Mastracchio, an experienced spacewalker, said he's more than ready to go out and make repairs, if deemed necessary by Mission Control.
"Any time you have something like this, it's good news, bad news," Mastracchio told The Associated Press on Friday.
"Of course, the bad news is the station's having problems and we have to go out and do a repair. The good news is we have the spare parts. We have the training. We have the skills and, of course, going out and doing a spacewalk is always very exciting -- yet very challenging."
One of two cooling loops on the space station shut down Wednesday after it got too cold. Flight controllers suspect a bad valve inside an external pump.
It's a serious situation that needs to be remedied quickly, officials said, in case there are additional failures.
On Friday, engineers in Houston spent a third day working on the issue. They want to see if they can solve the problem remotely, before ordering up a series of spacewalks to replace the entire pump. Three spacewalks were required to install this pump in 2010.
The space station cooling system uses ammonia to dissipate heat generated by on-board equipment.
The six astronauts have turned off all nonessential equipment to reduce the heat load, including some science experiments. That's resulted in extra free time for Mastracchio and his U.S., Japanese and Russian crewmates. Otherwise, life has not changed much a couple hundred miles up, he noted.
"We still have lights. We still have the toilet working. We still have food, and we're still very comfortable up here," Mastracchio said. "So the biggest problem is that we're just not taking the time during our schedule to do as much science as we normally would."
An upcoming delivery mission may be delayed because of the cooling system trouble. The commercial Cygnus capsule, currently scheduled for a Wednesday launch from Wallops Island, Va., almost certainly holds holiday surprises for the astronauts, along with the usual stash of supplies.
At least Mastracchio doesn't have to cram in any last-minute Christmas shopping, using the space station's Internet phone or online capability. The 53-year-old engineer said he bought presents for his wife before he rocketed into orbit at the beginning of November for a six-month mission.
"Being away from your friends and family during the holidays is not optimal," he told the AP. "But if you have to be away from your family on Christmas, then this is sure not a bad place to be."
And children, stay tuned.
"We're always on the lookout for Santa up here," Mastracchio said with a smile.
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