THE BUSH administration's ambitious ballistic missile defence (BMD) plan, dubbed Son of Star Wars by its critics, aims to defend the American homeland, its forces abroad and its allies from missile threats by states of concern. There are three main dangers; long-range ballistic missile attack with weapons of mass destruction (WMD), short-range missile attacks such as from cruise missiles, and a terrorist act with a WMD or high-explosive weapon.

The massive increase in defence spending requested by the US administration for FY 2003 reflects realisation of the threat of missile proliferation. Spending on missile defence is up by about 50 per cent to $7.7bn a year to pay for research on technical approaches for firing interceptors or lasers from land, ships, aircraft or space platforms and for striking enemy warheads at every stage of flight, from just after launch to the final seconds before impact.

For the second year in a row Senate efforts to cut funding from national missile defence (NMD) resulted in a compromise - Pentagon is to spend the contested money as it wished. The question about missile defence no longer will be whether or not to pursue and deploy it, but how and when. After 9/11, Washington identified a strategic opportunity to link NMD strongly to the war on terrorism. No sooner had Bush mentioned the axis of evil in his State of the Union address, he was telling the world "we will develop and deploy effective missile defenses to protect America and our allies from sudden attack... The United States of America will not permit the world's most dangerous regimes to threaten us with the world's most destructive weapons".

This was compounded by warnings in August 2002 of the possibility that terrorist groups or rogue states could use rudimentary cruise missile technology to attack US installations or the American homeland. Capable of taking off from ships close to shore and manoeuvring below radar scanners or behind terrain, cruise missiles present a particular problem as potential platforms for delivering nuclear, biological or chemical (NBC) warheads. Nevertheless, after 9/11 US intelligence realised that non-missile WMD attacks were a far greater threat than missile-delivered WMD attacks. Senator Joseph Biden, who chairs the US Senate Foreign Relations Committee, said "[BMD] will not protect us from cruise missiles. It will not protect us from something being smuggled in... from an atom bomb in the rusty hull of a ship coming into a harbor... or from anthrax."

The current layered programme is based on ground-based interceptors (GBI), that can strike enemy missiles at 240km-plus distances. The most mature upper-tier system, Theater High-Altitude Area Defence (THAAD), is intended to deal with long-range theatre missile threats as they begin to emerge while local defences are provided with the Patriot Advanced Capability-3 (PAC-3) system. Pentagon planners envision the PAC-3 system providing low-tier defence in a multi-tiered, anti-missile network, with other systems intercepting targets at higher altitudes and longer ranges.

THAAD comprises a lethal hit-to-kill interceptor and the TMD ground- based radar (GBR) surveillance and tracking sensor. It will provide a unique capability for wide-area defence against ballistic missiles at high altitudes and long ranges. THAAD missiles are intended to collide with a target ballistic missile, rather than to destroy it by exploding nearby, as do fragmentation warheads. Final guidance to the target is provided by an infrared seeker on the kill vehicle. Interception of a hostile ballistic missile is intended to be outside or high in the atmosphere. THAAD range is to be about 200km horizontally and 150km vertically.

The Ground-based Midcourse Defence Segment (GMDS) programme is a system of land-based interceptors aimed at hitting warheads during their midcourse phase, that is after they have soared out of the atmosphere and while they are arcing through space. This system is the most developed of all.

A proposed system of ship-based interceptors would be targeted at missiles in their boost and ascent phases. The US Navy's fleet of 61 Aegis-equipped cruisers and destroyers, designed to counter aircraft and cruise missiles, could provide ready platforms for combating ballistic missiles and so be equipped in just a few years and for a fraction of the cost of a land-based system. But the Pentagon would have to develop a much faster interceptor than the navy's latest, the Standard Missile 3, that is intended to go against medium-range missiles.

Now that the US has given notice to Russia of its withdrawal from the Anti-Ballistic Missile (ABM) Treaty, and that Russia no longer views the US BMD as a threat, tests have resumed. The most recent being a successful intercept during the midcourse phase of the target warhead's flight in October 2002, the fifth successful intercept, and the fourth consecutive in seven flight tests since October 1999 for the GMD programme. This system-level test successfully demonstrated hit-to-kill technology to intercept and destroy a long-range ballistic missile target although a further test in December failed when the device did not separate from its booster rocket.

In addition to the exoatmospheric kill vehicle (EKV) locating, tracking and intercepting the target resulting in its destruction using only body-to-body impact, it also demonstrated the successful integrated operation of space and ground-based sensors and radars, as well as the battle management, command, control and communications system (BMC3) function to detect the launch of the target missile and cue an early-warning radar to provide more detailed target-location data.

Prime areas of concern include accuracy and coping with countermeasures such as decoys and electronic disablers to disrupt launch and flight. Also, the point of transfer from one anti-missile system to the next poses vulnerabilities, e.g. when THAAD turns off and PAC-3 is deployed. Missile defence advocates believe the optimal solution lies in the use of interceptors or lasers fired from space-based platforms. The Pentagon has been trying to develop an airborne weapon consisting of a chemical laser mounted on a modified Boeing 747 jetliner. The first test shot is scheduled for late 2004.

The US depends on British support for Bush's BMD plans as the UK hosts the European Ground Relay Station for the future Space-Based InfraRed System (SBIRS), due to enter service from 2006. SBIRS low and high infrared satellites include 24 in low-Earth orbit capable of tracking ballistic missiles throughout their post-boost ascent and mid-course phases outside the Earth's atmosphere by detecting their residual heat against the cold of space.

In the case of a missile launched from the Middle East against North America, Fylingdales in north Yorkshire would be the primary early warning radar. Tony Blair has signalled he will approve a Fylingdales upgrade when Washington makes a formal request. For such a programme spread over several years, there is always the problem of interrupted testing due to dips in funding, as happened during the 1990s. As with the war on terrorism, the US needs co-operation from its allies and needs to read the intentions of missile-capable rogue states. On 18 November the US warned its allies that time was running out for them to join in with its BMD plans.