By Irene Klotz
CAPE CANAVERAL, Fla., Dec 15 (Reuters - Astronauts aboard the International Space Station are preparing for possible spacewalks this week to repair the outpost's failed cooling system, NASA said on Sunday.
One of two ammonia cooling systems on the U.S. side of the station shut down on Wednesday after a problem developed with a valve inside a pump located outside the $100 billion laboratory.
The shutdown forced astronauts to turn off non-essential equipment, suspending some of the station's science experiments. The six-member crew was not in any danger.
The station, a permanently staffed research laboratory owned by 15 countries, flies about 250 miles above Earth.
NASA engineers on Sunday continued to assess options for fixing the valve, said agency spokesman Josh Byerly with the Johnson Space Center in Houston.
Meanwhile, space station flight engineers Rick Mastracchio and Michael Hopkins began preparing their spacesuits in case spacewalks were needed to replace the faulty pump, NASA said in a statement posted on its website on Saturday.
"It's a serious problem, obviously it's something we have to fix," Mastracchio said during an inflight interview on Friday.
"It's not something I'm worried about, though," he added.
Astronauts were called upon in 2010 to replace the pump that is now experiencing problems. That work required three spacewalks.
"The biggest challenge on this spacewalk, in my opinion, is the large fluid connectors that are connected to the pump module. But of course, we have a lot of tools if we have problems with those to fix that," Mastracchio said.
NASA on Saturday also decided to delay by at least one day the launch of an Orbital Sciences Corp Antares rocket from Wallops Island, Virginia, on its first cargo run to the station
Orbital Sciences is one of two firms hired by NASA to fly cargo to the station following the retirement of the space shuttles in 2011.
Orbital Sciences completed a successful test flight to the station in September and had been preparing to launch its first operational mission on Wednesday. That flight, the first of nine under a $1.9 billion NASA contract, will now occur no sooner than Thursday, NASA said.
Ongoing efforts to troubleshoot the station's failed cooling system could further delay the launch of Orbital Sciences' Cygnus cargo ship to as late as December 21, before NASA would reschedule the mission for January, the agency said.
(Editing by Vicki Allen)