The solar sail is a proposed form of space propulsion that utilizes pressure from the sun's photons to provide thrust. Photons reflect from a mirror attached to a payload, conveying momentum and allowing the sail and its accompanying payload to accelerate. Because photonic pressure is quite small compared to, for example, the thrust produced by a chemical rocket, viable solar sails must be very large and light. One design being studied by NASA would be approximately half a kilometer (.31 miles) wide. Proposed building materials include mylar and polyimide films, aluminum, and a new kind of carbon fiber.
Designs which produce the highest thrust-to-mass ratio were developed by MIT student Eric Drexler in his master's thesis. (Drexler is primarily known as the father of the field of nanotechnology.) Employing aluminum films about 30-100 nanometers thick, Drexler's designs would offer thrust-per-square-meter values about 10 times higher than most proposed designs, which employ plastics. The key to realizing Drexler's design would be space-based manufacturing facilities. The films for solar sails are too delicate to be folded, launched, and deployed.
No spacecraft has yet been built that employs solar sails as a primary method of propulsion, though photonic pressure has been used to make small changes to the courses of space probes. However, in 2004, Japan's aerospace agency, ISAS, successfully deployed 2 solar sail prototypes in low earth orbit. In 2005, a joint private project between Planetary Society, Cosmos Studios, and the Russian Academy of Science, launched Cosmos 1, the world's first solar sail spaceship. Because of the nonconventional nature of solar sail propulsion, big government agencies have been reluctant to invest in it, leaving development in the field to private entities.
Solar sails would be ideal for use in the solar system, where the sun's rays are most intense. For missions outside of the solar system, planet-sized mirrors would need to be used to focus light energy precisely onto the solar sails. Though a massive engineering task, this may one day prove to be the easiest way to accelerate a spacecraft to a substantial fraction of the speed of light. Well-designed solar sails could travel in directions aside from directly away from the sun, by tilting the sail at an appropriate angle. To improve the speed of solar sails, the painted solar sail has been proposed, a hypothetical sail which would be coated in chemicals designed to vaporize throughout the spacecraft's journey, thereby providing additional thrust.