CAPE CANAVERAL, Fla. – Half of 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 later this week. Two of the Americans on board were already scheduled to conduct a spacewalk Thursday for routine maintenance, though 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.

The cooling system is critical for onboard 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.

On board are three Americans and three Russians.