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...