Christmas Day spacewalk? Astronauts will go outside to fix space station. (+video)

Astronauts aboard the International Space Station are now scheduled to conduct three spacewalks to replace a malfunctioning coolant pump on the station's exterior.

Three spacewalks over five days, with a final outing Christmas Day that ties the bow on a badly needed space-station repair job?

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This might not have been the way NASA astronauts Rick Mastracchio and Air Force Col. Mike Hopkins originally planned to spend Christmas. But that's the schedule they face now that the agency has decided to replace a malfunctioning coolant pump on the station's exterior.

Mission managers made the call on Tuesday afternoon, determining that it was more prudent to replace the pump with one of three spare units the station carries than continue pursuing a work-around engineers had devised for bringing the pump back into full service.

The pump sends ammonia through one of two external cooling loops designed to remove heat from the station's interior as well as from equipment on the station's exterior.

On Dec. 11, ground controllers noticed that the fluid was too cool. Temperatures were sufficient to continue cooling equipment outside the station's modules. But the ammonia also circulates through a heat exchanger inside the station, accepting the excess heat from a water-based loop that keeps hardware, labs, and living spaces in the station cool.

Engineers isolated the problem to a flow-control valve used to regulate the ammonia's temperature and devised a way to use a different valve to do the job.

"The engineering teams did just an amazing job of sorting through all kinds of options to try to recover the valve and look at other ways to manage the flow," said Michael Suffredini, the International Space Station program manager.

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Christmas Day spacewalk? Astronauts will go outside to fix space station. (+video)