THE congressional decision to cap deployment of the controversial MX missile at 40 or 50 symbolizes the trend from immobile land-based intercontinental missiles to more-mobile offensive missiles, such as the planned Midgetman and missiles now aboard submarines. Advocates of President Reagan's Strategic Defense Initiative -- the so-called ``star wars'' program -- seek ultimately to shift the emphasis in deterrence from offensive to defensive weapons. As part of its consideration of the military budget, the House of Representatives made the appropriate move this week by voting to set a firm deployment limit at 40 and refusing money to produce missiles beyond the 42 previously approved. The two extra missiles would be used for testing and spare parts. Later this summer, conferees from House and Senate will try to reconcile the House plan with the Senate's, which would permit the Pentagon to build 12 more and limit deployment to 50.
The two main arguments cited on behalf of the MX have not been convincing, and even some supporters have been only lukewarm toward the missile. Despite the MX's greater power and accuracy than its land-based predecessors, it is hard to see that it would improve the security of the United States, because it is potentially vulnerable in its fixed silos. Soviet missiles are not as accurate now as their American counterparts, but as they gain accuracy they can be expected at some point to have the capacity to destroy MXs in their silos. Mobile missiles, whether on land or under the sea, are much more difficult to track and destroy.
Similarly unconvincing is the view that the MX is necessary as a bargaining chip in the current nuclear arms control negotiations at Geneva. It is more probable that the space-based Strategic Defense Initiative was the prospective US weapon system that helped persuade the Soviets to come to the table.
When President Reagan inherited the MX missile system, he sought a force of 200 MXs. As congressional doubts grew about the missile's effectiveness, the total requested was lowered, first to 100 and then, reluctantly, to the 50 approved by the Senate.
Three months ago Congress voted to approve 21 missiles, bringing the number to 42; at the time several members of Congress said it was unlikely that many more would be authorized.
The fact that 42 MXs were approved despite the grave doubts about the missile's usefulness indicates the difficulty of stopping production of a major weapons system once it has begun. But the new congressional action to limit deployment shows that even large weapons systems can be halted if it is agreed they are not appropriate to the designated task.
It is now time for the US to move forward, as Congress has done, with the debate over the MX essentially completed. In the military sphere both the Midgetman and submarine-based missiles appear much less vulnerable than the MX. Preferable to any additional weaponry would be a verifiable treaty on nuclear arms.