Multiple independently targetable reentry vehicle

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The MIRVed U.S. Peacekeeper missile, with the re-entry vehicles highlighted in red.

A Multiple Independently targetable Re-entry Vehicle, or MIRV is a collection of nuclear weapons carried on a single intercontinental ballistic missile (ICBM) or a submarine launched ballistic missile (SLBM). Using a MIRV warhead, a single launched missile can strike several targets, or fewer targets redundantly. By contrast a unitary warhead is a single warhead on a single missile.

Technicians secure a number of Mk-21 re-entry vehicles on a Peacekeeper MIRV bus.

The military purpose of a MIRV is fourfold:

Provides greater target damage for a given missile payload. Radiation (including radiated heat) from a nuclear warhead diminishes as the square of the distance (called the inverse-square law), and blast pressure diminishes as the cube of the distance. For example at a distance of 4 km from ground zero, the blast pressure is only 1/64th that of 1 km. Due to these effects several small warheads cause much more target damage area than a single large one. This in turn reduces the number of missiles and launch facilities required for a given destruction level.
With single warhead missiles, one missile must be launched for each target. By contrast with a MIRV warhead, the post-boost (or bus) stage can dispense the warheads against multiple targets across a broad area.
Reduces the impact of SALT treaty limitations. The treaty initially limited number of missiles, not number of warheads. Adding multiple warheads per missile provided more target destruction for a given number of missiles.
Reduces the effectiveness of an anti-ballistic missile system that relies on intercepting individual warheads. While a MIRVed attacking missile can have multiple (3-12 on various US missiles) warheads, interceptors can only have one warhead per missile. Thus, in both a military and economic sense, MIRVs render ABM systems less effective, as the costs of maintaining a workable defense against MIRVs would greatly increase, requiring multiple defensive missiles for each offensive one.

The Russian government claimed to have developed the most advanced MIRV system as of 2006 for use in the Bulava missile.

[edit] Mode of operation

In a MIRV, the main rocket motor (or booster) pushes a "bus" (See illustration) into a freely-falling suborbital ballistic flight path. After the boost phase the bus maneuvers using on-board small rocket motors and a computerised inertial navigation system. It takes up a ballistic trajectory that will deliver a reentry vehicle containing a warhead to a target, and then releases a warhead on that trajectory. It then maneuvers to a different trajectory, releasing another warhead, and repeats the process for all warheads.

Minuteman III MIRV launch sequence:
1. The missile launches out of its silo by firing its 1st stage boost motor (A).
2. About 60 seconds after launch, the 1st stage drops off and the 2nd stage motor (B) ignites. The missile shroud is ejected.
3. About 120 seconds after launch, the 3rd stage motor (C) ignites and separates from the 2nd stage.
4. About 180 seconds after launch, 3rd stage thrust terminates and the Post-Boost Vehicle (D) separates from the rocket.
5. The Post-Boost Vehicle maneuvers itself and prepares for re-entry vehicle (RV) deployment.
6. The RVs, as well as decoys and chaff, are deployed during backaway.
7. The RVs and chaff re-enter the atmosphere at high speeds and are armed in flight.
8. The nuclear warheads detonate, either as air bursts or ground bursts.

Details are closely-held military secrets. The bus' on-board propellant limits the distances between targets of individual warheads. Some warheads may use small hypersonic airfoils during the descent to gain additional cross-range distance. It's possible the buses can release decoys to confuse interception devices and radars, such as aluminized balloons or electronic noisemakers.

Testing of the Peacekeeper re-entry vehicles, all eight (ten capable) fired from only one missile. Each line represents the path of a warhead which, were it live, would detonate with the explosive power of twenty-five Hiroshima-style weapons.

Accuracy is crucial, because doubling the accuracy decreases the needed warhead energy by a factor of four for radiation damage and by a factor of eight for blast damage. Navigation system accuracy and the available geophysical information limits the warhead target accuracy. Some writers believe that government-supported geophysical mapping initiatives and ocean satellite altitude systems such as Seasat may have a covert purpose to map mass concentrations and determine local gravitic anomalies, in order to improve accuracies of ballistic missiles. Accuracy is expressed as CEP (Circular Error Probable or Circle of Equal Probability). This is simply the radius of the circle that the warhead has a 50% chance of falling into when aimed at the center. CEP is about 90-100 meters for the Trident II and Peacekeeper missiles.