ISS astronauts performing critical spacewalk

Astronauts trying to repair valve on ISS

Published On: Dec 24 2013 09:21:09 PM EST   Updated On: Dec 21 2013 05:07:29 PM EST
CAPE CANAVERAL, Fla. -

Astronauts have stepped out on the first of a series of urgent repair spacewalks at the International Space Station.

The two Americans on the crew floated outside Saturday morning. They will disconnect an ammonia pump that contains a bad valve. The breakdown has crippled the space station's critical cooling system for the past 1 1/2 weeks.

The pump replacement is a huge undertaking attempted only once before, in 2010. NASA has ordered up three spacewalks to complete the job. The next one will be Monday.

Astronauts Rick Mastracchio (Muh-STRACK-ee-oh) and Michael Hopkins will go back out again Christmas Day, if any work remains.

The six-man crew had to turn off all nonessential equipment inside the orbiting lab following the malfunction on Dec. 11. Scientific research came to a near-halt.