Low Earth orbit - Revision history

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	{{Comparison satellite navigation orbits}}		{{Comparison satellite navigation orbits}}

	A '''low Earth orbit''' ('''LEO''') is an [[geocentric orbit\|Earth~~-centered orbit~~]] ~~near the planet, often specified as having~~ 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, with an altitude never more than about one-third of the [[Earth radius\|radius of Earth]].<ref>{{cite journal \|last1=Sampaio\|first1=Jarbas \|last2=Wnuk\|first2=Edwin \|last3=Vilhena de Moraes\|first3=Rodolpho \|last4=Fernandes\|first4=Sandro \|date=2014-01-01 \|title=Resonant Orbital Dynamics in LEO Region: Space Debris in Focus \|url=https://www.researchgate.net/publication/273597440 \|journal=Mathematical Problems in Engineering \|volume=2014 \|page=Figure 1: Histogram of the mean motion of the cataloged objects \|doi=10.1155/2014/929810 \|doi-access=free \|access-date=2018-07-13 \|archive-date=2021-10-01 \|archive-url=https://web.archive.org/web/20211001030902/https://www.researchgate.net/publication/273597440_Resonant_Orbital_Dynamics_in_LEO_Region_Space_Debris_in_Focus \|url-status=live}}</ref>		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, with an altitude never more than about one-third of the [[Earth radius\|radius of Earth]].<ref>{{cite journal \|last1=Sampaio\|first1=Jarbas \|last2=Wnuk\|first2=Edwin \|last3=Vilhena de Moraes\|first3=Rodolpho \|last4=Fernandes\|first4=Sandro \|date=2014-01-01 \|title=Resonant Orbital Dynamics in LEO Region: Space Debris in Focus \|url=https://www.researchgate.net/publication/273597440 \|journal=Mathematical Problems in Engineering \|volume=2014 \|page=Figure 1: Histogram of the mean motion of the cataloged objects \|doi=10.1155/2014/929810 \|doi-access=free \|access-date=2018-07-13 \|archive-date=2021-10-01 \|archive-url=https://web.archive.org/web/20211001030902/https://www.researchgate.net/publication/273597440_Resonant_Orbital_Dynamics_in_LEO_Region_Space_Debris_in_Focus \|url-status=live}}</ref>

	The term ''LEO region'' is also 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.		The term ''LEO region'' is also 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.
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	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 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 an LEO orbit because their highest altitude (or [[apsis\|apogee]]) exceeds {{cvt\|2000\|km\|mi}}. [[sub-orbital spaceflight\|Sub-orbital]] objects can also reach the LEO region but are not in an 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.		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 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 an LEO orbit because their highest altitude (or [[apsis\|apogee]]) exceeds {{cvt\|2000\|km\|mi}}. [[sub-orbital spaceflight\|Sub-orbital]] objects can also reach the LEO region but are not in an 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==		==Orbital characteristics==
	The mean orbital velocity needed to maintain a stable low Earth orbit is about {{convert\|7.8\|km/s\|mph}}, but reduces for higher orbits. Calculated for a circular orbit of {{convert\|200\|km}} it is 7.79 km/s, and for {{convert\|1500\|km}} it is 7.12 km/s.<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 [[delta-v]] needed to achieve low Earth orbit starts around 9.4 km/s. [[Atmospheric drag\|Atmospheric]] and [[gravity losses]] associated with launch typically adds 1.3–1.8  km/s to the LEO orbital velocity.{{Clarify\|reason=7.8 + 1.3 = 9.1 Km/s is inconsistent with "starts at" 9.4 km/s.\|date=September 2021}}<ref>{{Cite book\|title=How Spacecraft Fly\|last=Swinerd\|first=Graham\|publisher=Praxis Publishing\|year=2008\|isbn=978-0387765723\|pages=103–104}}</ref>		The mean orbital velocity needed to maintain a stable low Earth orbit is about {{convert\|7.8\|km/s\|mph}}, but reduces for higher orbits. Calculated for a circular orbit of {{convert\|200\|km}} it is 7.79 km/s, and for {{convert\|1500\|km}} it is 7.12 km/s.<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 9.4 km/s.

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

	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, by definition, ~~in free fall~~, ~~since there~~ is no force ~~holding it up~~. As a result ~~objects~~ in orbit, ~~including~~ people, experience ~~a sense of~~ [[weightlessness]].		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 both, the [[Gravity\|gravitational force]] and the [[centrifugal force]] balance out each other.{{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, spacecrafts in orbit continue to stay in orbit, and people inside or outside such crafts continuously experience [[weightlessness]].

