Image: International Space Station
In this NASA graphic of the International Space Station, the failed cooling system pump module is located on the main truss, just to the right of center.
updated 8/6/2010 6:45:39 PM ET 2010-08-06T22:45:39

Astronauts aboard the International Space Station are gearing up for a major cooling system repair job Saturday, one that has forced NASA mission planners on Earth to dramatically alter plans for a spacewalk this week.

NASA space station managers have cleared American astronauts Douglas Wheelock and Tracy Caldwell Dyson to go ahead with a seven-hour spacewalk repair Saturday to replace an oven-sized ammonia pump on the space station's right side.

The decision came after a week of round-the-clock analysis and spacewalk planning to draw up new plans for Wheelock and Caldwell Dyson, who were originally scheduled to perform a completely different spacewalk Thursday.

The pump, which failed July 31, is designed to push liquid ammonia through the right side of the space station. Its unexpected shutdown knocked out half of the space station's cooling system.

Dan Hartman, NASA's space station manager of integration and operations, called the frenetic week of emergency spacewalk planning "remarkable" and was amazed at how the space station support team "turned on a dime," according to NASA spokesperson Kyle Herring.

Space station astronauts were never in any danger from the malfunction, but they did have to switch off many systems, and power down the backups for some others, in order to keep the orbiting laboratory from overheating. (Graphic: Space Station's Cooling System Problem Explained)

Double spacewalk repair
It will actually take two spacewalks to completely replace the faulty cooling system pump on the International Space Station.

On Saturday, Wheelock and Caldwell Dyson will replace the stricken ammonia pump with one of four spares stored on the station's exterior for just such an occasion. That spacewalk is set to begin at about 7 a.m. ET.

But the new ammonia pump won't be activated until a second spacewalk, currently scheduled for Wednesday. It is during that second excursion that the astronauts are expected to hook up the five power and ammonia connections to fully activate the new pump.

Each pump weighs 780 pounds and is 5 1/2 feet long by 4 feet wide. They are about 3 feet tall. Caldwell Dyson has said the pumps are about the size of a laundry dryer.

Station cooling system troubles
NASA has long known that the space station's cooling system could pose a major problem if it malfunctioned. It is one of 14 major potential failures engineers have prepared repair plans for in advance just in case they occurred during the space station's lifetime, station managers have said.

"We're in a good position to go solve this problem," space station program manager Mike Suffredini told reporters this week. "It's a significant failure It's one we need to go after."

Wheelock and Caldwell Dyson will seal themselves inside the space station's Quest airlock overnight in a procedure known as a "camp out" among spacewalk planners. During the camp out, the airlock is kept at a lower pressure than the rest of the space station to help purge spacewalkers' bodies of nitrogen as a safety measure to prevent decompression sickness better known as the bends while working inside their NASA spacesuits.

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. Click here for space station mission updates and's NASA TV feed.

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