The results of the Geneva summit prove that President Reagan's often lonely approach to dealing with the deadly issue of nuclear weapons is the right one. The final statement endorsing "the principle of 50 percent reductions in the nuclear arms of the U.S. and the U.S.S.R. appropriately applied" was an extraordinary achievement.

Even the combined foreign policy wizardry of Nixon and Kissinger could produce only a controlled, but fairly high, rate of growth in the nuclear arsenals of the two superpowers. And the most radical proposal the international left has seriously proposed in the last decade was a mere freeze on existing nuclear stockpiles. Yet here were the two great nuclear powers endorsing the idea of a one-half reduction.

The weeks and months ahead will tell whether our negotiators have the skill and patience to fashion an arms control agreement that is in our national interest. In the meantime there is something we can and should do that would reduce the degree to which the United States is at risk from nuclear attack and would also give added momentum to the arms control talks. We need to build an insurance missile defense. It is something the Soviet Union has already done.

Under the terms of the ABM Treaty signed in 1972, both the United States and the Soviet Union have the right to build a limited missile defense system with up to 100 interceptor missile launchers. Almost immediately after the signing of the treaty, the Soviet Union began to deploy interceptor missiles around Moscow. Today they have the only operational missile defense system in the world.

Almost 100 interceptor missiles, mostly brand-new SH-08s, tipped with nuclear warheads, stand poised to blast off at a moment's notice. Because they intercept their targets high above the earth's surface, they can effectively protect a very large part of the Soviet Union from a limited missile attack. They cannot protect the Soviet Union from the combined assault of hundreds or thousands of nuclear missiles, but they can and do guarantee Soviet rulers that they need never fear the chilling assault of a nuclear missile launched accidentally, or being held hostage to a threat from a Third World power that has managed to build a few nuclear ICBMs.

In the early 1970s we began to build a defensive missile site to defend our regular missiles stationed in Grand Forks, N.D. In 1975 we stopped and tore down what had been built. Today we have nothing. If a nuclear missile or two, for whatever reason, were launched tow today, we would be helpless.

Take the worst case. A Soviet missile targeted on Washington is fired accidentally. The Soviets frantically notify the White House on the hot line. The North American Defense Command in Colorado tracks the incoming missile with its sophisticated computers and informs the Pentagon that it will strike the city in 14 minutes. What should our response be? Do we answer with a nuclear attack of our own, even though the Soviets are apologizing profusely for their mistake? Do we passively wait while our country is "decapitated?" What would you do if you were president?

The real answer is that we should not even have to contemplate such questions and all their ramifications. The Soviets don't. If we mistakenly fired a nuclear missile at Moscow, they could tell us not to worry and blast it harmlessly out of the sky with one of their interceptor missiles. The tragedy of our situation today is that we cannot do the same.

Last year the U.S. Army successfully tested an interceptor missile that proved we could destroy an incoming ICBM more than 100 miles up in space and that we could do it by simple collision -- without using a nuclear warhead. And that is something the Soviets cannot do. Today the technology is on U.S. shelves that would enable us to build an effective limited missile defense system, one totally permitted under the ABM Treaty, that could ensure the United States against the possibility of the most terrible accident in history.

The fact that we have not built any missile defense must be incomprehensible to the Soviets. Building and deploying a limited missile defense now would place us on a par with the Soviet Union in terms of operational missile defenses. Beyond that it would give the American people a chance to live in a nuclear world where at least you could protect yourself from an accidental attack.

And if both the United States and the Soviet Union possessed an insurance missile defense system, all of us just might find that we had created a measure of stability that we have not had since the Soviet Union developed nuclear weapons.