Astronauts complete spacewalk, fix computer

Astronauts Rick Mastracchio and Steven Swanson returned to the Quest airlock aboard the International Space Station at 11:32 a.m. EDT (GMT-4), ending a short but successful one-hour 36-minute spacewalk to replace a faulty external computer.

The start of the spacewalk, known as U.S. EVA-26, came just five hours after the departure of a Russian Progress supply ship that undocked from the Zvezda command module at 4:58 a.m. After moving about 300 miles ahead of the station, the uncrewed Progress will return and re-dock Friday to test an upgraded navigation and rendezvous system planned for future vehicles.

But the spacewalk was the focus for the station's NASA crew and flight controllers at the Johnson Space Center in Houston. The goal was to replace an external multiplexer-demultiplexer, or MDM, a 49-pound computer mounted in the central S0 segment of the station's solar power truss that failed April 11.

Less than an hour into a planned 2.5-hour spacewalk, Mastracchio and Swanson pulled a failed external computer from its rack in the central S0 truss segment of the International Space Station's solar power truss Wednesday and installed a replacement, making quick work of a critical repair.

Before unbolting the computer, Mastracchio reported there were no obvious signs of any damage in the area and examining the faulty "black box" after it was removed, he said everything looked good with no evidence of anything out of the ordinary. While additional troubleshooting will be needed to make sure, it would appear that an internal component failure of some sort was responsible for the original malfunction.

With the new computer in place, flight controllers sent commands to power it up and then began a series of diagnostic tests to verify its performance before uplinking the latest software. The faulty computer was carried back inside the station for troubleshooting and repairs.

"Your R & R was successful, we have a good MDM," Hansen called. "It's in diagnostic mode as expected."

"Oh, wonderful," one of the spacewalkers replied. "Fantastic," said the other.

"It's a pretty simple EVA in terms of the overall work we have to do," said Glenda Brown, the lead spacewalk planner at the Johnson Space Center in Houston, before the spacewalk. "It's just coming out from the airlock, driving three bolts to remove the failed unit, removing it, temp stowing it and installing the new one. Very straight forward."

On April 11, the backup computer, MDM EXT-2, failed to boot up properly and after reviewing telemetry, flight controllers concluded it was no longer useable. The computer was launched in place with the S0 truss segment in 2002 and operated flawlessly until the malfunction occurred.

While the failure had no immediate impact on station operations, the loss of redundancy put the outpost one failure away from major problems across a wide variety of systems.

Given the critical nature of the systems in question -- solar array positioning mechanisms, a robot arm transporter, coolant loop telemetry -- replacing a failed command MDM was on NASA's "big 12" list of quick-response spacewalk repairs.

But in this case, NASA managers opted to press ahead with launch of the SpaceX Dragon cargo ship before replacing MDM EXT-2. Procedural changes were implemented to ensure good solar array positioning even if a problem later developed with MDM EXT-1.

While NASA normally includes a variety of secondary objectives and "get-ahead" tasks in spacewalk timelines to take full advantage of the crew's time outside, only one minor item was added to the MDM replacement EVA. That's because NASA is still working through an extensive spacesuit analysis triggered by a potentially catastrophic water leak during a spacewalk last summer.

Engineers believe the water backup was caused by contamination that clogged a coolant loop filter, allowing water to make its way into European Space Agency astronaut Luca Parmitano's helmet. Since then, new filters have been installed, the coolant water has been flushed and replaced and new pump assemblies installed.

In addition, Mastracchio and Swanson installed water-absorbing pads in their helmets and plastic straw-like tubes running from their helmets down into the body of their suits to provide an emergency breathing path in case all else fails. The modifications were developed for a pair of "contingency" spacewalks last December.

Mastracchio was wearing suit No. 3011, the same extravehicular mobility unit, or EMU, that Parmitano used, while Swanson was wearing EMU No. 3005. A fresh spacesuit was carried up aboard a SpaceX Dragon cargo ship that was berthed Sunday, along with additional spare parts.

"The anomaly is due to contamination that was likely introduced by a filter that was used to clean the (cooling) system," said space station Program Manager Mike Suffredini. "We have since scrubbed all the suits' water lines, basically we've flushed the water three times in all three of the suits on orbit and in the cooling lines in the station that provide water to the suits.

"In addition to that, we've replaced the filters, we have replaced the fan pump sep that got clogged up in EMU 3011...and we just did the fan pump sep R & R for suit 3005."

But until the failure investigation is complete, NASA will only conduct contingency spacewalks to carry out critical repairs. Two such spacewalks were carried out last December by Mastracchio and Mike Hopkins to replace an ammonia coolant pump assembly. Today's EVA was NASA's third since the leak.

"We have been working diligently to understand the cause of the anomaly and to recover from that," Suffredini said. "In the process, we have done a thorough review of all our processes and procedures and our hazard reports, and we established a goal for ourselves to have all of that work done before we did a planned EVA.

"However, the vehicle keeps flying and occasionally we have contingency EVAs. We've done (two) of those already, to do a pump module replacement, and we're prepared to do this one for this EXT MDM that has failed."

The U.S. segment of the space station includes 46 computers, 24 of which are mounted in the lab's solar power truss. One of those external MDMs is used to command the others while a second unit serves as a "hot backup," ready to take over in case of any problems that might derail the primary computer.

This was the 179th spacewalk devoted to station assembly and maintenance since construction began in 1998, the second so far this year, the ninth for Mastracchio and the fifth for Swanson.

With today's spacewalk, 114 astronauts and cosmonauts representing nine nations have now logged 1,123 hours and 28 minutes of space station EVA time, or 46.8 days.

Mastracchio moves up to No. 5 on the list of most experienced spacewalkers, with 53 hours and 4 minutes of EVA time across his nine excursions while Swanson's total now stands at 27 hours and 58 minutes.

"Thank you, everyone," Swanson called. "Great work."