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.<br/> <br />It's called the Solar Impulse and its battery is charged by energy collected from 12,000 solar cells built into the wings.<br/> <br />The Solar Impulse can reach an average speed of 43 miles per hour.<br/> <br />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.<br/> <br />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.
