Favorable winds weren’t the only thing that helped Swiss psychiatrist Bertrand Piccard and co-pilot Brian Jones pull off the first nonstop round-the-world balloon flight in 1999. The trip also required burning nearly four tons of propane fuel, a fact that never sat well with the environmentally
conscious adventurers.


So now Piccard has dreamed up a greener—and far gutsier—aviation milestone to conquer: circling the globe in a solar-powered plane. “It would be the purest way to fly,” he says. Pure, yes; easy, no, notes engineer Paul MacCready