Two crew members from the International Space Station took a major step toward replacing a critical piece of the orbiting laboratory's cooling system today, following a failed attempt Sunday.
During a 7 hour, 26 minute spacewalk, Tracy Caldwell Dyson and Army Col. Douglas Wheelock removed a coolant pump on the outside of the station, clearing the way for another spacewalk Monday to install an on-board spare.
The pump failed unexpectedly on July 31, cutting in half the station's capacity to shed heat generated by its electrical systems, laboratory experiments, as well as six active astronauts.
The six crewmembers aboard the station were never in danger, NASA officials have emphasized. But the outage forced mission managers to significantly curtail research activities on the orbiting lab.
"Lots of smiles down here guys," came the word from mission control as Dr. Caldwell Dyson and Colonel Wheelock sat in the air lock after the spacewalk.
A major fix-it project
Swapping the coolant pumps represents one of the 14 most difficult maintenance jobs station crews face. Spacewalks ordinarily take weeks to plan because they require detailed choreography. But the urgency of returning the station's cooling system to full capacity prompted planners to accelerate the process for a repair job astronauts had trained for with only the broadest of brush strokes.
For instance, last night, engineers were still working on procedures governing the use of the station's robotic arm for today's effort. The broad-brush plan for removal of the pump that astronauts used in training had assumed that the arm wouldn't be available.
Clamping an astronaut to the end of the robotic arm, then having him hold a 780-pound pump steady while a crew member inside the station moves the two to the spot where planners want to deposit the pump – that's a procedure planners would rather not develop at the last minute, mission officials said.
But they did.
"There are so many facets to a major change out that you just don't really get to tackle" in the planning stage "unless you train for it at a very detailed level, which we hadn't done yet," said Michael Suffredini, NASA's space station program manager, during a post-spacewalk briefing.
How first spacewalk went wrong
Prior to the pump's failure, the two astronauts were scheduled to undertake a spacewalk for other purposes last Friday. Planners quickly changed the agenda and finally settled on Sunday for the initial attempt at removing the failed pump.
But in what amounted to an 8 hour, 3 minute outing – far longer than planned – Wheelock and Caldwell Dyson were thwarted by coolant that leaked from the last of four coolant lines they had to disconnect to remove the pump.
The leak prompted mission planners to halt the effort and forced the duo to undergo decontamination procedures to ensure no ammonia remained on their suits when they reentered the station.
The station's two cooling systems use ammonia to carry heat from heat exchangers in the station to radiators that extend from the station's long truss, which also supports the wing-like solar panels that supply electricity to the facility.
During today's spacewalk, the remaining coolant line was as balky as it was on Sunday. It took extra elbow grease to disconnect it. But this time, it didn't leak ammonia as it had Sunday.
Missions managers initially planned to have astronauts install the replacement pump this coming Sunday, Aug. 15. But they pushed the effort to Monday to give the crew and the team on the ground a brief breather from the intense planning they've performed over the past several days.
If Monday's spacewalk ends with a new pump ready to activate, planners say they may put off a fourth, clean-up-the-worksite spacewalk for up to several weeks to allow people on orbit and on the ground to catch their collective breath and more carefully think through procedures for the follow-up spacewalk.