If a nuclear attack were launched on America, it could involve hundreds of missiles carrying thousands of warheads, each travelling at up to 4 miles a second towards targets they would reach within 30 minutes of launch. To protect themselves, the US have therefore developed their Strategic Defense Initiative (SDI) or 'Star Wars' program.

A major part of this program is to develop lasers that will shoot down enemy missiles within five minutes of launch. If it was left any later than this, then defense becomes much more difficult because the missile releases up to ten seperate warheads and many decoys, greatly increasing the number of targets that have to be hit.

Lasers destroy their targets by directing onto them an intense beam of energy which travels at the speed of light - 186,000 miles a second. The simplest method of destruction is to focus a beam of infrared radiation on a missile so that it burns a hole in the rocket casing, causing fuel to escape so stopping the missile from reaching orbit. Another possiblity is directing the beam to disrupt the rocket's electronic guidance system.

The US are developing a chemical laser in which hydrogen and fluorine react together to form hydrogen fluoride, which is a corrosive gas or liquid which can be made to release a powerful burst of infrared radiation. The laser is focused and aimed by prisms and mirrors. A chemical laser of sufficient power, at least 25 megawatts, could destroy a missile almost 2,000 miles away.

The lasers would attack their targets from battle stations in space, a few hundred miles above the Earth. However, a total of about 100 stations would really be needed to give the US the possibility of complete protection, and getting that many in space would dwarf any previous space project.

Just the hydrogen fluoride needed to fuel the lasers would weigh about 2000 tons! Think of the costs for that kind of payload. How would such a project be funded? A possible alternative to that might be to base the lasers on land. The difficulty then though is that the atmosphere would disperse the laser beam, making it impossible to focus on the missile's skin. Putting the lasers on top of high mountains would reduce distortion, obviously because their would be less atmosphere to penetrate. Advanced optical techniques designed to counteract the dispersive effect of the atmosphere may also help.

Even so, no more than a tenth of the power of the laser could be expected to reach the target, which means that the lasers fired from Earth would need to be very powerful. They would probably need to have the power of around 400 megawatts each. This is the equivalent electricity consumption of a medium-sized city, and 1000 times more powerful than any laser known to exist today.

Mirrors would have to keep the lasers locked onto the missile for several seconds before it would be destroyed. Even if this were achieved, the enemy could probably still defeat the laser by putting a heat shield around the missile or by making it spin so that the beam could not be focused on the same spot long enough to burn a hole. The Star Wars program has also been developing lasers which produce X-rays rather than a beam of light. These X-rays are produced in a single pulse rather than a continuous beam (a pulse laser). The source of the X-rays is a small nuclear explosion. When the pulse of X-rays hits the enemy missile they are absorbed by it's skin, vaporising it and blowing the missile apart. Because X-rays are rapidly absorbed by the atmosphere they would also have to be fired from space when both the laser, and the missile it was attacking, had risen above the atmosphere. This is at least 50 miles above the Earth.

The idea is not to station the lasers permanently in space, but to launch them only when satellite observations show that an enemy attack is already under way. The X-ray lasers would be launched from submarines, and would then be quickly boosted into orbit where they would be aimed and fired accordingly and automatically.