EVER since the advent of ballistic missile technology it has been a dream of Pentagon hawks to create an impenetrable shield over the US.

The less outlandish theories of how best to knock out an enemy warhead, described by many as akin to shooting down a bullet, have ranged from lasers through space-based wire meshes to missiles.

Efforts to realise the dream have, however, largely foundered on the technical difficulty of locating, tracking and hitting a target moving several times faster than the speed of sound. Ronald Reagan’s Star Wars project envisaged space-based laser defences, but current technology makes anti-missile missiles the system the most likely to work.

The closest the US has come to creating its protective umbrella are the National Missile Defence (NMD) interceptor bases in California and Alaska. In tests the system has had some success but with only 20 silos and an inconsistent hit rate critics doubt that it could be relied upon to destroy a single warhead. It is thought to be easily beaten by the use of decoys and chaff.

Like most of the schemes designed to provide the shield it relies on missiles to knock out enemy warheads before they reach their target. The first attempt to build such a missile shield, the Nike-Zeus project deploying nuclear warheads, was cancelled in 1961 after it failed to find a way of tracking Soviet missiles and distinguishing them from decoy balloons and chaff.

Missile defence networks work on one of three models, depending on the flight phase of the enemy missile in which interception should occur. Boost-phase interception seeks to destroy the missile during launch, midcourse interception attacks it while it is gliding towards its target and terminal-phase interception aims to stop it as it descends.

The US is developing all three, but the interceptors of its headline NMD project are all designed to target enemy missiles during midcourse.

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