Explanation:

In orbital mechanics, decay is a process that leads to gradual decrease of the distance between two orbiting bodies at their closest approach 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, it burns, explodes, or otherwise breaks up in its 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. Causes of Orbital Decay includes Atmospheric drag, Tidal effects, Mass concentration, light and thermal radiation and gravitational radiation