Editing Low Earth orbit

{{Short description|Orbit around Earth between 160 and 2000 km}}
[[File:ISS-44 Milky Way.jpg|thumb|upright=1.6|A view from the [[International Space Station]] in a low Earth orbit (LEO) at about {{cvt|400|km|mi|abbr=on}}, with yellow-green [[airglow]] visible at Earth's [[horizon]], where roughly at an altitude of {{cvt|100|km|mi|abbr=on}} the [[Boundary of space|boundary between Earth and outer space]] lies and flying speeds reach [[orbital speed|orbital velocities]].]]

A '''low Earth orbit''' ('''LEO''') is an [[geocentric orbit|orbit around Earth]] with a [[orbital period|period]] of 128 minutes or less (making at least 11.25 orbits per day) and an [[orbital eccentricity|eccentricity]] less than 0.25.<ref>{{cite web |url=https://www.space-track.org/#/recent |title=Current Catalog Files |access-date=July 13, 2018 |quote=LEO: Mean Motion > 11.25 & Eccentricity < 0.25 |archive-date=June 26, 2018 |archive-url=https://web.archive.org/web/20180626190758/https://www.space-track.org/#/recent |url-status=live}}</ref> Most of the artificial objects in [[outer space]] are in LEO, peaking in number at an altitude around {{cvt|800|km|mi|abbr=on}},<ref name="l496">{{cite thesis | last=Muciaccia | first=Andrea | title=Fragmentations in low Earth orbit: event detection and parent body identification | date=2021 | doi=10.13140/RG.2.2.27621.52966 | page=}}</ref> while the farthest in LEO, before [[medium Earth orbit]] (MEO), have an altitude of 2,000 kilometers, about one-third of the [[Earth radius|radius of Earth]] and near the beginning of the [[Van Allen radiation belt#Inner belt|inner Van Allen radiation belt]].

The term ''LEO region'' is used for the area of space below an [[altitude]] of {{cvt|2000|km|mi}} (about one-third of Earth's radius).<ref name="UNOOSA">{{cite web |date=September 2007 |title=IADC Space Debris Mitigation Guidelines |url=http://www.unoosa.org/documents/pdf/spacelaw/sd/IADC-2002-01-IADC-Space_Debris-Guidelines-Revision1.pdf |publisher=INTER-AGENCY SPACE DEBRIS COORDINATION COMMITTEE: Issued by Steering Group and Working Group 4 |quote=Region A, Low Earth Orbit (or LEO) Region – spherical region that extends from the Earth's surface up to an altitude (Z) of 2,000 km |access-date=2018-07-17 |archive-date=2018-07-17 |archive-url=https://web.archive.org/web/20180717154257/http://www.unoosa.org/documents/pdf/spacelaw/sd/IADC-2002-01-IADC-Space_Debris-Guidelines-Revision1.pdf |url-status=live}}</ref> Objects in orbits that pass through this zone, even if they have an [[apogee]] further out or are [[sub-orbital spaceflight|sub-orbital]], are carefully tracked since they present a collision risk to the many LEO satellites.

No [[human spaceflight]]s other than the lunar missions of the [[Apollo program]] (1968-1972) have taken place beyond LEO. All [[space station]]s to date have operated [[geocentric orbit|geocentric]] within LEO.

