Orbital decay

Orbital decay is the gradual decrease in an orbit’s size or altitude as energy is lost or perturbations alter the orbit. It can end in impact, atmospheric entry, or a tighter orbit.

Atmospheric drag: A resistive force exerted by an atmosphere on an object moving through it. It removes energy from low-orbiting satellites, with stronger effects as atmospheric density rises.
Satellite orbit: The trajectory of an object bound gravitationally to a planet or other central body. Decay is a change in this broader class of trajectories, especially in low Earth orbit.
Orbital migration: A change in an orbit’s size or shape caused by interactions with other bodies or surrounding material. Migration includes outward as well as inward movement; orbital decay specifically emphasizes shrinkage or loss.
Sputnik 1: The first artificial Earth satellite, launched by the Soviet Union in 1957. Its orbit gradually lowered under atmospheric drag before it reentered Earth's atmosphere.
Space weather: Changes in the Sun and the solar wind that affect conditions in interplanetary space and near Earth. Solar activity expands Earth's upper atmosphere, changing drag and making satellite decay harder to predict.
Tidal acceleration: The transfer of angular momentum between a rotating body and an orbiting companion through tidal forces. Depending on spin and orbit, tidal exchange can move an orbit inward or outward.
Satellite graveyard orbit: A disposal orbit above the operational region used to reduce interference from retired geostationary satellites. Operators use it instead of allowing valuable geostationary spacecraft to drift into operational orbits.
Orbital stability: The persistence of an orbit under small disturbances over a specified timescale. A stable orbit resists significant change, unlike one undergoing sustained decay.
Phobos: The larger and closer of Mars’s two natural satellites. Tidal interactions are driving Phobos toward Mars, where it is expected to break up or collide.
Thermosphere: The high atmospheric layer above the mesosphere, strongly affected by solar radiation. Its variable density is a leading uncertainty in forecasts of low-Earth-orbit lifetimes.

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