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Ballistic missile defense
From Citizendium, the Citizens' Compendium
Ballistic missile defense (BMD) includes a range of technologies and military doctrines intended to defeat, or reduce the damage caused by, an enemy attack with ballistic missiles. While a ballistic missile could be intercepted in any of the three phases of its flight:
- boost: powered flight into a suborbital trajectory, typically over an area of land or sea either under the enemy's control or that the enemy does not consider contested (e.g., ocean from which a submarine-launched ballistic missile would be fired);
- midcourse: unpowered "coasting" through space or extremely high atmosphere, from which the reentry phase will be triggered by gravitation or a programmed maneuver such as firing retrorockets to slow the trajectory;
- terminal defense: interception of the reentry vehicle(s) as they reenter the atmosphere, at extremely high speed, from the midcourse phase;
each of these phases presents unique challenges for interception of the missile or its warhead. Some BMD systems, recognizing the complexity of each phase, may "layer" defenses, combining, for example, midcourse intercept with terminal defense against those warheads not destroyed in midcourse.
BMD divides into two major classes:
- Theater BMD, against short- to intermediate-range missiles. Depending on the range of the missile, the curvature of the earth may not hide much of the flight path from defensive sensors;
- National BMD, against intercontinental-range missiles, a significant part of the flight path of which will be hidden, from target-area sensors, by the curvature of the earth.
While early BMD interceptors had nuclear warheads, no known current system uses them. Nuclear warheads have been rendered impractical due to bans against testing in the atmosphere and space. Improvements in guidance also makes the large area of effect of a defensive nuclear burst unnecessary; the kill mechanisms of current systems are either kinetic (i.e., direct collision with the warhead) or directed energy (e.g., laser heating of the warhead).
Mechanisms to destroy enemy missiles are not limited to anti-ballistic missiles, although those are the most common that have been deployed. Directed energy weapons, especially lasers, are under active development. Autocannon have been proposed for terminal defense of point targets, and may be of increased interest against very short range missiles and unguided rockets, as, for example, on the borders of Israel.
Theater BMD systems
Interceptors include the U.S. Navy RIM-161 Standard SM-3, the U.S. Army THAAD and MIM-104 Patriot‎, and the joint U.S.-Israeli Arrow 2. Lasers and autocannon are being considered as final defense.
Radars used include the shipboard AN/SPY-2 and the U.S. Army AN/TPY-2.