== Defining characteristics ==
A wide variety of sources<ref>{{Cite web|title=Definition of LOW EARTH ORBIT|url=https://www.merriam-webster.com/dictionary/low%20earth%20orbit|access-date=2018-07-08|website=Merriam-Webster Dictionary|language=en|archive-date=2018-07-08|archive-url=https://web.archive.org/web/20180708162215/https://www.merriam-webster.com/dictionary/low%20earth%20orbit|url-status=live}}</ref><ref>{{Cite web|title=Frequently Asked Questions|url=https://www.faa.gov/space/additional_information/faq/#s1|access-date=2020-02-14|publisher=FAA|language=en-us|quote=LEO refers to orbits that are typically less than 2,400 km (1,491 mi) in altitude.|archive-date=2020-06-02|archive-url=https://web.archive.org/web/20200602021356/https://www.faa.gov/space/additional_information/faq/#s1|url-status=live}}</ref><ref>{{Cite news|last=Campbell|first=Ashley|date=2015-07-10|title=SCaN Glossary|language=en|publisher=NASA|url=https://www.nasa.gov/directorates/heo/scan/definitions/glossary/index.html#L|access-date=2018-07-12|quote=Low Earth Orbit (LEO): A geocentric orbit with an altitude much less than the Earth's radius. Satellites in this orbit are between 80 and 2000 kilometers above the Earth's surface.|archive-date=2020-08-03|archive-url=https://web.archive.org/web/20200803122937/https://www.nasa.gov/directorates/heo/scan/definitions/glossary/index.html#L|url-status=live}}</ref> define LEO in terms of [[altitude]]. The altitude of an object in an [[elliptic orbit]] can vary significantly along the orbit. Even for [[circular orbit]]s, the altitude above ground can vary by as much as {{cvt|30|km|mi}} (especially for [[polar orbit]]s) due to the [[flattening|oblateness]] of [[figure of the Earth|Earth's spheroid figure]] and local [[topography]].  While definitions based on altitude are inherently ambiguous, most of them fall within the range specified by an orbit period of 128 minutes because, according to [[orbital period|Kepler's third law]], this corresponds to a [[semi-major and semi-minor axes|semi-major axis]] of {{cvt|8413|km|mi}}. For circular orbits, this in turn corresponds to an altitude of {{cvt|2042|km|mi}} above the mean radius of Earth, which is consistent with some of the upper altitude limits in some LEO definitions.

The LEO region is defined by some sources as a region in space that LEO orbits occupy.<ref name="UNOOSA" /><ref>{{Cite news|others=David Hitt : NASA Educational Technology Services, Alice Wesson : JPL, J.D. Harrington : HQ;, Larry Cooper : HQ;, Flint Wild : MSFC;, Ann Marie Trotta : HQ;, Diedra Williams : MSFC|date=2015-06-01|title=What Is an Orbit?|language=en|work=NASA|url=https://www.nasa.gov/audience/forstudents/5-8/features/nasa-knows/what-is-orbit-58.html|access-date=2018-07-08|quote=LEO is the first 100 to 200 miles (161 to 322&nbsp;km) of space.|archive-date=2018-03-27|archive-url=https://web.archive.org/web/20180327095840/https://www.nasa.gov/audience/forstudents/5-8/features/nasa-knows/what-is-orbit-58.html|url-status=live}}</ref><ref>{{Cite news|last=Steele|first=Dylan|date=2016-05-03|title=A Researcher's Guide to: Space Environmental Effects|language=en|page=7|work=NASA|url=https://www.nasa.gov/connect/ebooks/researchers_guide_space_environment_detail.html|access-date=2018-07-12|quote=the low-Earth orbit (LEO) environment, defined as 200–1,000 km above Earth's surface|archive-date=2016-11-17|archive-url=https://web.archive.org/web/20161117060640/http://www.nasa.gov/connect/ebooks/researchers_guide_space_environment_detail.html|url-status=live}}</ref> Some [[highly elliptical orbit]]s may pass through the LEO region near their lowest altitude (or [[apsis|perigee]]) but are not in a LEO orbit because their highest altitude (or [[apsis|apogee]]) exceeds {{cvt|2000|km|mi|0}}. [[sub-orbital spaceflight|Sub-orbital]] objects can also reach the LEO region but are not in a LEO orbit because they [[atmospheric entry|re-enter the atmosphere]]. The distinction between LEO orbits and the LEO region is especially important for analysis of possible collisions between objects which may not themselves be in LEO but could collide with satellites or debris in LEO orbits.

