This story was updated at 7:20 a.m. EDT.
The spacewalking crew of NASA's space shuttle Atlantis began the third and final spacewalk of their mission early Friday to wrap up a battery upgrade service call on the solar arrays outside the International Space Station.
It will be the final spacewalk conducted by Atlantis astronauts while it is docked at the station because this STS-132 mission is expected to be the orbiter's last planned flight before the shuttle fleet's retirement.
The shuttle Atlantis crew woke Friday morning at 1:50 a.m. EDT (0550 GMT). The spacewalkers began their excursion ? called an extravehicular activity (EVA) in NASA parlance ? at 6:27 a.m. EDT (1027 GMT), and stay outside about 6 1/2 hours. They will exit out of the station's Quest airlock.
Mission specialists Garrett Reisman and Michael Good are following up two previous spacewalks on this mission to complete the job of installing new batteries on the left-most edge of the station's backbone-like truss.
Good and fellow mission specialist Stephen Bowen began the chore on Wednesday's spacewalk, when they got ahead of schedule and installed four of six batteries, leaving only two more for this final excursion.
"Originally we thought we'd have to perform three battery changes but because of the excellent work the crew did yesterday in our second spacewalk? that allows us an opportunity to get ahead," lead shuttle flight director Mike Sarafin said Thursday.
The batteries are ungainly, weighing about 375 pounds (170 kg) each. The spacewalkers will remove new ones from a cargo carrier in the shuttle's payload bay, and then swap them out with the aging units currently on the station.
In addition to the battery work, the spacewalkers will remove a robot arm tool called a grapple fixture from the shuttle's payload bay. For now, the astronauts will take it back inside the station with them; it will be installed during a later spacewalk on the station's Russian Zarya module.
"So when we repressurize and go back into station it won?t be just Garrett and I in that airlock," Good said in a preflight interview. "We?ll have this great big grapple fixture in there with us so hopefully there?ll be room for everybody in there."
Mission Control has also planned a medley of various get-ahead tasks for Good and Reisman to do that will help outfit the station for the future.
"On EVA 3 it?s kind of a clean-up day," Good said. "It?s to get everything done that we had hoped to get done on the whole mission, so whatever?s left."
The spacewalkers will install a hose for the ammonia coolant system on the station's exterior, and replace some errant tools in outside toolboxes.
Mission specialist Piers Sellers will help out from inside the orbiting lab by operating the station's robotic arm.
Overall, Atlantis' mission is going very well, the astronauts said.
"We are all absolutely thrilled at how this mission is going so far," Atlantis commander Kenneth Ham said Thursday. "We're just going to stay focused and hopefully get through the next few days."
The crew plans to undock from the space station on Sunday and land back at NASA's Kennedy Space Center in Cape Canaveral, Fla. on Wednesday, May 26.
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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 6:45 a.m. EDT.