WASHINGTON, Dec. 17 -- U.S. space agency NASA said Tuesday that a series of spacewalks will be necessary to repair a faulty cooling system on the International Space Station.

Under the current plan, two U.S. astronauts, Rick Mastracchio and Mike Hopkins will venture outside the space station on three separate days of December -- 21, 23 and 25 -- to remove a pump module that has a failed flow valve. They will replace it with an existing spare that is stored on the orbiting lab, NASA said in a statement.

The pump to be replaced is part of the station's external cooling loop that shut down Wednesday due to low temperatures. The space station has two external cooling loops, which circulate ammonia outside the station to keep both internal and external equipment cool.

NASA said that each of the three spacewalks will begin at 7:10 a.m. EDT (1210 GMT) and is scheduled to last six and a half hours.

Meanwhile, a Dec. 19 launch of Orbital Sciences' Cygnus cargo resupply mission to the space station is now delayed until January, NASA said, adding that the postponement "will allow ample time for the station crew to focus on repairing" the faulty pump module.