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, burns, explodes, or otherwise breaks up in its atmosphere, 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.
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:
- lower altitude → denser atmosphere → increased drag → increased heat → usually burns on re-entry
Orbital decay thus involves a positive feedback effect, where the more the orbit decays, the lower its altitude drops, and the lower the altitude, the faster the decay. Decay is also particularly sensitive to external factors of the space environment such as solar activity, which are not very predictable. During solar maxima the Earth's atmosphere causes significant drag up to a hundred kilometers higher than during solar minima.
Atmospheric drag exerts a significant effect at the altitudes of space stations, space shuttles and other manned Earth-orbit spacecraft, and satellites with relatively high "low earth orbits" such as the Hubble Space Telescope. Space stations typically require a regular altitude boost to counteract orbital decay (see also orbital station-keeping). Uncontrolled orbital decay brought down the Skylab space station, and (relatively) controlled orbital decay was used to de-orbit the Mir space station. Orbital boosts for the International Space Station (ISS) are regularly needed, and are one limiting factor for the length of time the ISS can go between visits from transit spacecraft.
Regular orbital boosts are also needed by the Hubble Space Telescope, though on a longer time scale, due to its much higher altitude. However, orbital decay is also a limiting factor to the length of time the Hubble can go without a maintenance rendezvous, the most recent performed successfully by STS-125, with space shuttle Atlantis launching May 11, 2009.
Uneven mass distributions (known as mascons) of the primary body will perturb orbits over time, and extreme distributions can cause orbits to be highly unstable. This effect has been discovered on the Moon, which has no atmosphere, but nonetheless has only four "frozen orbit" inclination zones where a lunar satellite can stay in a low orbit indefinitely. Lunar subsatellites were released on the last three Apollo manned lunar landing missions in 1971 and 1972; the subsatellite PFS-2 released from Apollo 16 was expected to stay in orbit for one and a half years, but lasted only 35 days before crashing into the lunar surface. In 2001, the mascons were mapped and the frozen orbits were discovered.
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An orbit can also decay by tidal effects when the orbiting body is large enough to raise a significant tidal bulge[clarification needed] on the body it is orbiting and is either in a retrograde orbit or is below the synchronous orbit. The resulting tidal interaction saps momentum from the orbiting body and transfers it to the primary's rotation, lowering the orbit's altitude until frictional effects come into play.
Light and thermal radiation
Small bodies of the Solar System also experience an orbital decay.
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Gravitational radiation is a hypothetical mechanism of orbital decay. It is negligible for orbits of planets and planetary satellites, but shall be noticeable for systems of compact objects where its existence is suggested by some observations.