Doug Wheelock and Tracy Caldwell Dyson are conducting the third spacewalk to replace a failed ammonia pump module on the station's S1 Truss. They set their spacesuits to internal battery power at 5:20 a.m. CDT, starting their excursion 35 minutes earlier than planned.
Their goal is to install a spare pump module, replacing the unit that failed on July 31 that resulted in one of the station's two coolant loops to shut down. The failed pump was removed from the truss during the spacewalk last Wednesday.
The spare to be installed was delivered to the station in July 2006 on the STS-121 mission and was mounted to a spare parts platform adjacent to Quest from where Monday's spacewalk is being staged.
Wheelock and Caldwell Dyson will maneuver the spare pump from the External Stowage Platform 2 to the S1 truss, where they will then mechanically attach it by driving a set of four bolts and mating five electrical connectors. At that point, if all goes as planned, ground controllers 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 temporarily maintaining 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, followed by the connection of three other fluid lines to permit the pump to finally be filled with coolant from an Ammonia Tank Assembly.
If time permits, the spacewalkers will close out their work by attaching a cable extension from Quest to an existing power cable on the Unity node. The extension is required to avoid interference when the Permanent Multipurpose Module is mated to the earth-facing side of Unity during the STS-133 mission in November.
The cable installation was one of the tasks scheduled for the original spacewalk Wheelock and Caldwell Dyson were to have conducted before the pump module failed two weeks ago.