Editing Atmospheric entry (section)

{{Short description|Passage of an object through the gases of an atmosphere from outer space}}
{{Redirect-multi|3|Reentry|HIAD|IRVE|other uses|reentry (disambiguation)|and|hiad (disambiguation)|and|irve (disambiguation)}}
{{Use mdy dates|date=September 2021}}
{{Use American English|date=January 2019}}
[[File:Entry.jpg|thumb|[[Mars Exploration Rover]] (MER) [[aeroshell]], artistic rendition]]

'''Atmospheric entry''' (sometimes listed as V<sub>impact</sub> or V<sub>entry</sub>) is the movement of an object from [[outer space]] into and through the gases of an [[atmosphere]] of a [[planet]], [[dwarf planet]], or [[natural satellite]].  Atmospheric entry may be ''uncontrolled entry,'' as in the entry of [[astronomical object]]s, [[space debris]], or [[bolide]]s.  It may be ''controlled entry'' (or ''reentry'') of a spacecraft that can be navigated or follow a predetermined course.  Methods for controlled atmospheric ''entry, descent, and landing'' of spacecraft are collectively termed as ''EDL''.

[[File:Reentry video as Orion returns from Artemis I.webm|thumb|Video of [[Orion (spacecraft)|Orion]]'s [[Non-ballistic atmospheric entry|skip reentry]] on [[Artemis 1]], showing the entire reentry process unedited from space to splashdown]]
Objects entering an atmosphere experience [[Drag (physics)|atmospheric drag]], which puts mechanical stress on the object, and [[aerodynamic heating]]—caused mostly by compression of the air in front of the object, but also by drag. These forces can cause loss of mass ([[ablation]]) or even complete disintegration of smaller objects, and objects with lower [[compressive strength]] can explode.

Objects have reentered with speeds ranging from 7.8&nbsp;km/s for [[low Earth orbit]] to around 12.5&nbsp;km/s for the [[Stardust (spacecraft)|Stardust]] probe.<ref name=stardust/>   They have high kinetic energies, and atmospheric dissipation is the only way of expending this, as it is highly impractical to use [[retrorocket]]s for the entire reentry procedure.  Crewed space vehicles must be slowed to subsonic speeds before parachutes or air brakes may be deployed. 

Ballistic warheads and expendable vehicles do not require slowing at reentry, and in fact, are made streamlined so as to maintain their speed. Furthermore, slow-speed returns to Earth from near-space such as [[Space diving|high-altitude parachute jumps from balloons]] do not require heat shielding because the gravitational acceleration of an object starting at relative rest from within the atmosphere itself (or not far above it) cannot create enough velocity to cause significant atmospheric heating.

For Earth, atmospheric entry occurs by convention at the [[Kármán line]] at an altitude of {{Convert|100|km|mi nmi|abbr=in}} above the surface, while at [[Venus#Atmospheric entry|Venus atmospheric entry]] occurs at {{Convert|250|km|mi nmi|abbr=on}} and at [[Mars atmospheric entry]] occurs at about {{Convert|80|km|mi nmi|abbr=on}}. Uncontrolled objects reach high velocities while accelerating through space toward the Earth under the influence of Earth's [[gravity]], and are slowed by friction upon encountering Earth's atmosphere. Meteors are also often travelling quite fast relative to the Earth simply because their own orbital path is different from that of the Earth before they encounter Earth's [[gravity well]]. Most objects enter at [[Hypersonic speed|hypersonic speeds]] due to their [[sub-orbital spaceflight|sub-orbital]] (e.g., [[intercontinental ballistic missile]] reentry vehicles), [[Orbit|orbital]] (e.g., the [[Soyuz (spacecraft)|Soyuz]]), or [[hyperbolic trajectory|unbounded]] (e.g., [[meteor]]s) trajectories. Various advanced technologies have been developed to enable atmospheric reentry and flight at extreme velocities. An alternative method of controlled atmospheric entry is [[buoyancy]]<ref>{{cite web |url=http://www.jpaerospace.com/atohandout.pdf |title=ATO: Airship To Orbit |publisher=JP Aerospace |access-date=December 14, 2013 |archive-date=October 13, 2013 |archive-url=https://web.archive.org/web/20131013190704/http://jpaerospace.com/atohandout.pdf |url-status=live }}</ref> which is suitable for planetary entry where thick atmospheres, strong gravity, or both factors complicate high-velocity hyperbolic entry, such as the atmospheres of [[Venus]], [[Titan (moon)|Titan]] and the [[giant planet]]s.<ref>{{cite book|title=Unmanned Spacecraft Meeting 1965|first=F.|last=Gross|publisher=American Institute of Aeronautics and Astronautics|doi=10.2514/6.1965-1407|chapter=Buoyant Probes into the Venus Atmosphere|year=1965}}</ref>