NASA astronaut Garrett Reisman, STS-132 mission specialist, participates in the mission's first spacewalk outside the International Space Station on May 17, 2010.
This story was updated at 7:30 a.m. EDT.
Two shuttle Atlantis astronauts are floating outside the International Space Station Wednesday on a spacewalk to fix a snagged cable on their spacecraft and install new solar array batteries on the orbiting laboratory.
Mission specialists Michael Good and Stephen Bowen are making the excursion, which should last about seven hours. It is the second of three spacewalks scheduled for Atlantis' 12-day STS-132 mission, its last planned mission before the shuttle is retired.
The astronauts began the spacewalk at 6:38 a.m. EDT (1038 GMT), about an hour earlier than previously planned, in order to squeeze in the repair of a snagged cable on a shuttle sensor system.
The six astronauts aboard space shuttle Atlantis woke this morning a little after 2:20 a.m. EDT (0620 GMT) to the song "Start Me Up" by the Rolling Stones, played especially for mission specialist Piers Sellers.
"We are awake, and ready for another day," he radioed down to Mission Control.
Atlantis astronauts discovered the cable issue when they tried to complete an inspection of their orbiter's heat shield Saturday, their first full day in space after launching Friday afternoon. The sensor camera normally used for the task wouldn't tilt fully because one of its cables was pinched by another piece of hardware on the shuttle. The astronauts were able to complete most of the scan using a backup camera.
Mission managers are hoping that 20 to 30 minutes of work by Bowen at the start of Wednesday's spacewalk will free the cable and fix the camera.
"That should restore that capability so that the camera and its sensors are available to the orbiter as required," said lead station flight director Emily Nelson during a Tuesday briefing.
Once the cable issue is resolved, the spacewalkers can move on to the main goal of today's spacewalk, or extravehicular activity (EVA).
"EVA 2 really is dedicated to one thing and that?s changing out six batteries at the end of the port truss," Bowen said in a preflight interview. The batteries are located on the P6 truss area all way out on the left side of the station's backbone.
Shuttle Atlantis carried up a set of six new batteries to replace the aging units currently on the station. They have a design life of six-and-a-half years, but the ones currently on the station have working even longer.
Good and Bowen will aim to swap out three on Wednesday, and Good and mission specialist Garrett Reisman will replace the last three on a follow-up spacewalk Friday.
The job is more complicated than it sounds, because each battery weighs a hefty 375 pounds (170 kg). They have a design life of 6 1/2 years, but the ones currently on the station have working even longer.
"These aren?t double A's," Good said. "These are big. They are about the size of a big suitcase, maybe two suitcases."
The work is part of a quest to outfit the space station with spares and backups so it can continue functioning in top shape after NASA's space shuttle fleet is retired later this year. While Russian Soyuz spacecraft will continue ferrying astronauts up to the lab, and a group of unmanned vehicles will make cargo runs, none of them has the cargo-carrying capacity of the space shuttles with their massive payload bays.
NASA currently hopes to keep the space station running through at least 2020, a decade after the shuttles are due to retire.
While the spacewalk is the main goal for Wednesday, some Atlantis astronauts have a full plate of other interior tasks to move equipment and outfit the station.
"Meanwhile inside the station we're going to have a busy day as well," Nelson said.
The spaceflyers plan to carry out some robotic arm maneuvers to relocate hardware around the outside of the station, and will complete other station maintenance tasks.
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SPACE.com is providing complete coverage of Atlantis' STS-132 mission to the International Space Station with Senior Writer Clara Moskowitz and Managing Editor Tariq Malik based in New York. Click here for shuttle mission updates and a link to NASA TV. Live spacewalk coverage begins at