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Spacewalk underway at International Space Station to fix leak

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In a spacewalk, Astronauts Chris Cassidy and Tom Marshburn replaced a pump component that has been gushing ammonia coolant, NASA reported Saturday.

The spacewalk got underway at 5:44 a.m. PDT and was expected to last more than six hours, according to NASA. The astronauts removed and replaced the faulty pump controller box on the far port truss of the International Space Station. No evidence of a continued ammonia leak was evident as NASA restarted the faulty coolant loop.

Live footage on NASA TV showed the two working in painstakingly slow motion in alternating darkness and light as the station circled Earth. Cassidy used power and hand tools to retrieve a spare controller box, while ground managers instructed him about such things as how many foot-pounds of torque to apply to bolts, and how long it would be until they had sunlight.

Amid the sober and complex choreography, Commander Chris Hadfield tweeted commentary and trivia. "Chris and Tom are safely outside ISS, hard at work in the vacuum of space, looking for signs of the ammonia leak. THAT was a busy AM inside," he wrote early in the walk.

"Do you hear how different their voices sound at the suit's low pressure? Turns out in the 4.2 psi oxygen you can't whistle -- air's too thin," he wrote later.

The leak, observed Thursday, comes from a pump that circulates ammonia coolant to one of eight solar panel systems that provide electricity to the station. That coolant loop has caused trouble for some time, and crew members attempted to locate and repair a leak in November, according to International Space Station program manager Michael Suffredini.

The spacewalk was the 168th for the International Space Station but was considered precedent-setting because of how rapidly the unplanned repair task unfolded. Although the six-member crew of Expedition 35 was in no danger, and there are seven other solar panels to provide power, NASA officials wanted to get the system back in operation to avoid anomalies in the power supply that could affect experiments and other tasks planned for the mission. 

[For the record, 9:13 a.m., May 11: An earlier version of this post gave an incorrect name for this space station mission. It is Expedition 35, not Explorer 35.]

The truss where the pair were working, called P6, was launched to the station in November 2000 and is one of the oldest components of the station's backbone. It was relocated from its original installation position to the far left side of the station during a Discovery mission in 2007. 

The spacewalk is the third time Cassidy and Marshburn have worked in tandem outside the station. 

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