Astronauts Rick Mastracchio and Steve Swanson replaced a failed computer on the International Space Station's power truss Wednesday, efficiently racing through a short spacewalk to restore full functionality to a critical control network.

Astronaut Steve Swanson hangs on the space station's truss in this view from a helmet-mounted camera on Rick Mastracchio's spacesuit. Photo credit: NASA TV/Spaceflight Now
 
The astronauts switched their spacesuits to internal battery power at 9:56 a.m. EDT (1356 GMT) and exited the space station's Quest airlock, heading toward the S0 truss segment at the center of the outpost's structural backbone.

Swanson carried a bag containing a fresh external multiplexer-demultiplexer as Mastracchio unbolted a faulty computer that stopped responding to commands April 11.

The external multiplexer-demultiplexer, or MDM, relays commands between computers and systems outside the space station, such as the solar array rotary joints, thermal coolant loops, the robotic arm's mobile transporter, and other functions.

The failed MDM was in backup mode when it failed, and a primary computer continued working.

The 51-pound computer box was launched inside the S0 truss in April 2002.

Less than an hour into the spacewalk, Mastracchio had removed the suspect MDM and driven three bolts to affix the fresh unit in its housing on the S0 truss. An initial test of the computer showed it was healthy.

"Your R&R was successful," radioed astronaut Jeremy Hansen from mission control in Houston. "We have a good MDM. It's in diagnostic mode as expected."

"Fantastic," one of the spacewalkers replied.

Photo credit: NASA
 
Swanson moved on to cut two lanyards draped over a door leading to power distribution modules inside the S0 truss, clearing the way for the space station's Canadian-built Dextre robot to replace the modules if needed in the future.

No other tasks were assigned to the spacewalkers Wednesday.

After an incident in which water filled astronaut Luca Parmitano's spacesuit helmet on an excursion in July, officials have barred spacewalks for routine maintenance tasks, only approving urgent EVAs to replace or repair critical failed systems.

Wednesday's spacewalk was one of the "big 12" EVAs astronauts train to accomplish when major failures occur outside the space station.

Mastracchio and Swanson returned to the Quest airlock. The start of repressurization of airlock marked the end of the spacewalk at 11:32 a.m. EDT (1532 GMT), putting the spacewalk's elapsed time at 1 hour, 36 minutes.

The replacement of the MDM ensures space station ground controllers have full insight and command capability for several key systems on the outpost's truss.

Before Wednesday's spacewalk, flight director Brian Smith said the changeout of the failed MDM for a spare computer box was a high priority.

"There are 46 computers on just the U.S. portion of the space station, and 24 of those are external," Smith said. "Of those 24, this MDM controls 12 of them, so it's very important."

Photo of a multiplexer-demultiplexer like the one replaced Wednesday. Photo credit: NASA
 
Planning for Wednesday's spacewalk was tricky because occurred the same week as the arrival of a SpaceX Dragon cargo craft Sunday and a technology demonstration using a Russian Progress supply ship, which undocked from the space station at 4:58 a.m. EDT (0858 GMT).

The Progress spaceship backed away to a safe distance from the space station to test upgrades to its Kurs radar navigation system, leading to its return to a docking with the complex Friday.

Such complex operations as the Dragon arrival, the Progress rendezvous exercise, and a spacewalk are typically scheduled with time for breaks in between.

Wednesday's spacewalk was the 179th EVA in support of space station construction and maintenance since December 1998. It was the ninth spacewalk of Mastracchio's career and the fifth for Swanson.

Follow Stephen Clark on Twitter: @StephenClark1.