Astronauts Christopher Cassidy and David Wolf were on a spacewalk today to replace the space station's four 9-year-old batteries with the new nickel-hydrogen batteries that weigh 370 pounds apiece.

The spacewalk got the astronauts on a considerable 150 feet trek from their hatch. Halfway through the process of replacing the batteries, Cassidy's spacesuit reported a high concentration of carbon dioxide, indicating a problem with the evacuation of the gas.

"Hey, like I said, we were looking at the LIOH (lithium hydroxide) trend in Chris's suit," Aki Hoshide radioed from mission control around 2:52 p.m. "We do see it trending up. This is probably not by working hard, but it seems like the canister itself is experiencing some problems. So at this point, we would like to start cleaning up. How copy so far?"

"I think I copied, the canister may be having problems, it's not a metabolism-related issue?" Wolf asked.

"And that is correct."

"Interesting," Cassidy said.

"Um hm," Wolf agreed. "Do you see the possibility of an imminent canister failure? We're cleaning up, by the way."

"And yeah, it's not an imminent failure," Hoshide said. "We do still have some time, we just wanted to make sure that you guys are back in the airlock.

Eventually mission control instructed Cassidy to return into the hatch immediately, as the CO2 levels got dangerously close to NASA's safety limits.

The Endeavour astronauts plan two final spacewalks, during which the remaining two batteries will be installed.