Editing Orbital decay (section)

===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>