Wedged into the tiny cockpit of the Solar Impulse 2, an airplane powered entirely by sunlight, a Swiss adventurer will soon begin an unprecedented journey across the Pacific Ocean.

André Borschberg will take off from a runway in Nanjing, China, on the seventh leg in a planned 12-part circumnavigation of the globe. Once airborne, the former fighter pilot will spend the better part of a week aloft, steering round the clock toward Hawaii at altitudes of up to 30,000 feet. He will fly far from the consolation of islands, landing strips, or easy rescue across a desolate expanse of ocean similar to the one in which Amelia Earhart disappeared 77 years ago.

The journey, scheduled to begin Tuesday weather permitting, is expected to last at least five days and possibly up to seven as the lightweight, custom-built aircraft battles oceanic winds and weather on the 5,077-mile passage to Honolulu.

If successful, Borschberg’s flight will be the first ever made across such a vast expanse by a solar-powered plane, and it will set a new bar for transport using clean technologies.

Solar-Powered Plane Continues Round-the-World Journey

“It’s going to be the moment of truth,” Borschberg said recently. “We’ve done a lot of tests, but we’ve never done this.”

With a laugh he added, “I’ll tell you, the engineers are quite nervous.”

The journey was designed by Borschberg and his partner-in-adventure, the Swiss psychiatrist Bertrand Piccard, to push the limits of solar flight and highlight the potential of renewable energy.

Piccard conceived the idea more than 12 years ago when he nearly ran out of fuel during an attempt to make the first round-the-world flight in a gas-powered balloon. Ultimately he succeeded, completing the circuit in 19 days. But by the end of his historic voyage Piccard vowed never to let fuel limit him again.

“I wanted to be able to fly forever,” he said.

Today, after years of research and experimentation, the Solar Impulse 2 is built to do exactly that, at least in theory. With a wingspan of 236 feet, wider than that of a Boeing 747 passenger jet, and a long, slender fuselage composed of ultra-light carbon, the plane’s form follows its function.

A skin of more than 17,000 photovoltaic cells runs along the wing tops and down the spine of the plane, and this network powers all of its systems, from onboard computers and cockpit cameras to its four electric motors. The cells also feed four highly-efficient batteries, which store enough energy during the day to power the aircraft through the night.

Planned Route of the Solar Impulse 2 JOHN TOMANIO, NG STAFFSOURCE: SOLAR IMPULSE

The multi-day ocean crossing to Hawaii will provide the first major real-world trial of Solar Impulse 2, and this test will be followed almost immediately by a second, when Piccard takes the yoke and flies east to Phoenix, Arizona, without landing.

Piccard and Borschberg themselves will also be tested as they each spend days alone in a cockpit about the size of a car trunk, sitting on a seat that doubles as a toilet. They will be unable to stand, though the seat does recline, and will sleep only briefly, catnapping when the flying is calm.

“It’s the first time we’ll be so long in the airplane,” said Borschberg, who for many years flew fighter jets in the Swiss air reserves. “Each cycle of day and night will be different. There will be a lot of information coming at us.”

View Images André Borschberg, an entrepreneur and former Swiss fighter pilot, founded the Solar Impulse project with Bertrand Piccard in 2004. He will pilot the plane on its most difficult leg to date, a nearly week-long nonstop flight from Nanjing, China across the Pacific to Hawaii. Photograph by Philip Cheung, National Geographic

The men began their journey in March, lifting off from a military airport in Dubai and heading east to Oman and Myanmar, to India and China. During stops for maintenance and rest, Piccard and Borschberg and a team of support staff worked to spread their clean technology message, hosting talks for local communities and opening their hangar to thousands of curious visitors.

So far the aircraft’s systems have performed well during flights of up to 20 hours, the pilots said. Small technical problems—an ill-fitting oxygen mask, the failure of solar cell, a misfiring alarm system in the cockpit—have been irritating, but easily fixed by the team’s ground crew.

A more persistent challenge for the pilots have been strong crosswinds. Such winds rarely deter heavier, more powerful aircraft, but they can have outsized effects on the slow-flying, lightweight Solar Impulse 2, pushing it off course or forcing changes to the flight plan as the pilots hope to complete their Pacific travel before the start of monsoon season.

During their travel across Asia, crosswinds delayed takeoffs and landings several times. Piccard was even shoved backward by crosswinds on his approach to the airport in Chongqing, China.

“I almost disappeared from the air traffic controller’s radar because I was flying backward,” he said.

The takeoff for Hawaii will send the plane and its pilots into a new dimension of variables, where the weather may be more extreme and harder to predict, and minor malfunctions can’t be easily fixed. Exhaustion, too, will dog the men and may be the heaviest burden of all.

But Piccard and Borschberg said they had spent years planning for the risks and searching for ways to stack the odds in their favor.

With the German navy they had parachuted into the cold waters of the North Sea and practiced untangling themselves from twisted cords and hauling themselves into a life raft. With yoga instructors and medical doctors they had designed meditation schemes and nutrition plans to keep them alert and healthy.

And, during each leg of the journey, a mission control team based in Monaco will monitor the plane’s progress and provide regular weather reports. Former aviators—some of them old military friends of Borschberg’s—will also be on hand to offer advice and support.

View Images The plane is entirely powered by its 17,248 solar cells arrayed across its wings, spine, and tail. The cells must collect and store enough sunlight during the day to power the plane through the night. Photograph by Revillard, Solar Impulse/Rezo.ch

Should Borschberg or Piccard need to ditch, he will take with him down to the sea a parachute, a life raft, food, and a radio. After that the plan is to call for rescue, either by way of some friendly navy, or by the luck of a passing ship.

For Piccard, such possibilities have long been woven into a family tradition of adventure. In 1960, Bertrand Piccard’s father, Jacques, descended in a bathyscaphe to the deepest parts of the Pacific. In 1931 his grandfather, Auguste, floated into the stratosphere aboard a balloon equipped with a pressurized passenger capsule.

View Images Bertrand Piccard, along with co-pilot Brian Jones, became the first to circumnavigate the globe in a balloon without stopping in 1999. The trip inspired him to begin designing a plane that could conceivably fly forever on sunlight. Photograph by Philip Cheung, National Geographic

Auguste knew Amelia Earhart. Bertrand Piccard remembers seeing them together in an old photograph.

“I’m a little jealous of my grandfather for that,” he said. “She was such a beautiful person. I will have a thought for her when I’m flying over.”

Borschberg said before his flight that he was eager to see how weather, technology, and human endurance would combine in the days ahead. He had spent more than a decade thinking about this flight. Dreaming of it, and trying not let the dream become an obsession.

“Being humble and being a little bit detached is important,” he said. “To not be too target-focused. I am more excited about what’s coming than I am nervous. I believe we have a great airplane.”

For the next several days he will put that faith into practice over the earth’s greatest ocean, flying by light alone. At a certain point he knows his thoughts will turn toward that first glimpse of green on the blue and the soft shudder of his wheels touching down. He said he would try not to want it too much.