Space shuttle Discovery rolls to the launch pad this morning. Credit: NASA-KSC

The 4.2-mile journey from NASA's cavernous Vehicle Assembly Building to the oceanside launch complex began at 1:58 a.m. EDT (0558 GMT) and took more than 10 hours to complete. The trip took longer than expected because of an over heated bearing that prevented NASA's Apollo-era crawler-transporter from maintaining its normal 1 mph top speed.

While engineers are confident Discovery and its new tank will be ready to go when July 13 rolls around - the launch team has five full days of contingency time built into the processing schedule - final clearance will hinge on the results of a critical meeting June 24 to assess the threat posed by ice impacts during ascent.

NASA had hoped to launch Discovery in mid May, but the long-awaited flight was delayed after a fueling test because of concern about ice buildups around a flexible liquid oxygen feedline bellows assembly. In addition, two of four hydrogen fuel sensors operated intermittently and a pressure relief valve in the hydrogen section of the tank cycled more often than expected.

NASA managers ultimately ordered engineers to move Discovery back to the VAB attachment to an external tank and boosters originally slated for the second post-Columbia mission. The new tank is equipped with a heater to minimize ice buildups around the feedline bellows.

Before the rollback, however, a second fueling test was conducted May 20, confirming the hydrogen vent valve's unusual behavior. Engineers believe the valve cycling was associated with a jet-like device called a diffuser that injects helium into the tank to help maintain the proper temperature and to pressurize the tank for flight. A new dual-screen diffuser was used in Discovery's original tank and managers decided to switch back to the original single-screen design for the new tank.

As for the hydrogen sensors, which operated normally the second time around, engineers now believe whatever caused problems during the first tanking test was corrected during extensive post-test troubleshooting.

NASA managers decided June 6 to forego a third tanking test, saying they were confident the new tank would behave normally during Discovery's countdown. Even though the hydrogen pressurization system begins operating just two minutes before liftoff, the countdown can be safely stopped if any problems develop.

Wanting to avoid any such last-minute surprises, however, some engineers argued in favor of a third tanking test to make sure the analysis is correct. But senior managers decided to press ahead. Any serious problems with the tank almost certainly would delay the flight beyond the July 13-31 launch window regardless of when they were discovered.

Against this backdrop of engineering troubleshooting, an independent panel assessing NASA's implementation of post-Columbia safety upgrades is nearing the end of its work, waiting for the results of the June 24 debris verification review to close out a pair of lingering open items.

Even though the new tank is equipped with additional heaters to prevent ice buildups in critical areas, questions remain about the threat posed by ice shaking off the tank during launch from stand-off brackets that hold a pressurization line near the tip of the fuel tank.

While engineers believe the tank will not shed any foam insulation debris big enough to cause a catastrophic Columbia-class disaster, the data are less clear cut about the threat posed by ice.

The debris verification review will set the stage for a final meeting by the Return to Flight Task Group on June 27 and 28, followed by a two-day flight readiness review at the Kennedy Space Center June 29 and 30.

Additional coverage for subscribers:
VIDEO: FINAL PREPS ARE MADE FOR ROLLOUT QT
VIDEO: DISCOVERY BEGINS ITS JOURNEY TO THE PAD QT
VIDEO: THE SHUTTLE EMERGES FROM THE ASSEMBLY BUILDING QT
VIDEO: DAY BREAKS OVER KENNEDY SPACE CENTER QT
VIDEO: HOT TRANSPORTER BEARING CAUSES PROBLEMS QT
VIDEO: DISCOVERY NEARS PAD 39B AFTER THE LONG JOURNEY QT
VIDEO: TRANSPORTER CLIMBS UP THE LAUNCH PAD RAMP QT
VIDEO: DISCOVERY ARRIVES AT ITS LAUNCH COMPLEX QT
MORE: SHUTTLE RETURN TO FLIGHT VIDEO LISTING
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