Quasisatellites

Quasisatellites are small bodies in near-Earth or planetary orbits that share approximately the same orbital period around the Sun as a planet with which they are associated. In a heliocentric orbit, a quasi-satellite is in a 1:1 mean-motion resonance with the planet, meaning it completes roughly one orbit around the Sun for every orbit of the planet. They are not gravitationally bound to the planet; instead, they remain in the planet's vicinity due to the resonance.

In a reference frame rotating with the planet, a quasi-satellite appears to gyrate around the planet and can trace a loop that keeps it near the planet for extended times. The orbit typically has a semi-major axis close to that of the planet, with moderate eccentricity; inclinations vary widely. Quasi-satellites are one type of co-orbital motion, alongside Trojan (L4/L5) and horseshoe configurations. A quasi-satellite can transition, under perturbations from the Sun and other planets, into a horseshoe orbit or dissolve into a distant path, so the quasi-satellite state is often temporary, lasting years to centuries.

Earth has identified quasi-satellites among near-Earth asteroids. They are of interest for dynamical studies and potential mission targets because of their proximity and the unique geometry of the relative motion. The study of quasi-satellites informs understanding of planetary resonance and the distribution of small bodies in near-planet space.