Just how far the U.S. has fallen behind the Russians in space is underlined by Soviet plans to send two unmanned vehicles in July to one of the moons of Mars - the first in a series of unmanned Mars expeditions.

By contrast, the American space program remains grounded. And the U.S., once the unchallenged leader in unmanned exploration of the planets, has not launched a planetary mission in 10 years.Budget cuts, and the diversion of all efforts into the space shuttle program, have effectively shut the U.S. out of planetary exploration. And the Challenger disaster did the rest.

As a result, there is talk about an American-Soviet joint manned venture to Mars. But worries about loss of U.S. technological secrets has delayed this prospect.

Before President Reagan went to the Moscow summit, the idea of a joint mission was on the agenda, yet no new cooperative missions came out of the talks. However, continuing budget problems may eventually force the U.S. into cooperation with the Soviets.

NASA is using data from its 1975 Viking mission to Mars to help the Russians identify possible landing sites on Phobos, one of the red planet's two moons. As payment, the U.S. will have access to all information gathered by the Soviet mission.

The Russian probes will hover over Phobos, taking measurements of the small moon. Remote-control landers will be dropped to the surface to analyze soil and carry out other experiments. Later Soviet missions will seek to use mobile vehicles on the surface of Phobes and even collect some samples to be shot back to Earth with small rockets.

While it is embarrassing for the once-mighty U.S. space program to be in the position of a poor relation, a cooperative Mars project would save perhaps half the cost of such an undertaking.

And even more important than the dollars is the opportunity for such a space project to build bridges on Earth between America and the Soviet Union.