Countdown clocks began ticking early Monday for the shuttle Atlantis' blastoff Thursday on a high-priority military mission to launch a top-secret spy satellite in the second post-Challenger flight.

With little fanfare, NASA test director Jerry Crute issued the traditional "call to stations" at 12:01 a.m. EST (10:01 p.m. MST) to kick off Atlantis' 43-hour countdown, which sources said includes 36 hours of built-in "hold" time."We started right on time, right on the money," said NASA spokesman George Diller. "So we're pressing on. Nothing is amiss, so I guess that's a good sign."

Blastoff, planned for around 7 a.m. EST (5 a.m. MST) Thursday, according to sources, will mark the 27th flight in the shuttle program, the second since Challenger's destruction in 1986, and only the third fully classified Department of Defense mission ever conducted.

Atlantis' crew - commander Robert "Hoot" Gibson, co-pilot Guy Gardner, Richard "Mike" Mullane, Jerry Ross and William Shepherd - was scheduled to fly to the Kennedy Space Center Monday afternoon to begin final flight preparations.

The countdown began as two cosmonauts and a Frenchman, launched Saturday from the Soviet Union, sailed through the final hours of a two-day orbital chase to link up with three cosmonauts aboard the Mir space station. When Atlantis blasts off Thursday, 11 men will be in orbit at once, tying the all-time space population record.

Unlike the Soviet flight, which is being conducted in the glare of international publicity, Atlantis' flight will be carried out in strict secrecy to hamper Soviet efforts to learn details about the shuttle's spy satellite payload.