Shuttle Flight Called Off Just Before Launch : Space: A faulty computer forces NASA to scrub the secret military mission with 31 seconds to go. The liftoff already had been delayed three days.

The scheduled launch of the space shuttle Atlantis, already delayed three times by an astronaut’s illness and foul weather, was scrubbed again early today with 31 seconds left in the countdown when a problem developed in a ground computer.

NASA engineers raced against a six-minute clock--the maximum time the flight could be delayed within the final seconds of the countdown--in hopes that they could fix the malfunctioning computer, a range safety monitor that can destroy the shuttle if it strays too far off course and threatens populated areas.

Space agency officials said they were assessing whether the failed computer, and the weather, would permit a fifth launch try early Monday morning.

Seconds before NASA engineers ran out of time, the delay in the countdown had caused temperatures in the shuttle’s main engines to drop too low to permit launch, driving a final nail in plans to proceed with liftoff.

National Aeronautics and Space Administration workers had pumped 500,000 gallons of fuel into Atlantis’ external tank Saturday to ready the shuttle for flight. Once in orbit on a secret mission, the shuttle had been expected to put a photo-imaging spy satellite into orbit.

A crew of five military astronauts was aboard Atlantis when the mission was scrubbed. It would have been the 34th flight of a U.S. space shuttle and the sixth shuttle flight entirely dedicated to a Defense Department mission.

Commander John O. Creighton’s scratchy throat and congestion had delayed the shuttle liftoff twice--on Thursday and Friday mornings--even as gusty winds and thunderstorms threatened to cause postponements. By Friday, NASA flight surgeons had cleared Creighton for flight, but torrential rains swept across Florida and grounded the shuttle on its launch pad for a third day in a row.

Not since the Apollo 13 mission of April, 1970, had an astronaut’s illness delayed a launch. NASA officials said Creighton’s illness was not “anywhere near significant enough” to prompt his replacement. But they added that the space agency no longer trains back-up crews and would have had to scramble to find a new commander for Atlantis had Creighton’s illness been more serious.

At Kennedy Space Center alone, each day’s delay costs $247,000 in labor and materials, officials said.

While the Defense Department has shrouded Atlantis’ payload in secrecy, experts said that the shuttle was to carry a $500-million satellite called the KH-12 into space. The “keyhole” satellite would be the sixth in a constellation of U.S. craft designed to photograph and relay images of military and other installations, according to John Pike, a space expert with the Washington-based Federation of American Scientists.

Gleaning information from the time and angle of the shuttle’s launch, as well as from the rough weight of its payload, Pike and others are able to judge the purpose and capabilities of Atlantis’ cargo.

According to Aviation Week and Space Technology, an authoritative industry magazine, the 37,300-pound satellite also will carry eavesdropping equipment.

From its projected orbit, the satellite is expected to come within photographic view of points as far north in the Soviet Union as Leningrad. In addition to helping monitor compliance to arms control agreements, such satellites would play a role in helping locate potential targets for nuclear attack.