Quotes of the Day

In a hangar outside Zurich, engineers are paring away at the obstacles to a very 21st century challenge: flying a plane around the world powered by nothing but the rays of the sun. If the Solar Impulse project goes to plan, in 2011 a gangly aircraft with the wingspan of an Airbus A380 and the weight of a compact car will attempt to circle the globe in about a month at an average speed of 43 m.p.h. (70 km/h), landing only five times along the way.

This odyssey is not merely an epic adventure. "The achievement won't be just to go around the world," says the man behind the project, Bertrand Piccard, "but to encourage a complete paradigm shift on how we use energy." Piccard, a 49-year-old Swiss psychiatrist and aeronaut, knows a thing or two about high-altitude derring-do. In 1999, he and a partner, Brian Jones, became the first people to circumnavigate the earth in a balloon, but it rankled Piccard that doing so required burning nearly four tons of propane gas. "The balloon flight was a personal dream, the last thing to write in the history books before the end of the 20th century," says Piccard, whose father and grandfather were legendary ballooning and diving adventurers. "This project is more difficult and more useful: we want to share a state of mind."

That prospect is inspiring enough to have attracted $67 million — about two-thirds of the project's overall budget — from a trio of high-profile sponsors: Swiss watchmaker Omega, part of the Swatch Group, which brings expertise in both miniaturization and efficient energy use; Deutsche Bank, which is keen to green its investment portfolio; and Belgian chemical and materials group Solvay, which backed the exploits of Piccard's grand-father, Auguste, when he co-piloted a balloon beyond the stratospheric altitude of 50,000 ft. (15,240 m) in 1931.

Engineers and environmentalists will watch Solar Impulse with interest, since it offers a rigorous testing platform for extracting maximal power from minimal energy. A recently unveiled prototype, HB-SIA, which will begin flight-testing next year before the larger plane is built for 2011, is a marvel of optimization. Its 200 ft. (61 m) wingspan is covered with photovoltaic cells, which convert the sun's rays into roughly the same amount of energy needed to light a large Christmas tree. That solar power drives four electric engines, and loads four lithium batteries — a quarter of the aircraft's total weight of 3,300 lbs. (1,500 kg) — which allow the plane to continue flying through the night. Project director and co-pilot André Borschberg says that while labs around the world are developing lighter and more effective batteries, those currently available impose severe limits on the plane's weight. "With twice the battery capacity, we'd have a different plane," he says. And perhaps a more comfortable one: HB-SIA's pilot will sit in an unheated, unpressurized cockpit, in which he'll encounter -76ºF (-60ºC) temperatures at high altitude. In order to lighten the plane's load further, Boschberg has already gone on a diet.

Given its weight and power source, the Solar Impulse design can't handle turbulence, rain or even heavy clouds. During the day the HB-SIA is expected to climb to 28,000 ft. (8,500 m) so it can preserve battery power after sunset by gliding down to 10,000 ft. (3,000 m) at night. For as much as one-third of the night, says Piccard, the plane will be able to fly its descending course without engine power. But once it reaches its nighttime cruising altitude, the burden of powering the plane will fall to the batteries alone.

The challenge, says Piccard, is to keep going until the next sunrise before the batteries are empty: "We have very little margin of error from night into day. Each dawn will be a moment of incredible suspense." For the 2011 flight, he and Boschberg will do alternating stints of five days and five nights between landings. A day on the ground spent charging in the sunlight should be enough to get the plane back into the air the next morning for another stage in its globe-girdling journey.

It's a delicate enterprise, complicated by meteorological challenges and the ungainliness of a plane this big and light. Even Piccard doesn't envision solar planes replacing today's airliners anytime soon, but that's not the point. To reduce emissions, he believes, aviation will eventually need to wean itself from fossil fuels. "To make reasonable use of any alternative," he says, "we have to become lighter and more aerodynamic to reduce consumption." Solar Impulse promises to generate an array of futuristic insights — and some old-fashioned thrills along the way.

• JAMES GRAFF
• A visionary team in Switzerland is developing a plane that can fly day and night on solar power