Editing Geosynchronous orbit

{{short description|Orbit keeping the satellite at a fixed longitude above the equator}}
{{Good article}}
{{Use mdy dates|date=February 2020}}
[[File:Geosynchronous orbit.gif|thumb|upright=1.3|Animation (not to scale) showing geosynchronous satellite orbiting the Earth]]

A '''geosynchronous orbit''' (sometimes abbreviated '''GSO''') is an Earth-centered [[orbit]] with an [[orbital period]] that matches [[Earth's rotation]] on its axis, 23 hours, 56 minutes, and 4 seconds (one [[sidereal day]]). The synchronization of rotation and orbital period means that, for an observer on Earth's surface, an object in geosynchronous orbit returns to exactly the same position in the sky after a period of one sidereal day. Over the course of a day, the object's position in the sky may remain still or trace out a path, [[Analemma#Of geosynchronous satellites|typically in a figure-8 form]], whose precise characteristics depend on the orbit's [[Orbital inclination|inclination]] and [[Orbital eccentricity|eccentricity]]. A circular geosynchronous orbit has a constant altitude of {{convert|35786|km|mi|0|abbr=on}}.<ref name=sdc20150426>{{cite news |last=Howell |first=Elizabeth |title=What Is a Geosynchronous Orbit? |url=https://www.space.com/29222-geosynchronous-orbit.html |work=Space.com |access-date=15 July 2022}}</ref>

A special case of geosynchronous orbit is the [[geostationary orbit]] (often abbreviated ''GEO''), which is a circular geosynchronous orbit in Earth's [[equatorial plane]] with both inclination and eccentricity equal to 0. A satellite in a geostationary orbit remains in the same position in the sky to observers on the surface.<ref name=sdc20150426 />

[[Communications satellite]]s are often given geostationary or close-to-geostationary orbits, so that the [[satellite antenna]]s that communicate with them do not have to move but can be pointed permanently at the fixed location in the sky where the satellite appears.<ref name=sdc20150426 />

== History ==
[[File:Clarke sm.jpg|thumb|upright|The geosynchronous orbit was popularised by the science fiction author [[Arthur C. Clarke]], and is thus sometimes called the Clarke Orbit.]]
In 1929, [[Herman Potočnik]] described both geosynchronous orbits in general and the special case of the geostationary Earth orbit in particular as useful orbits for [[space station]]s.<ref>{{cite book |last=Noordung |first=Hermann |url=https://commons.wikimedia.org/w/index.php?title=File%3AHerman_Poto%C4%8Dnik_Noordung_-_Das_Problem_der_Befahrung_des_Weltraums.pdf&page=102 |title=Das Problem der Befahrung des Weltraums: Der Raketen-Motor |publisher=Richard Carl Schmidt & Co. |year=1929 |location=Berlin |pages=98–100 |format=PDF}}</ref> The first appearance of a geosynchronous [[orbit]] in popular literature was in October 1942, in the first [[Venus Equilateral]] story by [[George O. Smith]],<ref name="VE">"(Korvus's message is sent) to a small, squat building at the outskirts of Northern Landing. It was hurled at the sky. ... It ... arrived at the relay station tired and worn, ... when it reached a space station only five hundred miles above the city of North Landing." {{cite book |last=Smith |first=George O.|author-link=George O. Smith |title=The Complete Venus Equilateral |date=1976 |publisher=[[Ballantine Books]] |location=New York |isbn=978-0-345-28953-7 |pages=3–4 |url=https://books.google.com/books?id=lj8H3R4J5GUC&q=squat}}</ref> but Smith did not go into details. British [[science fiction]] author [[Arthur C. Clarke]] popularised and expanded the concept in a 1945 paper entitled ''Extra-Terrestrial Relays – Can Rocket Stations Give Worldwide Radio Coverage?'', published in ''[[Wireless World]]'' magazine. Clarke acknowledged the connection in his introduction to ''The Complete Venus Equilateral''.<ref name="VEintro">"It is therefore quite possible that these stories influenced me subconsciously when ... I worked out the principles of synchronous communications satellites ...", {{cite book |url=https://archive.org/details/arthurcclarkeaut00mcal/page/54 |page=54 |title=Arthur C. Clarke |first=Neil |last=McAleer |year=1992 |isbn=978-0-809-24324-2 |publisher=Contemporary Books}}</ref><ref name="clarke"/> The orbit, which Clarke first described as useful for broadcast and relay communications satellites,<ref name="clarke">{{cite magazine |first=Arthur C. |last=Clarke |author-link=Arthur C. Clarke |url=http://www.clarkefoundation.org/docs/ClarkeWirelessWorldArticle.pdf |title=Extra-Terrestrial Relays – Can Rocket Stations Give Worldwide Radio Coverage? |date=October 1945 |magazine=[[Wireless World]] |pages=305–308 |access-date=March 4, 2009 |archive-url=https://web.archive.org/web/20090318000548/http://www.clarkefoundation.org/docs/ClarkeWirelessWorldArticle.pdf |archive-date=March 18, 2009}}</ref> is sometimes called the Clarke Orbit.<ref>{{cite web |publisher=[[NASA]] |url=http://www2.jpl.nasa.gov/basics/bsf5-1.php |title=Basics of Space Flight Section 1 Part 5, Geostationary Orbits |access-date=August 25, 2019 |editor=Phillips Davis}}</ref> Similarly, the collection of artificial satellites in this orbit is known as the Clarke Belt.<ref>{{cite magazine |url=http://web.mit.edu/m-i-t/science_fiction/jenkins/jenkins_4.html |title=Orbit Wars: Arthur C. Clarke and the Global Communications Satellite |last= Mills |first=Mike |magazine=The Washington Post Magazine |date=August 3, 1997 |pages=12–13 |access-date=August 25, 2019}}</ref>

