Wind power is the world´s fastest-growing energy source. Existing capacity worldwide is approaching 50,000 megawatts-roughly equivalent to that of 50 nuclear power plants. But there are problems with this seemingly benign wellspring of pollution-free electricity. Aside from being noisy, the whirling turbines interfere with television reception and are generally considered terrestrial eyesores rendered useless when the wind stops. Bryan Roberts, an engineer at the University of Technology in Sydney, Australia, has a solution: Instead of erecting wind turbines on the ground, float them in the jet stream, a screamingly fast current of air that circles the globe, fluctuating between altitudes of 15,000 and 45,000 feet.