Space shuttle Atlantis approaches launch pad 39B this morning. Photo: NASA-KSC

NASA rolled space shuttle Atlantis from Kennedy Space Center's cavernous Vehicle Assembly Building to launch pad 39B today in preparation for blastoff July 12 to deliver a $164 million airlock to the international space station.

The planned rollout Wednesday was aborted shortly after the shuttle emerged from the 52-story building due to lightning in the area. Officials ordered the mobile transporter carrying the shuttle be reversed, backing the spaceship into the safe confines of the VAB.

The first rollout attempt began at 1:54 a.m. EDT, but was halted about 2:35 a.m. when lightning was detected 15 miles away. NASA rules state the rollout can't occur if there is lightning within 20 miles.

Today's trek to the pad began at about 2:45 a.m. The 4.2-mile trip took about eight hours to complete.

The space agency finally cleared Atlantis for flight on Tuesday evening after concluding the problems with the international space station's robotic arm wouldn't impact the airlock installation mission.

The cause of intermittent communications dropouts between the arm's backup control unit and the Shoulder Pitch joint is believed to be a suspect computer chip in the joint's electronics.

The problem only happens when the arm functions on its backup power and commanding system. The primary has worked flawlessly.

During recent tests the problem did not reoccur, and managers determined there was enough confidence the arm will continue to work to press ahead with the Atlantis flight.

Even so, engineers are still working on a software patch to mask over the troublesome computer chip, preventing the problem from manifesting itself in the future.

NASA had considered bumping Atlantis' mission to late-September and allowing the next flight of shuttle Discovery to proceed first with liftoff in early August to deliver the Expedition Three crew to the station and return Expedition Two back to Earth.

Atlantis was supposed to fly June 14, but has been grounded while the robotic arm troubleshooting has played out.