Washington: Indian-American astronaut Sunita Williams and her nine companions spent a quiet Sunday at her new home in space preparing for a fourth space-walk to try once more to retract a sticky solar array.

It is a relatively quiet day up there at the International Space Station, Mission operations representative Phil Engelauf said. "Things are going relatively well onboard as the crews are finishing up some of the transfer activities and doing the final preparations" for the walk.

The fourth space-walk is scheduled to begin at 12:17 a.m. IST Tuesday (6:47 p.m. GMT Monday) and may last as long as six and a half hours depending on how much time it takes to retract the array.

The decision to add the fourth space-walk was made even as Williams and veteran space-walker Robert Curbeam succeeded in retracting all but 11 bays of the array during her first and the mission's third space-walk that lasted seven hours and 31 minutes.

Working on the stubborn panel for two hours Saturday after they finished rewiring the station, the duo shook it by hand, and ground controllers got it two-thirds retracted before time ran out on the space-walk.

The panel retracted far enough by electronic command to allow another set of panels installed in September to rotate and track the sun, but full retraction is needed to smooth the way for future construction missions.

Officials in Houston said the National Aeronautics and Space Administration (NASA) would use lessons learned wrestling with the panel on the next shuttle mission, which is scheduled to retract the other half of the troublesome panel.

Curbeam, who will be on his record fourth outing and Mission specialist Christer Fuglesang will first work on the array panel grommets and will then resume the shaking activities if necessary.

Mission managers said they were "very confident" that crews, with the help of space-walkers, would complete folding the 110-foot (33-metre) array into a 20-inch (50 cm-high) box so it can be relocated in future space station construction.

Snags reeling in the panel are not a complete surprise. It is wafer-thin and folds like a set of window blinds, though much more carefully. It had sat unfurled in space for six years, enduring alternating blistering heat and frigid cold.

Another objective of the fourth space-walk is to collect additional information that could prove useful when the opposite side of the array is retracted on the next Discovery mission in March.

Curbeam and Fuglesang will complete the day's preparations by spending the night in the Quest airlock for the pre-space-walk campout.

During the campout, the pressure will be lowered in the airlock to the pressure normally found on earth at 10,000 feet (3,000 metres) above sea level. The procedure protects against decompression sickness as Curbeam and Fuglesang go to the even lower pressure of spacesuits on Monday.

With the added space-walk, the Discovery crew's mission to supply and continue building the station, due for completion in 2010, has been extended by a day.

Undocking is now set for 3:39 a.m. IST Wednesday (10:09 p.m. GMT Tuesday) with landing targeted for 2.25 a.m. IST Saturday (8:55 p.m. GMT Friday) at the Kennedy Space Centre, Florida after 13 days in space.