"The result of that could be an orbit that is significantly different than the one planned and we may have our navigation predictions at the tracking stations that are incorrect," McNamee said from mission control at Jet Propulsion Laboratory in Pasadena, Calif. That would mean that controllers were searching for signals from the wrong position, he said.

Another possibility is that receivers on Earth have been tuned to the wrong frequency, which would cause them to miss communications from the orbiter, said Richard Cook, operation project manager for the orbiter. The spacecraft was supposed to send signals back to Earth when it emerged from behind the red planet minutes after completing its critical orbit-insertion burn that would have locked it into orbit around Mars.

While admitting some nervousness, controllers have stressed that the spacecraft will probably be found.

"It is likely that the spacecraft is in orbit around Mars and we are currently trying to establish contact with it," said Richard Zurek, project scientist for the Mars Surveyor '98 mission, which includes the climate orbiter and the Mars Polar Lander, which is still on its way to Mars.

The spacecraft has software programs that allow it to sort out a lot of its own problems, Zurek said. "It's a pretty capable spacecraft, it can take care of itself. It doesn't need to talk to us all the time, so if it's in orbit around the planet we still have hopes that we can get commands into it to configure it properly for the next pass close to the atmosphere," he said.

Controllers are taking several approaches to find the orbiter. "First, we're trying to broaden the search from the ground-based antennas that are looking for this signal in the cosmic haystack. The second thing is, we're trying to send commands back up to the orbiter, to call home, in effect, or to get ready to do a maneuver on it's next pass close to the atmosphere."

Whatever happened, it is unlikely that the craft passed low enough in the martian atmosphere to destroy it, Zurek said. The orbiter was designed to pass through the atmosphere once each orbit for the next two months in a technique called aerobraking.

At this point, Zurek said, the worst case scenario is that an extremely low pass through the atmosphere damaged some communications components the orbiter needs to communicate with Earth.

The communications problem raises questions about the impact a total loss of the orbiter would have on the Mars Polar Lander's mission. The climate orbiter is supposed to serve as a communications and data-relay station for the lander, which is scheduled to reach Mars on Dec. 3.

The lander is capable of communicating directly with Earth, but it can send data much more efficiently through an orbiting relay station. Loss of the climate orbiter would not compromise the lander's mission, Zurek said.

"The Mars Surveyor program is resilient to the loss of any one spacecraft," he said. "Certainly we were looking for a type of climate and weather data for this particular spacecraft, and that would be a loss. However the Mars Polar Lander, for instance, will be able to carry out its surface mission even if we don't establish contact with the orbiter again."

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