Re-entry vehicles

A multiple independently targetable reentry vehicle (MIRV) is a ballistic missile payload containing several thermonuclear warheads, each capable of being aimed to hit a different target. By contrast, a unitary warhead is a single warhead on a single missile. An intermediate case is the multiple reentry vehicle (MRV) missile which carries several warheads which are dispersed but not individually aimed. Only the United States, United Kingdom, Russia, France, Israel, and China are known to currently possess MIRV missiles.

The first true MIRV design was the Minuteman III, introduced in 1970, which held three smaller W62 warheads of about 170 kilotons in place of the single 1.2 megaton W56 used in the earlier versions of this missile. The smaller power of the warhead was offset by increasing the accuracy of the system, allowing it to attack the same hard targets as the larger, less accurate, W56. The III was introduced specifically to address the Soviet construction of an anti-ballistic missile (ABM) system around Moscow; MIRV allowed the US to overwhelm any conceivable ABM system without increasing the size of their own missile fleet. The Soviets responded by adding MIRV to their R-36 design, first with three warheads in 1975, and eventually up to ten in later versions. MIRV has been part of every strategic missile design since then.

The introduction of MIRV led to a major change in the strategic balance. Previously, with one warhead per missile, it was conceivable that you could build a defense that used missiles to attack individual warheads. Any increase in missile fleet by the enemy could be countered by a similar increase in interceptors. With MIRV, a single new enemy missile meant that multiple interceptors would have to be built, meaning that it was much less expensive to increase the attack than the defense. This cost-exchange ratio was so heavily biased towards the attacker that the concept of mutual assured destruction became the leading concept in strategic planning and ABM systems were severely limited in the 1972 Anti-Ballistic Missile Treaty in order to avoid a massive arms race.

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