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. Likewise, the solution to dealing with massive numbers of warheads was to use faster computers and automated radars, allowing many interceptors to be in flight simultaneously. Zeus was being developed just as digital computers were experiencing a massive improvement in performance through parallel processing. Radar systems were likewise introducing the first phased array radar (Passive electronically scanned array) systems. Combining the two would allow hundreds of warheads and interceptors to be tracked and controlled at once. As long as the interceptor missile was not significantly more expensive than the ICBM, which was likely given their relative sizes, overwhelming such a system would not be feasible in a weapons system. Taking these factors into consideration, ARPA outlined four potential approaches to a new ABM system. The first was Nike Zeus in its current form. The second was Zeus combined with a new radar system. The third included new radars and computers. Finally, the fourth, or X, plan called for all of these changes, as well as a new short-range missile. As the shorter range missile would overlap with Zeus, X also called for Zeus to be modified for even greater range as "Zeus EX". After considerable debate, the decision was made to cancel the existing Zeus deployment and move ahead with the X plan. Testing The first test-launch of the Spartan, as the X plan came to be called, occurred at Kwajalein Missile Range on 30 March 1968. Survivors The Air Defense Artillery museum at Fort Sill, and Safeguard park at Fort Bliss, have Spartan missiles on display. Photo gallery See also Sprint (missile) Nike-Hercules missile Nike Zeus Related lists List of military aircraft of