Intercontinental ballistic missiles

An intercontinental ballistic missile (ICBM) is a guided ballistic missile with a minimum range of 5,500 kilometres (3,400 mi) primarily designed for nuclear weapons delivery (delivering one or more thermonuclear warheads). Similarly, conventional, chemical, and biological weapons can also be delivered with varying effectiveness, but have never been deployed on ICBMs. Most modern designs support multiple independently targetable reentry vehicles (MIRVs), allowing a single missile to carry several warheads, each of which can strike a different target.

Early ICBMs had limited precision (circular error probable) that allowed them to be used only against the largest targets such as cities. They were seen as a "safe" basing option, one that would keep the deterrent force close to home where it would be difficult to attack. Attacks against (especially hardened) military targets, if desired, still demanded the use of a more precise manned bomber. This is due to the inverse-square law, which predicts that the amount of energy dispersed from a single point release of energy (such as a thermonuclear blast) dissipates by the inverse of the square of the distance from the single point of release. The result is that the power of a nuclear explosion to rupture hardened structures is greatly decreased by the distance from the impact point of the nuclear weapon. So a near-direct hit is generally necessary, as only diminishing returns are gained by increasing bomb yield.

Second- and third-generation designs (e.g., the LGM-118 Peacekeeper) dramatically improved accuracy to the point where even the smallest point targets can be successfully attacked.

ICBMs are differentiated by having greater range and speed than other ballistic missiles: intermediate-range ballistic missiles (IRBMs), medium-range ballistic missiles (MRBMs), short-range ballistic missiles (SRBMs) and tactical ballistic missiles (TBMs). Short and medium-range ballistic missiles are known collectively as theatre ballistic missiles.

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