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 on Saturday morning.
They will disconnect an ammonia pump that contains a bad valve.
The breakdown has crippled the space station's critical cooling system.
The pump replacement is a huge undertaking, attempted only once before, in 2010.
NASA has ordered up three spacewalks to complete the job, with the next to begin on Monday.
Astronauts Rick Mastracchio and Michael Hopkins will go back out again Christmas Day, if any work remains.
The six-man crew had to turn off all non-essential equipment inside the orbiting lab following the malfunction on December 11, when scientific research came to a near-halt.
