orbital decay

Orbital decay is a gradual decrease in the radius of an orbit, bringing the orbiting object closer to its host. Various mechanisms produce it:

• Artificial satellites, especially those in low Earth orbits are significantly effected by atmospheric drag, a reason for stationkeeping. This is likely the most common usage of the term.
• The inward radial drift presumed to happen during planet formation is somewhat similar and fits the definition of orbital decay. Orbital decay can result from electromagnetic radiation (EMR) (e.g., from the host body) such as in the Yarkovsky effect and the Poynting-Robertson effect.
• Gravitational waves (GWs) cause the orbital decay that produces the compact object mergers that resulted in GW detections, and causes the detected orbital decay of the Hulse-Taylor Binary.
• Tidal forces can cause orbital decay (or the inverse, depending upon circumstance), which is presumed to happen to some moons.

The term inspiral is used for the path of bodies in the case of a decided orbital decay, particularly used for their last few orbits before colliding, each orbit smaller than the previous.