Secret computer studies show that the existing U.S. cruise missile would not have a chance of penetrating the Soviet Union's sophisticated defense system, a revelation acutely embarassing to President Carter and threatening to the prospective SALT II agreement.

The studies, conducted jointly over the summer by a private contractor and the Pentagon, found that a scheduled "live" test would result in the Tomahawk cruise missile's being shot down by U.S. defenses. Consequently, the Defense Department some two weeks ago canceled the "live" test and substituted a "dead," or simulated, tets. That was intended to sidestep severe embarrassment for the weapon that became strategically crucial when Carter shelved the B-1 bomber.

But word has filtered out of the Pentagon, giving ammunition to Capitol Hill Critics of the Carter defense policy. The new strategic arms limitation agreement that is being negotiated in Geneva becomes more vulnerable than ever to criticism that it gives the Soviet Union a dangerous advantage.

A Defense Department spokesman told us there was no computer study made and that there will be "live" tests of the Tomahawk. But our sources at the Pentagon reaffirmed in detail the story of the cruise missile crisis.

The President's unexpected decision against B-1 production transformed the cruise missile from a theater to a global weapon. The Tomahawk, the only existing cruise missile, was developed as a sea-launched weapon but eventually will be launched from heavy bombers. In this manner, it has become a critically important U.S. strategic weapon.

The Tomahawk was to be tested beginning Dec. 6 at Nellis Air Base in Nevada against the U.S. Hawk air defense system on a "live" basis - the surface-to-air missile actually sent against the cruise missile (which would be launched from a slow-flying aircraft). But the computer studies showed that the Hawk radar would locate the Tomahawk and a surface-to-air Hawk missile would shoot it down.

The implications are unnerving. The Hawk is similar to the Soviet SA-3 system, which the Russians consider obsolete the peddle to their client countries. If the Tomahawk cannot get past the SA-3, what chance would it have against the far more advanced Soviet SA-10 that is now guarding the Russian homeland?

The decision was made to scrub the "live" test, firing the Tomahawk but not actually dispatching the Hawk surface-to-air missile - thereby saving the glamour weapon the indignity of being shot down. Instead, the test will be simulated by computer in a "dead" test.

Rep. Jack Kemp of New York, an important Republican voice on defense, plans to take the the House floor to accuse the Defense Department of "rigging" a test. At the least, experts believe, a simulated test always poses the temptation of self-deception.

Actually, there have been precursors of the Tomahawk's vulnerability. The radar of the F-15 has picked up a Tomahawk in flight. Testing of the Tomahawk against radar aircraft scheduled through next April at the China Lake and Point Mugu naval test sites in California now becomes the source of apprehension at the Pentagon.

Although the cruise missile team has boasted that its weapon presents radar a cross-section the size of a seagull, that may be too big. Further reducing the cross-section or increasing the missile's speed would require major changes. Nor is there room on the cruise missile for anit-radar countermeasures; the miniaturized motor and warhead take up all the limited space.

"I'm very much afraid," one technical expert told us, "that the cruise missile is about one weapon generation away from being able to penetrate Soviet defenses." Other experts believe a swarm of Tomahawks could overrun Soviet air defenses; but that would require thousands of cruise missiles, a number neither planned for production nor permitted under the proposed SALT II treaty.

Yet, without a B-1 bomber, Soviet superiority in heavy missiles would provide all the more lopsided a strategic advantage if the cruise missile cannot penetrate Soviet defenses. Therefore, even though it lost the fight for the B-1, the Air Force is desperate for a penetrating bomber and is pushing for a remodeled FB-111 (the old TFX) as a substitute.

In the absence of a penetrating bomber, the Tomahawk's ability to get by even obsolete U.S. defenses is of the most intense interest. If it cannot pass a "live" test, the credibility of the entire U.S. strategic-arms policy is in doubt.