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

	Equatorial low Earth orbits (ELEO) are a subset of LEO. These orbits, with low inclination to the Equator, allow rapid revisit times of low-latitude places on Earth and have the lowest [[delta-v]] requirement (i.e., fuel spent) of any orbit, provided they have the direct (not retrograde) orientation with respect to the Earth's rotation. Orbits with a very high inclination angle to the equator are usually called [[polar orbit]]s or [[Sun-synchronous orbit\|Sun-synchronous orbits]].		Equatorial low Earth orbits ('''ELEO''') are a subset of LEO. These orbits, with low inclination to the Equator, allow rapid revisit times of low-latitude places on Earth and have the lowest [[delta-v]] requirement (i.e., fuel spent) of any orbit, provided they have the direct (not retrograde) orientation with respect to the Earth's rotation. Orbits with a very high inclination angle to the equator are usually called [[polar orbit]]s or [[Sun-synchronous orbit\|Sun-synchronous orbits]].

	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.		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" ~~orbits~~ began to be seen in [[regulatory agency\|regulatory]] filings. These orbits, below about {{Cvt\|450\|km\|mi\|-1}} ~~and referred to as "'''VLEO'''"<!-- bolded per [[WP:MOSBOLD]] for redirect target -->~~, 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\|url=http://arxiv.org/abs/2007.07699\|journal=[[Progress in Aerospace Sciences]]\|volume=117\|pages=100619\|doi=10.1016/j.paerosci.2020.100619\|arxiv=2007.07699\|bibcode=2020PrAeS.11700619C\|s2cid=220525689\|access-date=2021-03-29\|archive-date=2021-03-19\|archive-url=https://web.archive.org/web/20210319141337/https://arxiv.org/abs/2007.07699\|url-status=live}}</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>		In 2017, "very low Earth orbits" ('''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\|url=http://arxiv.org/abs/2007.07699\|journal=[[Progress in Aerospace Sciences]]\|volume=117\|pages=100619\|doi=10.1016/j.paerosci.2020.100619\|arxiv=2007.07699\|bibcode=2020PrAeS.11700619C\|s2cid=220525689\|access-date=2021-03-29\|archive-date=2021-03-19\|archive-url=https://web.archive.org/web/20210319141337/https://arxiv.org/abs/2007.07699\|url-status=live}}</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==		==Use==
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	==Notes==		==Notes==
	{{reflist\|group=lower-alpha}}		{{reflist\|group=lower-alpha\|30em}}

	==References==		==References==

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{{short description|Orbit around Earth with an altitude between 160 and 2,000 km}}
{{Comparison satellite navigation orbits}}

A '''low Earth orbit''' ('''LEO''') is an [[geocentric orbit|Earth-centered orbit]] near the planet, often specified as having 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, with an altitude never more than about one-third of the [[Earth radius|radius of Earth]].<ref>{{cite journal |last1=Sampaio|first1=Jarbas |last2=Wnuk|first2=Edwin |last3=Vilhena de Moraes|first3=Rodolpho |last4=Fernandes|first4=Sandro |date=2014-01-01 |title=Resonant Orbital Dynamics in LEO Region: Space Debris in Focus |url=https://www.researchgate.net/publication/273597440 |journal=Mathematical Problems in Engineering |volume=2014 |page=Figure 1: Histogram of the mean motion of the cataloged objects |doi=10.1155/2014/929810 |doi-access=free |access-date=2018-07-13 |archive-date=2021-10-01 |archive-url=https://web.archive.org/web/20211001030902/https://www.researchgate.net/publication/273597440_Resonant_Orbital_Dynamics_in_LEO_Region_Space_Debris_in_Focus |url-status=live}}</ref>

The term ''LEO region'' is also 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.

All crewed [[space station]]s to date have been within LEO. From 1968 to 1972, the [[Apollo program]]'s lunar missions sent humans beyond LEO. Since the end of the Apollo program, no [[human spaceflight]]s have been beyond 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 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 an LEO orbit because their highest altitude (or [[apsis|apogee]]) exceeds {{cvt|2000|km|mi}}. [[sub-orbital spaceflight|Sub-orbital]] objects can also reach the LEO region but are not in an 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.

==Orbital characteristics==
The mean orbital velocity needed to maintain a stable low Earth orbit is about {{convert|7.8|km/s|mph}}, but reduces for higher orbits. Calculated for a circular orbit of {{convert|200|km}} it is 7.79 km/s, and for {{convert|1500|km}} it is 7.12 km/s.<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 [[delta-v]] needed to achieve low Earth orbit starts around 9.4 km/s. [[Atmospheric drag|Atmospheric]] and [[gravity losses]] associated with launch typically adds 1.3–1.8  km/s to the LEO orbital velocity.{{Clarify|reason=7.8 + 1.3 = 9.1 Km/s is inconsistent with "starts at" 9.4 km/s.|date=September 2021}}<ref>{{Cite book|title=How Spacecraft Fly|last=Swinerd|first=Graham|publisher=Praxis Publishing|year=2008|isbn=978-0387765723|pages=103–104}}</ref>

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

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, by definition, in free fall, since there is no force holding it up. As a result objects in orbit, including people, experience a sense of [[weightlessness]].

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

Equatorial low Earth orbits (ELEO) are a subset of LEO. These orbits, with low inclination to the Equator, allow rapid revisit times of low-latitude places on Earth and have the lowest [[delta-v]] requirement (i.e., fuel spent) of any orbit, provided they have the direct (not retrograde) orientation with respect to the Earth's rotation. Orbits with a very high inclination angle to the equator are usually called [[polar orbit]]s or [[Sun-synchronous orbit|Sun-synchronous orbits]].