[[File:Orbitalaltitudes.svg|center|700px]]

==Orbital characteristics==
The mean orbital velocity needed to maintain a stable low Earth orbit is about {{convert|7.8|km/s|mi/s|sigfig=2|abbr=on}}, which translates to {{convert|28000|km/h|mi/h|sigfig=2|abbr=on}}. However, this depends on the exact altitude of the orbit. Calculated for a circular orbit of {{convert|200|km|abbr=on}} the orbital velocity is {{convert|7.79|km/s|mi/s|sigfig=3|abbr=on}}, but for a higher {{convert|1500|km|abbr=on}} orbit the velocity is reduced to {{convert|7.12|km/s|mi/s|sigfig=3|abbr=on}}.<ref>{{Cite web|url=http://www.spaceacademy.net.au/watch/track/leopars.htm|title=LEO parameters|website=www.spaceacademy.net.au|access-date=2015-06-12|archive-date=2016-02-11|archive-url=https://web.archive.org/web/20160211202014/http://www.spaceacademy.net.au/watch/track/leopars.htm|url-status=live}}</ref> The launch vehicle's [[delta-v]] needed to achieve low Earth orbit starts around {{convert|9.4|km/s|mi/s|sigfig=2|abbr=on}}.

The pull of [[gravity]] in LEO is only slightly less than on the Earth's surface. This is because the distance to LEO from the Earth's surface is much less than the Earth's radius. However, an object in orbit is in a permanent [[free fall]] around Earth, because in orbit the [[Gravity|gravitational force]] and the [[centrifugal force]] balance each other out.{{efn|It is important to note here that “free fall” by definition requires that ''gravity'' is the only force acting on the object. That definition is still fulfilled when falling around Earth, as the other force, the ''centrifugal force'' is a [[fictitious force]].}} As a result, spacecraft in orbit continue to stay in orbit, and people inside or outside such craft continuously experience [[weightlessness]].

Objects in LEO orbit Earth between the denser part of the atmosphere and below the inner [[Van Allen radiation belt]].  They encounter atmospheric drag from [[gases]] in the [[thermosphere]] (approximately 80–600&nbsp;km above the surface) or [[exosphere]] (approximately {{cvt|600|km|-2|disp=or}} and higher), depending on orbit height.  Satellites in orbits that reach altitudes below {{cvt|300|km}} [[Orbital decay|decay]] quickly due to atmospheric drag. 

Equatorial low Earth orbits ('''ELEO''') are a subset of LEO. These orbits, with low [[orbital inclination]], allow rapid revisit times over low-latitude locations on Earth.  [[Retrograde and prograde motion|Prograde]] equatorial LEOs also have lower [[delta-v]] launch requirements because they take advantage of the Earth's rotation. Other useful LEO orbits including [[polar orbit]]s and [[Sun-synchronous orbit]]s have a higher inclinations to the equator and provide coverage for higher latitudes on Earth. Some of the first generation of [[Starlink]] satellites used polar orbits which provide coverage everywhere on Earth.  Later Starlink constellations orbit at a lower inclination and provide more coverage for populated areas.

Higher orbits include [[medium Earth orbit]] (MEO), sometimes called intermediate circular orbit (ICO), and further above, [[geostationary orbit]] (GEO). Orbits higher than low orbit can lead to early failure of electronic components due to intense [[radiation]] and charge accumulation.

In 2017, "[[very low Earth orbit]]s" ('''VLEO''') began to be seen in [[regulatory agency|regulatory]] filings. These orbits, below about {{Cvt|450|km|mi|-1}}, require the use of novel technologies for [[orbit raising]] because they operate in orbits that would ordinarily decay too soon to be economically useful.<ref>{{Cite journal|last1=Crisp|first1=N. H.|last2=Roberts|first2=P. C. E.|last3=Livadiotti|first3=S.|last4=Oiko|first4=V. T. A.|last5=Edmondson|first5=S.|last6=Haigh|first6=S. J.|last7=Huyton|first7=C.|last8=Sinpetru|first8=L.|last9=Smith|first9=K. L.|last10=Worrall|first10=S. D.|last11=Becedas|first11=J.|date=August 2020|title=The Benefits of Very Low Earth Orbit for Earth Observation Missions|journal=[[Progress in Aerospace Sciences]]|volume=117|pages=100619|doi=10.1016/j.paerosci.2020.100619|arxiv=2007.07699|bibcode=2020PrAeS.11700619C|s2cid=220525689}}</ref><ref name=pa20170303>{{cite news |last=Messier |first=Doug |url=http://www.parabolicarc.com/2017/03/03/spacex-launch-12000-satellites/ |title=SpaceX Wants to Launch 12,000 Satellites |work=Parabolic Arc |date=2017-03-03 |access-date=2018-01-22 |archive-date=2020-01-22 |archive-url=https://web.archive.org/web/20200122203256/http://www.parabolicarc.com/2017/03/03/spacex-launch-12000-satellites/ |url-status=live }}</ref>

