"For us all, the adventure starts," said Solar Impulse chief Bertrand Piccard as the team's mission control announced at sunset that they had decided to keep pilot Andre Borschberg flying in the darkness.

Sudden, strong winds at dusk disrupted the team's planning by pushing the plane to speeds up to 140 kilometres per hour (80 mph), eating into its time in the sun, he said at Payerne airbase in western Switzerland.

But, "The rest of the parameters are good," he said. "Which means that the team mission (control) can take the decision to go through the night."

The winds sped Borschberg towards the Alps and into potential turbulence, wiping out about an hour of extra sun-bathing to charge the plane's batteries at high altitude.

"The airplane had to eat its safety margin and start to descend one hour earlier than it could have done," said Piccard.

"So if there is one hour missing tomorrow morning, it could be that hour. That could be a crucial hour."

The prototype relies on the sun to power the four electric motors and charge the batteries, in theory storing enough energy to last through about seven to eight hours of darkness and land with a boost of sunlight after dawn.

It can also gently glide down in an emergency, Piccard said.

Borschberg was thrilled. "Conditions are really beautiful up here, I feel great," the former jet fighter pilot told AFP by radio as he cruised over the Jura hills in northern Switzerland during the day.

"I've been dreaming about this for seven years since we started the project, everybody on the team was looking forward to this very special day and I can tell you I'm really enjoying it," the 57-year-old said.

Confined to his seat in the narrow cockpit, the pilot snacked on high energy bars, homemade sandwiches, French rice pudding (riz au lait) and coffee, Piccard told AFP.

Borschberg had no automatic pilot and was in constant touch with the space-like mission control team to keep alert for what could go up to least 25 hours in the air.

He was flying at 6,500 metres (21,450 feet) as night fell, at a gentler speed of 23 knots (43 kmh).

The pilot's bodily functions and the plane's technical parameters were monitored second-by-second from the ground, while vibrating sleeves in his overalls were ready to wake him if the plane tilted more than few degrees.

The single-seater, shaped like a giant dragonfly, is clad with solar panels across a wingspan the size of an Airbus A340 airliner (63 metres), and can substantially change direction with a touch of the controls, pilots said.

The 12,000 solar cells and nearly half a tonne of batteries power four small electric motors and propellors -- the "power of a scooter", as the crew put it -- and weigh little more than a saloon car.

Solar Impulse took off into clear summer skies at 6:51 am (0451 GMT) on Wednesday.

"The goal is to take to the air with no fuel. The goal is to show that we can be much more independent from fossil energy than people usually think," explained Piccard.

The overnight flight is the first major hurdle for the project since it was set up seven years ago with the aim of ocean crossings, transcontinental and round the world flights by 2013 or 2014.

A first round-the-clock attempt was called off an hour before scheduled take-off last Thursday after an electronic component failed, but the aircraft has flown for up to 14 hours straight in daylight in recent weeks.

earlier related report
Green vision drives solar powered flight
Payerne, Switzerland (AFP) July 5, 2010 - When the Solar Impulse airplane took off on its pioneering round the clock flight Wednesday, the solar panels spread over a wing similar in size to that of an Airbus generated a tiny fraction of an airliner's power.

Each of the prototype's four solar-powered electric propellor motors produces barely more thrust than the flimsy petrol engine that helped the Wright Brothers make history with the world's first powered flight in 1903.

"We're rather far from commercial aviation," acknowledged Pascal Vuilliomenet of the Swiss federal institute of technology in Lausanne (EPFL), a technical university that is heavily involved in the solar powered aircraft.

Despite its limitations, the Swiss venture is attracting some of the best engineering brains around, industrial support, cutting-edge aviation technology and electronics, as well as the experience of an ex-astronaut, a former NASA test pilot and ballooning adventurers.

The 70-strong team are driven by the belief that a historic night flight fuelled by stored energy from the sun will be the first step in proving that more widespread use of solar energy is feasible today, not simply in aviation.

Team chief Bertrand Piccard believes that if an aircraft can fly day and night without fuel, no one could challenge the wider value of such technology in the household or at work.

"Solar Impulse is as much a message as an airplane," said Piccard Wednesday.

Flight control chief and former space shuttle astronaut Claude Nicollier told AFP that he does not see airliners running on solar energy in the foreseeable future.

"One of the engines of the Airbus A380 has about 25,000 kilowatts of power, here we have 6 kw (10 horsepower) on average with a huge set of solar panels on a huge wing, so it's orders of magnitude of difference," he explained.

Nicollier believes minor electronic equipment onboard airliners might become solar powered, but that the future for jumbo jet engines lies with new generation biofuels and subsequently hydrogen fuel cells.

His colleagues argue that the key to their prototype lies in highlighting and developing the technology: the solar panels, the batteries and the motors, as well as the ultra lightweight construction.

Vuilliomenet, the link man between the EPFL and Solar Impulse, said technological spin-offs from the components developed and refined with the prototype and its successor could reach the car industry.

Anil Sethi, chief executive of advanced "thin film" solar panel firm Flisom, said projects like Solar Impulse can feed into space flight or ground-borne transport.

"You'll never have flight powered by solar energy from Zurich to New York in a reasonable timeframe," added Sethi, although he did not rule out solar powered light aircraft for enthusiasts.

The single seater Solar Impulse prototype carries 12,000 solar cells across the wingspan of an Airbus A340 (63.4 metres) and weighs as much as an average family car (1,600 kg).

The 400 kg of lithium-polymer batteries have just enough capacity to store energy for an eight hour flight through pitch darkness in perfect conditions.

Piccard said the solar cells have gained in efficiency and become twice as thin in the seven years since the project was launched.

But he tacitly admits that technological limits have scaled back thoughts of a non-stop, solar-powered circumnavigation of the globe, leaving the crew to dream of a series of five day hops around the world.

"To make it non stop with a two seater we'd need a leap in technology to another level, and that's not in sight despite billions spent (worldwide) on research," said Piccard.

"The aim for industry research on batteries and solar panels now is to move from a linear growth in performance to exponential growth," he added.

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