HOUSTON — The U.S.-led International Space Station mission management team (MMT) deferred a “go/no go” decision Dec. 12 on whether to proceed with plans for the Dec. 18 launch of an Orbital Sciences Corp. resupply mission to the six-person orbiting science laboratory, while ground teams assess a faulty flow control valve in one of two external cooling loops.

The balky valve, housed in an external pump control module, forced NASA’s Mission Control to shut down noncritical systems tied to the crippled Loop A cooling system in three U.S. segment modules and shift other cooling responsibilities to the station’s Loop B backup on Dec. 11.

The MMT was assessing repair options, including a near-term U.S. spacewalk, the first since a July 16 incident in which the helmet of the NASA spacesuit worn by Italian astronaut Luca Parmitano flooded with water, forcing an abrupt end to the excursion. The window for Orbital Sciences’ Antares/Cygnus launch from the Mid-Atlantic Regional Spaceport on Virginia’s Eastern Shore extends to Dec. 18-21 and possibly Dec. 22, according to Kenny Todd, NASA’s ISS operations integration manager and chair of the MMT. The MMT deferred the “go/no go” launch decision it planned to make Dec. 12 until Dec. 16, but did not rule out acting sooner if troubleshooting of the crippled Loop A flow control valve warrants.

The ISS cooling system dissipates heat from the station’s many electronic systems, including life support and research hardware. Neither was affected by the latest cooling system restrictions.

The flow control valve regulates the temperature of ammonia circulating through an external ISS radiator that juts from the vaulting solar power truss as the coolant reaches a heat exchanger in the Node 2, or Harmony module. The Cygnus resupply capsule and its 3,230 lb. of cargo are to berth at Harmony once the freighter reaches the ISS. Harmony and two other modules in which noncritical systems were deactivated, Europe’s Columbus and Japan’s Kibo science modules, are cooled by water loops that reach the same heat exchanger.

The flow control valve shut the Loop A pump down on Dec. 11 when the ammonia temperature dropped too low for the water to circulate. The Loop A pump was restarted while ground controllers adjusted the valve to determine if the cooling loop could be recovered — at least temporarily. “This is a position we don’t want to be in long term. So the team is continuing to work through the fault tree of what might be going on with this particular flight control valve,” Todd said. “In the meantime, we have a good stable configuration and the crew is in good shape. They are continuing to go about their day-to-day activities.”

The ISS faced a similar issue in August 2010, when the Loop A pump motor failed, prompting a closely timed sequence of three difficult spacewalks by space station astronauts Doug Wheelock and Tracy Caldwell Dyson to replace a module housing the pump. The faulty flow control valve is housed in an identical pump module that would be replaced if a spacewalk becomes necessary. The ISS is equipped with three spare pump modules.

However, U.S. spacewalks were suspended following a troublesome July 16 outing by Parmitano and NASA’s Chris Cassidy. The spacewalk was halted abruptly when Parmitano’s helmet began to fill with water that massed around his eyes, ears and nose, forcing him to retreat to the safety of the U.S. airlock.

The Orbital Sciences mission represents the company’s first under a $1.9 billion, eight-flight Commercial Resupply Services contract reached with NASA in late 2008. The company qualified for the contract with a successful demonstration flight to the ISS flown under NASA’s Commercial Orbital Transportation Services program in September and October.

The latest Cygnus freighter is to follow a three-day trip to the ISS and remain berthed for 42 days.