Space Station Crew Tries Emergency Fix

A pair of crewmembers from the International Space Station are undertaking a tricky spacewalk in NASA's third effort to fix a system that helps keep critical systems on the orbiting platform from overheating.

Flight engineers Doug Wheelock and Tracey Caldwell Dyson exited the ISS at 6:20 a.m. EDT early Monday. Flight engineer Shannon Walker is guiding the spacewalkers from within the ISS, and is at the controls of the remotely operated Canadarm2, which also is playing a role in the operation.

Wheelock and Caldwell Dyson have made two previous spacewalks to work on the problem, including a a record jaunt of 7 hours, 26 minutes on Aug. 11. They made their first attempt on Aug. 7.

The ISS's current crew complement includes three U.S. astronauts and three Russian cosmonauts.

NASA said the problem began when a circuit breaker tripped on July 31, causing the failure of a pump that feeds an ammonia-based coolant solution to key systems and avionics instruments. Crewmembers attempted to reset the breaker and restart the pump immediately after the breaker trip, but the effort was not successful.

Despite the breakdown, NASA said the ISS remains in a "stable configuration," with backup systems working to ensure that critical components on the ISS don't overheat. It has said the problem does not pose any danger to the crew.

The ISS is carrying two spare pump modules that can be used as replacements.

NASA has been forced to reschedule tasks that were to be performed on spacewalks that have been preempted by the emergency repair operations.