A solar-powered plane is getting ready to hit the skies once again - this time, at night.

It will be the first ever manned night flight on a plane propelled exclusively by solar energy.

Solar Impulse will lift off from an airfield in Switzerland, on a sunny day sometime at the end of June.

It will then fly around, charging the solar cells on the plane's wings, in a bid to store enough energy for the electric motors to last until dawn.

The aim of the project's founders, Andre Borschberg and round-the-world balloonist Bertrand Piccard, is to show that a solar-powered craft is able to fly day and night - and eventually long-distance flights - without any fuel.

"These night flights in an aircraft propelled uniquely by solar energy are intended to demonstrate the potential of the renewable energies and the technologies that will gradually enable us to diminish our dependence on oil", said Dr Piccard.

The HB-SIA, as the plane is called, has a wingspan of 61m - comparable to a super-jumbo jet. But at just 1.5t, it is only the weight of a saloon car.

It first left the ground in December 2009 and has since performed a series of short daytime flight tests, dubbed "flea-hops", venturing no higher than 60cm (2ft) in altitude and 300m in distance.

The first full day flight was completed on 7 April.

Mr Borschberg will pilot the plane on its first night flight. If it proves a success, the Solar Impulse team will attempt to go even further.

The ultimate aim is to push the frontier of renewable solar energy. In two years' time, the plane will set off on its first manned transatlantic solar flight, followed in 2013 by an even more daring circumnavigation of the Earth.

No auto-pilot

With just a few weeks to go before the first 24-hour journey, Mr Borschberg told BBC News he was more excited than ever.

A Swiss Air Force pilot by training, he has flown many aircraft - including the Solar Impulse on a number of "flea-hops".

But this time, he said, will be like nothing he has ever experienced. Staying awake should not be a problem - many of us stay awake through the night at least once in our lives, he said.

"But the flight will require a lot of attention and concentration - the plane doesn't have an auto-pilot, it has to be flown for 24 hours straight."

He explained that he has been doing yoga to help stimulate the muscles and blood flow, something that will be necessary during the lengthy time in the cockpit.

The pilot said that the ultra-light aircraft is extremely sensitive to its environment, especially in the case of turbulence.

"But I don't think it's going to be risky because we have tested and prepared everything [thoroughly]," he added.

Mr Borscherg explained that it is possible to estimate how much power the plane has left and adjust to the situation accordingly - either by stopping the flight early and heading back to base or "taking a chance until the next sunrise and hoping that the energy will return to the solar generators early enough".

'Power of a scooter'

The pilot said that for him, the most exciting part of the venture is "being on the plane during the day and seeing the amount of energy increasing instead of decreasing as on a normal aircraft. That's what you can enjoy with this solar generator".

If the 24-hour flight is a success, it will be the first important step towards using renewable solar energy in a continuous manner.

"It is something very new. If we can do a day-and-night cycle once, we will be able to do it again and again. The next step would be to start flying long duration flights - and the first big milestone would be to fly over the Atlantic," said Mr Borscherg.

He explained that Solar Impulse will try to follow in the footsteps of the US aviator Charles Lindbergh, who in 1927 completed a solo non-stop flight from Roosevelt Field in New York's Long Island to Le Bourget Field in Paris, France, in the single-seat, single-engine monoplane Spirit of St Louis.

And in 2013, the solar craft will attempt an even more challenging endeavour - the first-ever manned solar-powered flight around the globe.

"We'd like to demonstrate that we could really reduce our energy consumption. We're flying with the power of a scooter - and if we can do it in the air, we can certainly do it on the ground as well," said Mr Borscherg.

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