Solar Impulse: Weather front halts sun-powered plane on marathon Pacific flight

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Swiss-made solar-powered plane Solar Impulse 2 took off from Nanjing Lukou International Airport in Nanjing, China to Hawaii on Saturday May 31, 2015.

(CNN) — The Solar Impulse 2 — diverted by weather from trying to fly all the way from China to Hawaii without using a drop of fuel — has landed instead in Japan.

The plane, which two pilots aim to fly around the world powered only by the sun, landed late Monday in Nagoya, about 320 kilometers (200 miles) west of Tokyo.

The trip to Hawaii was expected to be the toughest leg of the Solar Impulse’s round-the-world voyage. It was scheduled to take five days — and maybe a bit more — with just one pilot.

The 8,000-kilometer (4,971-mile) journey from Nanjing, China, to Hawaii — dubbed the “moment of truth” by alternating pilots Andre Borschberg and Bertrand Piccard — had already been delayed several times because of poor conditions over the Pacific.

But in the early hours of Sunday (Saturday afternoon ET), Solar Impulse embarked on the ambitious leg of the journey.

Swiss-made solar-powered plane Solar Impulse 2 took off from Nanjing Lukou International Airport in Nanjing, China to Hawaii on Saturday May 31, 2015.

Swiss-made solar-powered plane Solar Impulse 2 took off from Nanjing Lukou International Airport in Nanjing, China to Hawaii on Saturday May 31, 2015.

Mission Director Raymond Clerc said the route would see Solar Impulse fly over South Korea and northwest of Japan before heading out across the Pacific, and might take slightly longer than planned — potentially landing in Hawaii on the evening of Day 6.

But it wasn’t to be. On Monday morning the flight went into a holding pattern as a weather front blocked its path toward Hawaii.

“Yesterday we had the possibility to cross the weather front just before Hawaii on day 5. However, with the forecasts we now have, we don’t see this possibility anymore, which means that for the moment the road to Hawaii is blocked,” the team said in a emailed statement.

“We have asked Andre to stay where he is: it’s fine, the weather is good and the batteries are charging. During this time we will analyze where he will have to go to find a possibility to cross that front.”

Former fighter pilot Borschberg, who will be at the controls alone for the entire 130-hour flight, had expressed excitement for the flight after weeks of holdups.

Borschberg, who also flew from Abu Dhabi to Oman in March on the first leg of the plane’s 35,000-kilometer, five-month journey, said the latest journey would be “the flight of my life.”

He planned to spend the entire trip in the 3.8-cubic-meter cockpit, strapped into a special seat that serves both as bed (it reclines, allowing him to do essential exercises and to rest) and toilet.

At night, if there is no turbulence, Borschberg will be able to activate the autopilot and nap, but only for 20 minutes at a time.

He and Piccard have been trained in meditation and self-hypnosis, to allow them to concentrate for lengthy periods, and yoga to help them relax in the plane’s confined space.

Swiss-made solar-powered plane Solar Impulse 2 took off from Nanjing Lukou International Airport in Nanjing, China to Hawaii on Saturday May 31, 2015.

Swiss-made solar-powered plane Solar Impulse 2 took off from Nanjing Lukou International Airport in Nanjing, China to Hawaii on Saturday May 31, 2015.

Solar Impulse will be packed with enough food, water and sports drinks to meet Borschberg’s nutritional needs for a week, in case weather problems force it to stay in the air longer than expected.

The aircraft is also equipped with oxygen bottles, a parachute and a life raft in case it gets into trouble and Borschberg has to ditch midflight.

Piccard, who is due to fly the Atlantic leg of the journey later in the year, is open about the challenges they face.

In an interview with CNN last month, he said, “Maybe it will fail. Andre and I are very clear with ourselves, that maybe we’ll bail out.”

Solar Impulse has been stuck in China since March, when a planned overnight pit stop in Chongqing turned into three weeks on the ground there before it was able to fly on to Nanjing, the staging post for the mission’s most challenging flight.

The plan is that after a stopover in Hawaii, adventurer Piccard — who was part of the first team to circumnavigate the globe nonstop in a balloon in 1999 — will take the controls and fly to the U.S. mainland, landing in Arizona.

Solar Impulse will then cross the United States and the Atlantic, before returning to Abu Dhabi in July.

Solar Impulse’s 72-meter (236-foot) wingspan makes it wider than a Boeing 747, but the plane weighs just 2.5 tons, lighter than a large SUV. Its round-the-world mission is intended to raise awareness about the potential of solar power.