Where next for MX

Be it noted that the Congress of the United States has not nixed the idea of building a new generation of nuclear missiles to balance any advantage the Soviets may be thought to have in such weapons.

Congress has said loud and clear that it thinks the idea of mounting a hundred of such weapons in a ''dense pack'' sounds about as silly as it previously thought was Jimmy Carter's proposal for sending 200 of them around thousands of miles of race tracks in Utah and Nevada.

The race track idea was scuttled (or was buried - if you prefer a non-nautical figure of speech) by the Mormon Church. The members of that religious body, who are plentiful in those states and not without political influence there, decided that they did not want MXs in their neighborhood where they might attract unwelcome attention from similar missiles mounted in the Soviet Union.

The ''dense pack'' idea was politically easier for any neighborhood. A hundred could be mounted inside the area of an existing air base. And the idea was to put ''the pack'' in Wyoming which has the necessary air base and where there are plenty of existing land-based nuclear weapons (Minuteman type) in fixed silos. Besides the population of Wyoming is small. It is 4.8 per square mile, the second lowest in the American union. The only other state with lower density is Alaska at 0.54.

But ''dense pack'' is gone now simply because it did not sound plausible to a majority in Congress. After they turned it down they learned that their doubts were shared by three of the five members of the Joint Chiefs of Staff at the Pentagon. The majority of the ''chiefs'' thought the money could better be used for more and better conventional wea-pons.

But Congress is willing, and has said so, to continue to provide funds for research and development on a new strategic weapon. There is nothing super special about the MX version which they blocked. It was merely one version of a new experimental missile (what MX means). Other versions are on the drawing boards. The special merit in the vetoed version of MX was that it was big enough to put into space 10 warheads of 350 kilotons blast each, or 7 of 500 kilotons. By dropping to fewer warheads, higher blast powers can be reached.

The MX was the US Air Force answer to the Soviet SS-18 which is rated at 8 warheads of 900 kt. or 10 at 500 kt. A later version is believed to be on the way with a rating of 10 heads at 750 kt. each. Smaller versions can be built, but to get the same number of warheads at any given blast effect more launchers would be necessary.

The real question is whether to keep a land-based deterrent. A mobile version of MX could be built for trundling around the countryside on railway flatcars or trucks. But people generally might feel about these the same way the Mormons felt about the Carter or ''race track'' version of MX in their own back yard. Any land-based missile tends to attract undesirable attention.

It is to be noted in this connection that the US, the Soviet Union, and China are the only countries with fixed, land-based, long-range missiles. The British and French have put all of their long-range weapons in the air or at sea. The Japanese long ago decided that if they ever do have such weapons they will definitely be kept at sea or in the air - not on land.

The essential fact is that a mobile weapon is less vulnerable than a fixed base weapon.

There is no such thing as invulnerability any more.

Back in the early days of the nuclear age the US and the British possessed relatively invulnerable deterrents. They, having jointly pioneered the nuclear weapon, had missiles which could be dropped from long-range aircraft. Mounting them on intercontinental ballistic missiles came a little later. From 1945 with the Hiroshima bomb to 1949 when the Soviets set off their own first nuclear weapon, the US and Britain had a monopoly.

That is gone long since. The French and Chinese have long- and medium-range nuclear weapons. India and Israel probably have them, covertly. South Africa and Pakistan may have them. A dozen others could build them if they chose.

In this new condition any land-based weapon is vulnerable. And there are weapons which can be used against sea and airborne versions of the nuclear weapon. Nothing is totally immune to counterattack. But a missile in an airplane or in a submarine has a higher survivability rating than anything fixed in a silo on land.

Obviously, the act of Congress in nixing the US Air Force's favorite version of the MX has the effect of adding to the arguments which are pushing the American nuclear deterrent off the land and toward sea and airborne mounting.