The space shuttle Discovery today thundered into Earth orbit, a day later than planned, to begin an Air Force mission whose cargo, flight path, objectives and time in orbit have been stamped "top secret" by the Defense Department.

Carrying a five-man military crew, Discovery lifted off from Kennedy Space Center at 2:50 p.m. EST into cloudless Florida skies on the first secret mission in 15 shuttle flights. Liftoff time was not announced to the media until nine minutes before Discovery's three main hydrogen-fueled engines ignited on Launch Pad 39A.

A day late because sub-freezing temperatures Monday and Tuesday formed large layers of ice on Discovery's external fuel tank, the 100-ton spaceliner apparently departed on time today.

National Aeronautics and Space Administration officials would say only that it was within the launch "window" that lasted today from 1:15 to 4:15 p.m.

"The freezing temperatures gave us some concerns earlier in the week," launch director Robert Sieck said one hour after launch. "We waited a day, it was a lot warmer and we had no icing concerns whatever."

Amid a secretive atmosphere radically different from that surrounding previous shuttle launches, Discovery roared almost directly due east and away from the watchful eyes of what several sources described as a "small fleet" of Soviet trawlers and submarines offshore.

The sources said at least 12 Soviet surface vessels were three to five miles offshore. Space center public affairs officer Hugh Harris said, "The Air Force is responsible for surveillance of that area."

Told that on previous shuttle flights the Air Force reported Soviet ship movements offshore, Air Force Capt. Martin Hausner said, "Maybe that's what we used to do, but not anymore. That information is now classified."

During the ascent, not a word was heard publicly between ground controllers and the crew of Navy Capt. Thomas K. Mattingly, Air Force Lt. Col. Loren J. Shriver, Marine Lt. Col. James F. Buchli, Air Force Maj. Ellison S. Onizuka and Air Force Maj. Gary E. Payton.

All such communications are coded and transmitted between Discovery and the Air Force Mission Control Center at Sunnyvale, Calif., marking the first time the public will not hear from U.S. astronauts orbiting Earth.

Last month, The Washington Post reported that Discovery is carrying a "sigint" (signals intelligence) satellite that can intercept telemetry from Soviet missile tests.

The Associated Press reported that such satellites can monitor radio, ground-to-space communications and long-distance telephone calls made by microwave relay.

Every shuttle that has deployed a satellite has done so as soon as possible to avoid having to return with it to Earth if a mission is curtailed.

The Air Force has acknowledged that a solid-fueled engine called the inertial upper stage was to be used on this mission. It weighs 32,500 pounds and has always been used to take Air Force and NASA satellites into geosynchrous orbit.

The Air Force has said it will announce Discovery's return to Earth 16 hours before its landing, scheduled here.

Before liftoff, armed gate guards here told those accredited to cover the launch and contractors' personnel to produce driver's licenses so names on them could be matched with names on press passes. Vans were searched.

Contractors' personnel who traveled to the space center from around the country were told that they could not inform their families when to expect them home.

Photographers were denied access to the launch pad and had to set up "remote" cameras around Launch Pad 39B one mile away.

The number of invited guests was reduced to 200, compared with 13,000 on the last shuttle flight and as many as 100,000 for early shuttle launches.