NASA has decided to wait until Aug. 6 to conduct a spacewalk to replace a failed ammonia pump module on the International Space Station. The spacewalk had originally been planned for Aug. 5. Mission managers, program managers, flight controllers, engineers, astronauts and spacewalk experts made the decision the night of Aug. 2 after continuing to analyze and refine engineering requirements and reviewing the results of an underwater practice session, the space agency reported.
Expedition 24 astronauts Doug Wheelock and Tracy Caldwell Dyson currently are scheduled to start the repairs on the station's starboard (right) truss Aug. 6. NASA reported 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.
The space agency noted the station remains in a stable configuration and the crew is on a normal sleep shift and supporting a normal workday, but most of their planned activities this week have been canceled or deferred in order to support spacewalk preparations. 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 Aug. 9, NASA reported.
The pump failed July 31 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," a NASA report stated. "The station's crew worked with Mission Control to put the station in a stable configuration."
Each pump module weighs 780 pounds and is 5 feet, 6 inches by 4 feet by 3 feet. According to the mission rundown on NASA's site, the spacewalkers will need to disconnect and reconnect five electrical connectors, four fluid quick-disconnect devices, one fixed grapple bar and four bolts. The space agency also noted 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.
Wheelock, who will be designated EV1 (extravehicular crew member 1) and wearing a spacesuit bearing red stripes, will be making the fourth spacewalk of his career, while Caldwell Dyson, designated as EV2, wearing an unmarked spacesuit, will be making her first spacewalk. NASA TV coverage will begin at 6 a.m. and Wheelock and Caldwell Dyson will begin the spacewalk at 6:55 a.m. on Aug. 6.