[[File:Syncom 2 side.jpg|[[Syncom#Syncom 2|Syncom 2]]: The first functional geosynchronous satellite|thumb|left]]

In technical terminology, the geosynchronous orbits are often referred to as geostationary if they are roughly over the equator, but the terms are used somewhat interchangeably.<ref>{{cite book|chapter=Satellites and satellite remote senssing:{{vague|reason=check source todetermine if spelling error is our editor’s, or the source’s: one calls for ‘’[[sic]]’’ tagging; the other for mere correction.|date=October 2020}} --> Orbits |title=Encyclopedia of Atmospheric Sciences |edition=2 |year=2015 |pages=95–106 |last=Kidder |first=S.Q. |editor-first=Gerald |editor-last=North |editor-first2=John |editor-last2=Pyla |editor-first3=Fuqing |editor-last3=Zhang |doi=10.1016/B978-0-12-382225-3.00362-5 |publisher=Elsiver|isbn=978-0-12-382225-3}}</ref><ref>{{cite book |first=C.D. |last=Brown |year=1998 |url=https://books.google.com/books?id=vpilMLP7OHQC&pg=PA81 |title=Spacecraft Mission Design |edition=2nd |publisher=AIAA Education Series |page=81 |isbn=978-1-60086-115-4}}</ref> Specifically, '''geosynchronous Earth orbit''' ('''GEO''') may be a synonym for ''geosynchronous [[Near-equatorial orbit|equatorial orbit]]'',<ref>{{cite web |title=Ariane 5 User's Manual Issue 5 Revision 1 |url=http://www.arianespace.com/launch-services-ariane5/Ariane5_users_manual_Issue5_July2011.pdf |publisher=Ariane Space |access-date=28 July 2013 |date=July 2011 |url-status=dead |archive-url=https://web.archive.org/web/20131004215844/http://www.arianespace.com/launch-services-ariane5/Ariane5_users_manual_Issue5_July2011.pdf |archive-date=4 October 2013 }}</ref> or ''geostationary Earth orbit''.<ref name=NASA2001/>

The first geosynchronous satellite was designed by [[Harold Rosen (electrical engineer)|Harold Rosen]] while he was working at [[Hughes Aircraft]] in 1959. Inspired by [[Sputnik 1]], he wanted to use a geostationary (geosynchronous equatorial) satellite to globalise communications. Telecommunications between the US and Europe was then possible between just 136 people at a time, and reliant on [[high frequency]] radios and an [[Submarine communications cable|undersea cable]].<ref name=dm>{{Cite magazine|first=Jack|last=McClintock|date=November 9, 2003|url=http://discovermagazine.com/2003/nov/communications|title=Communications: Harold Rosen – The Seer of Geostationary Satellites|website=Discover Magazine |access-date=August 25, 2019}}</ref>

