The second of three spacewalks at the International Space Station proceeded ahead of schedule on Thursday, as two astronauts began disconnecting cables from a temporary power source to hook them up to a permanent one.

Astonauts Robert Curbeam and Christer Fuglesang, who performed Discovery's first spacewalk on Tuesday to install a two-tonne, $11-million US addition, were scheduled to begin the spacewalk at 3:15 p.m. ET but left a half hour early.

"I think I see I sunlight," Curbeam said as he began his spacewalk.

The two will reconfigure the power system controlling half of the space station by rewiring connectors from a temporary solar panel to the permanent solar arrays delivered in September.

Half the station was powered down hours in advance of the spacewalk, but critical systems, like computers, were switched to the other side temporarily.

NASA hopes the rewiring project will be a turning point for the station, as a permanent power source will accommodate more laboratories and lead ultimately to the completion of the space station.

"This is the major milestone" of the mission, said John Curry, NASA's lead flight director for the space station.

The new solar arrays will allow NASA and other space agencies to add two more labs and allow up to six crew members to live there, up from the current limit of three.

Saturday's scheduled spacewalk will accomplish the same task for the station's other side.

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 as 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 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 system handling the retraction mechanism. NASA managers expect to make a decision within the next couple of days.