The pump serves one of two cooling loops outside the space station that get rid of heat from the station's interior and from electrical systems on the station's exterior.
The space agency announced its decision Tuesday evening, after engineers uploaded a software patch in hopes of returning the pump to service, at least temporarily.
The pump, which circulates ammonia through its cooling loop, was taken off-line Dec. 11 after controllers noticed that the coolant was running too cold, a condition that could have damaged the heat exchanger through which coolant circulates inside the station.
Engineers traced the problem to a flow-control valve that was used to regulate the coolant's temperature. They found that they could use a valve in another part of the pump module as a temporary stand-in. But to use it effectively, the team needed to make changes in the pump's control software.
Engineers uploaded the software patch Tuesday, “and they were able to start controlling the temperature to a certain degree,” explains Josh Byerly, a spokesman at NASA's Johnson Space Center in Houston, in an e-mail. But NASA officials opted for the spacewalks to replace the pump with one of the spares the station carries on external storage platforms.
US astronauts Richard Mastracchio and Mike Hopkins are scheduled to exchange the balky pump for a fresh one during three 6-1/2-hour spacewalks. The spacewalks currently are scheduled for Dec. 21, 23, and 25. Japanese crew member Koichi Wakata will serve as the duo's crane operator – piloting the station's robotic arm from inside the station as the arm repositions the two units.
NASA's decision postpones Thursday's planned launch of a US resupply mission to the station. The launch of Orbital Sciences Corp.'s first commercial resupply mission is now scheduled to take place no earlier than Jan. 13, 2014.