Spacewalkers Doug Wheelock and Tracy Caldwell Dyson began their work on installing a spare ammonia pump module on the ISS at 6:20 am (1020 GMT) the US space agency said.

Flight Engineer Shannon Walker assisted the effort from inside the ISS by operating the station's robotic arm.

The ammonia pump failed unexpectedly on July 31 and was removed from the station during a spacewalk last Wednesday.

A first spacewalk to begin repairs on the cooling system took place on August 7 and lasted a record eight hours, three minutes.

Astronauts have been using a back-up cooling system, and NASA said conditions on the ISS are stable and the station's crew of three Americans and three Russians are not in danger.

earlier related report
Third Repair Spacewalk Set For Monday
After completing several days of analysis and refining procedures, International Space Station managers and mission operations personnel met Sunday and gave final approval for a third spacewalk Monday by Expedition 24 Flight Engineers Doug Wheelock and Tracy Caldwell Dyson to install a spare ammonia pump in the station's starboard truss as well as approval to restart and activate half of the station's cooling system that became inoperable two weeks ago due to a failure of the pump in the station's starboard truss.

The spacewalk Monday, which will be the third in the series of excursions to bring the station's cooling system back to full capability, is scheduled to begin at 5:55 a.m. Central time.

NASA Television coverage will begin at 5 a.m. CDT. It will be the 150th spacewalk in support of space station assembly and maintenance dating back to 1998, the sixth spacewalk of Wheelock's career and the third for Caldwell Dyson.

To prepare for the spacewalk, Wheelock and Caldwell Dyson will spend the night Sunday in the Quest airlock, beginning their "campout" to start purging nitrogen from their bloodstreams in advance of exiting Quest Monday morning for the planned 6 .5 hour excursion.

The goal of the spacewalk Monday will be to install the spare Pump Module into the S1 truss on the station, replacing the pump that failed July 31 causing one of two loops to shut down that provides cooling for the station's electronics. The failed pump was removed from the truss during the second spacewalk by Wheelock and Caldwell Dyson last Wednesday.

The spare to be installed Monday was delivered to the station in July 2006 on the STS-121 space shuttle mission of Discovery and was mounted to a spare parts platform adjacent to the Quest airlock from where Monday's spacewalk will be staged.

Wheelock and Caldwell Dyson will maneuver the spare pump from the external spare parts platform to the S1 truss, then will mechanically attach it by driving a set of four bolts before mating five electrical connectors.

At that point, if all goes as planned, flight controllers at Mission Control, Houston should see the replacement pump come to life by verifying electrical continuity and a brief spin up of the pump's driveshaft.

With the new pump checked out, Wheelock and Caldwell Dyson will then turn their attention to demating two fluid lines that are attached to a jumper box designed to temporarily maintain the proper pressure in the ammonia reservoir that feeds coolant to the pump itself.

Nitrogen will be vented from the replacement pump after the first fluid line is connected to the spare, followed by the connection of three other fluid lines to permit the replacement pump to finally be filled with ammonia from a coolant reservoir, called the Ammonia Tank Assembly.

Plans call for the revived Loop A of the station's cooling system to be temporarily powered off on Tuesday and the spare pump to be shut down for about an hour to allow flight controllers to reconfigure systems that have been cooled by Loop B since July 31.

The replacement pump and Loop A will then be powered back on to slowly bring up the cooling for power converter units, computer command routing systems, the Japanese Kibo Module, the European Columbus module and payload racks. Loop A should be fully operational once again by Thursday.

If time permits, Wheelock and Caldwell Dyson will close out the spacewalk by attaching a cable extension from Quest to an existing power cable on the Unity module that is required to avoid interference when the Permanent Multipurpose Module is mated to the earth-facing side of Unity on the STS-133 mission of the shuttle Discovery in November.

It was one of the tasks on the original spacewalk Wheelock and Caldwell Dyson were to have conducted before the Pump Module failure two weeks ago.

The failed pump will, for the time being, remain grappled to a payload attachment bracket on the starboard truss. No decision has been made as to whether a fourth spacewalk will be conducted by Wheelock and Caldwell Dyson to move it back to the external spare platform or defer that activity to a future spacewalk by a different crew.

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