Well, I didn't realize that the first solar space craft crashed into the ocean a few years ago. I just read that the second attempt is scheduled for 2010. The premise is simple, the sails are dark on one side and reflective on the other. As the craft is free in space without significant forces, the light bouncing from the reflective side and being absorbed on the dark side will add up over time to significant propulsion and speed.

If you have ever seen the toy shown above, this is the same process that will drive the craft. You can click the image for a better view. The toy is called a radiometer.

For more information on the Planetary Society, follow the related link.