Laser propulsion is a form of beam-powered propulsion where the energy source is a remote (usually ground-based) laser system and separate from the reaction mass. This form of propulsion differs from a conventional chemical rocket where both energy and reaction mass come from the solid or liquid propellants carried on board the vehicle.
The basic concepts underlying laser propulsion were first developed by Eugene Sanger and the Hungarian physicist Georgii Marx, with practical schemes being developed by Arthur Kantrowitz and Wolfgang Moekel in the 1970s.
Laser propulsion systems may transfer momentum to a spacecraft in two different ways. The first way uses photon radiation pressure to drive momentum transfer and is the principle behind solar sails and laser sails. The second method uses the laser to help expel mass from the spacecraft as in a conventional rocket. This is the more frequently proposed method, but is fundamentally limited in final spacecraft velocities by the rocket equation.
The forms described below are all of the second type, and could be described as thermal rockets. See Beam-powered propulsion for examples of the first type.