A pair of spacewalking astronauts tackled urgent space station repairs on Wednesday for the second time in five days, anxious to avoid the kind of toxic ammonia leak that disrupted their previous effort to restore full cooling.
The International Space Station has been operating with only half its usual cooling capability ever since an ammonia pump failed 1 1/2 weeks ago.
Science research is on hold and unnecessary equipment is off until the pump can be replaced.
The cooling system is crucial for keeping electronics from overheating.
Mission Control wished astronauts Douglas Wheelock and Tracy Caldwell Dyson good luck as they got started on what's considered to be one of the most challenging repairs in the 12-year history of the orbiting complex.
"Do good work out there today," Mission Control radioed. "Help us take care of this pump"
Flight controllers lowered the pressure in the disabled cooling line one of two identical loops prior to Wednesday's spacewalk.
That should help prevent ammonia from spewing out like it did during Saturday's spacewalk.
The leak developed around a jammed connector on an ammonia hose; the spacewalkers had to hammer the connector loose, then plug it back in to stop the stream of ammonia.
This time, the spacewalkers hoped to isolate the troublesome connector by closing off valves that are located upstream, and by venting out any residual ammonia.
Their objective on Thursday was to remove the broken 355-kilogram pump, about the size of a bathtub. A spare would be installed during a third spacewalk Sunday.
NASA originally figured two spacewalks would suffice. The jammed connector and ammonia leak on the first outing, however, set everything back.
