NASA has postponed a cargo mission to the International Space Station, a $100bn research lab that flies some 400km above Earth, and ordered a series of urgent spacewalks to fix a broken cooling line at the research outpost.
American astronauts Rick Mastracchio and Mike Hopkins will be sent outside the space station on three separate spacewalks, beginning on Saturday and ending on Christmas Day, the US space agency said on Tuesday.
A broken valve caused the failure on December 11 in one of the station's two external cooling loops, which circulate ammonia outside the station to keep both internal and external equipment cool, NASA said.
The breakdown forced the six-man crew to turn off all nonessential equipment aboard, including some science experiments. The crew was not in any danger, NASA said.
The broken pump - a bulky, 354kg bundle - was replaced by astronauts over three separate spacewalks in 2010.
This time, Mastracchio and Hopkins will retrieve a spare pump from a storage site outside the station and replace the faulty one.
"The next week will be busy with spacewalks so not much tweeting from here," Mastracchio said from space via Twitter, soon after the decision to launch the spacewalks was announced.
"Have not looked out the window in 4 days," he said in another tweet. "Too busy building space suits. Where did I put my gloves?"
Cargo run delayed
The repair job will sideline Orbital Sciences Corp's first cargo run to the International Space Station, which had been set to launch on Thursday from Wallops Island, Virginia.
Orbital Sciences is one of two companies hired by NASA to fly supplies to the outpost after the retirement of its 30-year space shuttle programme in 2011.
The flight, the first of eight under a $1.9bn NASA contract, will be rescheduled for no earlier than the second week of January, said NASA spokesman Josh Byerly, at the Johnson Space Center in Houston.
The spacewalk on Saturday will be the first by NASA crew members since July, when Italian astronaut Luca Parmitano almost drowned after his helmet filled with water.
NASA immediately aborted the spacewalk and suspended future attempts while the cause of the leak was under investigation. The investigation is still under way, but the spacesuits currently aboard the station have been cleared for use.
The space station also hosts one Japanese and three Russian crew members.
Russian Commander Oleg Kotov and Flight Engineer Sergey Ryazanskiy are scheduled to step out on an unrelated spacewalk on December 27, to install cameras and other equipment on a platform outside the Russian segment of the station.