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{{Short description|Orbit around Earth between 160 and 2000 km}}A low Earth orbit (LEO) is an orbit around Earth with a period of 128 minutes or less (making at least 11.25 orbits per day) and an eccentricity less than 0.25.WEB,www.space-track.org/#/recent, Current Catalog Files, July 13, 2018, LEO: Mean Motion > 11.25 & Eccentricity < 0.25, June 26, 2018,web.archive.org/web/20180626190758/https://www.space-track.org/#/recent, live, Most of the artificial objects in outer space are in LEO, with an altitude never more than about one-third of the radius of Earth (or about 2000 kilometers).JOURNAL, Sampaio, Jarbas, Wnuk, Edwin, Vilhena de Moraes, Rodolpho, Fernandes, Sandro, 2014-01-01, Resonant Orbital Dynamics in LEO Region: Space Debris in Focus,www.researchgate.net/publication/273597440, Mathematical Problems in Engineering, 2014, Figure 1: Histogram of the mean motion of the cataloged objects, 10.1155/2014/929810, free, 2018-07-13, 2021-10-01,web.archive.org/web/20211001030902/https://www.researchgate.net/publication/273597440_Resonant_Orbital_Dynamics_in_LEO_Region_Space_Debris_in_Focus, live, 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).WEB, September 2007, IADC Space Debris Mitigation Guidelines,www.unoosa.org/documents/pdf/spacelaw/sd/IADC-2002-01-IADC-Space_Debris-Guidelines-Revision1.pdf, INTER-AGENCY SPACE DEBRIS COORDINATION COMMITTEE: Issued by Steering Group and Working Group 4, 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, 2018-07-17, 2018-07-17,www.unoosa.org/documents/pdf/spacelaw/sd/IADC-2002-01-IADC-Space_Debris-Guidelines-Revision1.pdf," title="web.archive.org/web/20180717154257www.unoosa.org/documents/pdf/spacelaw/sd/IADC-2002-01-IADC-Space_Debris-Guidelines-Revision1.pdf,">web.archive.org/web/20180717154257www.unoosa.org/documents/pdf/spacelaw/sd/IADC-2002-01-IADC-Space_Debris-Guidelines-Revision1.pdf, live, Objects in orbits that pass through this zone, even if they have an apogee further out or are sub-orbital, are carefully tracked since they present a collision risk to the many LEO satellites.No human spaceflights other than the lunar missions of the Apollo program have taken place beyond LEO. All crewed space stations to date have operated within LEO.

Defining characteristics

A wide variety of sourcesWEB, Definition of LOW EARTH ORBIT,www.merriam-webster.com/dictionary/low%20earth%20orbit, 2018-07-08, Merriam-Webster Dictionary, en, 2018-07-08,web.archive.org/web/20180708162215/https://www.merriam-webster.com/dictionary/low%20earth%20orbit, live, WEB, Frequently Asked Questions,www.faa.gov/space/additional_information/faq/#s1, 2020-02-14, FAA, en-us, LEO refers to orbits that are typically less than 2,400 km (1,491 mi) in altitude., 2020-06-02,web.archive.org/web/20200602021356/https://www.faa.gov/space/additional_information/faq/#s1, live, NEWS, Campbell, Ashley, 2015-07-10, SCaN Glossary, en, NASA,www.nasa.gov/directorates/heo/scan/definitions/glossary/index.html#L, 2018-07-12, 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., 2020-08-03,web.archive.org/web/20200803122937/https://www.nasa.gov/directorates/heo/scan/definitions/glossary/index.html#L, live, define LEO in terms of altitude. The altitude of an object in an elliptic orbit can vary significantly along the orbit. Even for circular orbits, the altitude above ground can vary by as much as {{cvt|30|km|mi}} (especially for polar orbits) due to the oblateness of 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 Kepler’s third law, this corresponds to a 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.NEWS, 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, 2015-06-01, What Is an Orbit?, en, NASA,www.nasa.gov/audience/forstudents/5-8/features/nasa-knows/what-is-orbit-58.html, 2018-07-08, LEO is the first 100 to 200 miles (161 to 322 km) of space., 2018-03-27,web.archive.org/web/20180327095840/https://www.nasa.gov/audience/forstudents/5-8/features/nasa-knows/what-is-orbit-58.html, live, NEWS, Steele, Dylan, 2016-05-03, A Researcher’s Guide to: Space Environmental Effects, en, 7, NASA,www.nasa.gov/connect/ebooks/researchers_guide_space_environment_detail.html, 2018-07-12, the low-Earth orbit (LEO) environment, defined as 200–1,000 km above Earth’s surface, 2016-11-17,www.nasa.gov/connect/ebooks/researchers_guide_space_environment_detail.html," title="web.archive.org/web/20161117060640www.nasa.gov/connect/ebooks/researchers_guide_space_environment_detail.html,">web.archive.org/web/20161117060640www.nasa.gov/connect/ebooks/researchers_guide_space_environment_detail.html, live, Some highly elliptical orbits may pass through the LEO region near their lowest altitude (or perigee) but are not in a LEO orbit because their highest altitude (or apogee) exceeds {{cvt|2000|km|mi|0}}. Sub-orbital objects can also reach the LEO region but are not in a LEO orbit because they 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}}.WEB,www.spaceacademy.net.au/watch/track/leopars.htm, LEO parameters, www.spaceacademy.net.au, 2015-06-12, 2016-02-11,www.spaceacademy.net.au/watch/track/leopars.htm," title="web.archive.org/web/20160211202014www.spaceacademy.net.au/watch/track/leopars.htm,">web.archive.org/web/20160211202014www.spaceacademy.net.au/watch/track/leopars.htm, live, 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 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 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 over low-latitude locations on Earth. 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 orbits and Sun-synchronous orbits 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 orbits” (VLEO) began to be seen in 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.JOURNAL, Crisp, N. H., Roberts, P. C. E., Livadiotti, S., Oiko, V. T. A., Edmondson, S., Haigh, S. J., Huyton, C., Sinpetru, L., Smith, K. L., Worrall, S. D., Becedas, J., August 2020, The Benefits of Very Low Earth Orbit for Earth Observation Missions, Progress in Aerospace Sciences, 117, 100619, 10.1016/j.paerosci.2020.100619, 2007.07699, 2020PrAeS.11700619C, 220525689, NEWS, Messier, Doug,www.parabolicarc.com/2017/03/03/spacex-launch-12000-satellites/, SpaceX Wants to Launch 12,000 Satellites, Parabolic Arc, 2017-03-03, 2018-01-22, 2020-01-22,www.parabolicarc.com/2017/03/03/spacex-launch-12000-satellites/," title="web.archive.org/web/20200122203256www.parabolicarc.com/2017/03/03/spacex-launch-12000-satellites/,">web.archive.org/web/20200122203256www.parabolicarc.com/2017/03/03/spacex-launch-12000-satellites/, live,

