CAPE CANAVERAL, Fla. (AP) _ Atlantis, the fourth and possibly last space shuttle, makes its debut Thursday on a secrecy-shrouded military mission that reportedly will launch two satellites built to withstand nuclear assault.

The Air Force has said Atlantis and its five-man crew will be launched sometime between 10:20 a.m. and 1:20 p.m. EDT Thursday. The precise time will be disclosed nine minutes before the planned liftoff.

Throughout the flight, even less public information will be forthcoming from the Pentagon than was the case during the first all-classified Defense Department mission last January.

Even the length of the flight is a secret, but the landing will be announced 24 hours in advance.

Officials decline to reveal any details about the payload, but the Federation of American Scientists, citing public sources, reported last month the shuttle's cargo bay contains two Defense Satellite Communications System satellites, the advanced model known as DSCS-3.

DSCS-3 satellites are designed to be jam-proof, have been shielded against the electromagnetic effects of nuclear explosions and have a special transponder over which the president could transmit emergency messages to nuclear forces.

The Defense Department does not classify DSCS-3 satellites as secret, but as a matter of principal and precedent it has decided to place a secrecy label on most military flights of the shuttle. Officials said this policy will make it more difficult for Soviet spy ships that regularly operate off Cape Canaveral to monitor the launch and to learn the nature of any mission.

Reporters were denied the pre-launch interviews and news conferences that are normally available with NASA shuttle crews, and they will not be able to monitor space-to-ground communications.

On the January flight, the Air Force issued a statement every eight hours which said everything was going well aboard the shuttle Discovery. Those will be dropped on the upcoming flight. There will be announcements, however, if serious trouble develops.

The commander of the 21st shuttle mission is Air Force Col. Karol Bobko. The others are Air Force Lt. Col. Ronald J. Grabe, Marine Maj. David C. Hilmers, Army Lt. Col. Robert C. Stewart and Air Force Maj. William A. Pailes.

Atlantis is joining Columbia, Challenger and Discovery in the nation's fleet of space planes. It is the last one planned, although some congressmen have been pressing for a fifth vehicle to handle what they believe will be a heavy launch schedule in the 1990s when NASA constructs a permanent space station and the Pentagon has extensive plans for the reusable ships.

Documents in the public record disclose the DSCS-3 satellite is a vast improvement over the now-operational DSCS-2 series, that it has a six-channel communications transponder and measures 81-by-77-inches with a solar panel wingspan of 38 feet.

DSCS-3 provides superhigh-frequency communications for secure voice and high-data-rate transmissions. A specially designed antenna with electronically steerable beams is an anti-jamming feature, and it is the first military communications satellite built with materials designed to protect against the electromagnetic effects of a nuclear blast that an enemy might aim at it.

The Pentagon plans to launch four DSCS-3 satellites into stationary orbits 22,300 miles up to provide global coverage for ships, planes and ground troops of all services. Troops in the field will be able to communicate with the satellites through portable antennas only 33 inches in diameter. Two spare satellites also will be placed in space.