First spacewalk to repair station coolant system under way

American astronauts Doug Wheelock and Tracy Caldwell Dyson have begun the first of two spacewalks to repair a critical International Space Station coolant system.

The spacewalkers switched their spacesuits from station to battery power at 7:19 a.m. EDT and will soon float outside the Quest airlock.

This morning's spacewalk is expected to last about seven hours as Wheelock and Caldwell Dyson attempt to remove a failed pump module and replace it with a spare.

The size of an oven or clothes dryer, the bulky, 780-pound device pumps ammonia through a coolant loop serving the starboard side of the station's U.S. segment.

The pump failed a week ago, forcing the outpost's six residents to shut down a number of systems to prevent overheating. A second loop on the station's left side is working normally, but another failure without a backup loop available would be serious.

The failed pump is on the Starboard 1 truss segment a short distance from the Quest airlock, and the spare -- one of four -- is stored on a platform just below the airlock.

A second spacewalk targeted for Wednesday is expected to be needed to finish connecting four ammonia fluid lines, which may be stiff because they remain at full
pressure.

Shannon Walker, the third American on the station, which is also home to three Russian members of Expedition 24, will operate the station's 58-foot robotic arm from inside the Destiny Lab, maneuvering Wheelcok.

The spacewalk began about 25 minutes late after Caldwell Dyson had some trouble with the communications cap under her helmet. NASA says the problem was resolved.

This is the 148th spacewalk supporting space station assembly and maintenance, 120th to start from station airlocks and 15th by U.S. spacewalkers outside the station without a shuttle present.

It's the first spacewalk for Caldwell Dyson, and the fourth for Wheelock, who logged nearly 21 hours on the STS-120 shuttle mission flown by Discovery that relocated the Port 6 truss segment in late 2007.

In radio communications with Mission Control in Houston, Wheelock is designated EV-1, Caldwell Dyson is EV-2 and sometimes referred to by her initials "TC."

You can watch the entire spacewalk live on NASA TV -- click the box on the right side of this page to launch a viewer.

Here's how you can tell who's who: Wheelock's suit has red stripes on the legs and his helmet camera will show No. 19 in the lower right hand corner. Caldwell Dyson's suit is all white and her helmet cam is No. 20.