It is the basis of the national missile defense system that President Bush has ordered to be operational starting in 2004 to protect the United States from threats such as a long-range missile attack by North Korea.

But, experts said, just because the concept is simple does not mean it will work. Even with multiple tests due between now and 2004, the system likely will be put in place before it is determined whether it can deal with even basic countermeasures such as decoys to draw attention away from a true nuclear warhead, they said.

"I support the research. But I think that the system needs to demonstrate that it can work first," said Philip Coyle, a former assistant secretary of defense who helped evaluate the program under President Bill Clinton.

"None of the tests will present an opportunity to learn whether or not the system can discriminate between real targets and decoys and countermeasures," added Coyle, now a senior adviser for the Center for Defense Information think tank.

Bush and Defense Secretary Donald Rumsfeld were careful on Tuesday when they unveiled the deployment plan not to promise that the system would provide an impenetrable shield. "Better than nothing" is what Rumsfeld called it.

"It would be a very preliminary, modest capability, and you would be learning -- it would be in a testing and learning mode," Rumsfeld said. "But also, in the event it were needed, it would be able to provide you some limited capability to deal with a limited number of ballistic missiles."

Congress provided $16 billion for the program in the first two years of Bush's presidency. The Pentagon plans to seek $17.5 billion over the next two years. In the long run, the costs of the system could exceed $200 billion, according to the nonpartisan Congressional Budget Office.

The central element of the system is the plan to shoot down long-range missiles in mid-flight, long before they can bring devastation to their intended target. To deal with these, land-based interceptor missiles are set to be placed at Fort Greely in Alaska and Vandenberg Air Force Base in California.

The interceptors are made up of a booster rocket that shoots the missile into space, and a so-called "kill vehicle" that zeros in on an enemy warhead. The kill vehicle is released from the booster after leaving the atmosphere, and carries an infrared camera to help guide it to its target, and small rocket engines for maneuvering.

'HITTING A BULLET WITH A BULLET'

"That's what people refer to as hitting a bullet with a bullet. That's a difficult thing to do, but tests have shown that you can do that," said David Wright, a physicist and co-director of the Global Security Program for the Union of Concerned Scientists, which opposes deployment. "The problem is: that's not the hard part of developing one of these systems. The hard part is figuring out where the warhead is and what you should be shooting at."

There have been eight major tests of the system in which interceptors launched from Kwajalein Atoll in the Marshall Islands were intended to shoot down a dummy warhead aboard a modified Minuteman 2 intercontinental ballistic missile launched from Vandenberg Air Force base in California, about 4,800 miles away. Three failed and five succeeded.

The tests have not examined whether the system would be fooled by decoy warheads. Critics said any nation able to launch a long-range missile that could hit America would be capable of mounting countermeasures such as decoys to increase the chances that its warheads evade any defensive system.

One area of concern for the Pentagon is the interceptors' booster rockets. Two of the three test failures occurred because the kill vehicle did not separate from the booster.

"DOESN'T FLY RIGHT"

"I don't like where we are in terms of being developed with the boosters," said Lt. Gen. Ronald Kadish, director of the Pentagon's Missile Defense Agency. "I mean, we can't use an interceptor that doesn't fly right."

Kadish said his agency will concentrate in 2003 on coming up with a reliable booster rocket, noting that officials are looking at two possible alternatives.

Also missing is the high-resolution radar intended to be used to find incoming missiles, which has not yet been constructed. Defense Department officials plan to upgrade existing radars until the new radar system is ready.

"I think you have to acknowledge this thing has some capability against a simple threat. And the question becomes: Is that capability against a simple threat worth the money when you know that the threat may not be so simple," said Michael O'Hanlon, an expert at the Brookings Institution think tank.

"I would still tend to say under those circumstances that it's worth putting a small defense in place. But I think you have to keep your expectations in check."