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Kinetic Energy Hit-To-Kill Warhead
"The hit-to-kill or kinetic energy technology approach is based on the fact that when one object strikes another object at high speeds, a tremendous amount of destructive energy is released. The impact of an interceptor missile with an incoming tactical ballistic missile, aircraft, or cruise missile, can result in the total disintegration of both vehicles.
Such impact can literally vaporize even metals. In contrast, blast-fragmentation warheads may only redirect or break up the target vehicle. However, even with a large hit-to-kill interceptor, the effective impact window is relatively small"
Besides warhead can be made to explode at any altitude..it is not necessary for it to hit the ground to explode.