NASA managers terminated a spacewalk today when astronaut Chris Cassidy's carbon dioxide levels showed an upward trend due to a problem with his spacesuit's CO2 removal system. NASA managers said late Wednesday his CO2 levels never exceeded normal limits for crews inside the space station or the shuttle and that calling off the spacewalk early was simply a precaution.

Credit: NASA

"A spacesuit is a very small spacecraft and there's really not very much margin for error," said space station Flight Director Holly Ridings.

She said Cassidy, a former Navy SEAL making his first spacewalk, experienced no symptoms of hypercapnia, which include headaches, confusion and lethargy, and that his spacesuit will be equipped with a fresh canister of CO2-removing lithium hydroxide for a planned spacewalk Friday.

Cassidy and astronaut David Wolf were attempting to replace four of six batteries in the space station's oldest set of solar arrays when flight controllers because concerned about elevated CO2 levels in Cassidy's suit. At the time the spacewalk was called off, the astronauts had replaced two of the four batteries and were preparing to attach a third original battery to a storage pallet.

Instead, they left the third battery attached to a temporary fixture and returned to the Quest airlock module.

Normal CO2 levels in an astronaut's spacesuit are between 0.3 and 0.5 millimeters of Mercury. At its highest point today, Cassidy's suit had readings of 3 mm Hg. The shuttle and space station atmospheres are maintained at about 5 mm Hg. Spacewalk officer Kieth Johnson said a spacesuit's internal sensors will alert an astronaut if CO2 readings climb above 8 mm Hg and that symptoms of hypercapnia don't typically show up until CO2 concentrations reach 15 mm Hg.

Even so, "it became important that we move back to the airlock and put the crew in a configuration we understood based on that CO2 signature," Ridings said. "At no time was the crew in danger. The CO2 levels we saw inside the spacesuit were below what we manage to on the International Space Station and the space shuttle. I just came from talking to the flight surgeon and from talking to the crew and the shuttle commander, Mark Polansky. The crew is doing just great and they are ready to go out on the next EVA where we will (do) our four remaining battery R&Rs and complete this task."

Because one of the old 375-pound battery packs was left mounted on a flexible temporary stowage fixture, flight controllers told the shuttle-station crews not to use any exercise equipment that is not isolated from the station to avoid unwanted vibrations.

"We did leave the worksite in an unintended configuration," Ridings said. "We had temporarily stowed a battery kind of up above the worksite out on the integrated equipment assembly. The teams on the ground quickly assessed the impact of that configuration. It's very minor, we do have some additional exercise restrictions the crew will be under until we either A) have some more discussions on the ground or B) go back out on EVA-4 and resolve that work site."

Johnson said flight planners are re-assessing the crew's flight plan to re-prioritize the two remaining spacewalks. Cassidy and Thomas Marshburn will attempt to install the final four solar array batteries during a spacewalk Friday. Other high priority items include television cameras needed for the September approach of a new Japanese cargo ship.

Deployment of one or more payload storage mechanisms may be deferred to a future mission.

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