CAPE CANAVERAL, Fla. – Half the international space station’s cooling system suddenly shut down during the weekend, forcing the astronauts to power down equipment and face the likelihood of urgent spacewalking repairs.

After huddling Sunday, NASA managers gave preliminary approval for a pair of spacewalks, the first of which would take place late this week.

Two of the Americans on board were already scheduled to conduct a spacewalk Thursday for routine maintenance, although the repairs would supersede the original chores.

Officials stressed that the six occupants were in no danger and that the orbiting complex was in a stable situation.

The trouble arose Saturday night, when one of the two ammonia-fed cooling loops shut down. Alarms sounded throughout the sprawling outpost as the circuit breaker for the pump in that line tripped, causing the pump to stop working.

The cooling system is critical for on-board operations. The two ammonia lines ensure that all the station’s electronic equipment does not overheat and with one of those out of commission, there is no safeguard in case of a second failure.

Astronaut Tracy Caldwell Dyson hustled through the equipment shutdown procedures and, with crewmate Douglas Wheelock, installed a jumper cable to keep all the rooms cool.

Flight controllers tried to restart the disabled ammonia pump early Sunday, but the circuit breaker tripped again. No further repair attempts were planned, at least for now.

In fact, the astronauts were allowed to sleep in because of all the late-night disruptions.

Any repairs this week will involve replacing that ammonia pump, a difficult job that would require two spacewalks. Two spare pumps are stored on the outside of the station.

The first repair spacewalk likely would occur Thursday at the earliest, with the second excursion two or three days later. A final decision on whether to proceed with the task will be made after additional engineering review.

Among the equipment powered off for now: the GPS circuit, several power converters and a set of devices that route commands to various equipment.

On board are three Americans and three Russians.