Space Station Astronauts Set for Dramatic Spacewalk Repairs

Two spacewalking astronauts ventured outside the International Space Station Saturday to begin a major repair job on the outpost's main cooling system.

Clad in their NASA spacesuits, American astronauts Douglas Wheelock and Tracy Caldwell Dyson floated outside the station at 7:19 a.m. EDT (1119 GMT) to replace a large oven-size pump that moves liquid ammonia through the right side of the orbiting laboratory's cooling system.

It is the first of two unplanned spacewalks the astronauts will take to complete the complicated space station repair. A second spacewalk is set for Wednesday.

The ammonia pump failed July 31, knocking out half of the space station's cooling system and forcing astronauts to turn off some experiments and systems, as well as leave others without backups, in order to prevent the station from overheating. A tripped circuit breaker, likely caused by a power spike, caused the malfunction, station managers have said. [Graphic: Space Station's Cooling System Problem Explained]

Space station managers have said the pump malfunction is a major failure for the 12-year-old orbiting lab, enough so that engineers had already drawn up repair plans on the off chance it occurred.

The International Space Station uses liquid ammonia to cool its onboard systems by transporting waste heat to a network of radiators mounted to its main truss. There are two main cooling system loops ? Loop A and Loop B. The failed pump is in Loop A, while the other cooling loop remains operational.

Wheelock and Caldwell Dyson are expected to spend seven hours working to replace the station's stricken Loop A cooling system pump during their spacewalk. They will physically remove and replace the pump during the spacewalk, and plan to hook up the new pump's electrical and ammonia plumbing connections during the second spacewalk on Wednesday.

The failed ammonia pump is located on the station's right side truss and will be replaced with one of four spare pumps stored at the orbiting lab.

Each pump weighs 780 pounds (353 kg) and is 5 1/2 feet long (1.6 meters) by 4 feet wide (1.2 meters). They are about 3 feet tall (almost 1 meter). Caldwell Dyson has said the pumps are about the size of a laundry dryer.

Wheelock and Caldwell Dyson were initially slated to perform a different spacewalk on Thursday. That spacewalk was aimed at preparing the space station for future construction work. Those tasks have been postponed for now.

Saturday's spacewalk marks the first for Caldwell Dyson and the fourth for Wheelock.

• FAQ: International Space Station Cooling System Pump Failure
• Poll: Do We Need a New International Space Station?
• Graphic: Inside and Out: The International Space Station

NASA is broadcasting the International Space Station spacewalk repairs live from space on NASA TV. Click here for space station mission updates and SPACE.com's NASA TV feed.