Kim Jong Il is obviously uncomfortable. As tens of thousands of U.S. and South Korean troops staged an annual war-games exercise last week, he put North Korea's military on alert. The real pea under his mattress, though, could be four battle cruisers that ply the Sea of Japan, just over the horizon from the Dear Leader's beaches. These ships—two American, two Japanese—carry missiles capable of reaching North Korean nuclear-tipped rockets on their way to Japan, or even the satellite Kim has promised to put up any day now. U.S. Admiral Timothy Keating may have had these same missiles in mind when he threatened in late February to shoot down anything Kim felt emboldened to launch.
These four cruisers aren't the only ships that act as a de facto antimissile defense. The U.S. Navy has 73 Aegis ships around the world equipped with missiles that can reach space targets—whether the intercontinental ballistic missiles (ICBMs) that carry nuclear warheads or satellites that fly in low earth orbit. As the Obama administration shows signs of backing away from plans to put missile defenses in Poland and the Czech Republic, this fleet of "Aegis" cruisers, as they're called, may be called upon to take up the slack. U.S. Representative Ellen Tauscher, head of the House strategic forces subcommittee, praised recent progress on Aegis in hearings last month. "This was a major accomplishment that we should all take pride in," she said. "The same cannot be said of the long-range" ground-based missile defense. However, there are reasons to doubt that relying on Aegis will be an effective military strategy in the long run.
Compared with land-based missile defense, Aegis has the advantage of proximity. Ships can go, with minimal diplomatic hassle, wherever the threat is greatest. Kim's saber rattling, in fact, led the United States to supply Japan with Aegis equipment and know-how. Aegis, a combination of radars and interceptor missiles, was originally designed to defend battle cruisers against fighter jets. Technological improvements over the years gradually extended its range. The Bush administration funded a new interceptor—SM-3, for "standard missile"—capable of reaching the ICBMs Russia and China have and North Korea and Iran want. Tests suggest that it can fly fast and far enough to catch an ICBM shortly after leaving the atmosphere. That's an impressive feat, but experts caution that these tests were "scripted" and didn't take into account countermeasures an enemy might invoke. By the time a rocket leaves the atmosphere, it's almost impossible for an interceptor missile to tell the difference between the warhead and a decoy balloon. "If I were to throw a rock at you, but warn you ahead of time, you'd probably be able to deflect it," says Philip Coyle, former assistant deputy of defense in the Clinton administration and now an adviser to the Center for Defense Information in Washington, D.C. "But that's not to say you could get every rock thrown in the room, or in the whole country."
Tokyo is now developing a lighter, faster and more nimble version of the SM-3 that would come closer to hitting an ICBM at the end of its "boost phase," before it had time to throw up decoys. The new version, expected to be ready in a few years, will travel twice as fast as the current one, but still too slow by half, says MIT missile expert Theodore Postol. The Navy has an Aegis missile on the drawing board designed to attain such speeds, though funding has yet to be approved.
This missile wouldn't be a silver bullet either, says Postol. Even if the new interceptors hit their targets 100 percent of the time, they would still allow some warheads through. That's because the warhead occupies a small volume of the missile, usually at the tip, and interceptors aren't close to being able to sniff them out and make a direct hit. An airtight defense would require layers of redundancy—throwing lots of missiles at each ICBM—and could be countered easily by launching more ICBMs. "Missile defense encourages the enemy to do exactly what you don't what them to do—build more missiles," says Coyle. "I don't know if Kim is worried, but he shouldn't be." Postol argues that putting missiles on drone planes, which could shoot down on ICBMs while they're still rising off the launchpad, would work better than firing missiles from ships.
In one respect, Aegis is a completely effective weapon: it could easily take out low-flying military intelligence satellites. Does that confer a significant military advantage? Shooting down a nation's satellite would be so provocative it's hard to envision a scenario in which it would be a smart move. Besides, a hit on a 15-ton spy satellite would more than double the amount of space debris currently in orbit. That would make everybody uncomfortable.