Conventional wisdom at the time was that it would require too much [[rocket]] power to place a satellite in a geosynchronous orbit and it would not survive long enough to justify the expense,<ref>{{Cite book|url=https://www.caltech.edu/about/news/harold-rosen-1926-2017-53790|title=Harold Rosen, 1926–2017|publisher=Caltech|last=Perkins|first=Robert|date=January 31, 2017 |access-date=August 25, 2019}}</ref> so early efforts were put towards constellations of satellites in [[low Earth orbit|low]] or [[Medium Earth Orbit|medium]] Earth orbit.<ref name="lat"/> The first of these were the passive [[Project Echo|Echo balloon satellites]] in 1960, followed by [[Telstar 1]] in 1962.<ref>{{cite book|title=Beyond The Ionosphere: Fifty Years of Satellite Communication|year=1997|chapter-url=https://history.nasa.gov/SP-4217/ch6.htm |first=Daniel R.|last=Glover |editor=Andrew J Butrica|publisher=NASA |chapter=Chapter 6: NASA Experimental Communications Satellites, 1958-1995|bibcode=1997bify.book.....B}}</ref> Although these projects had difficulties with signal strength and tracking that could be solved through geosynchronous satellites, the concept was seen as impractical, so Hughes often withheld funds and support.<ref name="lat">{{Cite news|url=https://www.latimes.com/nation/la-na-syncom-satellite-20130726-dto-htmlstory.html|title=How a satellite called Syncom changed the world|first=Ralph|last=Vartabedian|newspaper=[[Los Angeles Times]] |date=July 26, 2013 |access-date=August 25, 2019}}</ref><ref name=dm/>

By 1961, Rosen and his team had produced a cylindrical prototype with a diameter of {{convert|76|cm|in}}, height of {{convert|38|cm|in}}, weighing {{convert|11.3|kg|lb}}; it was light, and small, enough to be placed into orbit by then-available rocketry, was [[Spin-stabilisation|spin stabilised]] and used dipole antennas<!-- As a physicist/engineer, I boldly changed the grammatical number of “antenna”, in the belief that a single antenna could not have been economically/logistically efficient. A reliable source should be sought, however. --> producing a pancake-shaped waveform.  <!-- “pancake-shaped waveform” is plainly nonsense:  Most likely what was intended is to insinuate that dispersion out of a preferred plane was limited; it‘s plausible that it reflects nothing more than our colleague misconstruing a graphic that was  intended to convey something entirely different. --><ref>{{cite web|url=https://nssdc.gsfc.nasa.gov/nmc/spacecraft/display.action?id=1963-031A|publisher=NASA|title=Syncom 2|editor=David R. Williams |access-date=September 29, 2019}}</ref> In August 1961, they were contracted to begin building the working satellite.<ref name=dm/> They lost [[Syncom#Syncom 1|Syncom 1]] to electronics failure, but Syncom 2 was successfully placed into a geosynchronous orbit in 1963. Although its [[inclined orbit]] still required moving antennas, it was able to relay TV transmissions, and allowed for US President [[John F. Kennedy]] to phone Nigerian prime minister [[Abubakar Tafawa Balewa]] from a ship on August 23, 1963.<ref name="lat"/><ref>{{Cite web|url=https://www.historychannel.com.au/this-day-in-history/worlds-first-geosynchronous-satellite-launched/|title=World's First Geosynchronous Satellite Launched|publisher=Foxtel|date=June 19, 2016|website=History Channel|access-date=August 25, 2019|archive-date=December 7, 2019|archive-url=https://web.archive.org/web/20191207144926/https://www.historychannel.com.au/this-day-in-history/worlds-first-geosynchronous-satellite-launched/|url-status=dead}}</ref>

Today there are hundreds of geosynchronous satellites providing [[remote sensing]], navigation and communications.<ref name=dm/><ref name=sdc20150426 />

