AP SCIENCE WRITER

LEE SIEGEL

PASADENA, Calif. (AP)—Engineers suspect Mars Observer’s main clock may be stuck, which could cause the spacecraft to fly right by the Red Planet, rather than orbit it, the project’s manager said Monday.

The announcement, by Glenn Cunningham, was a departure from NASA’s earlier statements that officials believed pre-programmed computer commands would automatically fire the spacecraft’s thrusters and place it into orbit around Mars on Tuesday afternoon.

Those commands won’t be carried out if the clock isn’t working, Cunningham said.

If the spacecraft is lost, it ‘‘would be a great blow to the planetary science community,’‘ Cunningham said.

Mission controllers waited nervously Monday to hear from Mars Observer, which suddenly went silent on Saturday.

Also Monday, controllers at the National Oceanic and Atmospheric Administration were trying to re-establish contact with the nation’s newest weather satellite. The satellite was launched in August and had worked fine until communication was lost Saturday, officials said. Two older weather satellites remain in operation.

Engineers were sending Mars Observer new commands to switch the spacecraft to a backup clock in an effort to restore communications and save the $980 million mission, which is run by NASA’s Jet Propulsion Laboratory.

With contact restored, engineers would assess Mars Observer’s condition, then send a new computer program to make sure it fires its thrusters and enters orbit at 1:42 p.m. Tuesday, Cunningham said.

The spacecraft’s planned trajectory was to have carried it within 150,000 miles of Mars by Monday morning.

Cunningham said earlier that engineers remain optimistic the spacecraft was temporarily silenced by a computer glitch and hadn’t blown up or gone off course in some catastrophic accident.

‘‘We go on the presumption these kinds of problems are caused by simple little things, probably not a whole conglomeration of exotic problems, but one simple little problem,’‘ Cunningham said.

Mars Observer was launched on its 11-month, 450-million-mile voyage from Florida on Sept. 25. It was designed to give scientists their best look yet at the planet that has long inspired the human imagination.

Controllers failed to hear from Mars Observer on Saturday evening after the spacecraft started carrying out pre-programmed commands to pressurize its thruster fuel tanks.

Cunningham said he doubted Mars Observer’s fuel tanks overpressurized and blew up because the spacecraft has backup regulators to prevent such an accident, and the chance of both failing ‘‘is very, very low.’‘

If the tanks ruptured or failed to pressurize, the spacecraft would be unable to fire its thrusters and it would fly uselessly past Mars.

‘‘If Mars Observer did fail catastrophically, that would be a disaster for NASA, the United States and the Mars exploration program,’‘ said Bruce Murray, vice president of The Planetary Society and a former Jet Propulsion Laboratory director.

After reaching Mars, the spacecraft was supposed to spend two months maneuvering into its final 234-mile-high orbit, looping almost over the poles. Then it would undergo a month of testing before spending 687 Earth days, or one Martian year, studying the planet’s landscape, weather and seasonal climate changes.

Mars Observer is the ninth U.S. spacecraft launched toward Mars. The twin Vikings reached the planet in 1976, after four of six Mariner missions also succeeded.

Before its breakup, the Soviet Union sent at least 15 and possibly 17 spacecraft toward Mars. Half of those missions failed completely.