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Nike-X was an anti-ballistic missile (ABM) system designed by the US Army to protect major cities in the United States from attacks by the Soviet Union's intercontinental ballistic missile fleet. The X in the name referred to its experimental basis, and was supposed to be replaced by a more appropriate name when the system was put into production. This never came to pass; the original Nike-X concept was canceled and replaced by a much lighter defense system known as Sentinel that used some of the same equipment. Nike-X was a response to the failure of the earlier Nike Zeus system. It was calculated that a salvo of only four ICBMs would have a 90% chance of hitting the Zeus base, whose radars could only track a few objects at the same time. This was fine in the late 1950s when the Soviets had only a few dozen missiles, but by the 1960s they would have hundreds and could afford to overwhelm Zeus. The attacker could also use radar reflectors or high-altitude nuclear explosions to obscure the warheads until they were too close to attack, making a single warhead attack highly likely to succeed. Nike-X addressed these concerns by basing its defense on a very fast, short-range missile known as Sprint. They would wait until the enemy warheads descended below the altitudes at which decoys or explosions had any effect, and then attack them in engagements lasting only a few seconds. Nike-X also used a new radar system and building-filling computers that could track hundreds of objects at once, and controlled salvos of many Sprints. Dozens of ICBMs would need to arrive at the same time in order to overwhelm the system. As the USSR's missile fleet continued to grow, the cost of implementing Nike-X grew into the tens of billions. Even then it did not provide a perfect defense, and tens of millions of Americans would still die in an all-out exchange. Robert McNamara felt the cost could not be justified and was skeptical of the system. Around this time, in the mid-1960s, a series of high altitude nuclear explosions revealed a new way to attack ICBMs that could be used at very long range, it was proposed to adapt Zeus to this role as the Zeus EX. As the team considered the various applications of the new missile, a number of deployment scenarios were studied where a limited number of interceptors might still be militarily useful. Among these, the I-67 concept suggested building a lightweight defense against very limited attacks. McNamara was as skeptical of any of these concepts as he was of the original Nike-X plans, but he was under intense pressure to deploy a system of any sort. When the Chinese exploded their first H-bomb in 1967, I-67 was promoted as a defense against a Chinese attack, and this system became Sentinel in October. Nike-X development, in its original form, ended. ==History== 抄文引用元・出典: フリー百科事典『 ウィキペディア（Wikipedia）』 ■ウィキペディアで「Nike-X」の詳細全文を読む スポンサード リンク
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