RONALD REAGAN inherits a new kind of missile gap when he becomes the 40th president of the United States next month.How he deals with it is the most significant question on the whole troubled front of strategic weaponry. There are dark perils and glittering opportunities.

To appreciate what Reagan is up against, consider what he will soon hear from the government intelligence analysts as they show him in their Strangelovian briefings the top secret satellite photos, view graphs and charts about the Soviet missile threat of the 1980s.

Today, Reagan will be told, the Soviets have nuclear warheads on the line powerful and accurate enough to destroy half of our land-based missiles standing silently on guard in underground silos in the Northwest. The tons of dirt and concrete are not enough to protect the missiles. A Soviet warhead bursting overhead could crack the silo so the missile could not be fired out of it. Soon, the analysts will state, the Soviets will have so many of these accurate warheads deployed that they could destroy almost the whole U.S. force of 1,000 Minute-man and 52 Titan missiles.

Worse yet from a domestic political standpoint, the Pentagon weapons chiefs will have nothing to offer the new president to close this gap between the U.S. and Soviet doomsday offenses during his first term.

The hard-to-hit land missile now under development, the MX (for missile experimental), will not be ready for duty until 1986, and probably later, given the usual slippages. Thus, this so-called "window of vulnerability" will be open for at least six years the way things stand now, no matter how much money Reagan spends on national defense.

How did President Carter and his team at the Pentagon allow this missile gap to open up? Were they guilty of malfeasance or nonfeasance in this crucial area of national security? William J. Perry, the Pentagon's widely respected research and development chief, admitted in an interview with The Washington Post that the Soviets surprised him and everyone else in the weapons community by demonstrating in missile tests of 1977, 1978 and 1979 that they were improving the accuracy of their existing SS18 and SS19 missiles rather than following their traditional pattern of waiting until they had their next generation of missiles in hand.

"I have to admit I was quite surprised," Perry said, more so than at any time in his four-year stewardship of U.S. weapons development. The indisputable intelligence on the Soviet tests "gave us a new sense of urgency; told us we were caught short." Early in 1977, before the Soviet tests began, Perry said he felt no need to rush the MX from its back-burner development position toward production. An independent group under Frank Press, director of the White House Office of Science and Technology Policy, had concluded before the tests, Perry said, that the Soviets did not yet have enough accurate warheads to imperil U. S. land missiles. "After the tests started, they looked at it again," said Perry of the Press group, "and we looked at it again. And we all agreed at that time that it [the threat to land missiles] could not be ignored."

This still leaves some skepticism on the missile vulnerability issue the Pentagon, including a New York Review of Books article, published on Nov. 20, which argued that government claims about intercontinental ballistic missile accuracy were a myth because of variables such as different gravitional forces which could pull the missile and then the warhead off course. Seymour L. Zeiberg, deputy for strategic systems at the Pentagon, read the article and called it inaccurate. "I was underwhelmed," said Zeiberg of the article's argumentation. For one thing, he said, such anomalies as gravitational pull have been plotted in detail with the help of satellites.

Unlike the missile gap that candidate John F. Kennedy deplored in 1960, and closed shortly after Inauguration Day as his Pentagon team conceded it could not after all be found, this one will not go away. What does this new president, who, like Kennedy, decried the other party's missile gap, do about closing it?

Here is the first part of Reagan's glittering opportunity: a fully certified hawk clearly and coolly explaining to the American people just where the nuclear arms race with the Soviet Union is taking them, and why. He can lay the groundwork for an arms control agreement he and the Senate could accept. Although it may sound strange to portray the hawkish, Republican conservative Reagan as a man who could deliver a new arms control agreement, it was, remember, another Republican, Richard Nixon, who negotiated the first strategic arms limitation treaty and got the Senate to ratify it. The Democratic presidents who came before and after him, Lyndon B. Johnson and Jimmy Carter, did not do as well.

After, or perhaps at the same time, Reagan explains to the public the Soviet threat to U. S. land missiles, he will have to say what he, as the new president, intends to do about it. He will collide with his campaign rhetoric if he merely announces he will continue with the MX program he inherited from President Carter. In his debate with the independent presidential candidate John Anderson on Sept. 21, Reagan called the Air Force's MX deployment blueprint "that fantastic plan of the administration to take thousands and thousands of square miles out in the Western states . . . We need the missile, I think, because we lack a deterrent to a possible first assault. But I am not in favor of the plan that is so costly."

