(CNN) — For more than a decade they dreamed of circling the globe, using only the energy from the sun to power their flight.
They even achieved what many said was impossible — a nonstop flight lasting almost five days and five nights over the open ocean, without a single drop of fuel.
But on Wednesday, the pilots of the Solar Impulse 2, the experimental plane attempting to fly around the world using only solar power, revealed that they are halting their journey, at least until 2016.
“We are disappointed, but not depressed,” pilot Bertrand Piccard tells CNN from Hawaii.
He laughs, but there is a tinge of sadness in his voice.
He says the unexpected ups and downs of their epic journey, which began in March over the desert sands of Abu Dhabi are “part of exploration and adventure.”
Since then, the Swiss psychiatrist and his long-time business partner, Swiss engineer Andre Borschberg, have taken turns flying solo across Oman, India, Myanmar, China, Japan, and the Pacific Ocean.
Now, their Hawaiian layover will last at least eight months.
The pilots estimate that’s how long it will take to replace and test the plane’s batteries, which they say overheated and were damaged beyond repair during the latest leg of the trip.
The University of Hawaii with the support of the Department of Transportation will host the airplane in its hangar at Kalaeloa airport. Post maintenance check flights will start in 2016 to test the new battery heating and cooling systems. The round-the-world mission will resume early April from Hawaii to the USA West Coast. From there Solar Impulse will cross the USA to JFK in New York before making the Atlantic crossing to Europe and then returning the point of departure in Abu Dhabi.
Longest solar-powered flight
History was written the moment Borschberg and the solar plane touched down near the shoreline of Oahu on July 3.
His flight clocked in at an astounding 117 hours and 52 minutes, setting a new record for the longest solo flight.
But it also presented considerable risks, and unexpected challenges, some of which the pilots are revealing for the first time.
First, two months of weather delays in China.
When Borschberg finally took off from Nanjing at the end of May, he spent less than two days in the air, before the team decided to divert to Japan because an unpredictable cold front was closing in.
While on the ground in Japan, one of the plane’s fragile wings was damaged in the wind and rain.
And an attempt to depart Nagoya after repairs were finished was scrubbed at the very last moment on the runway, once again, because of uncertain weather.
All of this set the stage for a flight the pilots called “the moment of truth,” which would determine, they said, whether flying continuously only on solar power could actually be done.
Borschberg finally took off from Japan early on the morning of June 29.
A few short hours into the flight, the former fighter pilot approached the “point of no return,” where weather would prevent the Solar Impulse 2, which flies about the same speed as a car, from returning to Japan.
And there was a big problem.
“Hardest, toughest decision”
“It was a nightmare,” says Piccard, who was in the team’s Monaco control center at the time.
It wasn’t because of the batteries.
With a five-day and five-night journey ahead of him, Borschberg had been planning to rest using breathing techniques, yoga, and regular 20-minute naps, while the plane flew at night on autopilot.
But the autopilot’s monitoring alarm system, meant to wake Borschberg if there was a problem, was malfunctioning, meaning he might not be able to sleep at all.
This did not go over well with the team’s technical experts.
“I had to work like hell with the engineers to avoid an explosion of the team,” Piccard remembers. “The engineers wanted Andre to come back to Japan.”
Borschberg says he managed to find a workaround using a separate system, but even with that fix, he and Piccard had to make the final call on whether or not to abort.
“This was the hardest, toughest decision I ever had to take in my life,” Borschberg recalls.
“It was very emotional. I really thought about my family. I really thought about my life. And I asked myself, ‘Do I have the right to do this?’ ”
Borschberg’s first night in the cockpit was sleepless.
It was only on Day 2 that a second, potentially more serious problem began to reveal itself.
Because the plane had undergone repairs in Japan, it needed to be tested on a maintenance flight.
However, air traffic control restrictions in Japan made a test flight impossible, so the team decided to perform the checks at the beginning of the mission to Hawaii.
Piccard was worried that there could be water still inside the aircraft that could freeze at high altitude.
For a plane with a wingspan larger than a Boeing 747, but only weighs about the same as a large SUV, any ice could be dangerous.
Borschberg started off with a steep climb approaching the plane’s maximum altitude, 8500 meters, then descended as he typically would do at night, while running off battery power.
When the experts gave him the thumbs up, he ascended once again to allow more than 17,000 solar panels to soak up the sun’s precious rays.
It is those back-to-back climbs, the pilots say, that strained the plane’s batteries, and caused them to overheat.
“The problem was the temperature never went down,” Piccard recalls.
Borschberg remembers feeling “not desperate but certainly nervous. If I had known that we had the risk with the batteries, I would not have continued.”
But at this stage of the journey, the pilots say making it to Hawaii, or ditching into the open ocean with a parachute and an inflatable lift raft, were the only options.
It was only once the plane touched down in Hawaii that engineers saw just how bad the damage was.
“We made a mistake with our batteries,” Piccard says. “It was a human mistake.”
Plans on hold
Inside a hanger at Kalaeloa Airport, engineers and technicians are now working to figure out how to cool the batteries.
It’s not the technology itself that was the problem, the pilots say, but how the batteries were insulated.
The team will order new parts, figure out how to integrate them, and then test them in flight. Piccard says that all of that will take at least a few months.
It’s a timeline that doesn’t work for the sun.
Because the days in the Northern Hemisphere are already getting shorter, completing the rest of the journey in 2015 isn’t an option.
If all goes to plan, Piccard will fly the next leg to Phoenix, Arizona, sometime in April.
Then, he and Borschberg will alternate flying across the U.S., the Atlantic, and eventually all the way back to Abu Dhabi, completing a 35,000 kilometer journey that is meant to inspire and draw attention to the power of clean technology.
Both pilots are feeling confident they can finish what they started, but neither will rule out that chance of more surprises ahead.
Piccard, who is descended from a long line of explorers, mused about the need to go find some fresh funding for the project, before reverting to one of his favorite quotes.
“If it was easy, someone else would have already done it.”