A proposed means of interplanetary or even interstellar propulsion. A large, low-mass surface is deployed in space. In addition to the flood of photons we know and bask in, the sun produces a 'solar wind', which is a stream of energetic particles flowing outwards from the sun. A large enough surface would absorb enough energy from the Sun's EM emissions and this flow to produce acceleration. Since there is no 'air resistance' in space, such a'sail' would just keep on going forever (theoretically) unless damaged, although the farther it gets from its energy source the slower the rate of acceleration. Some concepts for deep-space travel include the use of large lasers or masers on Earth or on a fixed base in the Solar System to provide those sails with an extra 'boost.'