NASA considers fourth spacewalk

A decision on whether to hold a fourth spacewalk to unstick a nitrogen valve could be announced at a 7:45 p.m. EDT flight director's update broadcast on NASA-TV, a NASA spokesperson said.

Today NASA engineers tried unsuccessfully to use remote controls to fix the stuck valve on the cooling system at the International Space Station.

The problem arose during a spacewalk Tuesday after mission specialists Rick Mastracchio and Clay Anderson installed a fresh coolant tank outside the station.

Ground controllers couldn't activate the new ammonia supply. If the problem can't be fixed in the next few days the coolant system that serves the station's starboard side might need to be shut down for a time.

The station will enter a roughly two-week period in which the sun's angle keeps one side of the outpost hot and the other side cold.

Temperature swings cause the station's ammonia coolant to expand and contract, a process managed by a bellows within the ammonia tank hardware that is fed by nitrogen. The bellows wouldn't work without normal nitrogen flow.

If the valve remains shut, the ammonia loop serving the starboard side of the station might need to be shut down, which would be a problem for the outpost's ability to maintain a full crew of six.

NASA officials have discussed extending the mission another day for a fourth spacewalk or having the station crew perform the spacewalk once Discovery leaves Saturday morning. It's scheduled to arrive at Kennedy Space Center Monday morning.