Solar sail

Solar sails (also called light sails, especially when they use light sources other than the Sun) are a proposed form of spacecraft propulsion. The spacecraft deploys a large, lightweight sail which reflects light from the Sun or some other source. The "radiation pressure" on the sail provides thrust by reflecting photons. Tilting the sail at an angle from the Sun can produce a thrust component perpendicular to the angle between the Sun and the spacecraft. Steering is usually with auxiliary vanes.

A number of demonstration projects have proven this method's feasibility. On a smaller scale, some unmanned spacecraft have been constructed with reflective panels that can be used to steer the spacecraft, to conserve fuel for maneuvering and attitude control. Solar collectors or sun shades can also serve as crude solar sails, and can help a spacecraft correct its attitude and orbit without using fuel.

It has also been proposed to use lasers to push solar sails, providing beam-powered propulsion. Given a sufficiently powerful laser and a large enough mirror to keep the laser focused on the sail for long enough, a solar sail could be accelerated to a significant fraction of the speed of light. To do so, however, would require the engineering of massive, precisely-shaped optical mirrors (wider than the Earth for interstellar transport), incredibly powerful lasers, and more power for the lasers than humanity currently generates. A potentially easier approach would be to use a maser to drive a "solar sail" composed of a mesh of wires with the same spacing as the wavelength of the microwaves, since the manipulation of microwave radiation is somewhat easier than the manipulation of visible light. The hypothetical "Starwisp" interstellar probe design would use a microwave laser to drive it. Microwave lasers spread out more rapidly than optical lasers thanks to their longer wavelength, however, and so would not have as long an effective range.

No solar sails have been successfully deployed as yet, but research in the area is continuing. The Russian space program has attempted to deploy at least one prototype test unit in Earth orbit.

See also spacecraft propulsion.