Endeavour Crew Finishing ISS Battery Swaps

Construction on the International Space Station continued July 24 as the Space Shuttle Endeavour crew engaged in a marathon 7.5-hour spacewalk to swap out batteries for the ISS' oldest set of solar arrays. The batteries store power collected by the solar wings and cost $3.6 million each.

The 16-day mission's fourth of five spacewalks was longer than originally planned after a July 22 spacewalk was cut short because of higher than normal carbon-dioxide levels in Mission Specialist Chris Cassidy's spacesuit. While spacewalking, astronauts carry a lithium hydroxide canister to scrub carbon dioxide from their oxygen system.

NASA stressed that Cassidy-a former Navy SEAL-was never in danger and blamed the carbon dioxide buildup on Cassidy's energetic start to the July 22 spacewalk.

Flight Director Holly Ridings said the canister doesn't work as well if an astronaut begins a spacewalk with a high metabolic rate.

"Chris is a Navy SEAL," Ridings said. "He's in great shape and so we really just needed to tell him, 'Hey, we know you can do this really well and really fast, but we need the [canister] to work right, so just slow down a little and take your time.' He took that with good humor."

Two of the six 370-lb. original batteries were changed out July 22 before NASA called Cassidy and Mission Specialist Tom Marshburn back to the Endeavour. By late afternoon July 24, Cassidy and Marshburn had completed replacing and installing five of the six batteries.

While the astronauts toiled in space July 24, an unmanned Russian spacecraft was launched from Baikonur Cosmodrome in Kazakhstan bearing 2.5 tons of supplies for the space station and its permanent six-person crew. The supplies on the cargo ship include 110 lbs. of oxygen, 463 lbs. of water, 2,718 lbs. of spare parts and science experiments, and 1,830 lbs. of propellant.

The Russian ship is expected to arrive at the ISS July 29, the day after Endeavour is set to undock from the ISS and head back to earth. Endeavour is due back at the Kennedy Space Center in Florida July 31.