HOUSTON, United States (AFP) -- Two U.S. astronauts will suit up and go on the third spacewalk of the Atlantis mission, putting the finishing touches on a new set of solar arrays on the International Space Station.

Astronauts Joe Tanner and Heidemarie Stefanyshyn-Piper, the only woman in the Atlantis six-member crew, will complete the series of spacewalks they initiated on Tuesday when they wired the new array-bearing structure attached to the ISS.

In an extra vehicular activity -- NASA talk for spacewalk -- expected to last about six-and-a-half hours, they will install radiator attachments to prevent overheating of the solar batteries that power the arrays.

The additional electricity furnished by the new arrays is expected to be sufficient to power the European Columbus laboratory and the Japanese Kibo laboratory, which are to be installed in later missions.

On Thursday, the solar arrays were stretched out like a giant, golden accordion.

It was the latest success on the 11-day space shuttle Atlantis mission launched Saturday to resume construction of the ISS, the first such mission since the 2003 shuttle Columbia disaster.

The unfurling fell behind schedule by a few hours when a software glitch interfered with the maneuvering of the arrays' rotation arm.

The first pair of array panels was opened at 5:26 am (1026 GMT) and the second set at 7:45 am (1245 GMT), said a commentator on the National Aeronautics and Space Administration's television service.

The new set of solar arrays measures 240 feet (73 meters) and will ultimately provide a quarter of the ISS's power once the orbiting laboratory is completed in 2010.

It will not be activated until the next scheduled shuttle mission, by Discovery, in December.

U.S. aerospace giant Lockheed Martin Corp. manufactured the new arrays as well as the identical original set. The 2000 installation of the first set was beset by problems.

On Tuesday, astronauts used a robotic arm to attach a 16-ton truss segment with the solar arrays, called P3/P4 and costing 372 million dollars, to the orbiting laboratory.

That step marked the resumption of construction of the ISS for the first time since the 2003 space shuttle Columbia disaster. The last construction mission was in 2002.

On Wednesday, two Atlantis astronauts went on a seven-hour spacewalk, the second of three in their mission, to remove launch restraints on the solar arrays that prevented damage when Atlantis lifted off from Cape Canaveral, Florida.

At the end of ISS construction in 2010, once the four sets of double arrays are in place, they will generate a combined 110 kilowatts of electricity, the equivalent to the consumption of 55 average households.

Atlantis, whose protective thermal shield was declared in perfect shape for reentry, is scheduled to land on September 20 at the Kennedy Space Center in Cape Canaveral, Florida.

The last ISS assembly work was in November 2002 after the Columbia accident forced NASA to work on improving flight safety.

Columbia was doomed by foam insulation that peeled off its external fuel tank during liftoff and pierced its heat shield, causing it to disintegrate as it returned to Earth in February 2003.