It's a plane powered by the sun --- and it took off on Friday from San Francisco for the first leg of an attempt to fly across the U.S. without any fuel.

It's called the Solar Impulse and its battery is charged by energy collected from 12,000 solar cells built into the wings.

The Solar Impulse can reach an average speed of 43 miles per hour.

Built with a single-seat cockpit, the plane is expected to reach Phoenix within 15 to 20 hours. It will also make stops in Dallas, St. Louis and D.C. --- pausing in each location to wait for nice weather.

With the wingspan of a jumbo jet and the weight of a small car, its developers are hoping this test model will whet the public's appetite for more clean energy planes.