==Use==
[[File:Sunrise To Sunset Aboard The ISS.OGG|thumb|Roughly half an orbit of the [[International Space Station]]]]

A low Earth orbit requires the lowest amount of energy for satellite placement. It provides high bandwidth and low communication [[latency (engineering)|latency]]. Satellites and space stations in LEO are more accessible for crew and servicing.

Since it requires less [[energy]] to place a satellite into a LEO, and a satellite there needs less powerful amplifiers for successful transmission, LEO is used for many communication applications, such as the [[Iridium Communications|Iridium phone system]]. Some [[communication satellite]]s use much higher [[geostationary orbit]]s and move at the same angular velocity as the Earth as to appear stationary above one location on the planet.

=== Disadvantages ===
Unlike [[geosynchronous satellite|geosynchronous satellites]], satellites in low orbit have a small [[field of view]] and can only observe and communicate with a fraction of the Earth at a given time. This means that a large network (or [[Satellite constellation|constellation]]) of satellites is required to provide continuous coverage.

Satellites at lower altitudes of orbit are in the atmosphere and suffer from rapid [[orbital decay]], requiring either periodic re-boosting to maintain stable orbits, or the launching of replacements for those that re-enter the atmosphere. The effects of adding such quantities of vaporized metals to Earth's [[stratosphere]] are potentially of concern but currently unknown.<ref>{{cite web | url=https://www.scientificamerican.com/article/space-junk-is-polluting-earths-stratosphere-with-vaporized-metal/ | title=Space Junk is Polluting Earth's Stratosphere with Vaporized Metal | website=[[Scientific American]] }}</ref>

===Examples===
* The [[International Space Station]] is in LEO about {{convert|400|to|420|km}} above the Earth's surface.<ref>{{cite web | url=http://www.nasa.gov/mission_pages/station/expeditions/expedition26/iss_altitude.html | title=Higher Altitude Improves Station's Fuel Economy | publisher=NASA | access-date=2013-02-12 | archive-date=2015-05-15 | archive-url=https://web.archive.org/web/20150515050746/http://www.nasa.gov/mission_pages/station/expeditions/expedition26/iss_altitude.html | url-status=live }}</ref> The station’s orbit decays by about {{cvt|2|km/month|mi/month}} and consequently needs re-boosting a few times a year.
* The [[Iridium satellite constellation|Iridium telecom satellites]] orbit at about {{convert|780|km|mi|abbr=on}}.
* [[Earth observation satellite]]s, also known as [[remote sensing]] satellites, including [[spy satellite]]s and other [[Earth imaging]] satellites, use LEO as they are able to see the surface of the Earth more clearly by being closer to it. A majority of artificial [[satellite]]s are placed in LEO.<ref>{{Cite web|title = NASA Earth Observatory|url = http://earthobservatory.nasa.gov/Features/OrbitsCatalog/|website = earthobservatory.nasa.gov|date = 2009-09-04|access-date = 2015-11-28|language = en|first = Riebeek|last = Holli|archive-date = 2018-05-27|archive-url = https://web.archive.org/web/20180527202627/https://earthobservatory.nasa.gov/Features/OrbitsCatalog/|url-status = live}}</ref> Satellites can also take advantage of consistent lighting of the surface below via [[Sun-synchronous orbit|Sun-synchronous LEO orbits]] at an altitude of about {{convert|800|km|mi|-1|abbr=on}} and near polar inclination. [[Envisat]] (2002–2012) is one example.
* The [[Hubble Space Telescope]] orbits at about {{convert|540|km|mi|abbr=on}} above Earth.
* [[Satellite internet constellation]]s such as [[Starlink]].
* The Chinese [[Tiangong space station]] was launched in April 2021 and currently orbits between {{convert|340|and|450|km|abbr=on}} above the Earth's surface. 
* The [[gravimetry]] mission [[GRACE-FO]] orbits at about {{convert|500|km|mi|abbr=on}} as did its predecessor, [[Gravity Recovery and Climate Experiment|GRACE]].

