HOUSTON - Imagine rewiring your house while living in it. Now imagine you're not even sure the new power system will work and you're 220 miles above the Earth. That's the scenario facing astronauts on the second and third spacewalks of the space shuttle Discovery's mission to the international space station, a two-part process scheduled for Thursday and Saturday.

Rewiring the station is an exciting turning point for NASA, one that graduates its orbital outpost from relying on an interim power system to establishing a permanent one that can begin accommodating more laboratories and lead ultimately to the completion of the space station.

"This is the major milestone," said John Curry, NASA's lead flight director for the space station. NASA and other space agencies will be able to increase the number of crew members aboard the space station — now at three — and other such expansions, Curry said. "Now you're limitless."

Astronauts Robert Curbeam and Christer Fuglesang, who performed Discovery's first spacewalk on Tuesday to install a two-ton, $11-million addition, were set to again step out of the space station Thursday, this time to reconfigure the power system controlling one-half of the space station. They'll do that by rewiring connectors from a temporary solar panel to the permanent ones.

Half the station will be powered down during this process, but critical systems, like computers, will be switched to the other side temporarily.

The third spacewalk will repeat this procedure, but on the flip side.

In both cases, NASA will for a short time lose the redundancy it likes to have in its systems.

Besides worrying whether the permanent power system will work as planned, NASA managers also will be looking to see whether its accompanying cooling system performs.

To get the station ready for this change, astronauts spent much of Wednesday retracting the temporary solar panel to give room for the new ones to begin rotating with the movement of the sun, generating as much power as possible for the station.

The retraction went far enough to allow the new panels to rotate, but it did not go all the way like NASA had hoped. Stubborn kinks in the accordion-like structure and slackness in the guide wire were the two main obstacles. Experts began meeting to explore fixes for that.

One option would be to schedule a fourth spacewalk so astronauts could manually help the panel retract. But managers cautioned that an additional spacewalk may be needed to troubleshoot any problems arising from the rewiring — the main task for this mission.

The half-retracted array is structurally stable and poses no risks in its current configuration. NASA could ask the space station residents to perform a spacewalk after Discovery returns to Earth in a week, or it may find a potential solution using the remote control that was commanding the retraction. NASA managers expect to make a decision within the next couple of days.

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On the Net:

NASA: http://www.spaceflight.nasa.gov

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