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Solar sails use photon pressure from the sun to propel a spacecraft, with designs using mylar, polyimide films, aluminum, and carbon fiber. MIT student Eric Drexler’s design offers 10 times higher thrust per square meter than most proposed designs. While no spacecraft has yet been built using solar sails as their primary propulsion, Japan’s aerospace agency and private entities have deployed prototypes. Solar sails would be ideal for use in the solar system, and planet-sized mirrors could be used for missions outside the solar system. Varnished solar sails have been proposed to improve speed.
The solar sail is a proposed form of space propulsion that uses photon pressure from the sun to provide thrust. Photons reflect from a mirror attached to a payload, transmitting momentum and allowing the sail and accompanying payload to accelerate. Because the photon pressure is quite small compared to, say, the thrust produced by a chemical rocket, vital solar sails must be very large and light. A design studied by NASA would be about half a kilometer (31 miles) wide. Proposed building materials include mylar and polyimide films, aluminum, and a new type of carbon fiber.
The designs that 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 nanotechnology field.) Employing aluminum film about 30-100 nanometers thick, Drexler’s designs would offer about 10 times higher thrust per square meter than most proposed designs, that use plastic. The key to making Drexler’s design happen would be manufacturing facilities in the space. Films for solar sails are too delicate to fold, flip and unfold.
No spacecraft have yet been built that use solar sails as their primary method of propulsion, although photon pressure has been used to make minor changes to the paths of space probes. However, in 2004, Japan’s aerospace agency, ISAS, successfully deployed 2 prototype solar sails into low Earth orbit. In 2005, a joint private project between the Planetary Society, Cosmos Studios and the Russian Academy of Sciences launched Cosmos 1, the world’s first solar sail spacecraft. Due to the unconventional nature of solar sail propulsion, large government agencies have been reluctant to invest in it, leaving field development 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 the solar system, planet-sized mirrors should be used to focus light energy right on the solar sails. While a huge engineering task, this may one day prove to be the simplest way to accelerate a spacecraft to a substantial fraction of the speed of light. Well-designed solar sails could travel in different directions from the sun, by tilting the sail at an appropriate angle. To improve the speed of solar sails, the varnished solar sail has been proposed, a hypothetical sail that would be coated in chemicals designed to vaporize during the spacecraft’s travel, thus providing additional thrust.
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