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Astronauts aboard the international space station will venture outside on three spacewalks to replace a malfunctioning pump that has shut down part of the station’s cooling system, NASA announced Tuesday.

The space agency also postponed until January the launching of a cargo supply ship, which had been scheduled to lift off Thursday.

The first spacewalk is Saturday, with the second Monday and the third next Wednesday.

Two loops of circulating ammonia cool equipment on the station. The problem started last Wednesday with a malfunction of a valve inside the pump, which is on one of the station’s exterior trusses.

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Flight controllers shut down that cooling loop. The remaining loop is sufficient for regulating the temperature of critical equipment, and NASA said there was no immediate danger to the six crew members.

NASA managers want both cooling loops to be operating before the cargo ship is sent into orbit.

During the spacewalks, each lasting 6½ hours, the NASA astronauts Richard Mastracchio and Michael Hopkins will remove the malfunctioning pump module, which had been installed three years ago, and replace it with a spare one.

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