HOUSTON — During a Feb. 28 spacewalk, Discovery astronauts Al Drew and Steve Bowen lugged a failed International Space Station (ISS) thermal control system pump to a stowage platform on the station’s U.S. Quest airlock, where it will await a return to Earth aboard NASA’s hoped-for STS-135 mission.
Technicians on the ground will perform a failure analysis when the unit returns from orbit. Retrieval of the 780-lb. pump assembly, which had been stowed on the station’s starboard solar power truss since early August, was the highest priority among a list of maintenance tasks tackled by Drew and Bowen during a spacewalk that began at 10:46 a.m. EST and ended at 5:20 pm. Discovery linked up with the orbiting science laboratory on Feb. 26, initiating 7-8 days of assembly activities, cargo exchanges and external maintenance. Drew and Bowen are scheduled to walk again on March 2, when they will attempt to vent the inactive pump of hazardous ammonia coolant.
The pump, which circulates ammonia through external radiators, failed on July 31, forcing a shutdown of half the station’s solar power-generation system. Station astronauts replaced the device with an onboard spare during early August spacewalks. But the cause of an apparent short-circuit within the pump remains a mystery. Without a fully functional thermal control system, the station is unable to generate sufficient solar power to sustain six full-time crewmembers and their science experiments.
During the spacewalk, Bowen lugged the pump from the August work site to the airlock while locked in a foot restraint on the tip of the station’s robot arm. “Great job,” radioed Nicole Stott, the Discovery astronaut who coordinated the outing, from inside the station. Two hours into the outing, the computer control post for the robot arm in the station’s Cupola viewing dome crashed, forcing operators Scott Kelly and Mike Barratt to head for a backup in the U.S. lab module. “I think we are back in business,” Barratt announced from the backup arm control post 45 min. later.
NASA intends to launch the 11-day STS-135 station supply mission between June 28 and late summer, using Atlantis. The flight would close out the shuttle program. However, the mission’s fate rests with the outcome of congressional deliberations on the 2011 budget.