People have long dreamed of perpetual flight, and the tipping point was reached earlier this month when a solar-powered drone stayed aloft for 48 hr. It showed that enough energy could be stored during the day to fly the aircraft at night, for at least several days, All that's needed are small, near-term improvements in technology to tip this feat into flights last-ing reliably for months. One application, albeit much harder, would be as a radio tower in the sky, giving line-of-sight access across a city.
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