Mission Specialist Robert Curbeam and Flight Engineer Sunita Williams are scheduled to exit into open space at 2:37 pm EST (1937 GMT) after several hours of preparations, US space officials said.

"The primary goal ... tomorrow is to finish the rewiring of the space station and getting the active thermo system up and running," said Kirk Shireman, deputy ISS program manager, told reporters.

But he said the space walkers will be asked to "go out and take a look at the solar array for us."

National Aeronautics and Space Administration officials decided to abandon, at least for the moment, attempts to unblock the solar array that remains stuck and may require a fourth space walk to fold it away.

Space shuttle Discovery and ISS astronauts on Wednesday only managed to fold the stubborn array half-way by remote control, and NASA engineers on Friday tried to determine why it remained stuck after repeated attempts at activation.

At one point, NASA engineers in Houston asked German astronaut Thomas Reiter to exercise vigorously for 30 seconds on the ISS's training machine, hoping his vibrations would cause the panels of the stubborn array to fold all the way.

Reiter, who after a five-month stay on the ISS will be replaced by Discovery astronaut Sunita Williams, tried several times to shake the array into action.

"I'm very sorry to hear that," said Reiter when told by the Mission Control Center in Houston, Texas, that all his efforts came to naught.

"We'll give you a silver medal for that," a NASA controller replied.

The solar array was stuck in a half-folded position, and while it needs to be fully retracted before it can be moved to another location on the ISS, its current position has allowed a new solar array to unfold and rotate properly.

ISS commander Mike Lopez-Alegria told reporters from the station that "a fourth EVA is one of the options still on the table," referring to extravehicular activity, NASA language for space walk.

Discovery astronauts on Tuesday and Thursday went on two spacewalks to attach a two-tonne truss segment and to rewire the ISS for the new, power-supplying solar array delivered in September by the shuttle Atlantis.

Despite the stuck solar array -- it has supplied electricity to the ISS for six years -- astronauts late Wednesday were able to activate the newer array so it could turn its photovoltaic cells to follow the sun.

Once it is fully installed and operational, it will double the ISS's electrical output. The station, when completed, will have a total of four solar arrays providing it with electricity.

The Discovery mission is part of 14 shuttle flights NASA has planned over the next four years to finish the ISS by 2010, when the shuttle fleet, down to three vehicles, is to be retired.

Discovery blasted off last Saturday from Cape Canaveral, Florida, in the first nighttime liftoff in four years. It docked on the station Monday.

NASA engineers said inspections carried out after takeoff and shortly before Discovery docked with the space station found no damage to the shuttle's heat shield.

Such inspections on the shuttles have become routine since the Columbia tragedy.

Columbia's heat shield was pierced by foam insulation that peeled off its fuel tank during liftoff, causing the shuttle to disintegrate during its return to Earth in February 1, 2003, killing all seven astronauts on board.

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