Image: Pool practice for spacewalk
Surrounded by divers, NASA astronauts Robert Satcher Jr. and Rick Sturckow practice maneuvers for a spacewalk underwater at Johnson Space Center's Neutral Buoyancy Lab.
updated 8/5/2010 1:09:25 PM ET 2010-08-05T17:09:25

NASA has delayed two spacewalks to fix a major cooling system failure on the International Space Station for the second time, pushing the start of the vital repairs to no earlier than Saturday.

American astronauts Douglas Wheelock and Tracy Caldwell Dyson had been planning to perform the first spacewalk Friday to replace a failed ammonia pump on the station's right side. That spacewalk, which was already delayed one day, will now begin Saturday at about 7 a.m. ET, with the second spacewalk set for no earlier than next Wednesday.

Mission Control radioed the spacewalk delay news to the station crew Thursday morning.

"Thank you for the word," Wheelock told Mission Control. "Thanks for the hard work. We know people are still working through the evening, and through the night, and we appreciate it."

The delays give NASA mission planners more time to finish refining the procedures and safety concerns for the pump repair spacewalks, NASA officials said. [Graphic: Space Station's Cooling System Problem Explained]

"What's really being done now is putting on the final touches," NASA spokesman Kyle Herring said during televised station mission commentary.

The space station's cooling system troubles began July 31, when a power spike tripped a circuit breaker, shutting down one of the two pumps that push liquid ammonia through the orbiting laboratory's cooling system. Attempts to reactivate the stricken pump were unsuccessful.

The failure forced astronauts to turn off many systems on the space station's U.S. segment and shut down the backup systems for some others, in order to reduce the amount of heat generated by equipment. The space station's working pump is currently operating normally to cool the systems left online.

NASA has repeatedly stressed that the space station crew is in no danger from the malfunction.

But mission managers do want to repair the cooling system pump swiftly in order to restore the space station to full operations. Astronauts on Earth have been practicing the spacewalk repair to help Wheelock and Caldwell Dyson prepare for their orbital work.

The malfunction is a major system failure, and one of 14 that NASA has prepared repair plans for on the off chance that it ever occurred. There are four spare ammonia pumps currently stored on platforms on the station's exterior.

The pumps are not small. They are bulky and are expected to be difficult to maneuver in the weightlessness of space.

Caldwell Dyson has said each pumps is about as large as a laundry dryer.

Each pump weighs 780 pounds (353 kilograms) and is 5.75 feet long (175 centimeters) by 4 feet wide (127 centimeters). They are about 3 feet tall (90 centimeters).

The International Space Station is currently home to six people: three Americans and three Russians.

NASA will broadcast the International Space Station spacewalk repairs live from space on NASA TV, with the first spacewalk slated to begin Saturday at 7 a.m. ET.

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