"Scientists working with a synthetic material 100-times thinner than a piece of paper are testing their theory that the sun can power interplanetary spacecraft. They believe that streams of solar energy particles called photons can push a giant, reflecting sail through space the way wind pushes sailboats across water." See space.com for more details.
Seriously, this thing is supposed to behave like an interplanetary schooner. From my understanding, it really only goes downwind, accelerating as it goes. It changes course by reshaping the sails, then lets the sun's gravity speed it up or slow it down. Very cool.