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CAPE CANAVERAL - Space shuttle astronauts left their ship for a fourth spacewalk today in a last-ditch effort to free a jammed solar wing panel on the International Space Station and complete every job Nasa wanted for its final flight of the year.
Already the shuttle crew, which reached the half-built orbital complex last Monday, has rewired the station's power grid, clearing the way for modules built by Europe and Japan to be attached to the station.
The only problem encountered so far involved the retraction of a six-year-old golden wing panel, or solar array, that is studded with solar cells to capture energy from the sun.
The panel is to be moved to another location next year. Before the transfer, however, the 110-foot(33-metre)-long span needs to be stored inside a storage box.
Repeated attempts over the past five days, including some hands-on work during the third spacewalk on Saturday, have left the panel still partly protruding.
Astronaut Robert Curbeam ventured out for a fourth time -- more than any other astronaut during a single shuttle mission. He was accompanied by Sweden's Christer Fuglesang, who also joined him on two of the mission's previous spacewalks.
The pair floated out the station's Quest airlock at 2 p.m. EST (7 p.m. British time) for what the US space agency hoped would be a quick fix of the jammed array.
The astronauts, however, are prepared to stay longer than the usual six-hour spacewalk, if necessary, having equipped their spacesuits with extra air purifiers and other supplies.
They also have wrapped their tools with insulating tape to minimize shock hazards, as the panel may have build-ups of static electricity. Crew mates taped the metal rings on the astronauts' spacesuits for the same reason.
Curbeam was to ride the station's robot arm to inspect the panel and poke free grommets that are snagged on wires guiding folded wing sections into a 20-inch-high storage box.
Fuglesang's job is to photograph the operation, as Nasa has another wing panel that needs to be retracted during its next shuttle mission, scheduled for March. He also will help loosen the wing sections by shaking the storage box, the same technique that Curbeam and station astronaut Sunita Williams tried, with some success, during Saturday's spacewalk.
The primary job during that previous outing was to finish rewiring the station's electrical system, which taps power from a new set of solar panels installed during the last shuttle mission in September.
While folding the solar panel is expected to be a relatively straightforward job, it set off a vigorous debate among Nasa managers. Some wanted to leave the work for the station astronauts or the next shuttle crew to handle; others approved the extra spacewalk but wanted to skip a final inspection of the shuttle's heat shield in order to preserve an extra day in space in case poor weather or equipment problems postpone landing.
The inspection, which will hunt for damage from any micrometeoroid strikes, is among the safety upgrades made after the 2003 Columbia accident.
In the end, Nasa made the unprecedented decision to give up one of its contingency days for landing, keep the inspection and add the spacewalk.
Touchdown is now targeted for 3:56 p.m. EST (2056 GMT) on Friday, a day later than planned. Nasa will mobilize backup landing sites in California and New Mexico, in addition to the prime site in Florida.
