The 236-foot wingspan of the Solar Impulse 2 is longer than a Boeing 747's, but the craft weighs only about 5,000 pounds—a flyspeck compared to the jetliner's 500,000-pound bulk. It has to be light, given how many solar cells you'd need running for an hour at noon to get the same amount of power as a gallon of gasoline (with these cells: about 1,400 square feet). 
Anna Pizzolante/REZO.CH
The 236-foot wingspan of the Solar Impulse 2 is longer than a Boeing 747's, but the craft weighs only about 5,000 pounds—a flyspeck compared to the jetliner's 500,000-pound bulk. It has to be light, given how many solar cells you'd need running for an hour at noon to get the same amount of power as a gallon of gasoline (with these cells: about 1,400 square feet).
Anna Pizzolante/REZO.CH 
FLIGHT PLAN When a glider descends, it’s converting altitude into distance. SI2 will take advantage of that, cruising on solar power at up to 28,000 feet during the day and making an unpowered descent at night to about 5,000 feet. The descent takes four hours, then the SI2 can spin the props for 10 hours on batteries. If it’s gloomy the next day, the plane—now below the clouds—won’t be able to recharge, and the pilot must land or ditch.
FLIGHT PLAN When a glider descends, it’s converting altitude into distance. SI2 will take advantage of that, cruising on solar power at up to 28,000 feet during the day and making an unpowered descent at night to about 5,000 feet. The descent takes four hours, then the SI2 can spin the props for 10 hours on batteries. If it’s gloomy the next day, the plane—now below the clouds—won’t be able to recharge, and the pilot must land or ditch.
SOLAR PANEL In an ideal world you'd use solar cells that were flexible, so they'd conform easily to the curved wing of the plane. But they're not as efficient, so the team opted for rigid panels instead. They coated them with a fluorine copolymer film, giving them enough bendability along with protection from UV light, which can reduce efficiency. 
Chris Philpot
SOLAR PANEL In an ideal world you'd use solar cells that were flexible, so they'd conform easily to the curved wing of the plane. But they're not as efficient, so the team opted for rigid panels instead. They coated them with a fluorine copolymer film, giving them enough bendability along with protection from UV light, which can reduce efficiency.
Chris Philpot 
PROPULSION SYSTEM The four ultra-efficient batteries together weigh about 1,400 pounds, a quarter of the craft’s total weight. Each of the 17.4-hp motors has a reduction gear to keep the propellers turning at 525 rpm, the most efficient speed for this plane. The Solar Impulse 2 will maximize power by cruising at 20 to 90 mph, depending on the altitude. But this e-plane is no albatross—it’s too delicate to take advantage of the thermals. Chris Philpot
PROPULSION SYSTEM The four ultra-efficient batteries together weigh about 1,400 pounds, a quarter of the craft’s total weight. Each of the 17.4-hp motors has a reduction gear to keep the propellers turning at 525 rpm, the most efficient speed for this plane. The Solar Impulse 2 will maximize power by cruising at 20 to 90 mph, depending on the altitude. But this e-plane is no albatross—it’s too delicate to take advantage of the thermals.
Chris Philpot 
WING This needs to be as light as possible without sacrificing rigidity and strength. Its three-piece wing spar is made of honeycombed cardboard sandwiched between carbon-fiber films and impregnated with a polymer that makes it light and strong. The underside of the wing is made of coated fabric to keep moisture out in the clouds where it belongs. Chris Philpot
WING This needs to be as light as possible without sacrificing rigidity and strength. Its three-piece wing spar is made of honeycombed cardboard sandwiched between carbon-fiber films and impregnated with a polymer that makes it light and strong. The underside of the wing is made of coated fabric to keep moisture out in the clouds where it belongs.
Chris Philpot 
COCKPIT The pilot will be alone in the cockpit for up to five days at a time. The seat reclines for naps and doubles as a toilet. An autopilot system will free the pilot from having to control the craft at every moment. The cabin is unheated and unpressurized, but it is insulated, and the ascent to 28,000 feet should be slow enough to prevent altitude sickness. There’s no ejection seat, but there is a parachute, life raft, and shark repellent. Chris Philpot
COCKPIT The pilot will be alone in the cockpit for up to five days at a time. The seat reclines for naps and doubles as a toilet. An autopilot system will free the pilot from having to control the craft at every moment. The cabin is unheated and unpressurized, but it is insulated, and the ascent to 28,000 feet should be slow enough to prevent altitude sickness. There’s no ejection seat, but there is a parachute, life raft, and shark repellent.
Chris Philpot 
HUMAN ELEMENT “Intelligent” fibers in the flight suit help smooth out temperature changes, directing heat toward the skin when it’s cold and absorbing heat when the cockpit warms up. Piccard and Borschberg will consume vacuum-packed, calorie-dense food, plus water and sports drinks. Plus, they each have their own approaches to sitting in cold, thin air for hours on end: Piccard, self-hypnosis; Borschberg, yoga and breathing exercises. Chris Philpot
HUMAN ELEMENT “Intelligent” fibers in the flight suit help smooth out temperature changes, directing heat toward the skin when it’s cold and absorbing heat when the cockpit warms up. Piccard and Borschberg will consume vacuum-packed, calorie-dense food, plus water and sports drinks. Plus, they each have their own approaches to sitting in cold, thin air for hours on end: Piccard, self-hypnosis; Borschberg, yoga and breathing exercises.
Chris Philpot 
MISSION CONTROL Only one guy is flying the plane, but his posse is huge: hundreds of support personnel, air traffic controllers, meteorologists, and routing experts, plus ground crew at every stop. A flight-monitoring and simulation system merges weather data with computer modeling to suggest course changes up to two days ahead of time. The journey is scheduled to take five months so that crews can exploit the best possible weather. Anna Pizzolante
MISSION CONTROL Only one guy is flying the plane, but his posse is huge: hundreds of support personnel, air traffic controllers, meteorologists, and routing experts, plus ground crew at every stop. A flight-monitoring and simulation system merges weather data with computer modeling to suggest course changes up to two days ahead of time. The journey is scheduled to take five months so that crews can exploit the best possible weather.
Anna Pizzolante If, one day, you find yourself driving an electric car that recharges its featherweight high-capacity lithium batteries with sunlight collected from super-efficient solar cells covering its carbon-composite body, you'll have Solar Impulse 2 to thank. It's a plane powered by nothing but sunlight, and this March, Bertrand Piccard and his copilot, engineer André Borschberg, will take it on a 12-leg, multiweek flight around the world. The point, Piccard says, isn't to start selling solar-powered planes—he just wants to show what might be possible. “When the Apollo astronauts went to the moon, it wasn't to launch tourism on the moon and open hotels and make money,” he says. “It was to inspire the world.”
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