Finally after their fourth unscheduled spacewalk, Discovery astronauts have successfully retracted a stubborn solar panel on the International Space Station [Images].

The solar wing is completely folded into its storage boxes, though one of the guide wires that run the length of the wing appears not to have reeled in all the way.

Applause rang out 0524 IST when the two ends of the array's boxes met for the first time since it was deployed in 2000.

It took veteran spacewalker Robert Curbeam and Christer Fuglesang more than three hours to fully retract the panel, freeing jammed guide wires and grommets to slowly fold the panel away before it is moved to another section of the station next year.

Astronauts extended Saturday's spacewalk by more than an hour in an effort to avoid venturing outside again, making significant process before light faded.

With NASA [Images] extending Discovery's mission for Tuesday's spacewalk, the shuttle is now forced to use one of two days of emergency fuel set aside to allow for landing delays due to bad weather.

Bad weather delayed Discovery's launch for two days last week, and the shuttle will now only have 24 hours of reserves for its fuel cells should weather delay its return to Earth. 

Discovery is now scheduled to undock from the station on Tuesday and astronauts will undertake a final heat shield inspection on Wednesday.

Discovery will then land at the Kennedy Space Center on Friday afternoon.

With only one bad weather day in reserve, NASA is expected to prepare backup landing sites at Edwards Air Force Base in California and White Sands, New Mexico.

NASA had been considering skipping the heat shield inspection if a fourth space walk was required -- despite insisting on additional safety precautions since damage to a fuel tank lead to the death of Columbia's seven astronauts during re-entry in 2003.

An unplanned initial inspection of the heat shield was undertaken as a precaution on Monday after sensors detected "very low" impact readings. After initial fears the shuttle had been damaged by space junk or a micrometeoroid, NASA engineers advised Discovery's crew it did not need to conduct an extra inspection.

The solar panel causing the problems is attached to the P6 tress segment. It is part of the station's backbone, but has been temporarily attached to one arm of the station since 2000 -- awaiting the P5 tress segment which astronauts installed during their spacewalk on Tuesday.

The P6 segment will be moved to the end of the P5 segment and the solar panel redeployed in 2007 -- completing one end of the station.