Multiple independently targetable reentry vehicle
A multiple independently targetable reentry vehicle (MIRV) is an exoatmospheric ballistic missile payload containing several warheads, each capable of being aimed to hit a different target. The concept is almost invariably associated with intercontinental ballistic missiles carrying thermonuclear warheads, even if not strictly being limited to them. An intermediate case is the multiple reentry vehicle (MRV) missile which carries several warheads which are dispersed but not individually aimed. All nuclear-weapon states except Pakistan[a] and North Korea are currently confirmed to have deployed MIRV missile systems. Israel is suspected to possess or be in the process of developing MIRVs.[4]
For the band, see M.I.R.V.
The first true MIRV design was the Minuteman III, first successfully tested in 1968 and introduced into actual use in 1970.[5][6][7] The Minuteman III held three smaller W62 warheads, with yields of about 170 kilotons of TNT (710 TJ) each in place of the single 1.2 megatons of TNT (5.0 PJ) W56 used on the Minuteman II.[8] From 1970 to 1975, the United States would remove approximately 550 earlier versions of the Minuteman ICBM in the Strategic Air Command's (SAC) arsenal and replace them with the new Minuteman IIIs outfitted with a MIRV payload, increasing their overall effectiveness.[6] As of 2017 the Minuteman III missile was converted to a single reentry vehicle system.[9][10] The smaller power of the warheads used (W62, W78 and W87) was offset by increasing the accuracy of the system, allowing it to attack the same hard targets as the larger, less accurate, W56.[8][11] The MMIII 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. While the United States phased out the use of MIRVs in ICBMs in 2014 to comply with New START,[12] Russia continues to develop new ICBM designs using the technology.[13]
The introduction of MIRV led to a major change in the strategic balance. Previously, with one warhead per missile, it was conceivable that one 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.
The military purpose of a MIRV is fourfold:
MIRV land-based ICBMs were considered destabilizing because they tended to put a premium on striking first.[17] The world's first MIRV—US Minuteman III missile of 1970—threatened to rapidly increase the US's deployable nuclear arsenal and thus the possibility that it would have enough bombs to destroy virtually all of the Soviet Union's nuclear weapons and negate any significant retaliation. Later on the US feared the Soviet's MIRVs because Soviet missiles had a greater throw-weight and could thus put more warheads on each missile than the US could. For example, the US MIRVs might have increased their warhead per missile count by a factor of 6 while the Soviets increased theirs by a factor of 10. Furthermore, the US had a much smaller proportion of its nuclear arsenal in ICBMs than the Soviets. Bombers could not be outfitted with MIRVs so their capacity would not be multiplied. Thus the US did not seem to have as much potential for MIRV usage as the Soviets. However, the US had a larger number of submarine-launched ballistic missiles, which could be outfitted with MIRVs, and helped offset the ICBM disadvantage. It is because of their first-strike capability that land-based MIRVs were banned under the START II agreement. START II was ratified by the Russian Duma on 14 April 2000, but Russia withdrew from the treaty in 2002 after the US withdrew from the ABM treaty.
MRV[edit]
A multiple re-entry vehicle (MRV) system for a ballistic missile deploys multiple warheads above a single aimpoint which then drift apart, producing a cluster bomb-like effect. These warheads are not individually targetable. The advantage of an MRV over a single warhead is the increased effectiveness due to the greater coverage; this increases the overall damage produced within the center of the pattern, making it far greater than the damage possible from any single warhead in the MRV cluster; this makes for an efficient area-attack weapon and makes interception by anti-ballistic missiles more challenging due to the number of warheads being deployed at once.[6]
Improved warhead designs allow smaller warheads for a given yield, while better electronics and guidance systems allow greater accuracy. As a result, MIRV technology has proven more attractive than MRV for advanced nations. Multiple-warhead missiles require both a miniaturized physics package and a lower mass re-entry vehicle, both of which are highly advanced technologies. As a result, single-warhead missiles are more attractive for nations with less advanced or less productive nuclear technology. The United States first deployed MRV warheads on the Polaris A-3 SLBM in 1964 on the USS Daniel Webster. The Polaris A-3 missile carried three warheads each having an approximate yield of 200 kilotonnes of TNT (840 TJ). This system was also used by the Royal Navy who also retained MRV with the Chevaline upgrade, though the number of warheads in Chevaline was reduced to two due to the ABM counter-measures carried.[6] The Soviet Union deployed 3 MRVs on the R-27U SLBM and 3 MRVs on the R-36P ICBM. Refer to atmospheric re-entry for more details.