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. Although pioneers like Paul MacCready had been building manned solar-powered aircraft since the 1970s, none were capable of flying after the sun had set, let alone across the Atlantic and Pacific for days at a time.

The obstacle is weight. To fly through the night, a plane must draw upon power from batteries charged during the day. But batteries hold far less energy per pound than a tank of jet fuel, so a plane must carry more weight in batteries to travel an equivalent distance. A heavier plane needs more energy to fly, which in turn requires even more battery power. Add a cockpit and pilot and the craft could be too heavy to even take off. That’s why solar-powered aircraft research has typically focused on unmanned vehicles, such as NASA’s flying-wing Helios.