Frozen orbit

A frozen orbit is an orbit whose eccentricity and orientation remain nearly constant over time because secular perturbations are balanced by suitable orbital elements.

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Secular perturbation theory: A method for describing the cumulative, long-term effects of perturbations on orbital elements. Frozen orbits arise when averaged perturbations cancel for selected orbital elements.
Lunar orbit: The path of a spacecraft or natural body around the Moon. Some lunar mission designs seek frozen orbits to reduce orbit-maintenance demands.
Orbital eccentricity: A dimensionless measure of how much an orbit differs from a circle. A frozen orbit keeps this measure nearly constant over long timescales.
Sun-synchronous orbit: A near-polar orbit whose orbital plane precesses at the rate needed to maintain a nearly fixed angle to the Sun. It deliberately preserves local solar time, whereas a frozen orbit targets stable shape and orientation.
J2 perturbation: The leading correction to a planet’s gravitational field caused by its equatorial bulge. Earth’s J2 effect drives the long-term changes that frozen-orbit elements can balance.
Mars orbiter: A spacecraft placed in orbit around Mars to observe or relay communications. Frozen-orbit geometry can support persistent coverage by Mars orbiters.
Orbital inclination: The angle between an orbit’s plane and a chosen reference plane. Inclination helps determine how gravitational perturbations affect orbital orientation.
Geostationary orbit: A circular equatorial orbit in which a satellite appears stationary above one point on Earth. Its defining stability is a fixed ground position, not frozen eccentricity and orientation.
Eccentricity vector: A vector pointing toward periapsis whose magnitude equals an orbit’s eccentricity. Freezing eccentricity and orientation means limiting the long-term change of this vector.
Lunar Reconnaissance Orbiter: A NASA spacecraft that has mapped the Moon from orbit since 2009. Its near-polar mapping orbit illustrates long-lived lunar orbital operations.