By James Dean
CAPE CANAVERAL, Fla. - Trouble with the International Space Station's cooling system could delay next week's planned launch of a cargo resupply mission and might require spacewalks to fix.
The station Thursday was operating with only one of its two ammonia coolant systems after one automatically shut down Wednesday apparently because of a valve problem.
That required nonessential systems to be powered down in several modules, limiting the amount of science research that could be performed.
The station's six-person crew was not in danger, but loss of the second coolant loop or other critical systems could present a crisis.
"While we're sitting at one loop, then I think we're somewhat vulnerable," said Kenny Todd, NASA's mission operations integration manager for the station program. "Our intention would be to try to move sooner rather than later to recover that functionality."
Station partners will meet Monday to review the timing of Orbital Sciences Corp.'s planned Wednesday launch from Virginia of an Antares rocket with an unmanned Cygnus cargo spacecraft, Orbital's first flight under a $1.9 billion resupply contract.
With some station systems now lacking backup, managers Thursday could not recommend the launch as planned, and they do not want to distract attention from the coolant issue.
Two ammonia coolant loops running outside the station dissipate heat generated from all the station's electronic systems.
Loop A shut down when it got too cold and risked freezing water that flows through heat exchangers.
Early investigation focused on a flow-control valve that Todd said acted as a "mixing valve" that helped regulate the ammonia's temperature.
Ground controllers Thursday sent commands to manipulate the valve and see how it responded.
If no near-term fix materializes, teams will begin planning spacewalks to remove and replace the 780-pound pump module that houses the valve, which cannot be replaced alone.
The pump module was installed during emergency spacewalks three years ago after the previous module stopped pumping coolant.
"That was a failure to be able to move the ammonia," Todd said. "What we're having here is a failure to be able to control the temperature of the ammonia."
Plans in 2010 called for two spacewalks - one to remove the pump module and another to install the spare - but three ultimately were required.
Before that repair, NASA said four spare pump modules were stowed outside the station.
If spacewalks are deemed necessary, they would look much the same as in 2010.
However, lessons learned from the most recent U.S. spacewalk, which was aborted when a spacesuit malfunction caused Italian astronaut Luca Parmitano's helmet to fill with water, will complicate the planning.
Todd said additional steps would be taken "to ensure that we've got the crew member protected."
A replacement part for at least one spacesuit was set to launch on Orbital's upcoming flight, but NASA did not say it was essential to performing spacewalks.
The swapping out of an ammonia coolant pump is one of 14 major repairs for which all U.S. crew members train before launching on six-month expeditions.
The current Expedition 38 crew includes Russian cosmonauts Oleg Kotov, Sergey Ryazanskiy and Mikhail Tyurin; Americans Mike Hopkins and Rick Mastracchio; and Koichi Wakata of Japan.