Today's potential threats to the U.S. are smaller countries such as Iran and North Korea that most likely have relatively few ICBMs, and lack the decoys and evasive navigation technology the Soviets had [source: Rosett]. On the other hand, their leaders are more belligerent, and there's also the danger that they could provide their nukes and missiles to non-state terrorists [source: Missile Defense Agency].
But whether you're trying to stop an intercontinental attack on the U.S. mainland or a short-range attack against a military base in South Korea, all missile warheads follow the same basic trajectory to their target. After being launched (the boost phase), they separate from the rocket booster and transition into a coasting midcourse phase in suborbital space, and then reenter the atmosphere in a terminal phase to descend upon their target [source: Rosett].
The initial boost phase might seem like the best time to knock down an enemy missile and warhead, because it's the time when the target is biggest and easiest to track. But it's difficult to get close enough to get a good shot. That's why U.S. missile defense focuses on the midcourse and terminal phases, even though warheads are a lot tougher to zero in upon [source: Rosett].
While ground-based interceptors in Alaska and California are poised to protect the U.S. mainland, its first line of missile defense is the Navy's Aegis ships, which are equipped with special radar and other systems to track enemy warheads, and powerful missiles capable of reaching them in suborbital space. Instead of trying to blow up the warheads, the U.S. defensive missiles are designed to collide with the warheads and smash them to bits -- a kinetic kill or "hit to kill" approach. Imagine a demolition derby in the sky, and you have the general idea [sources: Grier, Rosett].
Unlike the nuclear-tipped interceptors that the Pentagon envisioned a half-century ago, kinetic interceptors wouldn't actually detonate the warhead, only break it into pieces. That would eliminate the danger from nuclear fallout, though the pieces whizzing through space might become hazards for satellites and spacecraft [source: Global Security].
The big question is how well this would work. Critics of missile defense liken kinetic kill to hitting a bullet with a bullet, which makes it sound almost impossible to do [source: Rosett]. In tests, though, antimissile systems have done a bit better. Ground-based midcourse interceptors have managed to achieve a 50 percent kill rate, and ship-based Aegis systems nail dummy warheads about 80 percent of the time [source: Masters and Bruno]. But that's with careful preparation and advance notice; in a real attack, U.S. missile defenses might get 15 minutes' warning at best [source: Rosett]. And since even a single missile reaching its target would cause a catastrophe far bigger than 9/11, anything less than a 100 percent success rate wouldn't be good enough.