This "fantastic" and "costly" MX venture would total $34 billion in fiscal 1980 dollars, according to the Air Force; $56 billion by the General Accounting Office's reckoning; $100 billion, if you believe the estimate of the anti-MX group, SANE, a figure which includes operating costs. For between $34 billion and $100 billion, then, Reagan inherits a plan to build 200 giant MX missiles, each carrying 10 warheads of 335 kilotons accurate enough to destroy Soviet missiles buried underground. This gives MX "first strike" capability.

The Air Force's favored plan is to spread the 200 MX missiles out on valley floors in Nevada and Utah in a drag strip pattern. Each missile would periodically be hauled up and down on its own strip. The million-pound truck carrying the MX would turn off the strip and put the missile in garages off to the side, or pretend to do so. The missile could be kept hidden in any one of 23 cement garages, spaced 7,000 feet apart, along both sides of the gravel drag strip. Soviet gunners would never know, theoretically, which of the 23 garages held the missile. The Soviet gunners therefore would have to bracket each garage with two missiles to make sure of destroying the lone MX hidden in one of them, under the Pentagon's deep think of the unthinkables.

Since there would be a total of 4,600 garages, 200 missiles times 23 garages each, it would cost the soviets 9,200 warheads to destroy just 200 American MX missiles. That is a losing game, one the Kremlin will not try to play, according to Air Force leaders and other MX enthusiasts. If the Kremlin did try to win a warheads vs. shelters game, continues the Pentagon argument for MX, the United States could just keep building more garages for the same 200 missiles, drawing off more enemy warheads for relatively little cost. And if that did not prove to be enough to discourage the Soviets from trying to target all the garages the United States built, the Pentagon leaders figure the two superpowers would be in an all-out arms race. In that case, they say, the United States could tear up the antiballistic missile treaty of 1972 and put a new generation of anti-missile-missiles around the MX. This would be going back to trying to stop a bullet with a bullet.

But deploying the MX around the valleys of Nevada and Utah would be the Panama Canal of missile construction projects. By the Pentagon's own estimates, it would require digging up twice as much dirt as was excavated in building the Panama Canal; pouring twice as much cement as went into the Grand Coulee Dam; spreading the system over 8,000 square miles of government-owned land in Nevada and Utah; building 8,000 miles of roads for the missile; invading sparsely settle Western towns with thousands of constuction workers and their families, overwhelming schools, water supplies and inflicting extra crime and hell-raising on generally peaceful communities of the valley.

Critics hoot at the MX, calling it the missile age version of the French Maginot Line which was flanked by German troops in World War II. fGive up on land-based missiles, some are urging, and admit that pickle barrel accuracy requires putting more of the nuclear offense at sea in submarines or on surface ships. Retired Navy Capt. John E. Draim, for one, contends there is no reason the MX or any other missile could not be dumped off a submarine, barge or airplane and be launched on command from big lakes or the ocean on command. The Navy proved out the idea back in the 1960s, he said, by equipping missiles with floats and launching them out of the water under Project Hydra. It is time to give Hydra another chance, Draim contends. Moving nuclear missiles from land to sea would be like taking the cannon out of the village square, assert missile-to-sea advocates, thus keeping civilians out of the line of nuclear fire if war should come.

Carter's Pentagon responds that giving up on the land-based part of the U.S. missile submarines along with attacking bombers before they could take off. Besides that, even vulnerable land-based missiles like the Minutemen provide a hedge if the United States suddenly should find itself with submarine missiles and bombers that do not work. Psychologically, adds the Pentagon's Perry, "abandoning our ICBM force would concede an important perceptual advantage to the Soviets, a dangerouly misleading signal."

