One of my friends is a rocket scientist (aerospace engineer). I asked him about kill vehicles once, and he said that intercepting Russian ICBMs is a losing battle even if you can identify the ICBM and intercept it reliably. Russian ICBMs have inflatable decoy payloads in them that are balloons for all intents and purposes. Since ICBMs deploy their payload while in space, where there is no atmosphere, the decoy balloons behave just like the actual nuclear warheads, despite the huge difference in mass. Once they hit the atmosphere the decoy payloads slow down quickly, however you should target  the ICBM and its payload much earlier than this if you want to reliably defeat it. The decoys are much less expensive than the intercept vehicles, and the balloon decoys are light, so Russia can afford to put a whole mess of them in each ICBM.<p>*edited for clarity.