Bright and early on Dec. 24, NASA astronauts Rick Mastracchio and Mike Hopkins will head out of the International Space Station to continue an essential repair to the station's cooling system that began Saturday.
But first, they have to make a costume change.
The Christmas Eve spacewalk is the second in a series of emergency operations to replace a pump in one of the station's two external ammonia cooling loops. The system is responsible for keeping instruments both inside and outside the station from overheating.
On Saturday the two astronauts disconnected the refrigerator-size pump from four ammonia fluid lines, then moved it to a temporary storage site on the outside of the ISS. You can see highlights of that spacewalk, including lots of helmet-cam shots, in the video above.
The upcoming second spacewalk -- during which the astronauts will install a spare pump -- was originally scheduled to take place Monday, but NASA decided to push it back a day after a potential problem arose with Mastracchio's spacesuit.
Italian astronaut Luca Parmitano of the European Space Agency nearly drowned during a spacewalk this year when a water leak caused his helmet to fill with water. The two problems are not related, NASA said in a statement.
Mastracchio's spacesuit problems didn't arise until he was back on the station after Saturday's walk.
Temperatures outside the space station can vary widely, from as much as 200 degrees Fahrenheit in the sun to negative 200 degrees in the Earth's shadow, according to NASA spokesman John Ira Petty, so the astronauts' suits are equipped with an internal temperature-control system.
"They wear a cooling garment, kind of like long underwear with water running through it," said Petty, who works at the Johnson Space Center in Houston.
The temperature is controlled by a sublimator attached to the suit. When the astronauts were being repressurized inside the station Saturday, some of the water from the cooling garment may have gotten into the suit's sublimator. To be safe, the flight team in Houston decided to have Mastracchio switch to a backup suit for Tuesday's walk.
(Actually, Hopkins will be wearing the torso of the backup suit and giving the torso of his suit to Mastracchio. "The spare top was not a good fit for Mastracchio," Petty said.)
With the suit situation sorted out, the astronauts should be all set for their walk Tuesday. Although NASA had originally said the pump replacement would take three walks of about 6 1/2 hours each, they now think it may take only two.
"It is hard to say with confidence that any one thing is going to happen, though," Petty said. "We learn something every time we do something in space, and one of the first things we learn is that you never know exactly what will happen."
Tuesday's spacewalk is scheduled to begin at 4:10 a.m. PST.
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