This Plane Runs on Sun and Is About to Smash Some Records

One pilot, five days over the Pacific, no fuel

Solar Impulse 2 second flight
Test flights in Switzerland. Source: Solar Impulse / Revillard / Rezo.ch

Here we go.

The flight into Chongqing, China, happened at night. No moon. Pilot Bertrand Piccard nosed his plane into strong headwinds, and the turbulence was bad—not knock-you-up-and-down bad, but blow-you-off-the-runway bad. At one point the wind was so strong the plane was actually moving backward.    

A sudden cold front poses little threat to most planes, but the Solar Impulse 2 is no ordinary plane. It runs entirely off solar power, stored in four large batteries that allow it to fly through the night. Its solar-paneled wings stretch wider than a Boeing 747’s, but the ultra-light plane cruises at 40 miles an hour or so and carries just one traveler: the pilot.  

The First Solar Plane Flying Around the World

You’ve probably heard of Solar Impulse by now. The plane's two Swiss pilots—Piccard, 57, and partner André Borschberg, 62—have been breaking their own flight records for years in an effort to circumnavigate the planet using ​only energy from the sun. Their mission is about redefining the limits of current technology and testing the physical limits of the human pilot, who must survive for days at a stretch on little more than 20-minute naps and a bit of yoga (Borschberg) and self-hypnosis (Piccard).

Piccard landed the plane without incident in Chongqing, but the Solar Impulse team wasn't so lucky when Borschberg later tried to fly from China to Hawaii, the longest planned leg of the round-the-world trip. A shift in the weather forced the plane to the ground unexpectedly in Nagoya, Japan, where it was battered by winds. Solar Impulse 2 has spent the last two weeks in a mobile hangar waiting on wing repairs and an opening in the weather. 

The next flight will be the biggest trial yet, and if successful will set a new record for distance flown by an electric plane (roughly 4,000 miles) and duration in flight by a single pilot of any kind of plane (four to five days and nights). After starting its journey in Abu Dhabi in March, Solar Impulse now heads for the U.S.,​ requiring​ a harrowing leap over the vast Pacific. 

“We are so close,” says Borschberg, who will take the controls this week if the weather cooperates. “This is the moment of truth.”

The round-the-world journey started in March. Phoenix will mark the halfway point.
The round-the-world journey started in March. Phoenix will mark the halfway point.
Map data: Goolge / Illustration by Bloomberg
Two pilots, one plane. Borschberg and Piccard.
Two pilots, one plane. Borschberg and Piccard.
Source: Solar Impulse / Ackermann / Rezo

Big commercial planes will probably never run on solar power collected during flight. The physics of gravity and wind drag would require impossibly large wings. Bu​t if the solar panels stay on the ground, then a ​large battery-powered plane isn’t too far off. Airbus has already built an all-electric two-seater for short training flights and said a 90-passenger hybrid-electric plane that saves fuel and reduces pollution could be ready in 15 years. Boeing is also experimenting with hybrid engines. 

The basic technologies are available now. The solar cells covering Solar Impulse 2 are similar to panels affixed to rooftops. Its lithium batteries are equivalent to two Tesla Model S sedans.

Solar Impulse has an additional form of stored energy it can draw from: altitude. 

Catching solar rays above the clouds near the Nagano mountain region in Japan.
Catching solar rays above the clouds near the Nagano mountain region in Japan.
Source: Solar Impulse / Borschberg

Solar Impulse 2 starts the day aloft with nearly empty batteries and at a low altitude. As the morning sunlight bleeds like watercolors across the horizon, the solar panels get to work on three duties: propel the plane forward, charge the batteries, and climb. Over the course of the day, Solar Impulse 2 ascends to almost 28,000 feet, storing energy that it will convert to forward momentum at night, as the plane drifts back toward Earth. Altitude, to the Solar Impulse, is like a massive battery that doesn’t weigh anything.

The margins for error are tiny. The round-the-world journey must be completed by October, because as autumn days grow shorter in the Northern Hemisphere, the plane will have reached its limit for overnight endurance. Even now, a very cloudy morning means there won’t be enough energy stored up to make it through the night. If something goes wrong over the Pacific, it could require turning around, or worse, ejecting from the plane, inflating an emergency raft, and waiting for rescue.

