NASA set the countdown clock ticking Monday for returning Americans to space on Thursday aboard the shuttle Discovery.

"The launch countdown is now in progress," test director Terry Willingham announced promptly at 6 a.m. MDT. The countdown had been set to begin at midnight but was postponed when launch pad workers fell behind in preparations.NASA said that did not affect the goal of launching Discovery at 7:59 a.m. Thursday on the first U.S. manned space mission since Challenger exploded 32 months ago, killing its crew of seven,

"We're still on the timeline for launch on the 29th and that's the plan," launch director Bob Sieck said. The National Aeronautics and Space Administration had built 39 unprogrammed hours of contingency time into the countdown as insurance for last-minute problems.

Sieck said it was decided Sunday to allot eight hours of that time to complete the work of replacing panels and removing work platforms from around Discovery's engine compartment.

Closing out the aft end of the shuttle had been held up when low voltage readings were detected in an electrical circuit that ignites the explosive charges used to separate the shuttle from its fuel tank in flight. Engineers eventually determined the fault was in a ground circuit, which does not affect the flight.

Discovery's five astronauts planned to fly to the cape Monday afternoon from the Johnson Space Center in Houston. Once here, they will receive daily briefings, review their flight plan and undergo medical examinations.

Commander Rick Hauck, a Navy captain, pilot Dick Covey, an Air Force colonel, and the three mission specialists, George Nelson, Mike Lounge and Dave Hilmers, a Marine lieutenant colonel, all have flown on earlier shuttle flights.

During four days in orbit they are to release a $100 million communications satellite from Discovery's cargo bay, conduct 11 science and technology experiments and check out modifications made to the shuttle since the Challenger accident.