==== Former ====

* [[GOCE]] (2009-2013), an [[ESA]] gravimetry mission, orbited at about 255 km (158 mi).

* [[Super Low Altitude Test Satellite]] (2017-2019), nicknamed ''Tsubame'', orbited at {{cvt|167.4|km}}, the lowest altitude ever among Earth observation satellites.<ref>{{Cite news |date=2019-12-30 |title=Japan's low altitude satellite Tsubame registered in Guinness World Records |url=https://www.japantimes.co.jp/news/2019/12/30/national/japan-low-altitude-satellite-tsubame/ |access-date=2024-06-25 |work=The Japan Times}}</ref>

==== In fiction ====
* In the film ''[[2001: A Space Odyssey (film)|2001: A Space Odyssey]]'', Earth's transit station ("Space Station V") "orbited 300 km above Earth".<ref>{{Cite web |title=Space station from 2001: A Space Odyssey |url=https://www.esa.int/ESA_Multimedia/Images/2012/01/Space_station_from_2001_A_Space_Odyssey}}</ref>

==Space debris==
{{missing information|section|debris lifespan|date=August 2023}}
The LEO environment is becoming congested with [[space debris]] because of the frequency of object launches.<ref>{{Cite web|last=United Nations Office for Outer Space Affairs|date=2010|title=Space Debris Mitigation Guidelines of the Committee on the Peaceful Uses of Outer Space|url=https://www.iadc-home.org/documents_public/view/search_field/eNortjI2tlJy1CsuTcpKTS7RdtTLS8xNBVIlielAMjk_ryQ1r6RYyRpcMAypDUU~/page/1/id/126#u|access-date=October 19, 2021|website=Inter-Agency Space Debris Coordination Committee (IADC)}}</ref> This has caused growing concern in recent years, since collisions at orbital velocities can be dangerous or deadly. Collisions can produce additional space debris, creating a [[domino effect]] known as [[Kessler syndrome]]. [[NASA|NASA's]] Orbital Debris Program tracks over 25,000 objects larger than 10 cm diameter in LEO, while the estimated number between 1 and 10 cm is 500,000, and the number of particles bigger than 1 mm exceeds 100 million.<ref>{{Cite web |title=ARES {{!}} Orbital Debris Program Office {{!}} Frequently Asked Questions |url=https://www.orbitaldebris.jsc.nasa.gov/faq/ |access-date=2022-09-02 |website=NASA.gov |archive-url=https://web.archive.org/web/20220902133204/https://www.orbitaldebris.jsc.nasa.gov/faq/ |archive-date=2022-09-02}}</ref> The particles travel at speeds up to {{convert|17,500|mi/h|km/s km/h mi/h|abbr=on|order=out|sigfig=2|adj=ri0}}, so even a small impact can severely damage a spacecraft.<ref>{{Cite web |last=Garcia |first=Mark |date=2015-04-13 |title=Space Debris and Human Spacecraft |url=http://www.nasa.gov/mission_pages/station/news/orbital_debris.html |access-date=2022-09-02 |website=NASA.gov |archive-url=https://web.archive.org/web/20220908122950/https://www.nasa.gov/mission_pages/station/news/orbital_debris.html |archive-date=2022-09-08}}</ref>

==See also==
{{colbegin}}
* [[Comparison of orbital launch systems]]
* [[Geostationary orbit]] (GEO)
* [[Heavy-lift launch vehicle]] 
* [[High Earth orbit]]
* [[Highly elliptical orbit]] (HEO)
* [[List of orbits]]
* [[Medium Earth orbit]] (MEO)
* [[Medium-lift launch vehicle]]
* [[Specific orbital energy#Examples|Specific orbital energy examples]]
* [[Suborbital spaceflight]]
* [[Space domain awareness]]
* [[Van Allen radiation belt]]
{{colend}}
{{clear}}

==Notes==
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==References==
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