Decaying orbit

In orbital mechanics, decay is a process that leads to gradual decrease of the distance between two orbiting 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. The orbital decay can be caused by a multitude of mechanical, gravitational, and electromagnetic effects. For bodies in a low Earth orbit, the most significant effect is the atmospheric drag.

If left unchecked, the decay eventually results in termination of the orbit where the smaller object strikes the surface of the primary; or for objects where the primary has an atmosphere, the smaller object 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 comets), and so on.

Collisions and mergers of two stellar-mass objects usually produce cataclysmic effects; see stellar collision and gamma-ray burst.

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 km (90 mi).

Atmospheric drag at orbital altitude is caused by frequent collisions of gas molecules with the satellite. It is the major cause of orbital decay for satellites in low Earth orbit. It results in the reduction in the altitude of a satellite's orbit. For the case of Earth, atmospheric drag resulting in satellite re-entry can be described by the following sequence:

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