The Solar Impulse is a plane powered completely by the sun. Its wings, which are the size of a Boeing 747's, are covered in solar panels which generate and store electricity in batteries so the plane can fly 24/7 without ever needing to stop to refuel. The plane is currently on a journey across the United States, with the first leg being from Mountain View, CA to Phoenix, AZ.
The first leg of the journey will take about 20 hours to complete due to the plane being rather slow moving. In fact, the plane only moves at around 40 MPH. You can track the Solar Impulse on its journey via the live stream and site set up by the creators. The stream shows a Google Earth view of the terrain below the plane, while the site has current speed, direction, temperature, engine settings, and battery status.