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By Ian Williams

NANJING -- Solar Impulse is ready to take off. As is pilot Andre Borschberg.

It's now just a matter of waiting for the right weather window for the most ambitious and challenging part of the sun-powered plane's ambitious round-the-world journey –- five days and five nights across the Pacific to Hawaii.

“Is it going to work in the way we think its going to work? We just don’t know,” Borschberg told NBC News. “This one is the unknown."

Since April 21, home for the Solar Impulse has been a vast white purpose-built hangar near the perimeter of Nanjing Airport in China that has played host to a steady stream of well-wishers and other inquisitive visitors -– students, local officials and airport employees, all lining up for selfies with an aircraft whose mission is to demonstrate and promote clean energy.

“We need to be flexible, need to adapt, need to cope,” said Borschberg of the daunting journey ahead.

First, the weather has to be right. Solar Impulse cannot take off if cross-winds are greater than 4 knots, or 4.6 mph (or 7.4 km per hour). The team was stranded in the Chinese city of Chongqing for much of April because of rain and wind.

The man who will advise on that is Philippe Manuel, Solar Impulse's deputy flight director. During a visit by NBC News he demonstrated a simple wind gauge. As he held it high, it fluctuated between what felt like a pleasant three and six knots as the wind gusted.

“The breeze is great for a cocktail by the swimming pool,” he said. “But for Solar Impulse, this sort of breeze is too much.”

The aircraft weighs just 2.3 tons, about the same as a family car, but its wing-span is greater than that of a Boeing 747 Jumbo Jet.

Even periodically bringing the plane out of the hangar -– to sunbathe, as they call it, recharging its 17,000 solar cells –- is entirely dependent on the wind.

“Each time I look at the plane, I want to be in cockpit,” says Bertrand Piccard, Borschberg’s collaborator, who initiated the project 12 years ago.

The cockpit only has room for a single pilot, so they are taking it in turns to fly it, dividing up the legs of the journey. Piccard piloted the aircraft from Chongqing to Nanjing, and will be at the controls for the Atlantic crossing.

But the immediate challenge is for Borschberg to get it to Hawaii.

Last month, a bout of shingles forced him back to Switzerland for ten days, and he returned to Nanjing to a hero’s welcome -– cheers and thumping music from the 50-strong support team that is accompanying Solar Impulse around the globe with an almost evangelical zeal for its clean-energy message.

Borschberg, clearly moved by the reception, says he’s fully recovered and can’t wait to get airborne.

“This is going to be my home for almost a week,” he said, sitting in the cockpit.

During the daylight hours, the solar cells will charge, the power then used overnight.

“Every day we’ll climb to the altitude of Everest. Every night we’ll come down closer to the level of the ocean.”

Borshberg is a keen practitioner of yoga, which he will use to keep mentally and physically alert during his journey.

“Five days and five nights is difficult for the body. So physically you have to do some exercises. You have to stimulate the mind, the blood circulation and the organs –- and yoga is good for all three.”

He demonstrated how his chair fully reclines –- “a bit like a yoga mat."

He will also cat-nap. A series of eight short naps of twenty minutes per day. He says he has attuned his body to this -- though there is an alarm in his watch, just in case. And mission control in Monaco will no doubt be closely monitoring.

“The first goal is to have enough energy for the airplane. The second priority is the energy of the pilot,” Borschberg said.

The pilots have been full of praise for their Chinese hosts, who –- in their rhetoric, at least –- have embraced the idea of clean energy. Officials have been keen to identify with the Solar Impulse, even as the smoggy skies over China show how tough it will be to tackle the country’s chronic pollution from burning fossil fuels.

The Pacific crossing will be the seventh leg of an epic journey that started in Abu Dhabi on the ninth of March, and will finish in that same Emirate later this year after five more legs crossing the US, the Atlantic and Europe.

“Flying over water means no visual references, which usually helps in flying a plane, “ said Borschberg of the Pacific leg.

He will be carrying a parachute and life-raft, and he and Piccard received training in the North Sea with the German Navy.

“If you know you can cope with the most difficult scenarios, it helps you to be less worried,” he said.

During preparations for their round-the-world journey, they carried out computer simulations, and in that virtual world it all went smoothly. But forecasting is difficult. Reality can be different. The winds in China have been stronger than anticipated.

“The question is what did we not anticipate? What did we underestimate?”

Weather forecasting is particularly difficult for a five day flight, but they do have one big advantage should the landing conditions not be right when they approach Hawaii.

“We have no fuel problems, so we can wait,” Borschberg said with a smile.

And the most challenging part of the Pacific crossing?

“I’ll tell you afterwards!”