Anti-Ballistic Missile defense (ABM) is and has been an important geopolitical topic: from the earliest days of the Cold War, the USA and the USSR looked for ways to defend themselves from nuclear bombs, and the introduction of ballistic missiles as their main vector spurred major work in the ABM field. This led to the Nike, Sentinel and Safeguards projects in the US, which never provided any real protection against a Soviet attack. Their cost, as well as the introduction of multiple, independently-targeted warheads (MIRV) -which meant a single ICBM could launch a dozen of warheads, plus decoys – put a nail in the coffin of these developments. In 1972, the USSR and the US signed the ABM treaty, agreeing that only two ABM sites per country were allowed, with a hundred interceptors. The USSR chose to defend Moscow, and the chose US to defend an ICBM launch site and then quickly gave up on the idea.
Ronald Reagan revived the idea in the 80s, with the SDI (also known as “Star Wars”) project, which was supposed to include on-orbit lasers and interceptors to form a shield against Soviet missiles. It was controversial at the time, and due to the fall of the USSR these exotic concepts were never deployed and the SDI was cancelled. However, ABM programs survived in a less ambitious form and in 2001, George Bush withdrew from the ABM treaty, and the US started fielding large ICBM interceptors in Alaska and California, as well as smaller shipborne and air-transportable interceptors.
In 2018, Russia revealed several weapons intended to defeat ABM systems: a nuclear-powered cruise missile, a massive nuclear-powered torpedo and a hypersonic glide vehicle. This suggests that Russia takes the US interceptors seriously, despite the fact that simple countermeasures are thought to make finding the warheads among the decoys impossible. This article is a look into the technical issues of ICBM interception, and into the credibility of ABM defenses againt long-range missiles.
The target object to intercept is a reentry vehicle, which is deployed on a ballistic trajectory after the missile launching it has finished burning. It consists of a nuclear explosive, surrounded by a heat shield to survive atmospheric reentry at high speed, and typically has a conic shape and is around 1.5m long.
Warheads can be spun, so that they remain pointed in the right direction and do not start reentry too much on their side, which could destroy them or make them less accurate. If they are not perfectly balanced, the spin axis itself will slowly move along a cone.
Interceptions while the warhead is still on the missile are very challenging because the interceptor has to outrun it, so it has to accelerate very fast, and to be launched from quite close. In a typical scenario where ICBMs are carried by submarines at an unknown location far out at sea, or are stored in silos deep within the landmass of a country, this is not possible.