AP SCIENCE WRITER

LEE SIEGEL

PASADENA, Calif. (AP)—Engineers lost contact with the Mars Observer spacecraft as it was about to reach the Red Planet on a $980 million mission, but it is expected to orbit Mars despite the problem, NASA said Sunday.

By late Sunday, the spacecraft still hadn’t entered a ‘‘safe’‘ mode that would have aimed one of its antennas at Earth and re-established contact, said Glenn E. Cunningham, Mars Observer project manager at the agency’s Jet Propulsion Laboratory.

Engineers periodically radioed computerized commands in an attempt to get the unmanned spacecraft to send a signal to mission controllers and tell them its condition, he said.

If engineers fail to regain control of Mars Observer, it would be a costly and major setback for the National Aeronautics and Space Administration.

But Cunningham told The Associated Press: ‘‘I don’t think we’re going to lose this mission. I think we’ve had one of the typical little hiccups that affect all our missions because these are very complex pieces of hardware with many computer systems.’‘

Contact was lost Saturday evening as the spacecraft was automatically carrying out computerized orders to pressurize its propulsion system so it can fire its braking rockets and go into orbit around Mars on Tuesday afternoon, laboratory spokesman Bob MacMillin said.

Cunningham said he thought the spacecraft’s antennas pointed the wrong way or its transmitter failed to restart after pressurization. He said he doubted the fuel tanks ruptured or failed to pressurize, worst-case scenarios that would send the spacecraft hurtling uselessly past Mars.

The spacecraft, launched from Florida last Sept. 25, is programmed to automatically fire its thrusters and start orbiting Mars even if contact isn’t re-established by Tuesday afternoon, assuming the propulsion system was properly pressurized.

Mars Observer is supposed to go into a long elliptical orbit around the planet, then spend more than two months maneuvering into a near-polar circular orbit 234 miles above the Martian surface. Then, after a month of tests, it is to spend at least 687 Earth days—one Martian year—studying the geology, climate and weather of Mars while taking tens of thousands of pictures and other measurements.

Engineers lost touch with Mars Observer several times during its 11-month, 450-million-mile cruise from Earth. But in each instance, the spacecraft went into ‘‘safe’‘ mode within hours and contact was restored. Engineers have said they believed they fixed the computer programming glitches that caused those incidents.

In addition to carrying the most sophisticated science instruments ever put on a U.S. planetary spacecraft, Mars Observer also was supposed to help relay information from the Mars 94 Russian landing craft in 1995, and from a French exploration balloon and Russian robot rover during the Russian Mars 96 mission in 1997.

Its launch last year also was worrisome. It was carried into Earth orbit aboard a Titan III rocket, then was hurled toward Mars by a new Transfer Orbit Stage, or TOS, rocket. But the TOS failed to signal that it had ignited, leaving engineers without word for more than an hour before Mars Observer signaled that it was safe.

NASA’s Magellan spacecraft lost touch with Earth repeatedly after it started orbiting Venus in August 1990, but engineers solved the problem and Magellan successfully completed its primary mission to map the Venusian landscape by making radar pictures.

Mars Observer is the ninth U.S. spacecraft launched toward Mars and was to be the first American craft to reach the Red Planet since two Viking orbiters and their landers arrived in 1976. NASA earlier launched six Mariners toward Mars. Two of them failed.

Before its breakup, the Soviet Union sent at least 15 spacecraft toward Mars. Half of those missions failed completely.