America's ambitious National Missile Defence (NMD) programme to use the latest high technology systems to defend the continental USA against ballistic missiles is centred on a hundred batteries of missiles.

The central question facing the Pentagon's planners is not whether the Russians will amend international treaties governing missile defences but whether these missiles will work.

Prime missile candidate for the programme is the Theatre High Altitude Air Defence (Thaad) missile, which completed its second successful launch in June.

The programme is late and has been dogged by seven failures during the last few years, but now seems to be on track.

Technical problems

It should have been in service in 1997 but the technical problems and a lack of adequate funding caused severe delay.

The system is described officially as a medium-range, theatre defence missile; that means it has an effective range of about 150 km, has a high probability of hitting a target and is launched from a mobile launcher vehicle.

The NMD plan calls for a command post at Colorado Springs to have direct contact with a hundred missile launchers and an inventory of over 1,200 missiles.

The new Patriot passed its latest test in March

Although the most likely threat is from a small number of enemy missiles - perhaps 20 - the system could tackle a larger, saturation attack.

But it is unlikely to be fully effective and there will "leakers" which will have to be dealt with short range systems, like the Patriot and Hawk batteries.

These however are only found at key points like government, nuclear and military installations.

The result is clear - it is impossible to defend the whole of the USA against sustained ballistic missile attack but it is fool-hardy to ignore the threat completely.

Thaad is in competition with a US Navy concept to create an upper tier (altitude) system for warships called Navy Theatre-Wide (NTW) missile defence.

By June next year, the Pentagon will have to choose between Thaad and NTW if the White House's required deployment date of 2007 is to be fulfilled.

Unlike the billion-dollar SDI (Strategic Defence Initiative) programme, called Star Wars by the media, the NMD programme has new technologies like the hit-to-kill kinetic energy weapon, of which Thaad is the prime example.

High risk

In addition, the concept of layered defence is now being examined using satellites, airborne laser systems in converted Boeing 747 "Jumbo" airliners, warships and ground-based systems.

America's allies are happier with the NMD than SDI because it takes away the political and high-cost technology problems of space-based weapons.

But the physics remain - to prevent debris falling on the intended target, the incoming missile must be destroyed in the immediate post-launch or boost phase and that will need multiple shots, good target identification and early warning.

All of these technologies remain high risk.

If they do not work, the advocates of pre-launch strikes - attacking the launching sites - will win and that could be a short-cut to regional conflict.

But it is clear that diplomatic non-proliferation treaties and deterrence are no longer adequate, which means that a real system must be found and proven.