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.
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