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The pump is supposed to push ammonia coolant through the lines on the right side of the complex and prevent equipment from overheating. To cope with the failure, the six-person crew had to turn off all unnecessary equipment and halt science experiments a week ago.

The cooling line on the left side _ unaffected by the trouble _ has had to manage everything.

Although space station managers knew an ammonia pump would fail one day, they did not expect it to happen so soon in the 12-year life of the complex. The broken pump had been in operation since 2006.

Wheelock could find nothing wrong with the pump _ about the size of a bathtub _ and he saw no signs it had been hit by micrometeorites or other debris. Suffredini later noted that the connectors are prone to trouble, and that the leak is not entirely surprising.

Each pump is a boxy 5 1/2 feet by 4 feet by 3 feet and has a mass of 780 pounds. The new pump, an on-board spare, presumably will be installed on the second or third spacewalk.

NASA said the breakdown is serious but has not endangered the crew.

Saturday’s spacewalk was the first by Americans, without a shuttle present, since 2008.

The crew includes three Americans and three Russians. Caldwell Dyson has been on board since April, and Wheelock since June.