What Reagan's defense advisers are looking for right now is a quick fix for the missile gap -- a way to close the vulnerability before 1986, if the new president decides that the MX to become available then is the best long-term solution to land missile vulnerability after all. "We're giving serious consideration to Minuteman III redeployment," William Van Cleave, a principal military adviser to Reagan, told The Post. There already are 550 of these most modern of U.S. land missiles deployed, each with three highly accurate warheads. Van Cleave said he is studying the possibilitity of taking another 100 Minuteman III missiles out of the warehouse and deploying them. The resulting force of 650 Miniuteman III missiles would be moved around a new field of silos, like the MX shell game. Van Cleave said he is thinking about digging 10 new holes for each of the 650 Minutemen. He also is taking a fresh look "at the technical feasibility" of protecting land missiles with anti-ballistic missiles.

The Pentagon has already studied those alternatives in what Perry termed "exquisity detail" and found them wanting. Getting prime land away from farmers who own it, the Pentagon contends, would be tougher and take longer than getting the MX deployed on the land the U.S. government already owns in Nevada and Utah. The new generation of anti-ballistic missiles looks promising, Pentagon specialists say, but too risky to gamble on for protecting existing land missiles, to say nothing of the diplomatic and political uproar which would be generated if Reagan tried to tear up the 1972 U.S. Soviet treaty banning extensive ABM deployment.

Although Reagan has not endorsed the idea, a case can be made for giving up on land-based missiles and settling for only missile submarines and bombers -- a "dyad" of strategic nuclear forces rather than today's triad. But here again the stakes are too high to gamble that the Soviets will not find a way to detect our submarines in the next 20 years of the strategic arms race.

After hearing the arguments from Tonopah, Nev., which is slated to get the MX in its backyard under the Carter plan, to the Pentagon, and reading those excruciatingly dull articles the mafia of arms specialists write over and over again, I see two attractive options for Reagan before he commits himself to a full-blown MX program, or adopts the even more periolous course of announcing that American missiles will be launched if enemy warheads are seen on the way. A human or computer error could then start an incinerating World War III.

One option is for Reagan to tell Soviet leaders he will forego building the MX if they will reduce the number of ICBM warheads aimed at our existing missile force. If the United States could "be reasonably sure" of the numbers, Perry said 3,000 land-based Soviet warheads targeted on our Minutemen and Titans would be a "tolerable" risk. This would be down from today's threat of about 6,000 Soviet land-based warheads. The United States has a little over 2,000 warheads atop its land missiles aimed at Russia, but has the edge in the warheads carried on bombers and submarines.

The second option is what I will call the Perry plan for making surprise nuclear attack less tempting. Perry said he envisioned the scheme for a SALT III arms control agreement. There is no reason Reagan could not pursue the plan as a replacement for SALT II, however, Perry added.

"What I would push for is maintaining the survivability inherent in three different forces: bombers, land-based ICBMs, Trident submarines," Perry said. He would "harden" bombers against the effects of a nuclear explosion, start building the MXland missile and put the long-range Trident II missiles in submarines. "Having done that , we're in position to greatly reduce numbers. The more survivable forces we have, the fewer warheads we have to have. The think we're trying to hold constant is the number of surviving warheads."

In exchange for the Soviets going down to 2,000 or 3,000 ICBM warheads, the Pentagon weapons chief said it would be an acceptable risk for the United States to go down to 100 bombers carrying 20 cruise missiles each; halve the planned MX deployment scheme, settling for 100 rather than 200 missiles but still building 23 shelters for each MX; scrap the existing l,052 Minuteman and Titan missiles rather than keep hundreds of them on the line as the pending SALT II treaty allows, and build 12 Trident missile submarines to replace today's fleet of 41 Polaris and Poseidon boats. "That would cut our warheads in half but give us more security," Perry said. As for the Soviets buying such a deal, Perry reasoned that They ought to be as concerned about silo vulnerability as we are" and thus be willing to deploy fewer warheads on land and more at sea in submarines where they pose less of a first strike threat.

At the very least, Reagan, before plunging ahead with the MX, should assess the alternatives, including a new arms control agreement. No less an authority than Defense Secretary Harold Brown called for this in an interview with The Post: "They should set aside the campmaign rhetoric, ours and theirs," Brown said. "They should look at alternative programs, take a good hard look at them, and make their own decisions. It's part of the political tradition."