Space station astronauts tackle urgent repair job

Two space station astronauts have ventured out on a spacewalk to make urgent repairs.

Two space station astronauts have ventured out on a spacewalk to make urgent repairs.

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Shortly after sunrise Saturday, NASA astronauts Douglas Wheelock and Tracy Caldwell Dyson exited the International Space Station.

Their goal is to replace a broken ammonia cooling pump. Officials say it's some of the most challenging repair work ever attempted at the orbiting complex. The job is so difficult, in fact, that two spacewalks are needed.

The ammonia pump shut down last weekend and knocked out half of the space station's cooling system.

The pump is supposed to push ammonia through the lines and prevent equipment from overheating. Engineers have worked nonstop over the past week to come up with the emergency repair plan.