CAPE CANAVERAL, Fla. - The 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 key 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 onboard equipment.

Mission Control ordered the six-man crew to turn off some science experiments and other non-critical equipment; the powerdown continued Thursday. NASA officials stressed that the astronauts were safe and comfortable.

The suspect valve is inside an external pump that was replaced by astronauts three years ago. A software repair would be the easiest fix, a spacewalk the most complicated. The valve can't be reached so the entire pump would have to be replaced with one of the spares at the space station.