Fallout from US Antimissile Success ; Saturday's Display of US Defense Prowess May Harm Relations withRussia, China. Series: The Exoatmospheric Kill Vehicle, Tested for the First Time Saturday, Would Be the Centerpiece of a US National Missile Defense. Experts Say China and Russia Regard an NMD as a Threat and May Stockpile Nuclear Weapons. BY RAYTHEON

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When a missile hurtling through outer space was intercepted and destroyed over the Pacific on Saturday night, the successful test may have moved the United States a step closer to building a system to defend all 50 states against limited missile attacks.

In all likelihood, the GOP-run Congress will now use the test to pressure President Clinton to approve the deployment of a national missile defense (NMD) system, a decision he will make next summer.

Advocates say such a system is needed to protect the United States from an errant launch by Russia or China, or from a chemical, biological, or nuclear strike by "rogue" nations like Iran or North Korea, which are developing long-range missiles.

But critics say the threat is exaggerated, and they worry that this latest demonstration of America's technological prowess may deal a setback to global nuclear-disarmament efforts. Relations with China and Russia could sour, these experts warn, as those two nations fret that the US system may neutralize their strategic deterrents. As a result, Russia may reverse cuts in its huge nuclear force, and China may decide to enlarge its arsenal.

"What the world looks like 10 years from now is hard to say, but it would probably be a world that's got more nuclear weapons than it has now," warns John Pike, an expert with the Federation of American Scientists, a Washington-based arms-control organization.

In Saturday's $100 million test, a modified Minuteman missile, fired from California just after 10 p.m., was destroyed 140 miles above the earth by an interceptor known as the exoatmospheric kill vehicle, or EKV. The EKV was launched 4,300 miles away in the Marshall Islands in the Pacific Ocean. Fired 20 minutes apart, the projectiles were traveling about 8,000 miles an hour when they collided.

Moreover, the EKV was able to differentiate between the real target and two decoys, the kinds of countermeasures Iran and North Korea may be developing.

The 121-pound EKV contains no explosives. Instead, it destroys its target by ramming it at such a high speed that both disintegrate. Built by the Raytheon Corp., the device is lofted atop a missile, locks onto the heat emitted by its target, and is then steered into it by small rockets.

The test was a boost for the Ballistic Missile Defense Organization (BMDO), the Pentagon agency overseeing development of a limited missile-defense system and theater missile defenses (TMD) - systems intended to shield ground, naval, or air forces from missile attacks. The US has spent some $50 billion on the programs in the 20 years since President Reagan proposed a Strategic Defense Initiative to develop a shield against all-out nuclear attack. That vastly bigger effort has since been abandoned.

Proponents say the US needs both TMD and NMD systems, given the post-cold-war proliferation of long- and short-range missiles, and of the know-how to make nuclear, chemical, and biological weapons. …