Watch the EVA live at http://www.nasa.gov/multimedia/nasatv/index.html

Expedition 24 Flight Engineers Doug Wheelock and Tracy Caldwell Dyson switched their spacesuits to battery power at 7:19 a.m. EDT, signaling the start of the first of two spacewalks that will focus on removing the ammonia pump module that failed last Saturday and putting its replacement in place.

The repair tasks, which include removing the failed pump module from the S1 Truss and retrieving a spare from an external stowage platform, are expected to take about 6.5 to 7 hours. They are scheduled to complete installation and activation of the new pump module during the second spacewalk planned for Wednesday at 6:55 a.m.

Wheelock, designated as EV1, or extravehicular crew member 1, is wearing the spacesuit bearing the red stripes and conducting the fourth spacewalk of his career. Caldwell Dyson, designated as EV2, is wearing the unmarked spacesuit and making her first spacewalk. Flight Engineer Shannon Walker is operating Canadarm2, the station's robotic arm, and assisting the spacewalkers from inside the station.

Approximately two hours after the conclusion of the spacewalk NASA TV will broadcast a briefing from Johnson Space Center. The briefing participants will include Mike Suffredini, International Space Station program manager; Courtenay McMillan, Expedition 24 spacewalk flight director; and David Beaver, Expedition 24 spacewalk officer.

The first spacewalk was originally planned for Friday morning, but mission managers decided an additional day was necessary to finish working out all the spacewalk details. The additional time allowed for the final procedures to be uplinked to the station crew late Thursday and gave the crew one full day to review the plans that were developed by teams of flight controllers, engineers, and spacewalk and robotics experts. Managers also moved the second spacewalk to Wednesday to give the crew additional time to rest and prepare.

After the loss of one of two cooling loops July 31, ground controllers powered down and readjusted numerous systems to provide maximum redundancy aboard the orbiting laboratory. The International Space Station is in a stable configuration, the crew is safe and engineers continue reviewing data from the failed pump.

Source with additional links at http://www.nasa.gov/mission_pages/station/main/index.html