Washington: Space shuttle Discovery astronauts ran into difficulties while overseeing the complicated mechanical process of folding up an old solar panel on the International Space Station.

The folding of the 115-foot-long array was to set the stage for hooking up the ISS' permanent electricity-generating system during two spacewalks later this week.

The panel was having trouble easily folding and astronauts had to stop and start the process several times and had to re-extend the wing to get rid of wrinkles that developed in the folding process.

The folding of the left wing of a solar array that has been in use on the ISS for more than six years began around 1828 GMT.

After years of exposure in space, scientists had questioned whether the lightweight array would return to its original, compact shape. One NASA scientist compared the procedure to folding up a well-worn road map.

If the wing folds successfully it will allow astronauts to activate a rotating joint that should allow solar panels on another part of the station to follow the sun while the ISS moves through space.

The 73-metre-long rotating solar panels were installed during a shuttle mission in September.

The folding of the old solar panel and the rotation of the new panels should set the stage for Space Shuttle Discovery astronauts to rewire the station's power system, replacing a temporary power system operating since the space station went into orbit in 1998. Two more space walks are planned for Thursday and Saturday to rewire both halves of the station.

NASA has described Discovery's rewiring mission at the ISS as one of the most complex and difficult in the history of space flight.

The first space walk to install a new component on the ISS was completed successfully early Wednesday GMT.

At a press conference Tuesday, NASA officials said the shuttle's heat shield was cleared for a return flight to Earth and had not been damaged by a micrometeoroid.

Discovery lifted off late Saturday from the Kennedy Space Centre at Cape Canaveral, Florida for the construction mission to the ISS.

Discovery is on NASA's second working shuttle mission to the ISS since NASA returned to flight in summer 2005, after the shuttle fleet was grounded for two years following the 2003 Columbia disaster, which killed all seven crew members.

NASA spent most of the last year testing new safety systems. The September flight by the shuttle Atlantis marked the resumption of ISS construction.