Endeavour mission specialist Chris Cassidy works outside of the International Space Station to replace a set of old solar array batteries during the third spacewalk of STS-127 on July 22, 2009.
Credit: NASA TV
This story was updated at 9:15 p.m. EDT.
Two spacewalking astronauts replaced some of the oldest batteries on the International Space Station Wednesday, but a spacesuit glitch forced them to cut their orbital work short when it caused slightly elevated carbon dioxide levels.
Astronauts Dave Wolf and Chris Cassidy hoped to replace four of the station?s six oldest batteries during their nearly six-hour spacewalk. They were plowing through the tricky task when the glitch in the air-scrubbing gear in Cassidy's spacesuit forced them to end early.
At no point was Cassidy in danger, but once flight controllers noticed the elevated carbon dioxide levels in his spacesuit they decided to call off the spacewalk?s remaining tasks as a precaution. He immediately headed back to the station?s airlock to wait for Wolf.
?I guess we?re getting off early today,? Wolf said.
NASA said the problem appeared to be an issue with the air scrubbing filter in Cassidy?s spacesuit, not overexertion, and the astronaut did not feel any ill effects from the higher-than-normal carbon dioxide levels.
Those levels never increased beyond NASA?s acceptable limits for the space station or shuttle, mission managers said. Even so, the faulty filter will be returned to Earth to find out what went wrong, they added.
?Certainly, folks are very interested in understanding the root cause of the signature that we saw today,? lead space station flight director Holly Ridings told reporters late Wednesday.
Wolf and Cassidy left their battery replacement job only half finished, with two new batteries successfully installed. The rest will have to wait for the mission?s next spacewalk on Friday.
Wednesday?s spacewalk was the third of five excursions planned while NASA?s shuttle Endeavour is docked at the space station. The next one is set for Friday and also includes battery replacement chores.
?You did great out there,? astronaut Tim Kopra told the spacewalkers after their work.
Wolf and Cassidy spent five hours and 59 minutes toiling outside the station as it flew 220 miles (354 km) above Earth. They had hoped to work on the batteries for at least 6 1/2 hours.
Vital battery swap
The space station?s batteries are vital because they store the power generated by its expansive solar wings when the outpost flies over the Earth?s night side. The batteries in the solar wings at the very edge of the station?s left side have been in space since 2000, longer than any others on the outpost, so NASA wanted to change them before they died.
It was tricky work, with Wolf and Cassidy working at the port side edge of the space station?s backbone-like main truss, which is as long as an American football field. They had to perform a carefully choreographed shell game to remove the old batteries, move them by hand between the station?s edge and the tip of its outstretched robotic arm, and then make the return trip with the new batteries.
?I don?t want to make one false move here,? Wolf said during the first battery swap.
Each battery weighs about 367 pounds (166 kg) and is about the size of a refrigerator, the astronauts have said. They are designed to last 6 1/2 years. The ones being replaced by Endeavour astronauts have been in use on the station for nine years.
Before tackling the tough battery swap out, Wolf and Cassidy took care of some minor chores.
The spacewalkers removed four insulation covers from Japanese experiments that will be moved to the station?s new porch attached to Japan?s $1 billion Kibo laboratory. Cassidy tossed three of the covers overboard and packed the fourth for the return trip home.
The astronauts also finished adding some insulation around power lines that allow visiting shuttles to plug into the station?s power grid.
Wednesday?s orbital work marked the seventh career spacewalk for Wolf, who ended with 41 hours and 57 minutes of total spacewalking time. It was Cassidy?s first spacewalk, though he will venture back out on Friday and Monday with crewmate Tom Marshburn on the last two excursions.
Endeavour?s seven-astronaut crew is in the middle of a 16-day construction mission to deliver the station?s new porch, some spare parts and a new crewmember. The astronauts are slated to move the trio of Japanese experiments to the station?s new porch on Thursday.
Endeavour is slated to undock from the space station next week and return to Earth 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.