TO SUSTAIN the hopes that were raised at last week's summit, the Soviet Union and the United States need to work together on important scientific projects at the frontiers of knowledge. As a start, we should go to Mars together.

We should begin carefully, with unmanned missions. First, by 1994, we should cooperate in landing a mobile, unmanned space vehicle on Mars. Later, by 1998, we should cooperate in a mission to bring samples of the Martian planet back to earth. That is truly a challenge for the century. If all goes well with these missions, we could try to cooperate in landing men on Mars, maybe by the year 2001.

We should be realistic. If Americans are worried about transferring sensitive military technology to the Soviet Union, we should find ways to work cooperatively, short of fully-integrated missions. For example, we could each send payloads to Mars that would be launched separately from Earth but work together on Mars. I wouldn't exclude the more ambitious possibility of joint spacecraft orbiting Mars or landing on its surface. This would continue the sort of cooperation we showed in the 1975 Apollo-Soyuz docking. But it's probably premature to talk about such arrangements now.

The cost of these Martian missions would be manageable -- far below what our two countries now spend annually on nuclear arms. For the initial phase -- the Martian rover -- the total cost would be around $1 billion. The next mission, to gather Martian soil samples and return them to the Earth, would probably cost less than $5 billion. The final stage -- a major, manned expedition to Mars -- would cost $50 to $100 billion, assuming the dollar will be stabilized.

This joint space effort would not only broaden the dialogue that is now underway between our two countries, but also bring a new dimension to it. If our dialogue remains based on arms control, then only military aspects will be involved and we will have difficulty establishing the language of mutual understanding. Something stronger should exist.

I know what international scientific cooperation can mean, because I have benefited from it nearly all my professional life. Since the late 1950s, I have been part of the international scientific community. We compare research with our colleagues, exchange visits, develop friendships. My family learned to speak English, and now even my four-year-old grandson is bilingual. These contacts have made our entire lives different.

Scientific cooperation illustrates something that Freud said: When people possess something jointly, they have less chance to fight. Even the black humor of the nuclear age reflects this wisdom. Last year, in the midst of our Vega probe of Halley's comet, we had many important Americans coming to visit us in Moscow. We were told that one of the Americans was a member of a U.S. committee that selects targets to be attacked in a nuclear war. I said to him: "We hear you have 60 targets identified in Moscow. Can you do a favor for us in the spirit of scientific cooperation: Will you delete our space institute from this list?" He replied that during the working day, it would be taken off the list. What a boon to productivity! People wanted to work 24 hours a day.

My first job after graduating from the university in 1956 was at the Kurchatov Scientific Institute, one of our top laboratories. My specialty was plasma physics, and I began working on the controlled thermonuclear reactions known as "fusion." At that time, the whole area was completely classified, in both the Soviet Union and the United States, even though it had a peaceful goal of designing power stations. Everything we were doing we could discuss only in a small group that had clearance. We were very excited by what we were doing, but we had no idea what was going on elsewhere.

A tremendous breakthrough came in 1958, when we made our first major contact with Western scientists. It was at a conference on peaceful uses of atomic energy in Geneva, to which both sides had sent hundreds of scientists and engineers. You cannot imagine what that was like for us. To see Americans for the first time was like seeing extra-terrestials. What was even more amazing for me, as a young man, was to realize that these extra-terrestrials were speaking the same language of science that I was.

One American scientist, who later became a dear friend, delivered a lecture at this conference in Geneva. As we watched him scrawl equations on the blackboard, we Russians couldn't believe it. He was writing the same formulas we had brought with us in our suitcases from Moscow. My Soviet friends couldn't stop laughing. So many of our models and formulas were identical. In fact, maybe 90 percent of what we were doing was the same. But it was the last 10 percent that was most interesting.

The Americans had a concept they called "stellarator," which was modeled on the interior structure of stars. Our first thought was: God, these Americans are really smart!

But it turned out that we Russians understood some things better than the Americans. At that time, our American colleagues hadn't grasped the importance of "Tokamak," the name we give to the magnetic-field configuration that allows a hot plasma to be stable. Gradually, the Americans began to pay more attention to this, and today, this process is common in controlled-fusion programs in America.

Truly, science is international. The laws of nature don't know any borders.

