Christmas Eve spacewalk aims to finish crucial space station repairs

12 minutes ago

Nearly two weeks after a faulty coolant valve crippled the International Space Station, two NASA astronauts began a Christmas Eve spacewalk aimed at getting things back to normal.

Spacewalkers Rick Mastracchio and Mike Hopkins opened the airlock hatch and set the clock running for what's expected to be a six-hour-plus outing at 6.53.a.m. ET. They're due to install a refrigerator-sized coolant pump module with an assist from Japanese astronaut Koichi Wakata, who's operating the station's 58-foot-long (18-meter-long) robotic arm.

Tuesday's finale follows up on Saturday's successful spacewalk to remove the faulty pump module. A valve inside that apparatus failed on Dec. 11, forcing one of the station's two ammonia coolant loops to go offline.

The cooling system plays an essential role in keeping the onboard electronics from overheating. NASA had to shut down non-essential systems and switch other systems over to the second loop, reducing the station's safety margin in the process. If the other loop were to fail, that would spark an emergency that could have forced the six-man crew to abandon the station.

A similar situation required three difficult spacewalks in 2010, but this time around, the repairs have gone more quickly than expected. Aided by the robotic arm, Mastracchio and Hopkins are scheduled to pull a spare pump module out of storage, set it in place and get it hooked up on Tuesday.

'If the reconnections go as smoothly as the disconnections did, we should be able to do everything we need to do in less than the six and a half hours that's planned,' NASA spokesman Kelly Humphries told NBC News.

Worries about waterSafety concerns have added some extra twists to the repair operation: In July, Italian astronaut Luca Parmitano almost drowned when water from his spacesuit cooling system backed up into his helmet. The crew overhauled the spacesuits, and NASA pronounced them 'clean' - but as a precaution, the helmets were equipped with absorbent pads and emergency snorkels.

Neither Mastracchio and Hopkins reported any problems on Saturday, but they'll still have the pads and snorkels.

After Saturday's 5.5-hour spacewalk, Mastracchio mistakenly flipped a switch on his suit that may have sent water to a different cooling device known as a sublimator. In a worst-case scenario, that could have caused the device to freeze up during the next spacewalk.

As a result, Mastracchio's suit was set aside to dry out, Hopkins' suit was resized to fit Mastracchio, and a backup suit was fitted for Hopkins' use. The switch required an extra day of preparation - which is why the follow-up spacewalk is taking place on Tuesday instead of Monday, as originally scheduled.

During the spacewalk, Mastracchio and Hopkins will have to watch out for flying specks of ammonia frost. The coolant could be toxic if it's brought inside the station. Mastracchio reported that a few flecks leaked out on Saturday, but the ammonia was 'baked out' of his suit by exposure to sunlight.

Christmas Eve outingThis is the first spacewalk to be conducted on Christmas Eve since a Hubble repair operation in 1999. NASA commentator Rob Navias joked about the holiday angle in advance of the outing: ''Twas the day before Christmas, and all through this house, many were stirring.'

If Tuesday's spacewalk is successful, NASA could go ahead with a cargo resupply mission that had to be postponed due to the cooling system problem. The launch of Orbital Sciences' Cygnus cargo capsule is now scheduled for Jan. 7.

The station is currently at its maximum capacity of six live-aboard spacefliers. Three Russians round out the six-man crew: Oleg Kotov, Sergey Ryazanskiy and Mikhail Tyurin. Kotov and Ryazanskiy are planning a spacewalk of their own on Friday, to install new equipment on the station's Russian segment.

More about the spacewalks: Tune in NASA TV coverage via NBCNews.com, and tune in to TODAY for the latest on the spacewalk. Alan Boyle is NBCNews.com's science editor. Connect with the Cosmic Log community by 'liking' the NBC News Science Facebook page, following @b0yle on Twitter and adding +Alan Boyle to your Google+ circles. You can also check out 'The Case for Pluto,' my book about the controversial dwarf planet and the search for new worlds.