NASA's hopes for restoring communications with its silent Mars Observer spacecraft came down Monday to one mission controller's fervent prayer to the $1-billion space probe: "Talk to me, baby." But as of late Monday, the Observer had not answered.

In an effort to revive the spacecraft, NASA engineers nervously transmitted a series of electronic pleas over and over again to the probe, which is speeding toward a rendezvous with Mars today. NASA project officials said they did not know why there was no response.

Mars Observer project manager Glenn E. Cunningham at the Jet Propulsion Laboratory, which is running the mission, had his theories: The Observer's on-board clock might be broken, its radio could be overheated or its antenna askew. The probe might be absorbed in its own silent electronic catechism, deaf to all outside entreaties.

If they cannot attract its attention soon, National Aeronautics and Space Administration officials said the spacecraft--the first U.S. emissary to Mars in 17 years--will go spinning past Mars into space, never to be recovered.

The Observer, designed to provide the most detailed look at the mysterious fourth planet, is a pathfinder for an international armada of planetary probes to be launched toward Mars in the coming decades and an important step toward human exploration of the Red Planet in the next century.

"This would be a great blow to the planetary science community," Cunningham said. "If we never receive another signal from the spacecraft, we will never know what happened."

NASA officials did their best to maintain an air of stoic professionalism in the face of what they hoped was a temporary setback. Technical difficulties have plagued NASA's other recent planetary missions, but Observer had performed almost perfectly until Saturday when it fell silent.

On Monday, with less than 24 hours to go before the craft is to reach Mars, the tension at the lab was palpable.

"It is a very difficult period. I have been working on it for 12 years and it is almost there, almost within reach. And this happens," said Arden Albee, Mars Observer project scientist. Albee said that watching the huddled teams of engineers trying to restore communications over the weekend made him so anxious that he went home and ripped up his garden ivy.

"They have been trying to work out individual commands, then sets of commands, then anything they could hypothesize that could work, while being just as careful they don't do something bad."