A HIGH-powered, lightweight laser weapon that can be fitted to fighter aircraft to destroy missiles tens of kilometres away has been designed by DARPA, the Defence Advanced Research Projects Agency in the US.

Until now, lasers powerful enough to blow up missiles have been so big they can only be carried by large aircraft such as jumbo jets. For example, the Airborne Laser being developed by the US's Missile Defense Agency is designed to fit onto a Boeing 747 freighter aircraft to track and destroy ballistic missiles during their boost phase, although the weapon has yet to undergo flight tests.

But now DARPA says it has managed to shrink all the hardware for such a weapon so that it can fit under the wing of a fighter jet or piggyback on a vehicle to zap anything from ground-to-air and air-to-air missiles to rocket-propelled grenades.

The weapon, called the High Energy Liquid Laser Area Defence System (HELLADS), will weigh just 750 kilograms, including its cooling system, and will fit into a space of about 2 cubic metres, around the volume of a large refrigerator. Details of the design were revealed this month at the annual DARPATech conference in Anaheim, by project leader Donald Woodbury of the organisation's Tactical Technology Office in Arlington, Virginia.

The team, which is working with General Atomics of San Diego, California, has already built a prototype scaled model that is capable of producing a 1-kilowatt beam. They hope to have a 15-kW version finished and ready for testing by the end of the year, and a full-sized prototype capable of firing a 150-kW beam is due to be completed by 2007. "That's well within the design cycle of the Joint Strike Fighter," says Woodbury, referring to the next-generation US-UK fighter jet currently under development.

Precise details of the laser's design are classified, but New Scientist has learned that it involves a hybrid of two different types of laser, liquid and solid-state, each with their own advantages and disadvantages. Liquid lasers can fire a continuous beam, but require large cooling systems. Solid-state lasers are highly efficient but cannot be cooled in the same way, so they have to be pulsed to prevent them overheating, and that consumes more energy.

Woodbury's team claims to have combined the best of each, while shrinking the cooling system. "We've combined the high energy density of the solid-state laser with the thermal management of the liquid laser," he says.

But not everyone is convinced by DARPA's claims. Philip Coyle at the Center for Defense Information in Washington, says DARPA's goal of developing a lightweight tactical laser is not yet feasible. "These are still very much pie-in-the-sky concepts, and so far no high-power laser system has been developed for the US military with demonstrable military effectiveness under realistic operational conditions," he says.

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