The shuttle reached the launch pad after midnight following a 10½-hour, 4.2-mile journey from an assembly building.
The glacial, 1-mile-per-hour trip to the launch pad began at 2:05 p.m. EDT after a brief delay to discuss a hairline crack seen in the insulation covering Discovery's external fuel tank, reports CBS News Space Consultant Bill Harwood.
Engineers decided the two-inch-long crack did not require repair and a powerful Apollo-era crawler-transporter slowly hauled the shuttle and its mobile launch platform out of the cavernous Vehicle Assembly Building and into a brilliant spring sky.
Looking on were scores of space center employees, reporters, tourists aboard visitor's center buses and even the crew of the international space station, which passed almost directly overhead around 4:30 p.m. If all goes well, Discovery will dock with the station three days after launch on the 114th shuttle mission.
NASA later said the 1½-inch crack was high up on the shuttle in a spot where if foam flew off it would not likely hit the vehicle.
"It's a very, very tiny crack. Very, very narrow ... well within our experience base," said Mike Leinbach, shuttle launch director. "It was an acceptable condition for flight, so we rolled on out and we're going to fly with it just as it is."
After a delay of at least two hours, NASA then began moving Discovery from its assembly building to the launch pad.
Because of Columbia's disintegration over Texas in 2003 — and the deaths of all seven astronauts aboard — the tank has been extensively redesigned for Discovery's flight.
Rollout originally was planned for late March, but the 4.2-mile trip was held up because of minor processing snags and technical issues that used up most of the contingency time built into the schedule to handle unexpected problems at the pad.
By rolling to the pad Wednesday, NASA can still, in theory, launch Discovery as early as May 15. But any additional problems almost certainly will delay launch. NASA has until June 3 to get Discovery off the ground on the first post-Columbia mission or the flight will be delayed to at least July 13, the next available daylight launch opportunity to the international space station.
Discovery was moved on a 5.5 million-pound transporter, a huge platform on caterpillar tracks, along a specially built road that is almost as wide as an eight-lane highway. The transporter was graced with a banner signed by NASA employees that read, "We're Behind You Discovery."