Two U.S. astronauts successfully removed a stubborn cooling pump outside the International Space Station on Wednesday.

Doug Wheelock and Tracy Caldwell Dyson returned to the station after their 7-hour 26-minute spacewalk, where they were confined a bit longer in a secure airlock to make sure no dangerous ammonia had clung to their spacesuits.

During their sojourn, the two astronauts removed the defective pump on the outside of the orbiting ISS that has interfered with the station’s cooling system since July 31. A new pump will be installed in the coming days.

While disconnecting the pump, the two had to disconnect tubes that carry ammonia through the cooling system.

On Saturday, the hose leaked ammonia while they were trying to disconnect it, preventing the team from removing the faulty pump. But on Wednesday, ground control had reduced the pressure of the ammonia before the spacewalk, enabling the crew to avoid “time-consuming work,” a NASA official said during the live broadcast of the mission.

While the connector hose was “a bit stubborn” again on Wednesday, “Wheelock used a bit of muscle to eventually get it de-mated,” the official said.

The astronauts removed the faulty 350-kg cooling loop pump and put it in a special storage place outside the station after unplugging five electrical connections and removing four bolts.

In the coming days, Sunday at the earliest, ISS spacewalkers plan to go out again to install the replacement pump that is also stored outside.

The problem with the cooling system forced a shut-down of some systems aboard the ISS.

The cooling system is one of two used to keep electronics on the ISS from overheating. Alarms were triggered on July 31 signalling that the ammonia-based refrigeration system on the ISS had failed due to a power surge.

NASA officials stressed that the six crew members were in no danger, and all critical and many non-critical systems on the ISS were since operating as normal despite the malfunction.

The ISS could continue to operate without the cooling system indefinitely, but would be in trouble if the second ammonia cooling loop were to also fail before the first is replaced.

The U.S. space agency had known that the cooling unit would eventually need to be replaced after about 1,00,000 hours of use and had installed four extra units outside the ISS as a precaution.

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