Two astronauts quickly replaced a bad backup computer box and took on an extra task at the International Space Station on Wednesday during one of NASA's shortest spacewalks.

Within an hour, NASA astronaut Rick Mastracchio and Steve Swanson removed the old box, which failed to respond to commands on April 11, and installed a spare. Ground controllers reported that the new box, known as a multiplexer-demultiplexer or MDM, was working fine.

Mastracchio tucked the old box into an equipment bag and told Mission Control that he had "one MDM, slightly used."

"Nice and clean," Mission Control communicator Jeremy Hansen told the spacewalkers. "Good job."

Most spacewalks run for five or six hours, but this one lasted only an hour and 36 minutes arguably making it NASA's shortest glitch-free operation outside the space station.

An astronaut performs a repair job on the International Space Station during Wednesday's spacewalk.

Routine but critical task

The backup box is part of a redundant system that plays a part in controlling critical equipment on the station, including the solar arrays, a robotic rail car and the external cooling system. The primary computer box is working just fine, but NASA didn't want to go without a working backup any longer than necessary.

Replacing the box is considered one of the space station's "Big 12" routine maintenance tasks. Mastracchio and Swanson have been trained in advance for such jobs.

While Mastracchio switched the boxes, Swanson cut some dangling lanyards that had been blocking the way for the space station's Dextre robotic hand. In the future, Dextre might be able to perform maintenance tasks like the computer replacement without the need for a spacewalk.

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Spacewalking Repairmen Star in Juggling Act at Space Station