A plane powered entirely by the sun that flies at night. It sounds improbable, but Solar Impulse has just proven that it can be done. Following on from its maiden flight in April, the solar plane flew through the night in the skies above Switzerland this week, passing another important milestone on a journey that has already lasted seven years and is set to culminate in an attempt to fly around the world non-stop in 2012.

Solar Impulse HB-SIA has a wingspan of 63 meters (208 feet – think Airbus A340) yet at 1,600 kg, is just the weight of a car. It is covered in 11,628 solar cells, carries 400kg of batteries and is propelled by 4 x 10 HP electric engines.

The historic flight began in the early morning of July 7 when the aircraft took off from Payerne airbase. Piloted by CEO and co-founder of the Solar Impulse project André Borscherg, the plane spent the day gaining altitude and charging its batteries in preparation for the journey through the night. By 4:40 pm the batteries were fully charged and the the prototype had exceeded its specified maximum altitude of 8,500 meters, reaching a height of 8,700 meters.

“During the whole of the flight, I just sat there and watched the battery charge level rise and rise! Sitting in a plane producing more energy than it consumes is a fantastic feeling”, said André Borschberg , CEO and co-founder of the Solar Impulse project from the cockpit.

When night fell the aircraft began a a slow descent to an altitude of around 1,500 meters and continued on through the night using the energy stored in its batteries in what is both the longest and highest flight ever made by a solar plane. A fantastic achievement!

“This is a highly symbolic moment: flying by night using solely solar power is a stunning manifestation of the potential that clean technologies offer today to reduce the dependency of our society on fossil fuels!" added Bertrand Piccard, Initiator and President of Solar Impulse.

A second plane will be built to tackle the overall goal of completing a round-the-world solar flight slated for 2012/13.

Further details on the night flight plus Solar Impulse TV coverage can be found at the project site.