On a bright new wing

FLYING using only the power of the sun is an enticing prospect. But manned solar-powered aircraft are fragile and slow, like the ungainly craft which a Swiss team called Solar Impulse plan to fly around the world. Take the weight of the pilot out, however, and things change. A solar-powered unmanned aerial system (a UAS, more commonly called a drone) could fly long, lonely missions that conventional aircraft would not be capable of. Advances with the technology mean many such craft could be swarming into the sky.