For the last 15 years, I have worked in space research, and there I have found it even more natural to cooperate with American scientists.

We already have the foundation for working together in space research. The United States and the Soviet Union signed a treaty last April that opens the way for cooperation in 16 areas of space exploration, and it will probably take scientists on both sides a year or so to assess the possibilities. In many areas we envy the Americans for their outstanding achievements.But we are also proud of our own achievements, starting with the launching of Sputnik 30 years ago. We were proud of our Vega mission to intercept Halley's comet, and we were grateful that the United States helped us with that mission, by making available NASA's Deep Space Network.

Mars is the next great challenge in space exploration, and I propose that we should meet it together.

The first breakthrough would be to land a roving vehicle. This would be the next logical step after the American Viking program, which landed on Mars but didn't move around on the surface. Now we should try a land-mobile machine, with all the instrumentation that Viking had, and even more. The technical problem for this rover would be how to control it, given the difficulty in maintaining communications across the millions of miles that separate Earth and Mars. Even when Mars is at its closest position, it would take a radio signal a few minutes to reach the planet. But the rover would meet obstacles every few seconds. The answer would probably be to make the rover an "expert system," using the techniques of artificial intelligence, so that it could devise its own answers for how to maneuver on the Martian landscape.

One simple way to begin this process of landing a rover on Mars would be to cooperate in the missions that already are planned: the American Mars observer, which will be launched in 1992; and the Soviet orbiter, launched in 1994. These missions could exchange data, complement each other's research and coordinate measurements. We could also have some American scientific instruments on board our mission. As we gained confidence and trust, perhaps we could work toward a joint landing.

I would favor total cooperation in this rover project, because it would allow our two countries to divide up tasks so that each could concentrate on the things we can do best. On the rover mission, the Americans might do best in developing the artificial-intelligence system that would allow the vehicle to move around on Mars. I think we Russians might have the best technology for the rover itself. We used a prototype of our Mars rover recently to clear debris on the roof of the Chernobyl power station, and it worked, despite the hostile environment.

The next great challenge, after landing the rover, would be to send a mission that would bring back to Earth a sample of Martian soil. This would be a very complex task.

One approach to this mission would reduce American worries about transferring sensitive military technology. Both countries could send their spacecraft independently. They would land two different payloads on the surface of Mars. One payload would be the return rocket that would bring the samples back to earth. The other would be the rover itself. The two missions would complement each other, without requiring total integration.

There could be other safeguards against technology transfer, even if we used the same spacecraft. For example, if an American scientific instrument were to be launched on a Russian spacecraft, the sensitive hardware could be packaged in what we call a "black box" -- with entry ports, plugs and exchanges on the outside, but everything inside hidden. We are using this approach now with West European countries that are using our launch vehicles. This approach could be extended to much bigger payloads. You could even treat an entire spacecraft as a black box, and provide verification procedures to ensure that it wouldn't be tampered with.

Finally, let us aim to put an American and a Soviet on Mars. My sentimental goal would be to do it by the year 2001. Let us make it a space odyssey, as suggested by Arthur C. Clark. He has assured me that he will stay alive and in good shape until then. But perhaps a more realistic goal for landing men on Mars would be 2005.

We shouldn't rush the manned mission, or assume that it is the only kind of space exploration that matters. There is a foolish kind of romanticism that says the only grand vision of future space exploration is enormous spacecraft with men on board. But I think it is just as romantic to send clever robots into deep space. And it is more practical. After all, while we Americans and Soviets are building vast military arsenals, the Japanese are conquering the economic world with small robots.

Whatever we decide to do about Mars, the point is that our scientists should share their knowledge and communicate it to the public. It is becoming more important as we move the frontiers of science. We won't be able to solve the important issues on earth without bringing in the best brains.

There is powerful symbolism in the fact that as General Secretary Gorbachev and President Reagan were meeting last week in Washington, a group of space scientists from the two countries were meeting in Moscow to discuss future space missions. The formula for linkage between the Washington summit and Moscow meetings is clear. Humankind won't have much chance to explore other planets unless our own planet is saved from nuclear holocaust.

Roald Sagdeyev is director of Moscow's Space Research Institute and served last week as an adviser on space issues to General Secretary Gorbachev.