Nasa has ordered a series of urgent spacewalks to fix a broken cooling line at the International Space Station.

Station managers decided on Tuesday to send two American astronauts out as soon as possible to replace a pump that contains a faulty valve. It's a big job that will require at least two or three spacewalks. The first is scheduled for Saturday.

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

Half of the station's cooling system shut down last week. The six-man crew had to turn off nonessential equipment, including science experiments. While the astronauts are safe, Nasa wants the system back up to full strength, in case of another failure.