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RENEE MONTAGNE, HOST:

It's MORNING EDITION from NPR News. Good morning. I'm Renee Montagne.

DAVID GREENE, HOST:

And I'm David Greene. So if I were tell you there was a plane up in the night sky, flying on solar energy, would you believe me? Well, you probably should. Next month an odd looking plane will take off from an airfield in Mountain View, California and head east. It's called the Solar Impulse and it's the world's first solar powered plane capable of flying non-stop, all day, and yes, all night. As NPR's Steve Henn reports, its creators plan to fly the plane across the country this spring, by 2015 they want to fly a similar plane around the world.

STEVE HENN, BYLINE: I had seen pictures of this plane, the Solar Impulse, before I drove down to Moffett field a couple weeks ago to check it out, but really nothing prepared me for seeing it in person. Its wing span is larger than a 747 - but the entire plane weighs less than a car.

ANDRE BORSCHBERG: Yeah - that's right. Yes, that was that was the challenge.

HENN: Andre Borschberg is one of the creators of the Solar Impulse and one of its pilots. He says the wings are so large, in part, to generate lift.

BORSCHBERG: Then of course, you have big surface to integrate solar cells.

HENN: The idea to try and build a solar powered plane that could fly non-stop through the night, began with Andre Borschberg's partner, Bertrand Piccard. Piccard became internationally famous in 1999 after circumnavigating the world in a balloon. But on the trip, he very nearly ran out of propane.

BORSCHBERG: And he really asked himself if it could not be possible to get rid of this dependence. So that is how the idea started.

HENN: Design work on a plane that could fly without fuel began 10 years ago and today, standing under buzzing florescent lights inside a huge hanger, the Solar Impulse looks like some kind of giant space age egret on steroids. Its long, delicate wings are covered in photovoltaic cells. They tilt up at the end.

The plane has four electric engines and a narrow little cockpit that seats just one person. Everything about this design was dictated by the size of those wings and the energy that the sun striking them can produce.

BORSCHBERG: And when you know the surface you can easily calculate how much energy you can collect during the day. And then you take this energy and you have to use it over 24 hours because we are flying day and night. So if you know physics and aerodynamics, you know how much weight you can carry. And the weight which was allowed was not more than the weight of a car. So there was the starting point.

HENN: To keep the weight down they used a carbon fiber frame, as few batteries as possible, and stripped the cockpit bare. In this plane, the toilet is built into the pilot's seat.

BORSCHBERG: There is no heating. There is no pressurization. So we need an oxygen mask. So, yes, it's quite - it's more difficult and maybe less comfortable than flying in an airliner.

HENN: But the designers succeeded in creating plane capable of flying continuously - without using any fuel at all.

(SOUNDBITE OF AIRPLANE ENGINE)

BORSCHBERG: The contact with the external world is much more intense. So we can hear noise coming from the outside.

(SOUNDBITE OF RADIO)

BORSCHBERG: Thirty-thousand feet... (unintelligible) on the window.

The relationship with the environment - I mean with the atmosphere, with the winds, with the clouds, is much more intense with a normal airplane.

HENN: Two years ago, Borschberg kept this plane aloft over the Swiss Alps for 26 straight hours straight. Last summer he flew it from Spain to North Africa. The plane can only fly about 40 or 50 miles an hour. It so slow that sometimes when it turns into the wind it appears to float in place on a radar screen. Occasionally it'll even slide backward.

On long flights during the day the pilots will climb to almost 30,000 feet and then slowly descend during the night. Using gravity to help fly through the night is just one more way to conserve energy. But this plane is tricky to fly.

BORSCHBERG: We had to learn to do things very slowly, to wait to see the reaction, to be patient.

HENN: And careful - because the wings are so long and the plane is so slow, even a slight bank can cause the tip of the inside wing to loose lift.

BORSCHBERG: When we make turns with this airplane we need just a few degrees.

HENN: If you turn it too sharply, the Solar Impulse could tumble out of the sky. So designers created a unique physical warning system.

BORSCHBERG: Which vibrates on the on the arm of the pilot...

HENN: ...when a turn is to steep. When Piccard and Borschberg attempt to fly around the world, the pilot who flies over the Pacific will need to remain at the controls for four or five days straight. Borschberg recognizes that solar may never power commercial flights, but he said this flight was meant to challenge people's assumptions about what this technology can do. Steve Henn, NPR News, Silicon Valley.

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