Solar sails
There have already been several space experiments with solar sails although these have been mostly concerned with how to unfurl a sail in microgravity. However the solar sail principal has an important effect on some satellites, especially in geostationary orbit. The constant radiation pressure on their solar cell arrays tends to push them out of their intended orbits and so shorten their active life. Some satellites however have put the radiation pressure to use, using it to help turn the satellite without the need for propellant. Practical propulsion by solar sails will however have to wait for the future.
