IN a dramatic action, the Senate Armed Services Committee recently voted by a wide margin to reconstitute the mothballed ballistic missile defense site at Grand Forks, N.D. This deployment would be the first step in a larger plan for a comprehensive defense of the United States against a limited missile strike; it could eventually include multiple sites, many hundreds of ground-based interceptors, and space-based sensors and interceptors.The committee's decision was justified in large part, not by the threat of a massive Soviet nuclear attack, but instead by the threat of an accidental or unauthorized launch by the Soviets or a long-range ballistic missile attack by a third-world tyrant. At present, no third-world nation can strike the US with a long-range ballistic missile. This is not to say that this threat may not materialize in the future. Indeed it may, but the unquestioned ability of the US to respond in kind would probably preven t such an attack. Certainly this type of deterrence has worked with the Soviets for the past 40 years. In the meantime, we should redouble our efforts to stop the proliferation of ballistic missile technologies by cracking down on the transfer and sale of these weapons. A good place to start would be with China. The more realistic threat put forth by the Senate committee is that a Soviet land- or sea-based nuclear missile could be launched at the US by accident or on purpose, but not with the assent of Soviet political leaders intent on starting a nuclear war. This threat has been with us for a long time, but the regional unrest in the Soviet Union and concern over decentralization of power in the Kremlin have heightened concern. The committee suggests that the best way to deal with this possibility is to spend many billions of dollars to deploy a nationwide defensive system. It is true that we are naked to a missile attack that is unauthorized or accidental, and ultimately we may want to redeploy a defensive system at Grand Forks to protect against this threat. But our initial approach to solving this problem should focus on reducing the risk that such a launch would ever occur or limiting its effectiveness if it did. To this end we should discuss with the Soviets technical improvements that they could make to prevent an accidental launch of their missiles and insist on the deployment of in-flight destruct mechanisms on all nuclear ballistic missiles. Ironically, all the dummy nuclear missiles that we test and all of our space launch vehicles have destruct mechanisms, but the most dangerous missiles, those with live nuclear warheads, do not. The US does not attach destruct mechanisms to its intercontinental ballistic missiles and submarine-launched ballistic missiles because to do so would add to missile reliability complexity and it would be expensive to create the infrastructure to support these packages. Yet, when weighed against the cost of even a limited missile defense of the US, the infrastructure costs pale. It is interesting that in its bill the Senate committee urges the president to pursue immediate negotiations with the Soviets to amend critical restrictions of the Antiballistic Missile (ABM) Treaty in order to allow us to move forward with a nationwide defensive system. However, the committee would be well served by including in their recommendations to the president the suggestion that he engage the Soviets on the question of whether placing in-flight destruct mechanisms on nuclear missiles is in the s ecurity interests of both nations. Certainly there is no harm in discussing the option. Who knows, the Soviets might agree; they've surprised us in arms control negotiations before. On the other hand, to move forward rapidly with the construction of a new ABM site and time-constrained negotiations on amending the ABM Treaty may harm the progress that we have made in US-Soviet relations and strategic arms control. It certainly could not help Mikhail Gorbachev or other Soviet reformers who are already under attack by Soviet military hardliners for eroding Soviet security by giving away the store to the West. At present, we face a fail-safe situation, where an unauthorized or accidental launch by the Soviets could result in a circumstance where we trade the destruction of New York for the annihilation of Moscow. That need not be the case. The Senate Armed Services Committee is right to be concerned about this possibility, but we may be able to solve this problem in a more constructive and less costly fashion than the committee has suggested. We should begin negotiations immediately on a joint agreement to inc lude destruct mechanisms on all of our nuclear missiles. It could save this nation billions and better protect the interests of both countries.