In the wake of threats from the North Korean government, the United States is sending batteries of missiles to Guam and a warship (the USS John S. McCain) that can shoot down rockets fired from North Korea. As powerful as these defenses are, their effectiveness may lie more in their symbolic value than in how well they would stop missiles.

"The real value as a deterrent is to show we're interested in the region," Philip Coyle III, a former associate director for national security in the White House Office of Science and Technology, said in an email.

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"The Administration felt they had to do something to respond to the latest DPRK (North Korea) sword rattling, especially after North Korean officials mentioned Andersen Air Force Base in Yigo, Guam, among potential targets."

That isn't to say that the U.S. can't stop North Korean missiles. But missile defense is different depending on whether one is trying to stop a single ballistic missile or a barrage of them fired by North Korea. [North Korea Issues Hollow Threats Against U.S.]

How To Take Out A Missile

Modern missile defense is nothing like the "Missile Command" video game, in which players defend cities from incoming ballistic missiles by blowing them up just before they strike their targets. In real life, targeting incoming missiles is a lot harder, and missiles still need to be pretty close to their targets to destroy them, and they don't always manage to do that. The systems that the U.S. sent to the waters off South Korea and to Guam (a U.S. territory) are "kinetic kill" designs, which means the missile is fired and actually hits another missile, ensuring complete destruction of the warhead.

There are two ways to take out missiles: close to the target when they are in the "descent" or "terminal" phase of their trajectories, and farther off during launch or before the missiles re-enter the atmosphere. (Any ballistic missile going more than about 200 miles traces a high, arcing path that takes it out into space, albeit briefly).

The first method is best for protecting specific targets such as military bases because by the time the missile re-enters the atmosphere there isn't as much time to stop it. It's also much clearer where it is going. The second is better for protecting larger areas -- it's the idea behind many missile defense systems proposed during the Cold War to stop intercontinental missiles from the USSR.

Guam will be defended by a system called THAAD, for Terminal High Altitude Area Defense. The missiles from a land-based THAAD battery can steer toward and track down ballistic missiles in the final stages of descent -- still up to about 150 miles from the target. They can protect targets such as air bases, but they aren't designed to defend large swaths of territory. And there's one problem with THAAD: It hasn't been used in real battle yet. However, in October the U.S. Missile Defense Agency reported a successful test in which THAAD missiles hit four out of five targets.

The second method is best for protecting large swaths of territory because it hits the missiles as they launch, or just as they exit the atmosphere, when there is more time to respond.

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That's what the Aegis system does. It's the one that is on the destroyer USS John S. McCain. It has a larger defensive "footprint" than THAAD, meaning it can defend a larger area.

Aegis is designed to stop missiles when they are in "boost phase," shortly after launch, or before they re-enter the atmosphere after the engines are disengaged.

Both systems can work in concert, with THAAD using data from Aegis radar and satellites. And both systems use missiles that can maneuver to home in on attacking missiles.

Why not just use the Patriot system? Patriot missiles aren't really made for defending against longer-range weapons. The kind of missile a Patriot is most effective against has a range of up to about 600 miles. Such missiles aren't moving as quickly as those that can travel from the North Korea to Guam.

Missiles from a land-based THAAD battery can steer toward and track down ballistic missiles in the final stages of descent -- up to about 150 miles from the target. U.S. Army

Dealing With Decoys

To thwart antimissile systems, military strategists propose using decoys, though this hasn't actually happened yet. Most proposed decoy systems would be used above the atmosphere, and would be either a fake missile or simply a radar-reflective piece of metal.

Yousaf Butt, scientist in residence at the Monterey Institute of International Studies in California, noted that a decoy could even be a foil balloon. Outside the atmosphere, the decoys would follow the same general trajectories as the missiles that released them.

The THAAD systems are less vulnerable to being fooled, as they are targeting missiles in the last part of their flight where the targets are obvious.

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Aegis could, theoretically, be defeated by a system of decoys wherein the missile launches a set of them to confuse the Aegis system's targeting.

Butt said decoys work because they don't need to be as heavy as the "real" payload -- they can be relatively light bits of metal. He said it's also a relatively simple matter to just fill the sky with missiles, both decoys and not.

L. David Montague, principal at consulting firm LDM Associates, and retired president of the Missile Systems Division at Lockheed Missiles and Space Co., disagreed that decoys would be easy for North Korea to add to its missiles. He also said he doesn't think they would be that effective, because the Aegis radars have some ability to discriminate between targets.

The tactic that could work is to use multiple independent re-entry vehicles, or MIRVs. "The best decoy in the world is a real bomb," he said. That was why the U.S. and USSR developed them -- to defeat antiballistic missile systems. MIRVs, though, are even more complicated to build than ballistic missiles and the North Koreans haven't tested anything like it.

Are Nuclear Missiles A Danger?

Should North Korea launch missiles, they will likely carry only conventional warhead, which could certainly cause casualties in Seoul and do a lot of damage. To get an idea one can look at the Iran-Iraq war, in which both countries fired ballistic missiles at each other's cities, resulting in several thousand deaths. It was largely a terror tactic, though, since the casualties paled in comparison with those on the battlefield. During the first Persian Gulf War, Iraqi forces fired 42 Scud ballistic missiles into Israel, but only one Israeli and 42 U.S. National Guardsmen were killed by them.

So far, the evidence is that North Korea doesn't have a nuclear-tipped missile that could threaten any U.S. territory, nor does it seem to be able to put a nuclear bomb on something with shorter range. There are three reasons: first, designing a nuclear bomb small enough to fit on a missile is no easy task. Second, the longest-range missile North Korea has actually fired is the Musudan, with a range of about 1,800 miles. That can threaten Japan and South Korea, but not Guam. Third, North Korea has yet to test a re-entry vehicle capable of standing up to the stresses of hitting the atmosphere at sub-orbital flight speeds, without which any weapon will just burn up in the atmosphere. (Shorter-range missiles don't get as high and aren't moving as fast, so they don't contend with the high temperatures and duration that longer-range rockets do).

The only missile that could reach any U.S. territory, the Taepodong-2, has been tested only once, and it failed. Such a missile would have a range of 4,200 miles, enough to reach Alaska and Hawaii.

For all of these reasons, many experts don't take a North Korean missile attack on U.S. territories seriously.

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Coyle said he thinks North Korea's recent displays of belligerence are a means of bullying other countries to give it oil. He noted that in the agreements signed with Pyongyang in the 1990s, the U.S. agreed to deliver 500,000 tons of oil yearly at no cost and help them build a nuclear reactor for energy.

"North Korea is looking for leverage wherever they can get it," Coyle said. "But they don't have much more than bluff and bluster at their disposal. The DPRK desperately needs oil and gas energy resources, but has none. Zero. That's why in every negotiation they ask for promises of oil."

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