The alternative is to delay the launch to allow two ISS crew members to conduct two or three emergency spacewalks starting this weekend to replace the faulty pump.
Spare pumps are stored on the space station's truss – scaffolding the length of a football field. The truss supports the station's solar panels, radiators for station cooling, and other utilities, including two external cooling loops that transfer heat to the radiators.
The cooling-system malfunction on the space station occurred Dec. 11. Controllers noticed unusually low temperatures in ammonia circulating through one of the cooling loops. Left unchecked, the chilled coolant could have frozen water flowing through a heat exchanger inside the station.
That could have damaged the exchanger and leaked ammonia into the station, said Judd Frieling, the lead flight director for Expedition 38, the space station's current increment, during a televised update Tuesday.
Controllers were able to reroute cooling to the second external loop, but the transfer meant the crew had to shut down nonessential equipment in order to reduce the heating load on the second loop.
Engineers traced the problem to a malfunctioning valve designed to adjust the flow of coolant. The coolant's temperature should remain within an optimum range for cooling the station's interior and some of its exterior hardware.
During the past few days, engineers have come up with with a temporary work-around that could allow for the crew to avoid having to scrap the station resupply mission for emergency spacewalks.
The short-term fix involves manipulating another valve in the pump that isolates the loop's radiators. The isolation valve normally is either open or closed, but by adjusting the valve from the ground – activating it briefly, then shutting off power to it when it reached a partially closed position – controllers could get it to mimic the variable settings of the failed valve.
To avoid an immediate emergency spacewalk, however, the valve needs to be more responsive. Software engineers worked overnight Monday to design a software patch that would knock the response lag time down from between two- and three-tenths of a second when operated manually to about one-tenth of a second via onboard computer, Mr. Frieling explained.
“Hopefully by this afternoon, we'll be able to uplink it to the vehicle and start to re-learn how to manipulate that isolation valve such that we can control the temperatures of the external loop pretty reliably,” Frieling said Tuesday.
The results of applying the software patch and the impact on spacewalks and the resupply mission could be announced Wednesday morning, says NASA spokesman Josh Byerly.
In the meantime, US crew members Richard Mastracchio and Mike Hopkins have been testing their space suits and reviewing procedures for two or three spacewalks that would be needed to swap out the balky pump. If called for, the spacewalks could begin Friday or Saturday.
The station currently hosts a six-member crew led by station commander and Russian cosmonaut Oleg Kotov. The crew consists of two other cosmonauts, Mikhail Tyurin and Sergey Ryazanskiy, the two Americans, and Japanese astronaut Koichi Wakata. If spacewalks are needed, Dr. Wakata would operate Canadarm 2, a 56-foot-long robotic arm designed to support maintenance tasks outside the station.
Orbital Sciences Corporation also continued its preparations Tuesday to launch its resupply mission to the space station. This would be the company's first commercial resupply mission after a successful demonstration mission in October. The mission is scheduled to launch from Wallops Island on the Virginia coast at 9:19 p.m. Eastern Standard Time Thursday.
The company's Antares rocket is slated to loft an Orion cargo capsule carrying 2,780 pounds of supplies ranging from spacewalk tools and crew supplies to science experiments.
Orbital Sciences Corporation is one of two companies NASA tapped to resupply the station. The other, Space Exploration Technologies Corporation, already has flown two formal resupply missions and is slated to launch four more next year.