Poynting–Robertson effect

A dust grain orbiting the Sun intercepts sunlight and transfers some of the photons' momentum to the grain. The grain then reradiates absorbed energy. Because the grain is moving relative to the incoming light, the radiation force includes a small component opposing its orbital motion. This velocity-dependent component is called Poynting–Robertson drag. Solar radiation pressure also pushes outward, away from the Sun; it is a distinct, primarily radial component of the radiation force. Both effects must be considered when modelling a grain's path.

For a grain orbiting the Sun, Poynting–Robertson drag gradually reduces orbital energy and angular momentum. The resulting long-term evolution generally lowers the orbit's semimajor axis and circularizes its eccentricity. The force is weak compared with solar gravity and radiation pressure, but can matter over long timescales. In the geometric-optics regime, its importance relative to a grain's inertia increases as particle size decreases, though the detailed behaviour also depends on grain properties and orbit. The drag is one of several influences on interplanetary dust; planetary perturbations, collisions, and other radiative forces can also shape its evolution.

Interplanetary dust is continually supplied by sources such as comets and asteroids. Radiation forces affect the subsequent motion of this material and therefore enter models of dust streams and meteor showers. Poynting–Robertson drag can move grains inward from more distant heliocentric orbits, but it does not mean that every grain follows a simple path directly into the Sun: its actual fate depends on its size, composition, initial orbit, and competing processes. The same basic radiation-related effect can also apply to small particles orbiting other luminous bodies, including particles around planets, although illumination and shadowing complicate those cases.

John Henry Poynting discussed radiation's influence on small Solar System bodies in a 1903 paper. Howard P. Robertson later developed a relativistic treatment of radiation's dynamical effects in 1937. The effect is consequently named for both physicists. Its equations are used in celestial-mechanics models, while explanations of the force's physical interpretation have been discussed in the literature; careful treatments distinguish radiation pressure from the velocity-dependent change associated with orbital motion.