progress.
"All right. Give her a puff," Fischer radioed from mission control.
"That looked like that cleaned out a lot," Williams reported a moment later. "It's pretty clean compared to what it just looked like."
She then retrieved a second canister of nitrogen and directed a jet at the bolt assembly itself.
"I puffed at it," she said. " I didn't see much come out, but it does look cleaner, actually."
"You think that cleaned it out?" Fischer asked.
"Yes, I do, I saw a couple of little things come out of there."
"All right," Fischer said, "if this thing works we're going to name that tool 'Puff the Magic Dragon.'"
With Williams helping align the MBSU, Hoshide tried again to lock it in place. But again, the bolt refused to drive in.
"You know, at this point we're coming up with new ideas," Fischer said. "We've tried almost every backup we have on this stupid bolt."
While Hoshide was wrestling with the MBSU, Williams completed the second priority of the spacewalk, routing two long cables from a power distribution panel inside the solar array truss to the interface between the U.S. and Russian segments of the station. The cables will be needed next year when a new Russian lab module is attached to the Zvezda command module.
11:40 AM EDT, 08/30/12: Balky power unit pulled from truss
Japanese astronaut Akihiko Hoshide, anchored to the end of the space station's robot arm, ran into problems removing a 220-pound power switching unit out of its housing on the forward face of the lab's solar array truss. But he finally succeeded in freeing two tight bolts holding main bus switching unit No. 1 in place, clearing the way for its removal and replacement.
 |
| Akihiko Hoshide, at far left, anchored to the end of the space station's robot arm, moves a malfunctioning electrical distribution box -- MBSU 1 -- from its mount on the forward face of the lab's solar power truss to a storage platform where a spare is located. (Credit: NASA TV) |
After temporarily stowing the malfunctioning MBSU, Hoshide planned to remove the spare and move it back up to the solar array truss for installation. Fellow spacewalker Sunita Williams, meanwhile, was well ahead of schedule routing two long electrical cables that eventually will provide U.S. solar array power to a Russian laboratory module scheduled for launch late next year.
Tight bolts are not unusual on the space station, which undergoes extreme temperature swings as the lab repeatedly moves from sunlight to darkness and back again. Only two bolts held the MBSU in place, but Hoshide had problems getting both of them loose using an astronaut power tool. After multiple attempts at different torque settings, and making sure the box was properly aligned in its housing, Hoshide managed to get both bolts out.
After the power unit was pulled out of its mounting plate, Williams inspected the bolt housings and reported what appeared to be fine metal shavings in one of them. Williams was asked to inspect the bolts on the backside of MBSU 1 to see if there were any signs of cross threading.
"Yeah, I can see a similar type of pattern on this H2 bolt," she said. "Some galling and some metal shavings around the bolt. The threads are all still there, there are just small, hair-like metal shavings."
The spacewalkers then pressed ahead, temporarily mounting the power unit before retrieving the spare.
08:20 AM EDT, 08/30/12: Spacewalk begins
Floating in the Quest airlock, space station astronauts Sunita Williams and Akihiko Hoshide switched their spacesuits to battery power at 8:16 a.m. EDT (GMT-4) Thursday to begin a planned six-and-a-half-hour spacewalk.
The primary goals of the excursion -- the first for NASA since the space shuttle's retirement last year -- are to replace a balky power switching unit on the station's solar array truss and to install cables needed by a Russian lab module scheduled for launch next year. If time is available, the astronauts will carry out a variety of "get-ahead" tasks, servicing external cameras and installing a protective shield over the lab's forward docking port.
For identification Williams, call sign EV-1, is wearing a spacesuit with red stripes while Hoshide, call sign EV-2, is wearing an unmarked suit.
This is the 164th spacewalk devoted to station construction and and maintenance since assembly began in 1998, the third so far this year, the fifth for Williams and the first for Hoshide. Going into the EVA, 108 astronauts and cosmonauts have logged 1,027 hours and 38 minutes -- 42.8 days -- of station EVA time.
"There's a couple of things that are really critical on the spacewalk," Williams said in a NASA interview. "One of them is replacing an MBSU, main bus switching unit. We have four of them on the space station, one of them hasn't been working quite a hundred percent for probably the last eight or nine months.
"We've been talking about trying to get this guy replaced. It's nothing critical at the moment, it just decreases some of our redundancy, and, of course, with a humungous space station that we have and all the laboratories that are running and all the power that's coming from the solar arrays, we like to have as much flexibility as possible. So we'd like to replace that MBSU."
Power generated by the station's eight solar array wings, four on each end of the lab's main truss, is routed to four main bus switching units, all located in the central S0 truss segment. Two power channels feed into each 220-pound MBSU, which in turn deliver 160-volt array power to a pair of DC-to-DC converter units. The DDCUs step the primary power down to the 124 volts used inside the station.
The MBSUs are critical to station operation, providing grounding and allowing flight controllers or station astronauts to crosstie power channels or to isolate them as needed when problems develop. MBSU No. 1 is delivering power, but it no longer responds to commands or provides detailed diagnostic information.
The second major objective of Thursday's spacewalk is to route cables from the U.S. segment of the station to the interface with Russia's modules that will provide power from the station's U.S. solar arrays to a Russian laboratory module scheduled for launch late next year.
The multi-purpose laboratory module will be attached to the Earth-facing port of the Zvezda command module after the Pirs docking compartment currently attached is jettisoned next year.