LONDON (Reuters) - A U.S. Pentagon invention could make air combat resemble a battle scene from Star Wars, with a laser so small it can fit on a fighter jet, yet powerful enough to knock down an enemy missile in flight. The High Energy Laser Area Defense System (HELLADS), being designed by the Pentagon's central research and development agency, will weigh just 750 kg (1,650 lb) and measures the size of a large fridge. To date, such lasers have been so bulky because of the need for huge cooling systems to stop them overheating, that they had to be fitted to large aircraft such as jumbo jets, New Scientist magazine reported on Wednesday. But the Pentagon's Defense Advanced Research Projects Agency reckons it has solved the problem by merging liquid and solid state lasers to cut the size and weight by "an order of magnitude," according to its Web site. Liquid lasers can fire a continuous beam but need large cooling systems, while solid state laser beams are more intense but have to be fired in pulses to stop them overheating. "We've combined the high energy density of the solid state laser with the thermal management of the liquid laser," New Scientist quoted project manager Don Woodbury as saying. Dubbed the "HEL weapon" by its developers, a prototype capable of firing a mild one kilowatt (kW) beam has already been produced and there are plans to build a stronger 15-kW version by the end of the year. If everything goes according to plan, an even more powerful weapon producing a 150-kW beam and capable of knocking down a missile will be ready by 2007 for fitting onto aircraft. © Reuters 2005. All Rights Reserved.