The systems - how is it meant to work?

An incoming missile goes through three main phases:

Boost phase when it is firing up in to the atmosphere and its boosters are burning. This lasts from 1 - 5 minutes

Mid-course phase when the missile is flying outside the atmosphere through space. During this phase decoys and balloons may be deployed to confuse the Missile Defence system and make it hard to recognise the real missiles. This lasts from 10 - 15 minutes

Terminal phase when the missile re-enters the atmosphere and begins its descent towards its target This lasts for about 1 minute

A Missile Defence system on the scale that the US is trying to develop will use a variety of systems to try to intercept the incoming missiles. It will however, only be able to intercept a small number of missiles - not the hundreds that Reagan initially envisaged.

SYSTEMS

Systems that may be used include:

Interceptor Missiles located at key bases across the world and on Aegis destroyer battle-ships. These would most likely have to make a direct hit with the incoming missile - likened to hitting a bullet with a bullet - with both missiles travelling at thousands of miles and hour. Also being considered are nuclear- tipped interceptors that could get close to the incoming missile in space and then detonate. However, the explosion would risk knocking out the civilian and military satellites owned by the US and so is not favoured.

Air-Borne Lasers Lasers mounted on modified Boeing 747s, being developed now, that would be able to fire at and intercept the missile. This would be particularly useful at the boost phase, when it is best to intercept the missile (as i) it is firing its boosters and so is easier to detect and ii) it is in its initial stages and so will reduce risk of a successful attack) or at the terminal phase when things are getting risky. The technology is complex as on-board and ground-based sensors would be needed to detect the missile, which is travelling at 2/3 mile per second, and staff would also need to make adjustments to the laser to minimise laser beam interference from the atmosphere However, it is due out this decade.

Space-Based Lasers - also being developed now, although a bit further off than the air-borne laser. These work on a similar principle but would fire at the missile from space either when it is outside the atmosphere, or the laser could pierce the atmosphere and hit the missile during boost phase. Of course, this weapons could also be used to target other things on the planet.

Space-based kinetic hit-to kill technology - other space-based technology is being considered including devises that can be fired from space to collide with incoming missiles outside the atmosphere. This is currently quite a long way off as interceptor missiles have development priority.

A number of integrated systems are being developed that will work together to form the full global missile defence system. These include THAAD, Ground-Based Mid-Course, the Aegis system, the Patriot 3 system, MEADS and ARROW.

As well as the front line attack technology, there is a necessity for high-tec ground-based and space- based command and control centres and monitoring systems to ensure that information about trajectory and target are assessed correctly.

These include:

Updated Phased-Array Radars (at Thule in Greenland, Fylingdales in England, and a couple in the USA). These updated systems, used initially for Early warning of attack, will now be able to monitor and track incoming missiles and chart their trajectories.

X-Band Radars - The Phased Array Radars will still not be good enough to detail which of the objects in space are real missile and which are decoys sent up to confuse the system. As such, a more powerful X-band radar system, with radars probably located on ships at sea, will be used to detect and analyse the space objects

Space-Based Infra-Red System (SBIRS) and Space Tracking and Surveillance System (STSS) - A system of satellites at various distances from the Earth, again aimed at tracking and monitoring the missiles during key points of their flight. Data will be sent from these satellites to ground-relay stations on Earth (Menwith Hill in Yorkshire, England and Pine Gap in central Australia).

Ground Command and Control centres - These centres, in the US, will receive the data and control the overall system during the attack. They will have up to 20 minutes from launch to hit or it will be too late.