Solar Impulse awaits 'moment of truth'

It will be a test of man and machine. The Solar Impulse project is waiting to undertake its greatest challenge yet: flying non-stop from Nanjing in China to Hawaii in the Central Pacific.

For a passenger airliner, the 8,000km could be completed in 10 hours or so. But for this solar-powered, prop-driven, experimental aircraft, it could take 5-6 days and nights of continuous flight.

The plane will need the weather on its side, which is why the team is currently sitting tight in Nanjing, looking out for a sizeable, favourable window.

So far on its epic round-the-world quest to promote clean technologies, Solar Impulse has been restricted to short hops of about 20 hours' maximum duration.

To complete this seventh leg will involve smashing several aviation records - not least the longest-duration journey for a single-seater plane.

The Swiss entrepreneur and engineer Andre Borschberg, who will be at the controls, has supreme confidence in the technology, but he is in no doubt how tough the coming mission will be.

"It's more in the end about myself; it's going to be an inner voyage," he told BBC News.

"It's going to be a discovery about how I feel and how I sustain myself during these five or six days in the air."

Borschberg will be strapped to his seat for the duration, confined in a cockpit no bigger than a phone booth.

He can take little catnaps of 20 minutes or so, and he will be practising yoga to try to keep his body fresh. He must be ready to react at very short notice.

Although it has a wingspan of over 70m, Solar Impulse only weighs a couple of tonnes, and dealing with any turbulence will require the attention of all his 40 years of flying experience.

Solar Impulse: The journey so far

• LEG 1: 9 March. Abu Dhabi (UAE) to Muscat (Oman) - 441km; in 13 hours and 1 minute

• LEG 2: 10 March. Muscat (Oman) to Ahmedabad (India) - 1,468km; in 15 hours and 20 minutes

• LEG 3: 18 March. Ahmedabad (India) to Varanasi (India) - 1,215km; in 13 hours and 15 minutes

• LEG 4: 19 March. Varanasi (India) to Mandalay (Myanmar) - 1,398km; in 13 hours and 29 minutes

• LEG 5: 29 March. Mandalay (Myanmar) to Chongqing (China) - 1,459km; in 20 hours and 29 minutes

• LEG 6: 21 April. Chongqing (China) to Nanjing China - 1,241km; in 17 hours and 22 minutes

Every step of the journey will be monitored from the project's control room in Monaco.

Perhaps the support group under greatest pressure is the team of meteorologists. They have to find the window of perfect weather across the Pacific.

Modern forecasters have supreme skill in the 24-hour look-ahead, but the uncertainties grow the farther out you look.

The enemy is high cirrus because it will limit the amount of energy the plane can garner from its 17,000 solar cells.

At dusk, controllers will want to see Solar Impulse's four lithium-ion batteries completely topped up to be sure the plane has the ability to survive until sunrise the next morning.

"This is really important," said mission director Raymond Clerc.

"Even with 100% charge, by the next morning we would have only 10% or maybe 7%. So, if we go into the night with 90% charge, we are at zero the next morning."

Andre Borschberg: "It's going to be an inner-voyage"

Strategy is everything. A simulation team in Monaco will be constantly looking ahead, ingesting the latest weather reports and air-space constraints into a predictive model that will help find the optimum route.

The basic plan is to fly high during the day, up at 28,000ft, to present, hopefully, the solar arrays with a perfect blue sky.

Then, at night, Borschberg will descend to no lower than 3,000ft, conserving as much energy as possible. During some phases of this descent, the props may even be off. He'll be gliding.

And, yet, here is the battle. At high altitude, Solar Impulse will probably have a tail wind helping to take it east. But, the deeper the plane descends into the atmosphere at night, the more likely it is to encounter the trade winds blowing to the west. Progress during the dark hours could be very slow.

"But for us, time is not the first priority," said Clerc. "For us, energy is the first priority, and if we have to fly one more day and night, it's not a problem because we have all the equipment and supplies onboard for the pilot. It might be tough on the pilot, but technically it's not a problem."

A quick look at the map shows there are not really any practical landing possibilities east of Japan, should Solar Impulse get into difficulties.

In one or two extreme scenarios, the aircraft could make for Midway Atoll if the Hawaii goal looks totally out of the question.

And if not too far from China or Japan, Borschberg could also simply turn around. The team has, though, had to prepare for the possibility of ditching.

The pilot himself would not go down with the plane because of the risk of electrocution once in the water. Instead, Borschberg would bail out with a dingy and wait for a ship to come pick him up.

Bertrand Piccard, who has been sharing flying duties on the round-the-world journey, says there is strong faith in the ability of the vehicle to perform during what he calls its "moment of truth".

"We have a lot of redundancy, so if one electrical system fails we can reconfigure things very quickly. And we - as pilots - know how to do this by heart. We know the systems perfectly," he told BBC News.

It is Piccard who will take on the Hawaii to Phoenix leg after a short layover. This stint will not be quite as long - "only" four days and four nights of continuous flight.

Like his business partner, he too cites the inner-voyage as the motivation.

"This is the magic of adventure," he said.

"It's when you get out of your habits and routine, and you are able to discover a new inner-strength, new inner skills, and you see that you are able to function even better than before."