The first military mission of the space shuttle program ended this afternoon when Discovery landed at Kennedy Space Center, surrounded by the same secrecy in which the mission began.

The 100-ton spaceliner bearing Navy Commodore 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 landed flawlessly on the concrete runway here at 4:23 p.m. (EST).

Wearing their blue flight suits, the five astronauts emerged from Discovery's cabin, stepped down to the runway and walked together to a van that took them to the dispensary for a medical examination, without making any of the customary postflight remarks.

In an uncharacteristically brief statement, the National Aeronautics and Space Administration said "there were no performance problems" aboard Discovery that would have forced the astronauts back to Earth prematurely. Only early this morning did the Pentagon, which has cited security reasons in refusing to divulge information about the shuttle's military cargo, authorize NASA to announce that the shuttle was to land this afternoon.

Unconfirmed reports have said the shuttle's mission included launching a sophisticated surveillance satellite to eavesdrop on the Soviet Union.

The only other announcement today by NASA and the Air Force was that the inertial upper stage (IUS) engine used to carry the secret satellite into an unidentified orbit had worked as intended. The announcement was the only indication that the first shuttle mission flown for the Pentagon had been successful.

There was widespread concern in the Air Force and at NASA about this new workhorse engine, which had not been tested in space since it failed on a space shuttle flight in April 1983.

That mission almost cost NASA its $100 million Tracking Data and Relay Satellite, which the two-stage IUS engine was to boost into a permanent orbit 22,400 miles above the Earth to serve as a radio relay linking shuttles with ground control during the next 10 years. The second stage of the IUS misfired, sending the satellite into a tumbling orbit far short of its final destination above the Equator northeast of Brazil. It took months of periodically firing thrusters on the satellite to nudge it into proper orbit.

The success this time of the Boeing-built IUS was especially good news to NASA, which plans to use it on the 16th shuttle mission next month when Challenger is to take the second of three TDRS satellites into space. The three radio relays are intended to replace worn-out and expensive ground stations still being used to communicate in space.