On the morning of March 21, 1999, Bertrand Piccard and Brian Jones landed their balloon in the Egyptian desert, completing the first such nonstop flight around the world. Amid the celebrations, Piccard made a sobering discovery: The propane tanks needed to keep his balloon aloft were almost empty. “If the winds had been a little weaker over the Atlantic, I would have ditched,” he says. Piccard vowed then to devise a way to circumnavigate the globe using no fuel at all.

Beginning in May, Piccard and a partner will take turns flying a single-seat, solar-powered airplane from San Francisco to New York—a prelude to an around-the-world flight planned for 2015. Named HB-SIA (for Solar Impulse Alpha), Piccard’s plane defies conventional aviation wisdom. When he first told others about his dream, “almost everybody thought I was completely crazy,” he says.