WHEN the US air force began work on the Airborne Laser a decade ago (New Scientist, 18 June 1994, p 4), it was aiming to build a weapon that could stop ballistic missile launches by "rogue states". After 10 years and more than $2 billion spent, the project is still struggling to get anywhere.

It seemed at the time a far more realistic idea than the "Star Wars" programme proposed by Ronald Reagan a decade earlier. Reagan's plan to make nuclear weapons "impotent and obsolete" was a response to the cold war nuclear stand-off, in which thousands of US and Soviet nuclear weapons were poised for launch. As part of the programme, Reagan proposed to build a fleet of orbiting battle stations equipped with 5-megawatt lasers that would zap nuclear missiles as they rose out of the atmosphere.

Real-world technology fell far short. Each laser would have needed tonnes of ...