By Irene Klotz
CAPE CANAVERAL, Fla., Dec 17 (Reuters) - Astronauts aboard the International Space Station will begin a series of spacewalks this weekend to repair the orbital outpost's cooling system, delaying a cargo resupply flight until January, NASA said on Tuesday.
One of the station's two ammonia cooling systems shut down on Wednesday, forcing astronauts to power down unnecessary equipment and suspending some of the laboratory's science experiments. The six-member crew was not in any danger, NASA said.
Engineers on the ground tried to devise ways to bypass an apparently faulty valve inside a pump, located outside the station. But by Tuesday afternoon, with the cooling system still down, the U.S. space agency decided to have two astronauts aboard the station replace the pump with a spare.
Three spacewalks are planned to complete the work, the first of which is scheduled for Saturday by station flight engineers Rick Mastracchio and Michael Hopkins. Two more spacewalks are targeted for Dec. 23 and Dec. 25.
Mastracchio and Hopkins will retrieve a spare pump from a storage site outside the station and replace the faulty unit. Astronauts tackled the same job, which proved more difficult than anticipated, in 2010.
Repairing the station's cooling system will sideline Orbital Sciences Corp's first cargo run to the station, a $100 billion research lab that flies about 250 miles (400 km) above Earth.
Orbital Sciences, one of two companies hired by NASA to fly supplies to the outpost following the retirement of the space shuttles in 2011, had been preparing for a launch from Wallops Island, Virginia, on Thursday.
The flight, the first of eight under a $1.9 billion NASA contract, will be bumped to no earlier than the second week of January, said NASA spokesman Josh Byerly, at the Johnson Space Center in Houston.
The spacewalks will be the first by NASA crew members since July, when Italian astronaut Luca Parmitano had a spacesuit problem that caused his helmet to fill with water. NASA immediately aborted the spacewalk and suspended future outings while the cause of the leak was under investigation.
The investigation is still under way but the spacesuits aboard the station have been cleared for use.