HOW WOULD YOU explain this to a visiting Martian?

The Soviet Union is the overriding consideration in all the great issues of national security that face the United States. The Soviet Union has been open to foreign travel since 1956. The overwhelming majority of American political leaders, particularly members of Congress, has never visited the Soviet Union.

By the same token, only about 20 percent of the Soviet politburo, and a smaller fration of the communist party's central committee, have ever seen America.

During the last quarter of a century, each superpower has spent between $5 and $10 trillion on defense, most of it to prepare for war against the other -- approximately $30,000 for every citizen of both countries. You'd think they might have spent a little of this money on travel, if only to get a first-hand impression of the "enemy" that justifies all this spending -- and weaponry.

This failure to even see -- let alone know -- the enemy has bothered some of us for a long time. I wrote about it in the Outlook section in 1977. As long ago as 1970, the Senate passed a bill intended to encourage more visits in both directions, but the Nixon administration killed the legislatin in the House.

Even after President Nixon had himself visited both Moscow and Peking, it was the position of his administration that it was encouraging "scientists, sportsmen, tourists, artists and other non-political persons" to travel to the Soviet Union. Why not the political persons? Perhaps because, on both sides, the highest leadership looks with some hesitation on increased involvement in foreign affairs of their colleagues, political peers, and subordinates. Knowledge can be a complicating thing.

There are other obstacles to more travel by American politicians to the Soviet Union. It isn't easy to arrange a trip, and there is no organization to help out (the way, for instance, the National Committee on U.S.-China Relations would help organize a trip to or from the Peoples' Republic). The charge of junketing poses another sort of problem. Critics of the trips members of the House and Senate make during recesses apparently overlook the absence in Moscow of both nightclubs and naked ladies.

The effect of such travel would be salutary. For Americans, I am convinced, The net effect of travel to the Soviet Union can be summarized in the aphorism "trust them less, fear them less."

For hundreds of years, Western intellectuals have been disillusioned by the intellectual suffocation of Russian life. Western obvservers feel themselves, as one U.S. Ambassador put it, "in a profoundly different and strange civilization."

This is what produced those famous reversals of position of Nicholas deCustine in 1839 ("I do not blame the Russians for being what they are, I blame them for pretending to be what we are.") and Andre Gide in 1937 ("In the USSR, everybody knows beforehand, once and for all, that on any and every subject there can be only one opinion."). In 1948, a Soviet official told John Steinbeck:

"We are very tired of people who come here and are violently pro-Russian and who go back to the United States and become violently anti- Russian. We have had considerable experience with that kind."

On the other hand, the Russians' fear of war, and the weaknesses of Soviet society have, have tended to tranquilize those visitors most alarmed about Russian aggressiveness. Few visitors return from Russia thinking them "ten feet tall" or capable of superly timed feats of technological attack. Russia is not Japan -- a fact that is obvious when you are there, but which many of our leaders seem unable to appreciate from afair.

But those who do see the Russians up close seem to get the message.After a 1958 trip to Moscow Sen. John Stennis (D-Miss.) reported: "Frankly I was not prepared for what I saw." He doubted that "Russia now plans a direct military attack upon us." In 1974, Sen. William Roth (R-Del.) found a weak U.S.S.R. "If the current economic condition of the Soviet Union is any indication, communism is a highly inefficient system."

And there is much in the craw of each side that can only be understood by seeing it. For example, would you believe there is a museum in Moscow devoted to the American attack on Russia? You may wonder, as one senator did in hearings on the travelissue, "And have we ever attacked them?" Indeed we have -- and the Russians well remember the Allied Expeditionary Force in 1917, which they consider an attempt to "strangle Bolshevism in its cradle."

The Federation of American Scientists is now trying to convince more senators and more Soviet officials to visit the other superpower. It's long since time. In the last 25 years, only about two dozen senators have bothered to go to the Soviet Union just to look around. (Another 70 have made more formal visits for official purposes.) Of the 100 men and women now serving in the senate, 58 have not visited the Soviet Union. Of 435 House members, 77 percent have never been to Russia.

We did a survey of the senate in the late '70s and concluded that those who are most concerned about the Soviet threat are least likely to investigate it. In our survey, of 33 senators voting dovishly on five issues, 55% had been to the Soviet Union, while only 22% of those voting hawkishly had done so.

The situation may be comparable on the Soviet side. Probably the people most concerned about the American threat do not come here. Probably also those who do come go home "trusting us more and fearing us more." We are both more open than Russians can imagine from their life experience, and much more powerful and richer. So visits by Russian leaders to this country might actually enhance our security.

Letting some fresh air into the U.S.-Soviet quarrel will not cure all that afflicts it. But the history of that quarrel and the grotesque excesses of the arms race, suggest that, without fresh air little can be fixed.