NASA engineers, given time off for the Thanksgiving holiday, worked Saturday to ready the shuttle Atlantis for the start of its countdown to blastoff Thursday on a top-secret military mission.

The countdown was scheduled to begin at one minute past midnight Sunday - 2:01 a.m. MST Monday - leading to a launch attempt Thursday morning.The launch team worked Saturday to complete work in Atlantis's engine compartment and to finish up installation and checkout of explosive devices needed by the shuttle's self-destruct system.

"They're finishing up the ordnance installation," said NASA spokesman Richard Young. "This is mostly range safety type of stuff."

Like all large American rockets, Atlantis is equipped with a range safety system that would allow the ship to be blown up on radio command from the ground in the event of a catastrophic malfunction that otherwise could put an out-of-control shuttle over populated areas.

The goal of the 27th shuttle mission, the second since the 1986 Challenger disaster, is the deployment of a top-secret Pentagon spy satellite.

Because the mission is a classified Department of Defense flight, however, few details are being released publicly, including the exact launch time or any information about the payload.

But sources have said the countdown will begin at the T-minus 43-hour mark and that it features 36 hours of built-in "hold time," which would result in a launch around 7 a.m. Thursday if no adjustments are made in the holds.

The first day of the countdown is highlighted by the arrival of Atlantis's five-man crew - commander Robert "Hoot" Gibson, co-pilot Guy Gardner, Richard "Mike" Mullane, Jerry Ross and William Shepherd - at the Kennedy Space Center for final flight preparations.

All five are active-duty military officers and three, Gibson, Ross and Mullane, are shuttle flight veterans.

The astronauts are scheduled to arrive in Florida after a flight from the Johnson Space Center in Houston around 4 p.m. Monday when sources said the countdown was to enter a planned 8-hour hold.

The hold was scheduled to end at midnight Monday, setting the stage for engineers to load liquid oxygen and liquid hydrogen into on-board tanks to power Atlantis's electricity-producing fuel cells.

Another 8-hour hold was scheduled to begin Tuesday morning to allow time for engineers to make up any ground that might have been lost in the fuel cell loading exercise.

A 16-hour, 40-minute hold was expected to begin at midnight Tuesday, ending at 4:40 p.m. Wednesday when engineers planned to roll a rotating gantry away from Atlantis, exposing the ship to view as it will be for launch.

With the tank full, a weather briefing was planned during a two-hour hold early Thursday and if all goes well, Gibson and company are expected to begin boarding Atlantis shortly after 4 a.m.

Once Atlantis is safely in orbit, NASA will have no comment on the status of the shuttle or its crew until the landing time is announced 24 hours before touchdown.