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The crew aboard the International Space Station is struggling to fix a major problem. A pump in one of the station's cooling systems failed 10 days ago. Tomorrow, two crew members are scheduled to perform a spacewalk to replace the critical pump.

But as NPR's Joe Palca reports, the walk probably won't be enough to solve the problem.

JOE PALCA: The failed pump circulated ammonia through one of two cooling systems aboard ISS, the International Space Station. Without it, the cooling system is inoperable. And cooling is essential. Michael Suffredini is ISS manager. He spoke to reporters last week about the problem.

Mr. MICHAEL SUFFREDINI (Manager, ISS): All of the systems, other than some of the components out on the truss where the power modules are, get their cooling from this cooling system.

PALCA: Without it, all the equipment aboard the station would overheat and the crew quarters would be uninhabitable. There is another completely independent cooling system that's working just fine. But the failed pump means there's no margin for another failure.

Mr. SUFFREDINI: If we�lose the next cooling system, then we don't have the ability to cool most of the components on board ISS, and so that would be a significant challenge to the team to work through that.

PALCA: Significant challenge might be an understatement. The Russian modules that make up ISS have their own cooling systems, so the entire crew of six would have to live in there until one of the ammonia cooling systems could be repaired, and it'll be a tight squeeze.

Suffredini says NASA knew a pump might fail.

Mr. SUFFREDINI: This is an anomaly we knew someday would happen. It's an anomaly that we have trained for. It's an anomaly that we have planned for. So, I mean, we're in a good position to go solve this problem. It is a significant failure, though, and so it's one we need to go get after.

PALCA: NASA managers were hoping that two spacewalks would be sufficient to replace the pump with one of the four spares at the station, but the astronauts performing the first walk last Saturday found an ammonia leak that prevented them from removing the failed pump.

Suffredini spoke again to reporters Saturday afternoon after the spacewalk.

Mr. SUFFREDINI: Obviously we didn't get quite as far as wed hoped to today.

PALCA: The plan now is to try again tomorrow. Chief spacewalk officer David Beaver says there's still several things to disconnect before the failed pump can be removed.

Mr. DAVID BEAVER (Chief Spacewalk Officer): One fluid connection, there's also five electrical connections that need to be done after that's completed. And then there's four bolts that will need to be released.

PALCA: Then the new pump has to be bolted into place, all the lines connected and the old pump stowed away. Suffredini says it will most likely take a third spacewalk, and possibly more, to complete the repairs.

Mr. SUFFREDINI: We will get through this problem. The challenge is to get through this problem before the next problem hits the other cooling system.

PALCA: Losing the existing system isn't an idle concern: the pump on that system is the same age as the one that failed.

Joe Palca, NPR News, Washington.

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