US astronauts Rick Mastracchio and Mike Hopkins completed a 7.5 hours spacewalk to replace the faulty ammonia pump whose internal control valve failed earlier in December 2013, NASA said.

Following two spacewalks to replace a degraded pump module on the truss, or backbone, of the ISS, flight controllers in the Mission Control Center at NASA's Johnson Space Center in Houston successfully restarted the new pump.

The pump module controls the flow of ammonia through cooling loops and radiators outside the space station, and, combined with water-based cooling loops inside the station, removes excess heat into the vacuum of space.

The new pump now is considered fully functional, but it will take some time to fully reintegrate the pump and Loop A of the two-loop external cooling system, NASA said. Despite concerns about leaking spacesuits, astronauts reported no problems.

The suits have "functioned perfectly and have been bone dry" throughout the course of the spacewalk, NASA said. Expedition 38 Flight Engineers Hopkins and Mastracchio retrieved a replacement pump from an external stowage platform near the end of the station's backbone, and installed it during a 7 hour, 30 minute spacewalk on Christmas Eve, December 24.

Engineers at mission control sent a series of commands to the new pump module at the end of spacewalk to ensure that ammonia, an excellent thermal conductor, was flowing to the new pump module.

Main tasks of the spacewalk included the removal and installation of a spare pump module. The first task was to remove the spare pump module from the space station's External Stowage Platform-3.

Hopkins attached himself to the Canadarm2 and took a ride to the worksite. Mastracchio tethered himself to the station and translated to the S1 truss to assist his partner. While doing the connection work, the duo demated ammonia fluid lines from a jumper box that enabled ammonia flow during the repair spacewalks.

After experiencing some difficulty disconnecting a fluid line the spacewalkers reported seeing ammonia flakes escaping a valve. Once they were back in the Quest airlock the duo conducted ammonia decontamination procedures on their spacesuits, the space agency said.

All four fluid lines were successfully reconnected to the newly installed pump module restoring ammonia flow. Hopkins and Mastracchio completed electrical connections to the pump module, power was successfully restored to the ammonia pump module.

This was the 176th spacewalk in support of space station assembly and maintenance.


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