A beam of light carries both energy and momentum. The momentum of light results in a slight pressure on a surface exposed to sunlight that is known as photon pressure. When light reflects off a mirror, it pushes the mirror slightly. A spacecraft that uses this effect for propulsion is called a lightsail. One that specifically uses light from the Sun to push the sail is called a solar sail spacecraft.
Photon pressure is very weak. At the distance of Earth from the Sun, the pressure produced by sunlight on a mirror with an area of 1 square kilometer (247 acres, or about a third of a square mile) is slightly under 10 Newtons. This pressure would cause an acceleration of about a tenth of a centimeter per second per second on a spacecraft with a mass of 10,000 kilograms (roughly 10 tons). This is not a very high rate of acceleration, but because the mirror does not use up any fuel, the acceleration can be continuous, and speed will build up slowly. In an hour (3,600 seconds) the speed will build up to 3 meters per second (about 10 feet/second); in a day (86,400 seconds) the speed will build up to almost 80 meters per second (260 feet/ second); and in a year the speed will build up to 28 kilometers per second—over 96,000 kilometers (60,000 miles) per hour.
The characteristics of a solar sail spacecraft are extremely light weight, a very large sail area, and low but constant acceleration. Designs for a solar sail spacecraft use a sail that is made out of thin plastic (often Mylar or Kapton), with a thin coating of aluminum to make it reflective. The total sail thickness might be as little as 5 micrometers (1/4000th of an inch). A square meter of this type of sail will weigh only 7 grams (a quarter of an ounce). To keep the thin sail spread, a solar sail spacecraft will use lightweight spars, or else the sail will rotate so that centrifugal force keeps it extended.
The light pressure force on a sail, F, can be calculated from the Einstein relation:
F= 2P/c The force produced is equal to two times the power of light reflected, divided by the speed of light. (The factor of two assumes a perfectly reflecting mirror and is derived from the fact that the reflected light is reversed in direction, thus giving the sail a momentum of twice the photon momentum.)
The force of a solar sail need not be directly outward from the Sun. If the sail is tilted, a sideways force can be produced to increase or decrease the orbital velocity . If the orbital velocity is increased, the orbit moves outward from the Sun; if the velocity is decreased, the orbit moves inward toward the Sun.
Lightsails have been proposed as a propulsion system for missions to other stars because the fact that a lightsail does not need a fuel tank means that it can continue to accelerate for the extremely long period required to achieve a significant fraction of the speed of light. Since a mission to the stars would move through interstellar space far from the Sun, this type of lightsail-propelled starship would require a large laser to beam the light to push the sail. To make the lightest possible sail (and thus create the highest level of acceleration), proposed laser-pushed lightsails would be designed without the plastic sheet and would have only the thin reflective layer of the sail.
The pressure produced by light from the Sun should not be confused with the solar wind. The solar wind consists of a stream of charged particles (mostly protons ) emitted by the Sun. The solar wind also has a pressure, although because the density of the solar wind is very low, the pressure is also low. Solar-wind pressure is about one-tenth as strong as light pressure. The use of magnetic fields to sail on this solar wind pressure has been proposed. This is called "magnetic sail" propulsion or "minimagnetospheric plasma propulsion."
see also Power, Methods of Generating (volume 4); Solar Power Systems (volume 4).
Geoffrey A. Landis
Clarke, Arthur C. "The Wind from the Sun." In The Collected Stories of Arthur C. Clarke. New York: Tor, 2001.
Friedman, Louis. Starsailing. New York: John Wiley & Sons, 1988.
Mallove, Eugene, and Greg Matloff. The Starflight Handbook. New York: John Wiley & Sons, 1989.