Trouble with one of the International Space Station's external coolant loops, used to dissipate the heat generated by the lab's electronics systems, triggered an extensive powerdown late Saturday. NASA managers met Sunday and gave preliminary approval to a difficult two-spacewalk repair job, starting as early as Thursday, to restore the critical system to normal operation.

It is not yet clear what went wrong, but the ammonia pump module that is part of coolant loop A, mounted on the right side of the station's main power truss, failed around 8 p.m. EDT Saturday. A problem somewhere in the system caused a circuit breaker to trip, setting off multiple alarms and waking the crew.

With half the station's cooling gone, flight controllers were forced to shut down two of the station's four U.S. control moment gyroscopes, used to help maintain the lab's orientation in space, one communications channel, several solar power current converters and a variety of computer control boxes known as multiplexer-demultiplexers.