The shuttle Discovery's roll out to the launch pad now is scheduled for Monday, the Fourth of July, in a "big step" toward the first post-Challenger flight, a top NASA official said Friday.

"Come Monday morning, the Fourth of July, we plan and hope to see something out there on the pad that we haven't seen in quite some time, and that's Discovery," said James Harrington, shuttle operations director at the Kennedy Space Center.

"We are planning as of right now to have a roll out . . . at midnight Sunday night. That would put us out on the pad, hard down on the mounts, by 8 o'clock Monday morning."

Once on the launch pad, Discovery will face a major hurdle on July 24 when the ship's three liquid-fueled main engines are fired in a crucial unmanned 20-second test.