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" orbits began to be seen in [[regulatory agency|regulatory]] filings. These orbits, below about {{Cvt|450|km|mi|-1}} and referred to as "'''VLEO'''", 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|url=http://arxiv.org/abs/2007.07699|journal=[[Progress in Aerospace Sciences]]|volume=117|pages=100619|doi=10.1016/j.paerosci.2020.100619|arxiv=2007.07699|bibcode=2020PrAeS.11700619C|s2cid=220525689|access-date=2021-03-29|archive-date=2021-03-19|archive-url=https://web.archive.org/web/20210319141337/https://arxiv.org/abs/2007.07699|url-status=live}}</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]], satellites in LEO have a small [[field of view]] and so can observe and communicate with only a fraction of the Earth at a time. That means that a network (or "[[Satellite constellation|constellation]]") of satellites is required to provide continuous coverage. Satellites in lower regions of LEO also suffer from fast [[orbital decay]] and require either periodic re-boosting to maintain a stable orbit or launching replacement satellites when old ones re-enter.

===Examples===
* The [[International Space Station]] is in a LEO about {{convert|400|km|mi|-1|abbr=on}} to {{convert|420|km|mi|-1|abbr=on}} above 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> and needs re-boosting a few times a year due to orbital decay.
* 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.
* The Chinese [[Tiangong space station]] was launched in April of 2021 and currently orbits between about {{convert|340|km}} and {{convert|450|km}}.

; 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|url=https://www.esa.int/ESA_Multimedia/Images/2012/01/Space_station_from_2001_A_Space_Odyssey|title = Space station from 2001: A Space Odyssey}}</ref>

==== Former ====
* The Chinese [[Tiangong-1]] station was in orbit at about {{convert|355|km}},<ref>{{Cite web |url=http://www.people.com.cn/h/2011/1001/c25408-1418236438.html |title="天宫一号成功完成二次变轨" |access-date=2020-10-13 |archive-date=2011-11-13 |archive-url=https://web.archive.org/web/20111113130054/http://www.people.com.cn/h/2011/1001/c25408-1418236438.html |url-status=dead }}</ref> until its de-orbiting in 2018.
* The Chinese [[Tiangong-2]] station was in orbit at about {{convert|370|km|mi|abbr=on}}, until its de-orbiting in 2019.
* [[Gravimetry]] missions such as [[Gravity Field and Steady-State Ocean Circulation Explorer|GOCE]] orbited at about {{convert|255|km|mi|abbr=on}} to measure Earth's gravity field at highest sensitivity. The mission lifetime was limited because of atmospheric drag. [[Gravity Recovery and Climate Experiment|GRACE]] and [[GRACE-FO]] were orbiting at about {{convert|500|km|mi|abbr=on}}.

==Space debris==
{{Update|section|reason=2010 citation from wayback machine is severely outdated. The number of tracked objects is much larger and keeps growing every month|date=September 2021}}
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|url-status=live|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]]. The [[Combined Space Operations Center]], part of [[United States Strategic Command]] (formerly the United States Space Command), tracks around 8,500 objects larger than 10 cm in LEO.<ref>[http://www.vandenberg.af.mil/library/factsheets/factsheet.asp?id=12579 Fact Sheet: Joint Space Operations Center] {{webarchive|url=https://web.archive.org/web/20100203082814/http://www.vandenberg.af.mil/library/factsheets/factsheet.asp?id=12579 |date=2010-02-03 }}</ref> According to an [[Arecibo Observatory]] study, there may be one million dangerous objects larger than 2 millimeters in orbit,<ref>{{Cite web |url=http://www.astronomycafe.net/qadir/q1697.html |title=archive of astronomy: space junk |access-date=2009-04-15 |archive-date=2017-03-20 |archive-url=https://web.archive.org/web/20170320190907/http://www.astronomycafe.net/qadir/q1697.html |url-status=dead }}</ref> which are too small to be visible from Earth-based observatories.<ref>[http://eugen.leitl.org/tt/msg04534.html ISS laser broom, project Orion] {{webarchive|url=https://web.archive.org/web/20110728181940/http://eugen.leitl.org/tt/msg04534.html |date=2011-07-28 }}</ref>

==See also==
{{colbegin}}
* [[Comparison of orbital launch systems]]
* [[Geostationary]] orbit (GEO)
* [[Heavy lift launch vehicle]]
* [[High Earth orbit]] (HEO)
* [[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]]
* [[Ukrainian Optical Facilities for Near-Earth Space Surveillance Network]]
* [[Van Allen radiation belt]]
{{colend}}
{{clear}}

==Notes==
{{reflist|group=lower-alpha}}

==References==
{{reflist|30em}}
{{Include-NASA}}

{{orbits}}
{{People currently in space}}
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[[Category:Earth orbits]]
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