President Carter has embraced the controversial defense theory which claims that the Soviet Union will be able by the 1980s to destroy 90 percent of U.S. land-based missiles (ICBMs) in a surprise attack. The president has called this "our most pressing strategic problem." His solution was to order production of a mobile MX missile, the most lethal weapon in U.S. history.

Unfortunately, the president has chosen one of the slowest, most expensive and strategically destablizing solutions to be perceived vulnerability problem. The best possible choice would be to make the existing Minuteman ICBM force mobile aboard small submarines in U.S. coastal waters.

This proposal, known as the Shallow Underwater Missile System (SUM), offers dramatic strengths compared with land-based deployment of the MX. The 550 Minuteman III missiles could be carried by 138 small submarines, nearly identical to the German Type 600 submarine now in operation. This submarine force would patrol the U.S. coastal waters of the Atlantic and Pacific, which offer one-half-million square miles of protected, navigable area. The Minuteman III missiles aboard the submarines would continue to offer a wide range of strategic options, retain their present high degree of accuracy, while being able to stay in direct contact with strategic command centers in times of crisis. In short, the SUM proposal with the existing ICBM forces maintains all the positive features of the land-based MX proposal while retaining none of its most glaring weaknesses.

For example, defense specialist Richard Garwin recently indicated to aboard these 450-ton submarines. The first missile deployment could be achieved by 1984, with the entire force at sea by 1986. As a result, our ICBM force would be invulnerable at the very time the Soviet counterforce threat will become severe.

By contrast, even the best Air Force estimates conclude that the full MX force will not be deployed before September 1989 -- a decade from now.

But serious problems are working against the 1989 deadline. No fewer than 33 federal laws will bear on the problems of land acquisition alone. To build and operate the MX "racetrack" scheme, the fourth- largest city in Nevada will have to be built and maintained in the middle of the desert.

Construction will require some 600,000 tons of cement, 32 to 48 million tons of sand, 210 million gallons of liquid asphalt, 125 million gallons of petroleum fuel and 17.9 billion gallons of water. At a minimum, then, the president's decision to deploy the MX will cause an unnecessary, 3- to 4-year delay in solving what he terms "our most pressing strategic problem."

Even with the substantial environmental and construction problems to be addressed, the Air Force assures Congress that the MX system can be built for about $30 billion. Other estimates range as high as $50 billion.

By contrast, Garwin's estimate on the cost for a fully operational minuteman III force in the SUM concept is less than $12 billion. The billions saved by placing Minuteman III missiles at sea could be used to ease the burgeoning U.S. deficit or our dangerous dependence on foreign oil. The Air Force is already advancing the proposal that the United States may be forced to place an antiballistic missile (ABM) system around the MX site in the late 1980s to protect it against a large increase in Soviet warheads, an expensive addition to long-term MX costs. With the U.S. forces at sea, no such costly abandonment of the ABM treaty would be necessary. What's more, SUM would be easily verifiable and would not hold millions of U.S. citizens hostage to a Soviet counterforce attack.

Once deployed, the 200 MX missiles will increase the already immense explosive power of our ICBMs by over 400 percent, giving the United States the ability to destroy every Soviet ICBM site in a surprise attack. Even after such a counterforce strike, the United States would retain an overwhelming residual force equal to hundreds of thousands of Hiroshima bombs.

But the difference between a theoretical U.S. and Soviet first- strike capability is dramatic. Fully 70 percent of Soviet warheads rest on their land-based missiles. By contrast, some 25 percent of U.S. warheads are carried by our ICBMs. Put plainly, the preemptive strike posed by the MX against the Soviet arsenal would dwarf the much-vaunted Soviet threat against U.S. ICBMs.

In response, the Soviets will be forced to make one or more strategically dangerous decisions. Options could include launching ICBMs on warning of attack, which would greatly increase the tension in any crisis; abrogating the ABM treaty to deploy an anti-missile to protect their ICBMs; deploying their own mobile system, which may not be verifiable; or attaching the United States preemptively in a crisis for fear that the United States might use MX to destroy Soviet ICBMs in their silos. As a result, every international crisis would carry with it a heightened risk of nuclear war.

The present Minuteman III force poses no such first-strike capability. Deploying the force aboard coastal submarines will ensure the full range of strategic options short of a first-strike counterforce threat, while offering maximum protection from Soviet attack.

Finally, by protecting our missiles while not proceeding with the MX, the United States could accept a proposal considered by both during SALT discussions to establish a moratorium on new land-based missiles. This would be a major step toward a more comprehensive moratorium on deployment of additional strategic systems. Such a moratorium is essential if the United States is to exert the moral authority necessary to halt frightening proliferation of nuclear weapons among Third World nations. The SUM/Minuteman III alternative to the MX will save billions, enhance the protection of our strategic forces and offer hope that effective controls can finally be placed on the burgeoning arms race.