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In terms of mechanics of orbit, orbital decay is defined as a gradual decrease of the distance between two orbiting bodies at their closest approach through many orbital periods. The orbiting bodies can be a planet and its satellite, a star and any object orbiting it, or components of any binary system, etc. Orbital decay happens for a reason i.e. due to frictional mechanism which transfers the energy from the orbital motion.
If orbital decay is not checked, the decay could eventually results in termination of the orbit whereby the smaller object strikes the surface of the primary or for objects where the primary has an atmosphere, the smaller object usually burns, explode; and so on.
Causes of orbital decay are as follows:
Firstly, atmospheric friction at orbital altitude is caused by frequent collisions of gas molecules with the satellite which results in the reduction in the altitude of a satellite's orbit. It is one of the main cause of orbital decay for satellites in low Earth orbit (LEO).
Secondly, an orbital decay due to the tidal effect whereby the orbiting body is large enough to raise a significant tidal bulge on the body it is orbiting and is either in a retrograde orbit or is below the synchronous orbit. This eventually result in loss of momentum from the orbiting body and transfers it to the primary's body rotation, thus lowering the orbit altitude.
Thirdly, light and thermal radiation also causes orbital decay. Small objects in the Solar System also experience an orbital decay due to the forces applied by pressure of asymmetric radiation. Ideally, energy absorbed is equal to blackbody energy emitted at any given point, resulting in no net force. But the Yarkovsky effect states that the absorption and radiation of heat are not instantaneous, objects which are not terminally locked absorb sunlight energy on surfaces exposed to the Sun, but those surfaces do not re-emit much of that energy until after the object has rotated, so that the emission is parallel to the object's orbit which results in a very small acceleration parallel to the orbital path. However this gains significance for small objects over millions of years. The Poynting-Robertson effect is a force opposing the object's velocity caused by asymmetric incidence of light, i.e., aberration of light. For an object with prograde rotation, the effects of these two phenomenon will apply opposing, but generally unequal, forces causing orbital decay.
Fourthly, Gravitational radiation is another mechanism which can cause orbital decay. It is negligible for orbits of planets and planetary satellites (when their orbital motion is considered on time scales of centuries, decades, and less), but is apparently noticeable for systems of compact objects, as seen in observations of neutron star orbits. All orbiting bodies radiate some gravitational energy, hence no orbit is infinitely stable.
Fifthly, Electromagnetic drag also causes orbital decay. Satellites which are using an electrodynamic tether and moving through the Earth's magnetic field, creates drag force that could eventually deorbit the satellite.
Orbital decay is a phenomenon which makes it clear that all orbiting objects will eventually loose their orbital path. So nothing will remain in orbit forever. Our artificial satellites use booster to remain in orbit. But they too remain in orbit as long as there is fuel for the booster.
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