Astronauts in orbit and on the ground practiced Monday for a major repair job later this week at the International Space Station, struck by a massive cooling system failure.

The weekend malfunction knocked out half of the space station's cooling system, forcing the crew of six to turn off unnecessary equipment and halt scientific work to avoid any overheating.

NASA's space station program manager, Mike Suffredini, ranked the problem as one of the most serious in the 12-year history of the orbiting lab, but stressed the outpost could keep going indefinitely given the current situation. The fear is that the second cooling loop could shut down at any moment and leave the station in precarious shape.

For now, "everything the crew needs to survive, they're in good shape, all those systems are active," Suffredini told reporters Monday. "What we're talking about, really, is it would be a significant challenge if we suffered the next failure."

Two of the Americans on board — Douglas Wheelock and Tracy Caldwell Dyson — will venture out on a spacewalk to replace the pump Thursday. A second spacewalk will be needed to finish the job, probably Sunday.

The 780-pound pump is difficult to handle, and the astronauts will need to guard against any hazardous ammonia leaks.

Engineering teams have been working nonstop since the right-side cooling loop shut down Saturday night. A pump that drives ammonia coolant through those lines failed when a circuit breaker tripped.

The disabled pump has been at the space station since 2002 and operating fully since just 2006; it was a premature failure. The electrical short is believed to be internal to the pump. Engineers believe a new pump will solve the problem, but there is no guarantee, Suffredini noted.

Four spare pumps are on board.

"This is an anomaly we knew some day would happen," Suffredini said. "We're in a good position to go solve this problem. It is a significant failure, though, in terms of systems."

Wheelock and Caldwell Dyson trained for this type of repair job before they launched to the space station. They were going to take a spacewalk anyway Thursday to perform prep work for a shuttle visit in November; all those original chores have been pushed to the side.

A pair of astronauts in Houston took to a giant swimming pool Monday afternoon to rehearse the repair procedures. Another practice session was set for Tuesday. The spacewalk will be delayed until Friday if extra time is needed to prepare, flight director Courtenay McMillan said.

NASA officials repeated Monday that the astronauts are safe and the outpost is stable. But lots of equipment remains shut off: extra lights, heaters and science experiments.

If both cooling loops were to fail, the Russian side of the space station would have to carry the entire cooling load. The crew would have just enough time to attempt emergency repairs before, in all likelihood, abandoning ship in Russian Soyuz capsules to return to Earth.

The space station is meant to operate until 2020. Shuttles will stop visiting, though, early next year. Only two shuttle flights remain, and there is no room on board either Discovery or Endeavour to return the failed pump, Suffredini said. If a third and final mission is approved for next summer, the discarded piece could be returned for analysis.

Any additional spare pumps that might be needed in years to come, Suffredini said, could fit aboard a Japanese cargo carrier or commercial craft such as the type being designed by Space Exploration Technologies of California.



NASA: http://www.nasa.gov/mission_pages/station/main/index.html