Editing Orbital decay (section)

{{Short description|Process that leads to gradual decrease of the distance between two orbiting bodies}}
{{Redirect-distinguish|Decaying Orbit|Decaying Orbit (film)}}{{Redirect-distinguish|Inspiral|Inspiral (horse)}}
[[File:Altitude of Tiangong-1.svg|thumb|upright=1.3|Altitude of [[Tiangong-1]] during its final year of uncontrolled reentry.<ref name="cmse">{{cite web|title=Tiangong-1 Orbital Status|url=http://en.cmse.gov.cn/col/col1763/index.html|website=Official Website of China Manned Space|publisher=China Manned Space Engineering Office|access-date=1 April 2018|date=1 April 2018}}</ref>]]
{{Astrodynamics}}

'''Orbital decay''' is a gradual decrease of the [[distance]] between two [[orbit]]ing bodies at their closest approach (the [[periapsis]]) over many orbital periods. These orbiting bodies can be a [[planet]] and its [[satellite]], a [[star]] and any object orbiting it, or components of any [[binary system (astronomy)|binary system]]. If left unchecked, the decay eventually results in termination of the orbit when the smaller object [[collision|strikes]] the surface of the primary; or for objects where the primary has an atmosphere, the smaller object [[Atmospheric entry|burns, explodes, or otherwise breaks up]] in the larger object's [[atmosphere]]; or for objects where the primary is a star, ends with incineration by the star's radiation (such as for [[comet]]s).  [[stellar collision|Collisions of stellar-mass objects]] are usually accompanied by effects such as [[gamma-ray burst]]s and detectable [[Gravitational wave|gravitational waves]].

Orbital decay is caused by one or more mechanisms which absorb energy from the orbital motion, such as [[drag (physics)|fluid friction]], [[mass concentration (astronomy)|gravitational anomalies]], or [[electromagnetic interaction|electromagnetic]] effects. For bodies in [[low Earth orbit]], the most significant effect is [[atmospheric drag]].

Due to atmospheric drag, the lowest altitude above the [[Earth]] at which an object in a circular orbit can complete at least one full revolution without propulsion is approximately 150&nbsp;km (93&nbsp;mi) while the lowest [[perigee]] of an elliptical revolution is approximately 90&nbsp;km (56&nbsp;mi).