A solar sail is an interplanetary drive.

It consists of a large area (typically hundreds of square meters) of very lightweight material that is held at an adjustable angle to the suns rays, and by reflecting the light, obtains thrust.

The thrust obtained this way is incredibly low, and solar sails would have to spend many months or years travelling, but the main advantage is that they use no propellent at all.

They are only suitable for use when already in orbit, very much away from any atmosphere.