Engineers have checked out a fresh coolant pump installed on the International Space Station during a spacewalk Tuesday and begun restarting science experiments and other systems knocked offline Dec. 11 by a faulty ammonia valve, NASA officials said Thursday.

Astronaut Mike Hopkins on a spacewalk Tuesday to replace a degraded pump module on the International Space Station. Credit: NASA
 
The tricky repair work, accomplished on two spacewalks Saturday and Tuesday, brought the space station's external cooling system back to full capability and clears the way for a commercial resupply flight by Orbital Sciences Corp. in January.

Satisfied the new pump module was healthy, engineers at mission control in Houston reactivated the space station's coolant loop A early Wednesday to begin the process of turning back on experiments and other systems shut down after a valve malfunctioned Dec. 11, according to a NASA update Thursday.

The orbiting complex has two coolant loops to dissipate heat from the space station's avionics systems. The system works by flowing ammonia through heat exchangers, where the ammonia picks up heat from water that circulates through the space station's pressurized modules to collect thermal energy from heat-generating computers and other hardware.

Some of the space station's non-essential systems, including experiment hardware, were powered down after a valve inside the pump module malfunctioned Dec. 11. The valve regulates the flow of ammonia through coolant loop A, and the stuck valve caused the ammonia to become too cold.

Officials worried the cold ammonia would cause the water passing through the heat exchangers to freeze, so they shut off the flow of ammonia to the heat exchangers, removing vital cooling capability from a portion of the space station's internal systems.

Without cooling, mission control turned off the systems serviced by coolant loop A until they could regulate the temperature of the ammonia. After initially studying other ways of controlling the ammonia flow and temperature, engineers opted to send astronauts Rick Mastracchio and Mike Hopkins on a pair of spacewalks to remove the degraded pump module and replace it with one of several spares stored outside the space station.

Mastracchio and Hopkins unplugged and removed the pump module with the malfunctioning valve Saturday. During the Christmas Eve spacewalk, the astronauts moved a spare refrigerator-sized pump module into position on the space station's starboard truss before reconnecting ammonia and electrical lines.

The two spacewalks lasted nearly 13 hours.

Engineers at mission control in Houston switched on the new pump module Tuesday night. After initial checks, they allowed ammonia in coolant loop A to circulate through heat exchangers for the Destiny laboratory, the Harmony and Tranquility nodes and the Japanese Kibo laboratory.

The European Space Agency's Columbus laboratory, which was also affected by the coolant system shutdown, will reintegrated with coolant loop A next week when ground teams return from a holiday break, according to NASA.

The Dec. 11 valve malfunction delayed the launch of an Orbital Sciences Corp. commercial cargo mission to the space station. Originally planned for mid-December, the launch of the Orbital Sciences Cygnus resupply spacecraft on an Antares rocket is now scheduled for no earlier than Jan. 7.

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