Recent satellite photos of North Korea's primary missile test site, for example, indicate their program may be less developed than experts thought. No other country in the world shares the fear that North Korea is an imminent nuclear threat. Experts predict that the Chinese would respond to a missile shield by building more-sophisticated projectiles or putting multiple warheads on them. Both techniques would allow them to overwhelm a limited defense. Yet despite the debate about the effects of deploying a limited national missile-defense system, questions remain about whether the system is technologically feasible.
Problems are likely to arise, analysts say, when an incoming projectile is accompanied by decoys.
The decoys, for instance, could confuse the kill vehicle, which, after separating from the booster, has only seconds to determine the real target. In the first airborne test, which the Pentagon deemed a success, the kill vehicle first identified a decoy balloon and then hit the real target nearby, in part by chance. Defense officials acknowledge that the project is moving at an unusually fast pace. They also say their efforts are limited by political concerns - such as trying to work within the confines of the ABM treaty. In today's test, to be launched high over the Pacific Ocean, the system used to shoot down a dummy missile will be only a skeleton of the final product.