A plane that can fly on solar power, day or night, will make its way across the United States this summer — the first time the plane has attempted a cross-continental flight.

The Solar Impulse — built as a project of the Swiss Federal Institute of Technology, the brainchild of Bertrand Piccard and André Borschberg — has the wingspan of a 747 but only weighs as much a Honda Prius. It flies thanks to four turboprop engines powered entirely by batteries and solar panels.

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Borschberg told Discovery News that the although the plane could make the whole trip from California to New York in one go, the pilot cannot. The plane travels at 40 to 50 miles per hour, so a cross-country flight would take days. And since there’s only room for a single person in the cockpit, in part to save weight, and no autopilot, the trip will have to be broke up into five legs.

Weight was a big part of the design constraint. The designers needed to find material that was light but also strong. They settled on carbon fiber, which is lighter than any metals.

During its flight, Solar Impulse will maintain a specific altitude about 29,000 feet, where the engines operate with maximum efficiency. All things normal, the Solar Impulse could, Borschberg said, could travel for 20 hours. The longest commercial flight is somewhat less than that – 18.5 hours for the trip from Newark to Singapore (that route is scheduled to be discontinued). And eventually longer flights will be tested. The second iteration of the Solar Impulse will have the ability to house two pilots and support them for days at a time.

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It’s a very unusual aircraft to handle, Piccard said. “You have a tendency to overcompensate, because it reacts so slowly.” The plane banks – that is, tilts to the side — at a maximum of five degrees from the horizontal. That required a whole new instrument built just for the Solar Impulse, a bank indicator that was calibrated to show single degrees. Most aircraft instruments are much more crude, since ordinary planes typically bank more steeply in turns.

The solar panels are conventional silicon, with an efficiency of about 25 percent. While there are more efficient solar panels such as those used in the satellite industry, those designs are often too heavy, Borschberg said, as they tend to be encased in glass. And although the power is stored in batteries, the engines can run directly from the energy collected by the solar panels. In fact, the plane could be flown on an empty battery.

More than showcasing technology, Piccard and Borschberg said they hope to inspire people. “You fly a robot like this and nobody cares,” Piccard said. “When we fly it with a pilot people ask me to come to schools and speak. I give much better speeches than a robot.”

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