Astronauts to Change Space Batteries in Spacewalk

Two astronauts will head outside the International Space Station Wednesday to begin the delicate task of replacing old batteries at the very edge of the orbital outpost.

Spacewalkers Dave Wolf and Chris Cassidy will replace four of six old solar array batteries on the station?s on the port side during the tricky maintenance call, which is slated to begin at about 10:58 a.m. EDT (1458 GMT) and last more than six hours. It is the third of five spacewalks for astronauts while the shuttle Endeavour is linked to the station.

?We?ve always wrestled with how to do this battery change out,? Wolf told SPACE.com before flight. ?We?ve been wringing our hands for upwards of seven years on this task.?

A tricky chore

While it seems straightforward, the batteries are the oldest ones on the space station, which uses solar arrays to generate power. They are attached to the station?s Port-6 (P6) solar wing - the first ever launched to the outpost - and have been in space since 2000.

Simply reaching the work site is challenging since it sits at the end of the port side on the station?s backbone-like main truss. The space station?s metallic backbone is as long as an American football field and serves as the foundation for its four sets of solar arrays. Wolf and Cassidy will be working hundreds of feet away from the station?s core, with no quick way back.

?I think it?s the farthest you could get from the airlock hatch?since the airlock?s on the starboard side,? Cassidy said in a NASA interview.

All six of the new batteries to be installed are attached to cargo pallet that is currently perched at the tip of the station?s 57-foot (17-meter) Canadarm2 robotic arm. Astronauts moved the arm into position on Tuesday and will stretch it out to its full length today, with the battery carrier extended toward today?s portside worksite.

Moving the individual batteries has its own challenges. Wolf and Cassidy will have to shuffle the old and new batteries between their work site and the carrier on the robotic arm. Each battery weighs 367 pounds (166 kg) and is the size of a refrigerator.

?It?s a long, complicated choreography to change all these batteries,? Wolf told reporters before flight. ?They?re heavy, delicate, so there?s a lot of chance for error?it just all has to go right.?

Other tasks on tap

Wolf is a veteran NASA astronaut making his seventh career spacewalk with today?s excursion. He also serves the spacewalk branch chief in NASA?s astronaut office.

Cassidy, a U.S. Navy SEAL, will be making his first spacewalk during the orbital work and is making his first spaceflight on Endeavour?s construction flight to the space station. He became the 500th person to reach space when Endeavour launched toward the station last week.

In addition to their battery swap work, Wolf and Cassidy also plan to prepare a trio of new experiments for installation on the station?s Japanese porch. They are also expected to finish adding insulation to power lines that allow visiting shuttle like Endeavour to tap into the station?s power grid.

The last two of the six old batteries being replaced at the station will be swapped out during a Friday spacewalk - the fourth for Endeavour?s seven-astronaut crew, mission managers said.

The astronauts are currently in the middle of a 16-day mission to deliver the station?s new experiment porch and a new crewmember. They are slated to return home July 31.

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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.