anti-ballistic missile An anti-ballistic missile (ABM) is a surface-to-air missile designed to Missile defense, destroy in-flight ballistic missiles. They achieve this explosively (chemical or nuclear), or via hit-to-kill Kinetic projectile, kinetic vehicles, which ma ...

(ABM) defence the cost-exchange ratio is the ratio of the incremental cost to the aggressor of getting one additional warhead through the defence screen, divided by the incremental cost to the defender of offsetting the additional missile. For instance, a single new

ICBM An intercontinental ballistic missile (ICBM) is a ballistic missile with a range (aeronautics), range greater than , primarily designed for nuclear weapons delivery (delivering one or more Thermonuclear weapon, thermonuclear warheads). Conven ...

might require a single new ABM to counter it, and if they both cost the same, the cost-exchange ratio would be 1:1. Throughout the

Cold War The Cold War was a period of global Geopolitics, geopolitical rivalry between the United States (US) and the Soviet Union (USSR) and their respective allies, the capitalist Western Bloc and communist Eastern Bloc, which lasted from 1947 unt ...

, the cost-exchange ratio was almost always strongly in favor of the offense. Some of this has to do with the fact that an ICBM can be aimed at any target, which the defender cannot know in advance. To shoot that warhead down, the defender has to wait until it appears on

radar Radar is a system that uses radio waves to determine the distance ('' ranging''), direction ( azimuth and elevation angles), and radial velocity of objects relative to the site. It is a radiodetermination method used to detect and track ...

, which typically happens only a few hundred miles from the target. This means a single defensive missile cannot be used to counter a single warhead; ABMs have to be deployed in a spread-out fashion so that one can respond no matter where the warhead appears. Even if a single ABM is needed to shoot down that single new missile, that single new ABM would be needed to be added to multiple bases depending on their range. For short-range weapons like the Sprint, dozens are needed for every new Soviet warhead. Through the 1950s and 60s there were intense ongoing debates about the exact figures of the cost-exchange ratio. This ended with the introduction of multiple independently targetable re-entry vehicles, or MIRVs. MIRV allowed a single ICBM to launch multiple warheads, each attacking a different target. Now every new ICBM required dozens and dozens of new ABMs to counter it, swinging the cost-exchange ratio so dramatically in favor of the offense that it ended any debate on the topic. Consideration of cost-exchange ratios was influential in persuading the

United States The United States of America (USA), also known as the United States (U.S.) or America, is a country primarily located in North America. It is a federal republic of 50 U.S. state, states and a federal capital district, Washington, D.C. The 48 ...

and the

Soviet Union The Union of Soviet Socialist Republics. (USSR), commonly known as the Soviet Union, was a List of former transcontinental countries#Since 1700, transcontinental country that spanned much of Eurasia from 1922 until Dissolution of the Soviet ...

to sign the ABM Treaty. The topic was once again a consideration in the era of the

Strategic Defense Initiative The Strategic Defense Initiative (SDI) was a proposed missile defense system intended to protect the United States from attack by ballistic nuclear missiles. The program was announced in 1983, by President Ronald Reagan. Reagan called for a ...

, SDI or "Star Wars". In this case the defensive weapons attacked the ICBMs before they released their warheads, reducing the exchange ratio to one, although at a very high dollar cost. Some weapons, like the

Project Excalibur Project Excalibur was a Lawrence Livermore National Laboratory (LLNL) Cold Warera research program to develop an X-ray laser system as a ballistic missile defense (BMD) for the United States. The concept involved packing large numbers of expendab ...

system, completely reversed the ratio by attacking dozens of missiles at once, a single weapon thereby potentially destroying hundreds of warheads. Ultimately these technologies failed to mature and the system was abandoned with the ending of the Cold War.

References

* * {{cite journal , journal=Science , volume=231 , issue=4744 , title= Cost of space-based laser ballistic missile defense , date=21 March 1986 , pages=1387–1393 , first1=G , last1=Field , last2=Spergel , first2=D, pmid=17748077 , doi=10.1126/science.231.4744.1387 , bibcode=1986Sci...231.1387F , s2cid=28309749 Nuclear warfare Nuclear strategy Missile defense

See also

References