The first classified flight of the space shuttle was postponed last night for 24 hours, to Thursday afternoon, as another night of freezing temperatures in Florida threatened to cause severe icing of the craft's external fuel tank.

The flight had been scheduled for launch this afternoon with an all-military crew of five whose cargo, chores, flight path and mission length are secret.

Launch of the shuttle Challenger on its Air Force mission was rescheduled for between 1:15 and 4:15 p.m. Thursday.

Ice chunks on the huge external tank could have dropped off and damaged the shuttle if the flight had not been delayed, officials said.

"We've had a couple of hard freezes during the night down here, and we expect another one tonight," NASA public affairs officer Richard Young said yesterday. "Physical inspections are being performed to see if pipes carrying fuel and water to extinguish fires on the launch pad might have frozen shut."

Just about everything about the mission has been classified by the Pentagon. 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 did not conduct the traditional pre-launch news conference and will not hold one after landing.

The Air Force will not disclose how long the crew will stay in orbit and what flight path they will take on grounds that it would help the Soviet Union figure out what the mission entails and might even enable them interfere.

"We are working to deny our adversaries any information which might reveal the identity or mission of DOD [Defense Department] payloads," Brig. Gen. Richard F. Abel, Air Force director of public affairs, said in a recent news conference. "The more mission information they have, the easier it is for them to counter the capabilities of these payloads."

The Washington Post reported last month that Challenger will carry into orbit a "sigint" (signals intelligence) satellite that can intercept telemetry from Soviet missile tests. The Associated Press later reported that the satellite can pick up radio, ground-to-space communications and even long-distance telephone calls made by microwave relay. Defense Secretary Caspar W. Weinberger assailed The Post for "irresponsibility" but would not comment on the article's accuracy.

The Air Force so far has not filed a flight-path plan with the United Nations. It is not required to do so until "as soon as practicable," but usually the military space units of the United States and the Soviet Union file the equivalent of flight plans with the U.N. under the Convention on Outer Space Affairs.

The location of the two ships that will recover the burned-out solid-fuel rocket boosters that help loft Challenger into orbit is also secret.

"That information would help to reveal the azimuth that Challenger will follow into orbit," said Hugh Harris, public affairs officer for the National Aeronautics and Space Administration.

The Defense Department has said that on this mission it will test a rocket system that boosts a satellite into final orbit. The booster failed to operate properly when used almost two years ago.

Though the Air Force has not said so, deployment of the satellite is expected to take place on the first or second day in orbit. Every shuttle that has carried a satellite has deployed one in the first two days.

For the first time in 15 shuttle missions, there will be no public air-to-ground communications. All communications will be encrypted and transmitted between Challenger and the Air Force Mission Control Center in Sunnyvale, Calif.