With good weather expected, the shuttle Discovery and its seven-man crew were cleared Saturday for liftoff Sunday on a thrice-delayed "Star Wars" research mission now running two months behind schedule.

Five days after a faulty fuel pump sensor scuttled a launch try Tuesday, engineers at launch pad 39A planned to pump a half-million gallons of supercold liquid oxygen and liquid hydrogen rocket fuel back into Discovery's external tank late Saturday for a planned liftoff at 5:01 a.m. MDT Sunday.With no technical problems under study, NASA managers were optimistic about finally getting Discovery airborne, despite the threat of early morning fog.

NASA test director Albert Sofge told reporters agency managers cleared the shuttle for flight. "Everything is going smoothly and there's nothing but good news at this point."

Air Force meteorologists called for an 80 percent chance of early morning fog that would delay an on-time liftoff.

But the rising sun was expected to burn off any such fog as the morning progressed, and forecasters predicted a 90 percent chance of getting Discovery into orbit by the end of the ship's three-hour, 20-minute launch period.