CAPE CANAVERAL — Astronauts aboard the International Space Station dimmed the lights, turned off unnecessary equipment and put off science work Thursday as NASA scrambled to figure out what's wrong with a cooling unit.
One of two identical cooling loops shut down Wednesday when the line got too cold because of a faulty valve.
The system uses ammonia to dissipate heat from on-board equipment.
Mission Control ordered the six-man crew to turn off some science experiments and other noncritical equipment; the powerdown continued Thursday.
NASA officials stressed that the astronauts remained safe and comfortable.
The suspect valve is inside an external pump that was replaced by spacewalking astronauts three years ago. Flight controllers are looking at ways to fix the valve. A software repair would be the easiest option, a spacewalk the most complicated.
Three spacewalks were needed back in 2010 for the pump replacement.
The valve can't be reached so the entire pump would have to be replaced with one of the spares at the space station.
Kenny Todd, a space station manager, said the orbiting outpost is left "somewhat vulnerable" with only one good cooling line.
Two Americans currently are aboard the orbiting lab, as well as three Russians and a Japanese.