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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&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 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&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 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&nbsp;km/s, and for {{convert|1500|km}} it is 7.12&nbsp;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&nbsp;km/s. [[Atmospheric drag|Atmospheric]] and [[gravity losses]] associated with launch typically adds 1.3–1.8 &nbsp;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&nbsp;km/s, and for {{convert|1500|km}} it is 7.12&nbsp;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&nbsp;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&nbsp;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&nbsp;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==
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{{reflist|group=lower-alpha|30em}}


==References==
==References==
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