These wings work best when they haven't yet disintegrated.
These wings work best when they haven't yet disintegrated.
Source: Solar Impulse / Pizzolante

The career of Solar Impulse 1, the current plane’s predecessor, almost ended in disaster in 2013. It was the last day of a mission to cross the U.S., from Mountain View, California, to New York. Borschberg took off from Washington D.C. and headed out over the Atlantic Ocean. Everything seemed to be going well until the undercover of a wing tore loose. A passing helicopter took pictures of the plane’s underside and sent them to Solar Impulse Mission Control, then located in Switzerland. 

“The first feedback that I received from the engineers,” Borschberg says, “was that  they were surprised that the wing had not disintegrated yet.” 

It was a sunny day, but too turbulent for an emergency landing. Any attempt to bring the plane down before nightfall could shred the wing to pieces. Borshberg would have to ride out a nine-hour flight to New York. He mentally rehearsed the procedures for bailing out: how to exit the plane, how to open the parachute, how to land in the water and inflate the emergency raft.

“I thought: OK, if this happens, you can do it,” Borschberg says. “And therefore, if this happens, you better enjoy it, because you don’t have the chance to jump out of the cockpit every day.” He landed successfully at John F. Kennedy International Airport. 

The toilet is beneath the pilot's seat.
The toilet is beneath the pilot's seat.
Source: Piccard / Solar Impulse

Life on the Impulse is an experiment in confinement and solitude. The electric plane produces little noise save the slow whistle of propellers in the wind. Borschberg says when he lays the seat flat and closes his eyes for a 20-minute rest, suddenly the sounds of tiny creaks and rattles in the plane come alive "like living creatures.” The plane is quiet enough that when he flew over the Sea of Japan, he could hear other planes in the sky even though they were too far away to see.  

Sleep is the most difficult challenge. Autopilot allows for about eight 20-minute naps a day, a schedule of deprivation that takes its toll as the flight hours add up. Both pilots have gone only as long as three days and nights in a flight simulator, significantly less than what will be required by the next voyage. Borschberg, a mechanical engineer and tech entrepreneur, says that to prepare he sometimes slips into the nap schedule even when not actively training, including while on vacation with his wife and children. With the additional wakeful hours, he has taught himself to draw, but he says the reduced sleep hampers creative thought the longer it goes on. 

Bertrand Piccard comes from a family of explorers and adventurers. His father, Jacques Piccard, still holds the record for the deepest a human has ever traveled, to the Marianas Trench in the North Pacific Ocean in 1960. Bertrand's grandfather, Auguste, once held the record for highest altitude in a balloon, in the 1930s.

Dueling selfies: U.S. Navy Lieutenant Don Walsh (top left) and Jacques Piccard (bottom left) set the deep-dive record in the Marianas Trench in 1960. Fifty-five years later, Jacques's son, Bertrand (right), picked up this selfie stick during his stay in China, just after breaking the record for longest solar-powered flight. 
Dueling selfies: U.S. Navy Lieutenant Don Walsh (top left) and Jacques Piccard (bottom left) set the deep-dive record in the Marianas Trench in 1960. Fifty-five years later, Jacques's son, Bertrand (right), picked up this selfie stick during his stay in China, just after breaking the record for longest solar-powered flight. 
Source: NOAA via Bloomberg / Twitter
Repairsing a wing beneath the inflatable mobile hangar in Nagoya.
Repairsing a wing beneath the inflatable mobile hangar in Nagoya.
Source: Solar Impulse / Pizzolante

Piccard, Borschberg, and their financial backers have spent $150 million on Solar Impulse over 12 years (which Borschberg points out is a fraction of the cost to run a Formula One car racing team). Part of the Solar Impulse engineering team is already working on the next mission: to design high-altitude "platforms"—solar-powered vehicles that can stay aloft for years at a time, replacing expensive satellites.

The purpose of the whole enterprise, the pilots say, is to show how the capabilities of clean energy have surpassed our imaginations, and to inspire people to catch up to what's possible.

“It’s important to get out of our habits, out of our certitudes, out of our beliefs," Borschberg says as he prepares his mind, body and plane for this next record-breaking ascent. “Life is more interesting when we try new things." 

Near the team's Mission Control, in Monaco, a month before the adventure began.
Near the team's Mission Control, in Monaco, a month before the adventure began.
Source: Solar Impulse / Revillard / Rezo.ch
The trip starts, and ends, in Abu Dhabi.    
The trip starts, and ends, in Abu Dhabi.    
Source: Solar Impulse / Stefatou / Rezo.ch
The First Solar Plane Flying Around the World