Success! International Space Station astronauts replace vital pump.

Two space station astronauts hit pay dirt in orbit Monday after successfully replacing a vital pump to restore their spacecraft's cooling system, which has been limping along at half-power for more than two weeks.

Astronauts Douglas Wheelock and Tracy Caldwell Dyson wired up a bulky ammonia pump the size of a kitchen oven into the right side of the space station during the hours-long spacewalk. It was the third spacewalk for the astronauts to make the tricky repair 220 miles (354 km) above Earth. A fourth spacewalk is planned for later to clean up some final work.

"Game over!" Wheelock said as they wrapped up the pump replacement work. Mission Control used remote commands to start filling the pump with liquid ammonia and plans to start testing it on Tuesday.

The space station's cooling system has been running at half-strength since July 31, when an electrical short shut down one of two pumps that move liquid ammonia through the system. [Graphic: Space Station's Cooling System ProblemExplained]

The failure forced astronauts to turn off some experiments and systems, as well as leave others without backups, in order to prevent the station from overheating.

Critical pump failure

If the pump repair continues to check out, NASA plans to begin testing the space station's cooling system on Tuesday. The system could be fully reactivated on Thursday if everything goes well.

"It's not going to be an immediate process. There's not one switch we can flip, obviously," space station flight director Courtenay McMillan said last week.

NASA station managers have said the ammonia pump failure has been one of the most challenging repairs for the International Space Station ever attempted. The cooling system is so critical to station operations that a pump repair is one of 14 major failures for which NASA engineers have prepared emergency plans for in advance, they added.

While the space station crew tackled the problem in space, a team of engineers and astronauts on Earth worked round-the-clock to come up with the repair plans.

It took two spacewalks just to remove the broken pump from its housing. That disabled pump is parked in a temporary spot and must be moved to its final perch on a spare parts platform in yet another spacewalk.

There are four spare ammonia pumps on the space station, one of which was used for this repair. Each pump weighs 780 pounds (353 kg) and is 5 1/2 feet long (1.6 meters) by 4 feet wide (1.2 meters). The replacement pump used Monday was delivered in 2006.

Smooth spacewalk

Monday's spacewalk began at 6:20 a.m. EDT (1020 GMT), about 35 minutes ahead of time. The astronauts swiftly tackled their first chore: removing the new pump from a spare parts platform on the station's exterior.

"You know, when you're on the [spare parts platform] looking down at the Earth like this, it's like extreme hang gliding," Caldwell Dyson said.

One of four bolts securing the pump in place gave Wheelock some trouble when it refused to budge, but some extra elbow grease and tools did the trick.

"Come on now, bolt," Wheelock said as he freed the stuck bolt."Turning! Sweet!"

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. The broken pump was delivered to the space station in 2002 and was activated in 2006.

Spare parts key for station

The $100 billion International Space Station is currently home to six astronauts; three Americans and three Russians. Spare parts are a key concern since the station is slated to keep flying through at least 2020. NASA plans to fly two more shuttle missions (in November and February 2011) before retiring its three remaining space shuttles for good. A potential extra shuttle flight, which would launch next summer if approved, is also under discussion in Congress.

Once the space shuttle fleet is retired, NASA will rely on spacecraft from Russian to launch crews and cargo to the space station until American-built commercial spaceships or new government vehicles become available. Unmanned cargo ships built by the Japanese and European space agencies are also expected to keep the station stocked with supplies.

Monday's spacewalk was the fifth career spacewalk for Wheelock and the third for Caldwell Dyson. It was also the 150th spacewalk dedicated to space station assembly and maintenance.

The International Space Station has been under construction since 1998 by 15 countries and five different space agencies.

• Graphic: Inside and Out: The International Space Station
• FAQ: International Space Station Cooling System Pump Failure
• NASA: 4 Spacewalks Needed to Complete Space Station Repairs

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