Expedition 24 astronauts Doug Wheelock and Tracy Caldwell Dyson, both flight engineers, currently are scheduled to start the repairs Friday during the first of two spacewalks.

Mission managers, program managers, flight controllers, engineers, astronauts and spacewalk experts rescheduled the spacewalk, originally planned for Thursday, after continuing to analyze and refine engineering requirements, and reviewing the results of an underwater practice session.

Fellow astronauts Cady Coleman and Suni Williams spent Monday afternoon in the Johnson Space Center's Neutral Buoyancy Laboratory practicing underwater the tasks needed to restore the cooling loop over the course of two spacewalks.

Meanwhile, robotics experts are continuing to refine the procedures that will be used by Flight Engineer Shannon Walker to guide the station's robotic arm, Canadarm2, as she moves Wheelock into position to swap the failed unit with a spare unit currently stored on External Stowage Platform 2.

That spare parts carrier is attached to the Quest airlock that Wheelock and Caldwell Dyson will use to exit and reenter the station.

The station remains in a stable configuration. The crew is on a normal sleep shift, and supporting a normal workday, but most of the planned activities this week have been cancelled or deferred in order to support spacewalk preparations.

Wheelock, Caldwell Dyson and Walker reviewed spacewalk procedures Tuesday, taking a look at the activities to be performed during the excursion as well as familiarizing themselves with the steps needed to decontaminate the spacesuits should ammonia crystals seep out of the pump module connectors.

Also on Tuesday the station's Mobile Transporter was commanded to move to the Starboard 1 truss. With the Mobile Transporter in position, the ground team will be able to gather additional data to confirm power resources are sufficient for Canadarm2 to support the spacewalk.

Each pump module weighs 780 pounds and is 5 .5 feet long (69 inches) by 4 feet wide (50 inches), and is 3 feet tall (36 inches). The spacewalkers will need to disconnect and reconnect five electrical connectors, four fluid quick-disconnect devices, one fixed grapple bar and four bolts.

The spare pump module that will be used to replace the failed unit was delivered to the station on the STS-121/Utilization Logistics Flight-1 mission in July 2006.

The pump failed Saturday night after a spike in electrical current tripped a circuit breaker. When the 780-pound pump failed, it shut down half of the station's cooling system. Efforts to restart the pump, which feeds ammonia coolant into the cooling loops to maintain the proper temperature for the station's electrical systems and avionics, were not successful. The station's crew worked with Mission Control to put the station in a stable configuration.

NASA TV coverage will begin at 6 a.m. EDT Friday and Wheelock and Caldwell Dyson will begin the spacewalk at 6:55 a.m. In the first spacewalk, they will unbolt and remove the failed pump module, and install the spare. A second spacewalk to hook up a variety of electrical and fluid connections for the new pump module is targeted for Monday.

Wheelock, who will be designated as EV1, or extravehicular crew member 1, wearing the spacesuit bearing the red stripes, will be making the fourth spacewalk of his career, while Caldwell Dyson, designated as EV2, wearing the unmarked spacesuit, will be making her first spacewalk.

Wheelock and Caldwell Dyson originally were scheduled to perform a spacewalk to outfit the Russian Zarya module for future robotics work and prepare the station for the installation of a new U.S. permanent multipurpose module. However, because of the importance of restoring redundancy to the station's cooling and power systems, the two new spacewalks will be dedicated to the pump module replacement.

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