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.

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