Use

File:Sunrise To Sunset Aboard The ISS.OGG|thumb|Roughly half an orbit of the International Space StationInternational Space StationA low Earth orbit requires the lowest amount of energy for satellite placement. It provides high bandwidth and low communication 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 phone system. Some communication satellites use much higher geostationary orbits and move at the same angular velocity as the Earth as to appear stationary above one location on the planet.

Disadvantages

Unlike 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 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 unknownwww.scientificamerican.com/article/space-junk-is-polluting-earths-stratosphere-with-vaporized-metal/

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,WEB,www.nasa.gov/mission_pages/station/expeditions/expedition26/iss_altitude.html, Higher Altitude Improves Station’s Fuel Economy, NASA, 2013-02-12, 2015-05-15,www.nasa.gov/mission_pages/station/expeditions/expedition26/iss_altitude.html," title="web.archive.org/web/20150515050746www.nasa.gov/mission_pages/station/expeditions/expedition26/iss_altitude.html,">web.archive.org/web/20150515050746www.nasa.gov/mission_pages/station/expeditions/expedition26/iss_altitude.html, live, and needs re-boosting a few times a year due to orbital decay.
• The Iridium telecom satellites orbit at about {{convert|780|km|mi|abbr=on}}.
• Earth observation satellites, also known as remote sensing satellites, including spy satellites 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 satellites are placed in LEO.WEB, NASA Earth Observatory,earthobservatory.nasa.gov/Features/OrbitsCatalog/, earthobservatory.nasa.gov, 2009-09-04, 2015-11-28, en, Riebeek, Holli, 2018-05-27,web.archive.org/web/20180527202627/https://earthobservatory.nasa.gov/Features/OrbitsCatalog/, live, Satellites can also take advantage of consistent lighting of the surface below via 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 2021, and currently orbits between about {{convert|340|km}} and {{convert|450|km}}.
• The gravimetry mission GRACE-FO orbits at about {{convert|500|km|mi|abbr=on}} as did its predecessor, GRACE.

Former

• The Chinese Tiangong-1 station was in orbit at about {{convert|355|km}},WEB,www.people.com.cn/h/2011/1001/c25408-1418236438.html, “天宫一号成功完成二次变轨”, 2020-10-13, 2011-11-13,www.people.com.cn/h/2011/1001/c25408-1418236438.html," title="web.archive.org/web/20111113130054www.people.com.cn/h/2011/1001/c25408-1418236438.html,">web.archive.org/web/20111113130054www.people.com.cn/h/2011/1001/c25408-1418236438.html, dead, 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.
• GOCE, another gravimetry mission, orbited at about {{convert|255|km|mi|abbr=on}}.

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”.WEB, Space station from 2001: A Space Odyssey,www.esa.int/ESA_Multimedia/Images/2012/01/Space_station_from_2001_A_Space_Odyssey,

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.WEB, United Nations Office for Outer Space Affairs, 2010, Space Debris Mitigation Guidelines of the Committee on the Peaceful Uses of Outer Space,www.iadc-home.org/documents_public/view/search_field/eNortjI2tlJy1CsuTcpKTS7RdtTLS8xNBVIlielAMjk_ryQ1r6RYyRpcMAypDUU~/page/1/id/126#u, October 19, 2021, Inter-Agency Space Debris Coordination Committee (IADC), 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’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.WEB, 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/20220902133204www.orbitaldebris.jsc.nasa.gov/faq/ |archive-date=2022-09-02}} 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.WEB, Garcia, Mark, 2015-04-13, Space Debris and Human Spacecraft,www.nasa.gov/mission_pages/station/news/orbital_debris.html, 2022-09-02, NASA.gov,web.archive.org/web/20220908122950/https://www.nasa.gov/mission_pages/station/news/orbital_debris.html, 2022-09-08,

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
• Suborbital spaceflight
• Ukrainian Optical Facilities for Near-Earth Space Surveillance Network
• Van Allen radiation belt

{{colend}}{{clear}}

Notes

{{reflist|group=lower-alpha|30em}}

References

{{Use dmy dates|date=September 2022}}{{reflist|30em}}{{Include-NASA}}{{orbits}}{{People currently in space}}{{Authority control}}