NASA set the countdown clock ticking today 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 8 a.m. EDT as he signaled the traditional ``call to stations.'' That call assembled more than 100 controllers at their consoles. Early work included loading Discovery's computer software, servicing the main engines and other propulsion systems and removing work platforms from around the base of the spaceplane. ``We're looking forward to doing the shot and doing it right and launching on time,'' Frank Merlino, another test director, told a news conference shortly after the countdown began for the first U.S. manned space mission since Challenger exploded over the Atlantic, killing the crew of seven, 32 months ago. Thursday's forecast was for scattered and broken clouds, a wind of 10-15 knots from the southeast, temperatures of 85 degrees and widely scattered showers. ``Given the constraints NASA has, things look very, very good,'' said Senior Master Sgt. Mike Beeman, an Air Force weather spokesman. The countdown had been set to begin at midnight but was postponed when launch pad workers fell behind in preparations Sunday. NASA said that did not affect the goal of launching Discovery at 9:59 a.m. Thursday. ``We're still on the timeline for launch on the 29th and that's the plan,'' launch director Bob Sieck said Sunday. 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. ``We don't want those people to have to do that under the gun with the clock counting,'' he said. ``So we said, `Take all the time it takes and we'll delay the call to stations to accommodate that.''' Closing out the aft end of the spaceplane 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 here late this afternoon from their training base at the Johnson Space Center in Houston. Once here, they will receive daily briefings on launch preparations, review their flight plan and undergo medical examinations. Commander Rick Hauck, a Navy captain, and pilot Dick Covey, an Air Force colonel, planned to practice landings at a runway here in a jet outfitted to handle like the shuttle. The three mission specialists in the crew are 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. ``Because there are so many changes, we're treating it like a test flight of a new vehicle,'' Hauck said in a recent interview. The Rogers commission that investigated the Challenger accident said the direct cause was a leak at a joint between two segments of one of Challenger's two solid-fuel booster rockets. Engineers redesigned the booster, and made improvements to the orbiter, the main engines and the external fuel tank. In all, 56 major changes and more than 400 lesser ones were made. The program to redesign all three shuttles has cost about $2.4 billion to date. The commission report said cold weather, 36 degrees at launch, contributed to the Challenger accident by stiffening O-rings in the joint, preventing them from sealing gaps properly. Some engineers had argued with lower-level managers against launching in the cold, but their concerns never were relayed to top managers who made the launch decision. NASA has changed procedures to make certain concerns of any agency or contractor employee are relayed to top management. And a Mission Management Team, made up of 21 key NASA and contractor managers and engineers, has been established to monitor the final two days of the countdown. The team is headed by astronaut Bob Crippen, who has made four shuttle flights. With the help of team members, he will assemble all information in the final minutes of the count and will have sole authority to signal the decision to launch.
