them bringing the missile within the lethal radius of its warhead, sometimes within a few hundred meters. Cancellation In spite of Zeus' successful testing program and interceptions, it was becoming increasingly clear that the fully integrated system would not be effective in an actual operational scenario. This was due primarily to two problems; decoys shielding the warhead from detection until it was too late for interception, and the rapid increase in the number of deployed ICBMs which threatened to overwhelm the system. The former problem was becoming increasingly apparent beginning in approximately 1957. Missiles designed to carry a specific warhead began having increasing levels of excess throw-weight as warhead physics improved missile design, resulting in smaller and lighter warheads. Even a small amount of excess capacity could be used to carry radar decoys or chaff, which is very light weight, and would create additional radar returns that would act indistinguishably from those of the real warhead, in the airless, exo-atmospheric vacuum, of sub-orbital space; where the missile intercept was planned. In that environment it would be difficult to pick out the warhead. As long as the decoys spread out or the chaff blocked an area larger than the lethal radius of the 5 Megaton (Mt) interceptor (much smaller in space than in the atmosphere), several interceptors would have to be launched to guarantee the warhead would be hit. Adding more decoys was extremely inexpensive, requiring very expensive ABMs to be added in response. At the same time, both the US and USSR were in the midst of introducing their first truly mass-produced ICBMs, and their numbers were clearly going to grow dramatically during the early 1960s. Zeus, like Hercules and Ajax before it, used mechanically directed radar dishes that could track only one target and one interceptor at a time. It was planned that Zeus bases would actually consist of several launcher sites connected to a central control, but, even in this case, the site might be able to guide four to six missiles simultaneously. As the ICBM fleet numbered hundreds even before Zeus could become operational, it would be simple to overcome the defense by directing sufficient warheads over it to overwhelm its ability to guide interceptions rapidly enough. Nike X The solution to both of these problems was to improve speed of both the defending missiles, and the defensive system as a whole. Decoys are less dense than