Astronauts Douglas Wheelock and Tracy Caldwell Dyson Kick off Six-Hour Spacewalk

Astronauts Douglas Wheelock and Tracy Caldwell Dyson, floating in the Quest airlock module, switched their spacesuits to internal battery power at 8:27 a.m. to officially kick off a planned six-hour spacewalk. The primary goal of the excursion is to remove a shorted ammonia pump module so a replacement can be installed during a spacewalk Sunday.

Photo: Astronauts Douglas Wheelock and Tracy Caldwell Dyson Kick off Six-Hour Spacewalk

Astronauts Douglas Wheelock and Tracy Caldwell Dyson Kick off Six-Hour Spacewalk

The astronauts attempted to remove the faulty pump during a spacewalk Saturday, but one of four ammonia line quick-disconnect fittings leaked, presumably because of problems with an internal valve on the outboard side of the fitting, Spaceflight Now reports.

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.

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.

The space station is meant to continue working until 2020. NASA will have to rely on Russia and other countries for crew and cargo transport once the shuttle fleet is retired next year, according to The Associated Press.