Editing Orbital decay (section)

==Sources of decay<span class="anchor" id="Sources of Orbital Decay"></span>==

===Atmospheric drag===
{{further|Atmospheric drag}}
Atmospheric drag at orbital altitude is caused by frequent collisions of gas [[molecule]]s 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 maximum|solar maxima]] the Earth's atmosphere causes significant drag up to altitudes much higher than during [[solar minima]].<ref>{{cite arXiv|last1=Nwankwo|first1=Victor U. J.|last2=Chakrabarti|first2=Sandip K.|title=Effects of Plasma Drag on Low Earth Orbiting Satellites due to Heating of Earth's Atmosphere by Coronal Mass Ejections|date=1 May 2013|class=physics.space-phn|eprint=1305.0233 <!-- unsupported parameter |url=https://arxiv.org/abs/1305.0233 -->}} </ref>

Atmospheric drag exerts a significant effect at the altitudes of [[space station]]s, [[Space Shuttle]]s and other crewed 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,<ref>{{cite web|title=The Biggest Spacecraft Ever to Fall Uncontrolled From Space|author=Wall, Mike|date=May 5, 2021|url=https://www.space.com/13049-6-biggest-spacecraft-falls-space.html|publisher=space.com|access-date=April 29, 2023}}</ref> and (relatively) controlled orbital decay was used to de-orbit the [[Mir]] space station.<ref>{{cite web|title=20 Years Ago: Space Station Mir Reenters Earth's Atmosphere|date=March 23, 2021|url=https://www.nasa.gov/feature/20-years-ago-space-station-mir-reenters-earth-s-atmosphere|publisher=NASA|access-date=April 29, 2023}}</ref>

[[Reboost]]s for the Hubble Space Telescope are less frequent 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 having been performed successfully by [[STS-125]], with Space Shuttle ''Atlantis'' in 2009.  Newer [[space telescope]]s are in much higher orbits, or in some cases in solar orbit, so orbital boosting may not be needed.<ref>[http://hubble.nasa.gov/missions/sm4.html The Hubble Program – Servicing Missions – SM4<!-- Bot generated title -->]</ref>

=== Tidal effects ===

{{further|Tidal acceleration}}
An orbit can also decay by negative [[tidal acceleration]] when 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 saps angular momentum from the orbiting body and transfers it to the primary's rotation, lowering the orbit's altitude.

Examples of satellites undergoing tidal orbital decay are Mars' moon [[Phobos (moon)|Phobos]], Neptune's moon [[Triton (moon)|Triton]], and the extrasolar planet [[TrES-3b]].

===Light and thermal radiation===
{{main|Poynting–Robertson effect|Yarkovsky effect}}
Small objects in the [[Solar System]] also experience an orbital decay due to the forces applied by asymmetric radiation pressure. Ideally, energy absorbed would equal [[blackbody]] energy emitted at any given point, resulting in no net force. However, the [[Yarkovsky effect]] is the phenomenon that, because absorption and radiation of heat are not instantaneous, objects which are not [[tidal locking|tidally 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. This results in a very small acceleration parallel to the orbital path, yet one which can be significant 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, these two effects will apply opposing, but generally unequal, forces.

=== Gravitational radiation ===
{{main|Two-body problem in general relativity}}
[[Gravitational radiation]] is another mechanism of orbital decay. It is negligible for orbits of planets and planetary satellites (when considering their orbital motion on time scales of centuries, decades, and less), but is noticeable for systems of [[compact star|compact objects]], as seen in observations of neutron star orbits. All orbiting bodies radiate gravitational energy, hence no orbit is infinitely stable.

=== Electromagnetic drag ===

Satellites using an [[electrodynamic tether]], moving through the Earth's magnetic field, create drag force that could eventually deorbit the satellite.