Expedition 24 Flight Engineers Doug Wheelock and Tracy Caldwell Dyson switched their spacesuits to battery power at 6:19 a.m. CDT on Saturday, signaling the start of the first of two spacewalks that will focus on removing and replacing a failed ammonia pump module.
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 between six-and-a-half to seven hours.
Installation and activation of the new pump module is scheduled for a second spacewalk planned for Wednesday.
Wheelock, designated as EV1, or extravehicular crew member 1, is wearing a spacesuit with red stripes and conducting his fourth career spacewalk.
Caldwell Dyson, as EV2, is wearing an unmarked suit and making her first spacewalk.
ISS Flight Engineer Shannon Walker is operating Canadarm2, the station's robotic arm, and assisting the spacewalkers from inside the station.