Blogger update, 3 p.m.: NASA has confirmed successful installation of a third battery on the Port 6 truss, leaving one more to go today. No word yet on whether the spacewalkers, who are slightly behind schedule, will need to replenish their oxygen supplies. Today's lengthy spacewalk could be extended to allow two astronauts to replenish their oxygen supply.
Endeavour spacewalkers Chris Cassidy and Tom Marshburn are working to install the third of four fresh solar array batteries on an International Space Station girder.
Mission controllers in Houston will assess their work timeline and oxygen supply when that task is done.
The spacewalk began at 9:54 a.m. and was scheduled to last seven hours and 30 minutes.
Climbing to the Quest airlock for oxygen and back could add another hour, which would put the spacewalk near the top of the record books.
The longest spacewalk on record was performed in March 2001 by Jim Voss and Susan Helms, Patrick Air Force Base's commander from June 2006 until last fall. The spacewalk lasted eight hours, 56 minutes.
Flight directors said Thursday that if all four remaining batteries could not be replaced today, the work would be completed during a final spacewalk planned Monday.
The six old solar array batteries being replaced were launched in November 2000 and have outlived their expected 6.5 years of service.
They'll be returned to the ground by space shuttle Endeavour.
The new nickel hydrogen batteries should last at least the same 6.5 years.
Each of three battery pairs can store about 8 kilowatts of electrical power generated by two solar arrays wings on the Port 6 truss, providing power when the station is blocked from sunlight.
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