Spacewalkers Add New Batteries to Space Station
Endeavour astronaut Tom Marshburn works to replace old batteries on the International Space Station's Port 6 truss during a July 24, 2009 spacewalk, the 4th of the STS-127 mission.
Credit: NASA TV.

Two determined astronauts added a fresh set of vital batteries on the International Space Station Friday during an extra-long spacewalk made more so by the need to take things slow and steady.

Spacewalkers Chris Cassidy and Tom Marshburn spent more than seven hours replacing four batteries at the core of the space station?s oldest solar arrays, which sit at the very edge of the outpost?s left side.

It was the first spacewalk since a Wednesday excursion that was cut short by elevated carbon dioxide levels in Cassidy?s spacesuit. In that spacewalk, Cassidy apparently worked so hard at the start that he dried out the precious air-scrubbing canister used to clean his air.

At no point in that spacewalk was Cassidy in danger, but Mission Control urged that he and Marshburn take things slowly during Friday?s orbital work.

?Chris is a very motivated and very intelligent guy,? space station flight director Holly Ridings told reporters late Thursday. ?He?s a Navy SEAL. He?s in great shape. We really just needed to tell him ? just slow down a little and take your time.?

Flight controllers also gave periodic updates on the astronauts? spacesuit performance during the spacewalk. Late into the orbital work, Mission Control noticed Cassidy?s carbon dioxide levels were again rising slightly but he was not in any danger.

?How are you doing, Chris?? asked astronaut Tim Kopra, who choreographed the work from inside the shuttle Endeavour.

?I?m doing fine,? Cassidy called back.

Friday?s spacewalk was the fourth of five planned spacewalks for Endeavour?s 16-day mission to the International Space Station. The shuttle?s seven-astronaut crew has delivered a new crewmember for the station?s six-man crew and an experiment porch for Japan?s Kibo lab at the orbital outpost.

Space station?s new batteries

Despite their deliberately steady pace, Cassidy and Marshburn sped through their battery replacement work at the left edge of the space station?s main truss. The station?s truss serves as its backbone and is as long as an American football field.

?You guys are going so fast that we?re having a hard time keeping up here,? Kopra said.

The batteries are a critical part on the space station?s solar array power grid. They store power from the solar wings when the space station is flying in darkness over the Earth?s night side.

There are four sets of batteries on the station, one for each pair of solar wings, but they are only designed to last 6 1/2 years. Each battery weighs about 375 pounds (170 kg) and is about the size of a small refrigerator, astronauts have said.

The batteries on the station?s Port 6 truss - where Cassidy and Marshburn worked Friday - are part of the outpost?s oldest solar arrays, which launched in 2000 and have been in service for nine years. So NASA wanted spacewalkers to replace them before they died. The old batteries will return to Earth aboard Endeavour next week.

Friday's spacewalk began at 9:54 a.m. EDT (1354 GMT) and marked the 129th excursion dedicated to space station construction or maintenance. It was also the second career spacewalk for both Cassidy and Marshburn, who will also venture out on the fifth and final outing of their mission on Monday.

The spacewalkers started their work just hours after a Russian cargo ship began its own trip to the International Space Station. The unmanned space freighter Progress 34 launched early Friday to deliver 2 1/2 tons of cargo, including fresh fruit, spare parts and other supplies to the space station.

All 13 of the astronauts aboard the linked shuttle and space station will have a day off tomorrow to rest during their busy mission.

Endeavour commander Mark Polansky apparently told his crew, and Cassidy and Marshburn in particular, that the time off is much deserved.

?The boss says you?ve earned a day off tomorrow,? astronaut Dave Wolf told the spacewalkers as they returned inside the station.

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SPACE.com is providing continuous coverage of STS-127 with reporter Clara Moskowitz and senior editor Tariq Malik in New York. Click here for mission updates and SPACE.com's live NASA TV video feed.