Delay Will Give Additional Time For Refining Procedures

NASA has decided to wait until Friday to conduct a spacewalk to replace a failed ammonia pump module on the International Space Station.

Flight Engineers (L-R) Tracy Caldwell Dyson and Doug Wheelock prepare spacesuits in the ISS Quest airlock. NASA Photo

Mission managers, program managers, flight controllers, engineers, astronauts and spacewalk experts made the decision Monday evening after continuing to analyze and refine engineering requirements, and reviewing the results of an underwater practice session.

Expedition 24 astronauts Doug Wheelock and Tracy Caldwell Dyson currently are scheduled to start the repairs on the station’s starboard truss Friday. Fellow astronauts Cady Coleman and Suni Williams spent the 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 Expedition 24 flight engineer Shannon Walker to guide 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 today remains in a stable configuration. Most of the crew's planned activities this week have been cancelled or deferred in order to support spacewalk preparations.

Plans are in work to move the station’s Mobile Transporter into position on the Starboard 1 truss on Tuesday. With the Mobile Transporter positioned early, the team will be able to gather additional data to confirm power resources are sufficient to use the arm to support the spacewalk.

Each pump module weighs 780 pounds and is 69 inches long by 50 inches wide, and is 3 feet tall. 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.

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.

FMI: www.nasa.gov