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CAPE CANAVERAL, Florida (Reuters) - NASA cut short a spacewalk outside the International Space Station on Wednesday after carbon dioxide began building up in one astronaut's spacesuit, officials said.
The outing, by shuttle Endeavour's lead spacewalker David Wolf and rookie astronaut Chris Cassidy, was called off shortly before 4 p.m. (2000 GMT) as the astronauts were replacing batteries in the station's solar power system.
Two of four batteries were installed when Mission Control told the crew to head back to the airlock. They had been spacewalking for about six hours when Cassidy's suit began having problems.
Sensors showed a build up of carbon dioxide. The gas, a byproduct of breathing, typically is chemically removed by a canister of lithium hydroxide.
"It seems like the canister itself is experiencing some problems," astronaut Aki Hoshide radioed to the Endeavour crew from Mission Control in Houston.
"It's not an imminent failure," Hoshide added. "We just wanted to make sure that you guys are back in the airlock."
The spacewalk was the third of five planned during shuttle Endeavour's ongoing mission at the space station.
Also Wednesday tests on the fuel tank earmarked for NASA's next space shuttle launch in August show no problem with how the insulating foam is bonded to the tank, an agency spokesman said on Wednesday.
The test result could clear a hurdle for NASA's next shuttle launch.
NASA ordered tests of the fuel tank scheduled for use during the shuttle Discovery's launch to station after an unusually large amount of foam fell off the shuttle Endeavour's tank during its launch last week.
NASA has been fastidious about in-flight debris since losing the shuttle Columbia in 2003. A piece of foam that fell off Columbia's tank during launch smashed into the ship's wing, breaching a heat-resistant panel. The shuttle broke apart as it flew through the atmosphere for landing 16 days later, killing all seven astronauts aboard.
Most of the foam shed during Endeavour's climb into orbit on July 15 fell off so late during ascent that there was not enough atmospheric force for free-flying debris to slam into the ship and cause damage.
NASA, however, wants to be sure that whatever problem triggered the foam loss on Endeavour does not reoccur, especially while the shuttle is in the early phases of launch when debris impacts could do harm.
Engineers believe a bonding problem may have triggered the foam loss on Endeavour's tank.
Tests at the shuttle tank manufacturing plant outside New Orleans show normal adhesion of the foam on Discovery's fuel tank, said Steve Roy, a spokesman with the Marshall Space Flight Center in Huntsville, Alabama, which oversees shuttle fuel tank production.
"That's what we expected to see and that's what we saw," Roy told Reuters.
The issue is not over. NASA is considering taking additional samples from Discovery's tank foam for more tests, Roy added.
Meanwhile, the search for what caused Endeavour's foam loss continues.
"We have not determined the root cause yet," Roy said.
Editing by Jane Sutton and Eric Walsh