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====Real (non-equilibrium) gas model====
A non-equilibrium real gas model is the most accurate model of a shock layer's gas physics, but is more difficult to solve than an equilibrium model. The simplest non-equilibrium model is the ''Lighthill-Freeman model'' developed in 1958.<ref>{{cite journal |last=Lighthill |first=M.J. |title=Dynamics of a Dissociating Gas. Part I. Equilibrium Flow |journal=Journal of Fluid Mechanics |volume=2 |pages=1–32 |date=Jan 1957 |doi=10.1017/S0022112057000713 |issue=1|bibcode = 1957JFM.....2....1L |s2cid=120442951 }}</ref><ref>{{cite journal |last=Freeman |first=N.C. |title=Non-equilibrium Flow of an Ideal Dissociating Gas |journal=Journal of Fluid Mechanics |volume=4 |pages=407–425 |date=Aug 1958 |doi=10.1017/S0022112058000549 |issue=04|doi-broken-date=
When running a Gibbs free energy equilibrium program,{{clarify|date=August 2018}} the iterative process from the originally specified molecular composition to the final calculated equilibrium composition is essentially random and not time accurate. With a non-equilibrium program, the computation process is time accurate and follows a solution path dictated by chemical and reaction rate formulas. The five species model has 17 chemical formulas (34 when counting reverse formulas). The Lighthill-Freeman model is based upon a single ordinary differential equation and one algebraic equation. The five species model is based upon 5 ordinary differential equations and 17 algebraic equations.{{Citation needed|date=December 2017}} Because the 5 ordinary differential equations are tightly coupled, the system is numerically "stiff" and difficult to solve. The five species model is only usable for entry from [[low Earth orbit]] where entry velocity is approximately {{cvt|7.8|km/s|km/h mph}}. For lunar return entry of 11 km/s<!-- 36545 ft/s in NASA 1960s units -->,<ref>[https://ntrs.nasa.gov/archive/nasa/casi.ntrs.nasa.gov/19690029435.pdf Entry Aerodynamics at Lunar Return Conditions Obtained from the Fliigh of Apollo 4] {{Webarchive|url=https://web.archive.org/web/20190411091352/https://ntrs.nasa.gov/archive/nasa/casi.ntrs.nasa.gov/19690029435.pdf |date=April 11, 2019 }}, Ernest R. Hillje, NASA, TN: D-5399, accessed 29 December 2018.</ref> the shock layer contains a significant amount of ionized nitrogen and oxygen. The five-species model is no longer accurate and a twelve-species model must be used instead.
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==Environmental impact==
[[File:ISS-46 Soyuz TMA-17M reentry.jpg|thumb|A plume in Earth's upper atmosphere left behind by a Soyuz spacecraft having reentered]]
Atmospheric entry has a measurable impact on [[Earth's atmosphere]], particularly the [[stratosphere]].
Atmospheric entry by spacecrafts have reached 3 % of all atmospheric entries by 2021, but in a scenario in which the number of satellites from 2019 are doubled artificial entries would make 40 % of all,<ref name="h473">{{
Considering [[space sustainability]] in regard to atmospheric impact of re-entry is by 2022 just developing<ref name="b448">{{cite journal |
Suggested alternatives are the use of less polluting materials and by in-orbit servicing and potentially in-space recycling.<ref name="b448"/><ref name="p583"/>
==Gallery==
<gallery widths="200px" heights="150px">
Soyuz TMA-05M spacecraft reentry.jpg|Close up of reentry trail (Soyuz)
Soyuz TMA-05M capsule reentry.jpg|Early reentry [[Plasma (physics)|plasma]] trail (Soyuz)
File:Re-entry of Progress Spacecraft 42P - NASA Earth Observatory.jpg|[[Progress (spacecraft)|Progress]] during atmospheric entry over Earth
Space Shuttle reentry aboard flight deck.jpg|Space Shuttle cockpit view during reentry</gallery>
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