Think you have a productive Saturday ahead? Try this: NASA astronauts are getting up bright and early to hop out into space and fix an important cooling loop pump aboard the International Space Station.

The two spacewalkers, rookie Michael Hopkins and six-time veteran Rick Mastracchio, plan to replace a faulty pump in an external ammonia cooling loop that shut down last week after it got too cold.

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It’s going to be a complex job, spanning a planned three separate spacewalks – and the last one is set for Christmas Day. The astronauts will have to pull a pump module about the size of a refrigerator out of one part of the space station, carefully disconnect it, and then bring over another refrigerator-size spare to replace it. For this spacewalk, they’ll start disconnecting the faulty pump and start some work on the spare one.

This spacewalk is set for 4:10 a.m. Pacific Time, and NASA coverage starts an hour earlier. It would be the first U.S. spacewalk since Italian astronaut Luca Parmitano nearly drowned in his spacesuit during the summer. The suit has since been examined and repaired, officials said.

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