At the control centre in Monaco of the sun-powered Solar Impulse 2 plane, Swiss pilot and projet co-founder Bertrand Piccard explains to AFP why his colleague Andre Borschberg had to make an unplanned stop in Japan in the middle of an historic round-the-world voyage.

AFP: The Solar Impulse 2 took off Saturday from Nanjing (eastern China), and was supposed to arrive in Hawaii after six days and six nights. What happened?

PICCARD: When we left China, the weather conditions as far as Hawaii were okay. Then they got worse. We didn't expect to fly through an active weather front, with ice, rain and turbulence. It's a plane that flies slowly and is sensitive to turbulence, and it needs sun to be able to recharge its batteries.

On the fifth day of the flight the front got stronger. We will never be able to finish a round-the-world trip if we crash in the middle of the Pacific!

This world tour is maybe not going as fast as we would like, but this is not a race. The goal is to get there.

In terms of safety, it was much better to make an intermediate landing in Nagoya and to wait there for the weather to improve. It was the last place we could safely land. The little islands dotted here and there in the Pacific are by no means an alternative.

AFP: Failure or technical feat?

PICCARD: We're a little disappointed to not have been able to fly non-stop from China to Hawaii. But a plane that flies 40 hours without fuel, only on solar power, is already extraordinary. We're extremely happy with the performance of this plane. The technical feasibility is there. This plane shows what we can do with clean technology.

Maybe people will finally realise that a round-the-world trip on solar power has never been done, and that it's not easy. It's a big adventure. We're trying to do something for the first time ever, and historic firsts are never easy. It often takes multiple attempts. We'll see if we succeed this year. The team is very motivated.

Solar energy is a good alternative. In some places, you need wind, geothermal energy, biomass and hydropower. Solar is not always the panacea. What we want is to promote clean technologies.

The engines of the Solar Impulse offer 97 percent efficiency, which is incredible. That's only a three-percent loss. With a car engine, there's 73-percent loss. We're now pushing the limit to the max to show what's possible.

AFP: Is the idea of a solar commercial flight completely utopian?

PICCARD: Currently, it's hard to see how we could have 200 passengers aboard a solar-powered plane. At the same time, when the Wright brothers made their first powered flight 112 years ago, they also didn't have the technology to fly passengers.

You need pioneers to pave the way and manufacturers to optimise and develop the rest!

Japan airport readies for Solar Impulse unscheduled stop
Nagoya, Japan (AFP) June 1, 2015 - An airport in Japan was readying Monday to welcome Solar Impulse 2 on an unscheduled stop after mission controllers decided the weather was not right for the sun-powered plane to cross the vast Pacific Ocean.

The record-breaking aircraft was set to touch down in central Japan around 1400 GMT, the team said on its website.

"On my way to Nagoya disappointed for not continuing but very thankful to the Japanese authorities for their support," pilot Andre Borschberg Tweeted.

The seventh leg of the round-the-world journey was set to take 62-year-old Borschberg on a six-day, six-night flight from the Chinese city of Nanjing across the Pacific to Hawaii, a distance of some 8,500 kilometres (5,250 miles).

The super-lightweight plane, which is covered in solar panels, had spent much of the day in a holding pattern over the Sea of Japan as organisers examined forecasts on the projected flight path.

"Weather deteriorating over Pacific, decision taken for intermediate landing in Nagoya and wait for better conditions," tweeted Bertrand Piccard, the initiator of the mission.

"When we took off from China it was quite clear we could cross the front," Piccard said on a live video posted on YouTube.

"It was almost easy, I would say, the weatherman was very confident.

"Now the window has closed. The (weather) front is too thick, too big. The plane would have to go through big layers of cloud.

"The only safe decision is to stop in Nagoya, wait a few days before carrying on."

The flight from China had already notched up Solar Impulse 2's first overnight leg, with the aircraft relying solely on batteries charged by the sun's energy.

- Promote green energy -

The flight from Nanjing to Hawaii was scheduled to be the longest section of the maiden solar-powered global circumnavigation, an attempt to promote green energy.

The journey began in Abu Dhabi in March and was originally intended to be for 12 legs, with a total flight time of around 25 days.

Speaking on Saturday hours before the departure from Nanjing, Borschberg told reporters the plane could land in Japan in case of technical problems, but the open ocean offers no such possibility.

"In case of emergency, we have Japan on the way, so we have identified airports where we could stop but this only really is in case of very difficult technical problems," Borschberg previously said.

"As soon as we leave this part of the world, then afterwards we are in the open sea. There is no way to come back."

Failure could mean a parachute descent into the ocean, hundreds of kilometres from rescue.

No ship is trailing the plane since it travels far too fast for a maritime vessel to keep up, even though its maximum speed of 140 kilometres an hour is much slower than conventional jet aircraft.

Solar Impulse 2 is powered by more than 17,000 solar cells built into wings that, at 72 metres (236 feet), are longer than those of a Boeing 747 and approaching those of an Airbus A380 superjumbo.

The plane is the successor to Solar Impulse, which managed a 26-hour flight in 2010, proving its ability to store enough power in lithium batteries during the day to keep flying at night.

Ridiculed by the aviation industry when it was first unveiled, the venture has since been hailed around the world, including by UN chief Ban Ki-moon.