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. There are two main approaches: off-board, where the laser source is external to the spacecraft, and onboard, where the laser is part of the spacecraft's propulsion system. Off-board laser propulsion, which includes laser-powered launches and laser light sails, eliminates the need for the spacecraft to carry its own energy source. Onboard laser propulsion involves using lasers in nuclear fusion or ionizing interstellar gas for propulsion.
History The basic concepts underlying a photon-propelled "sail" propulsion system were developed by Eugene Sanger and the Hungarian physicist György Marx. Propulsion concepts using laser-energized rockets were developed in the 1970s by Arthur Kantrowitz and Wolfgang Moekel, with a variant using laser ablation pioneered by Leik Myrabo. An exposition of Kantrowitz's laser propulsion ideas was published in 1988. 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. Thus, the first uses the laser for both energy and reaction mass, while the second uses the laser for energy, but carries reaction mass. Thus, the second is fundamentally limited in final spacecraft velocities by the rocket equation.
Laser-pushed lightsail A laser-pushed lightsail is a thin reflective sail similar to a solar sail, in which the sail is being pushed by a laser, rather than the sun. The advantage of lightsail propulsion is that the vehicle does not carry either the energy source or the reaction mass for propulsion, and hence the limitations of the Tsiolkovsky rocket equation to achieving high velocities are avoided. Use of a laser-pushed lightsail was proposed initially by Marx in 1966, as a method of interstellar travel that would avoid extremely high mass ratios by not carrying fuel, and analyzed in detail by physicist Robert L. Forward in 1989. Further analysis of the concept was done by Landis, Mallove and Matloff, D. G. Andrews and others. A large diameter beam is required so that only a small portion misses the sail due to diffraction, and the spacecraft must have strong pointing stability capabilities so it can tilt its sails fast enough to follow the center of the beam. These requirements increase in importance as mission complexity increases, such as when moving from interplanetary to interstellar missions, and when moving from flyby missions to one-way landing missions and then to return missions. The laser may alternatively consist of a large phased array of small devices that receive their energy directly from solar radiation. The laser-pushed sail is proposed as a method of propelling a small interstellar probe by the Breakthrough Starshot project. Another method of moving a much larger spacecraft to high velocities is by using a laser system to propel a stream of much smaller sails. Each alternative mini sail is slowed down by a laser from the home system so that they collide at ionising velocities. The ionising collisions could then be used to interact with a powerful magnetic field on the spacecraft to provide a force to power and move it. An extension of the idea is to utilize nuclear materials on the mini sails. These materials would undergo fission or fusion, greatly increasing the magnitude of the imparted force. However, this approach would require much higher collision velocities compared to non-nuclear implementations.
Photon recycling Metzgar and Landis proposed a variant on the laser-pushed sail, in which the photons reflected from the sail are re-used by re-reflecting them back to the sail by a stationary mirror; a "multi-bounce laser-based sail." This amplifies the force produced by recycling the photons, resulting in considerably higher force produced from the same laser power. There is also a multi-bounce photonic sail configuration which uses a large Fresnel lens around a laser generating system. In this configuration the laser shines light on a probe sail accelerating it outwards which is then reflected back through the Fresnel lens to be reflected off a larger more massive reflector probe going in the other direction. The laser light is reflected back and forth many times improving the force transmitted but importantly allows the large lens to remain in a more stable position as it is not greatly influenced by the laser lights momentum. An optical cavity allows greater re-use of photons, but keeping the beam in the cavity becomes much more challenging. An optical cavity can be made with two high-reflectance mirrors, forming a Fabry–Pérot optical resonance cavity in which any small movement of mirrors would destroy the resonance condition and null photonic thrust. Such optical cavities are used for gravitational wave detection as in LIGO, for their extreme sensitivity to the movement of mirror. Bae originally proposed to use photon recycling for use in a nanometer accuracy formation flight of satellites for this reason. Bae, however, discovered that in an active optical cavity formed by two high-reflectance mirrors and a laser gain medium in between, similar to the typical laser cavity, photon recycling becomes less sensitive to the movement of mirrors. Bae named the laser thruster based on the photon recycling in an active optical cavity Photonic Laser Thruster (PLT). In 2015 his team demonstrated the number of photon recycling up to 1,540 over a distance of a few meters and photonic thrusts up to 3.5 mN with the use of a 500 W laser system. In a laboratory demonstration, a Cubesat (0.75 kg in weight) was propelled with PLT.
Laser energized rocket There are several forms of laser propulsion in which the laser is used as an energy source to provide momentum to propellant that is carried on board the rocket. The use of a laser as the energy source means that the energy provided to the propellant is not limited by the chemical energy of the propellant.
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