Astronauts work to replace faulty coolant pump during Christmas Eve spacewalk

INTERNATIONAL SPACE STATION - Two American astronauts are floating high above Earth this morning during a planned spacewalk.

Rick Mastracchio and Mike Hopkins floated out of the International Space Station's airlock just before 4 a.m. local time, for the six-hour Christmas Eve spacewalk.

The goal of today's spacewalk is to install a replacement ammonia pump module in one of the station's cooling systems.  A valve in the pump assembly malfunctioned Dec. 11, partially disabling the system and forcing flight controllers to power down non-essential systems.

More importantly, the malfunction left the station one failure away from a more drastic powerdown and possible evacuation if the lone operational coolant loop broke down.

The two astronauts removed the problematic pump during a five and a half hour spacewalk on Saturday.  Today, they plan to install a spare unit.  If the spare works as expected, the space station can be restored to normal operations.

To watch the live spacewalk, click here.