Decoys are less dense than warheads, though with the same aerodynamics. Therefore, they are subject to more deceleration when they begin their reentry of the upper atmosphere. The warhead, which is dense and streamlined, experiences less deceleration from air resistance, eventually passing by the decoys. The rate at which this happens depends on the types of decoys used, but the warhead will have passed even advanced types of decoy by the time it is . At this point the warhead is vulnerable to attack, but is only 5 to 10 seconds from its planned detonation (air burst or ground burst). To address these issues, a very high speed missile was required. Zeus was simply not fast enough to perform such an attack; it was designed for interceptions lasting about two minutes.