Urgent spacewalk repairs for station ordered by NASA

CAPE CANAVERAL, Fla. -- NASA has ordered a series of urgent spacewalks to fix a broken cooling line at the International Space Station.

Station managers decided Tuesday to send two American astronauts out as soon as possible to replace a pump that has a bad valve. It's a major job that will require three spacewalks -- Saturday, Monday and on Christmas Day.

"The next week will be busy with spacewalks so not much tweeting from here," NASA astronaut Rick Mastracchio tweeted from space soon after the decision was announced.

The spacewalks are taking priority over the launch of a supply ship from Virginia. The commercial delivery had been scheduled for this week, but is delayed until at least mid-January.

Half of the station's cooling system shut down last week, forcing the six-man crew to turn off all nonessential equipment, including some science experiments. Because of the valve failure, one of the two cooling lines became too cold.

The space station cooling system, which runs ammonia through the lines, is critical for dispelling heat generated by onboard equipment.

While the astronauts are safe and comfortable, NASA wants the system back up to full strength, in case of another failure that could leave the outpost more vulnerable than it is now.

Flight controllers tried in vain to fix the valve remotely, then came up with a plan to use another valve to regulate the temperature. Some success was reported, and for a while, engineers thought the space station could limp along with the short-term solution. But on Tuesday, managers opted for spacewalks now.

This is the same pump that was replaced by a two spacewalking astronauts in 2010. Three spacewalks were needed.

Mastracchio and astronaut Michael Hopkins trained for just such a repair before going into orbit. They have been prepping all week, just in case of such a decision.

Orbital Sciences Corp., meanwhile, will stand down from its planned Thursday night launch of its Cygnus cargo ship from Wallops Island, Va.

The station crew includes three Russians and one Japanese, aside from the two Americans.