Astronauts prepare to rewire space station

After a successful first spacewalk, the crew aboard the space shuttle was set to spend much of Wednesday preparing to rewire the international space station, one of their most important tasks.

One of the station's solar panels needs to be retracted to make room for another panel that will begin rotating for the first time after the rewiring takes place. The panels follow the sun's movement to maximize the amount of power they can generate for the orbiting space lab.

But retracting a solar array isn't easy: it can lose its original flatness, just like a once-crisply folded map after it's been spread out on a car's dashboard. NASA managers are wary about putting too much pressure on the array while trying to get it back in its box.

"A map never goes back to the way you bought it. It just doesn't," said John Curry, NASA's lead flight director for the space station.

Though NASA has developed contingency plans for various scenarios, managers aren't sure how the process will play out.

The rewiring is a vital step in the growth of the space station. Reconfiguring the power system will give the station the capability to provide electricity to laboratories that will be added to the structure in upcoming years.

Discovery's astronauts have already completed one of the main tasks on the 12-day mission. Spacewalkers Robert Curbeam and Christer Fuglesang, along with robotic arm controllers Joan Higginbotham and Sunita Williams, installed a 2-ton, $11-million addition to the space station truss.

Williams has also officially replaced German astronaut Thomas Reiter to become the newest space station resident. Reiter will return with the Discovery crew after a five-month stay in space.

Two more spacewalks will be dedicated to rewiring the station from the current temporary power system to the permanent one.

"This is a major milestone for the program," Curry said.

The array to be retracted has been part of the station's temporary power system. It will be moved to its permanent position on a later shuttle mission.

Also, NASA plans to fill cooling loops with ammonia. The pumps on those loops are scheduled to be turned on after the rewiring so the liquid can dissipate heat from electronics in the new power grid.

Discovery is scheduled to return to Earth on Dec. 21. NASA managers cleared the space craft for the flight back, saying dings on its left wing and belly were not of concern. The space agency has been particularly wary of damage to the shuttle's heat shield since the Columbia disaster in 2003. A gash on that vehicle allowed hot gases to penetrate Columbia during its re-entry to Earth, killing the seven astronauts aboard.