A pioneering flight around the world will use nothing but sunshine for fuel. In the dusty peach dawn of a desert day the Solar Impulse 2 airplane took flight at 11:12 PM Eastern time on March 8 from the United Arab Emirate of Abu Dhabi on the first leg of a bid to fly around the world exclusively powered by electricity generated from sunlight.

At a top speed of 45 kilometers-per-hour the single-seat airplane flew to Muscat in neighboring Oman over roughly 10 hours, touching down at roughly 2:13 PM Eastern time, after a few hours spent circling and waiting for the right weather conditions to land. The plane is an upgraded version of the original Solar Impulse, which flew across the U.S. in 2013; both planes were built by the Solar Impulse group, led by Swiss adventurers Bertrand Piccard and Andre Borschberg.

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Borschberg piloted this first leg of at least 12 that will circle the planet, and either he or Piccard will pilot all of the various legs over the next several months. The 10-hour flight did not require some of the exotic meditation, yoga and self-hypnosis techniques that will be required for later, much longer legs, including at least five days and nights as the plane crosses portions of the massive Pacific Ocean. Twenty-minute naps will have to do for refreshment during those epic treks, and a computer system—plus light-flashing goggles—will be in place to wake the pilot should something untoward occur. In fact, the pilot will wear an armband that vibrates whenever the airplane exceeds more than 5 degrees of bank angle, because anything beyond that could cause the airplane to lose lift. The pilot will also be crammed into an unheated, unpressurized cockpit of 3.8 cubic meters, room enough for little more than a reclining couch seat, which also has a removable bottom for a toilet. For the flights across the U.S., the pilots had to rely on empty water bottles and will power.

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The plane itself is a marvel of engineering, with a 72-meter wingspan—longer than that of a Boeing 747.

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The primary structural component is carbon-fiber sheets that weigh just 25 grams per square meter, or roughly three times lighter than a similar sized piece of paper. That carbon fiber is used sparingly in structural spots where forces push on the airplane.  But the interior of the wings, the fuselage and other areas are empty to save even that tiny bit of weight, co-pilot Piccard explained.

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Atop those wings as well as on the body and even the tail of the plane are 17,248 solar cells as thin as a human hair that generate electricity as the plane flies, some of which is stored in four lithium polymer batteries. Those batteries take over powering the plane’s four electric motors at night, which spin the two propellers under each wing. All told the plane weighs 2,300 kilograms and the four batteries are the heaviest passengers, weighing in total 633 kilograms. Making the plane required 12 years of calculations, computer simulations, building and testing, according to Piccard, along with some $140 million.
 
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During a full day's flight the airplane will climb as high as 8,500 meters by mid-afternoon before coasting down slowly at 15 meters per minute to a low altitude of 1,500 meters at night. That plan saves energy before the next dawn by gliding instead of using powered flight. Piccard and Borschberg used a similar strategy on the flights across the U.S. The two adventurers hope to raise awareness that such pollution-free flight is possible and to inspire incorporation of solar materials into more common aircraft and elsewhere. "All the technologies we are using on this plane could be used everywhere," Piccard said.
 
After a brief pit stop in Muscat, the plane will cross the Arabian Sea for Ahmedabad, India, on the morning of March 10, weather permitting. For all its engineering marvels, the plane is not built to cope with inclement weather. The itinerary also includes stops in Mandalay, Myanmar; Chongqing and Nanjing in China; as well as Hawaii, Arizona and New York City among other destinations. All told, the airplane will cover some 35,000 kilometers before touching back down in Abu Dhabi in about five months, again depending on weather and other delays. All of those kilometers of flight will be powered by photons from the sun. "We are not in a hurry, you know," Piccard said. "This plane is more about demonstrating the incredible that is possible rather than speed to destination."

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