Although most populated land locations on the planet now have terrestrial communications facilities ([[microwave]], [[fiber-optic]]), which often have latency and bandwidth advantages, and telephone access covering 96% of the population and internet access 90% as of 2018,<ref>{{cite web|url=https://www.itu.int/en/mediacentre/Pages/2018-PR40.aspx |title=ITU releases 2018 global and regional ICT estimates |publisher=[[International Telecommunication Union]] |access-date=August 25, 2019 |date=December 7, 2018}}</ref> some rural and remote areas in developed countries are still reliant on satellite communications.<ref>{{cite news |last=Thompson |first=Geoff |title=Australia was promised superfast broadband with the NBN. This is what we got |url=https://www.abc.net.au/news/2019-04-23/what-happened-to-superfast-nbn/11037620 |publisher=[[Australian Broadcasting Corporation|ABC]] |date=April 24, 2019 |access-date=August 25, 2019}}</ref><ref>{{cite news |publisher=[[CNET]] |url=https://www.cnet.com/news/in-rural-farm-country-forget-broadband-you-might-not-have-internet-at-all/ |title=In farm country, forget broadband. You might not have internet at all. 5G is around the corner, yet pockets of America still can't get basic internet access. |first=Shara |last=Tibken |date=October 22, 2018 |access-date=August 25, 2019}}</ref>

== Types ==

=== Geostationary orbit ===
{{main|Geostationary orbit}}
[[File:Geostat.gif|thumb|The geostationary satellite (green) always remains above the same marked spot on the equator (brown).]]
A geostationary equatorial orbit (GEO) is a circular geosynchronous orbit in the plane of the Earth's equator with a radius of approximately {{convert|42164|km|mi|0|abbr=on}} (measured from the center of the Earth).<ref name="smad"/>{{rp|156}} A satellite in such an orbit is at an altitude of approximately {{convert|35786|km|mi|0|abbr=on}} above mean sea level. It maintains the same position relative to the Earth's surface. If one could see a satellite in geostationary orbit, it would appear to hover at the same point in the sky, i.e., not exhibit [[diurnal motion]], while the Sun, Moon, and stars would traverse the skies behind it. Such orbits are useful for [[telecommunications satellite]]s.<ref>{{cite web|url=https://www.esa.int/Our_Activities/Telecommunications_Integrated_Applications/Orbits |title=Orbits |publisher=[[ESA]] |access-date=October 1, 2019 |date=October 4, 2018}}</ref>

A perfectly stable geostationary orbit is an ideal that can only be approximated. In practice the satellite drifts out of this orbit because of perturbations such as the [[solar wind]], [[radiation pressure]], variations in the Earth's gravitational field, and the [[gravity|gravitational]] effect of the [[Moon]] and [[Sun]], and thrusters are used to maintain the orbit in a process known as [[orbital station-keeping|station-keeping]].<ref name="smad">{{cite book|title=Space Mission Analysis and Design|publisher=Microcosm Press and Kluwer Academic Publishers |editor1-first=Wiley J. |editor1-last=Larson |editor2-first=James R. |editor2-last=Wertz |bibcode=1999smad.book.....W |last1=Wertz |first1=James Richard |last2=Larson |first2=Wiley J. |year=1999|isbn=978-1-881883-10-4}}</ref>{{rp|156}}

Eventually, without the use of thrusters, the orbit will become inclined, oscillating between 0° and 15° every 55 years. At the end of the satellite's lifetime, when fuel approaches depletion, satellite operators may decide to omit these expensive manoeuvres to correct inclination and only control eccentricity. This prolongs the life-time of the satellite as it consumes less fuel over time, but the satellite can then only be used by ground antennas capable of following the N-S movement.<ref name="smad"/>{{rp|156}}

Geostationary satellites will also tend to drift around one of two stable longitudes of 75° and 255° without station keeping.<ref name="smad"/>{{rp|157}}

=== Elliptical and inclined geosynchronous orbits ===

[[File:Qzss-45-0.09.jpg|left|thumb|A quasi-[[zenith]] satellite orbit]]

Many objects in geosynchronous orbits have eccentric and/or inclined orbits. Eccentricity makes the orbit elliptical and appear to oscillate E-W in the sky from the viewpoint of a ground station, while inclination tilts the orbit compared to the equator and makes it appear to oscillate N-S from a groundstation. These effects combine to form an [[analemma]] (figure-8).<ref name="smad"/>{{rp|122}}

Satellites in elliptical/eccentric orbits must be tracked by steerable [[ground station]]s.<ref name="smad"/>{{rp|122}}

====Tundra orbit====
{{main|Tundra orbit}}

The Tundra orbit is an eccentric geosynchronous orbit, which allows the satellite to spend most of its time dwelling over one high latitude location. It sits at an inclination of 63.4°, which is a [[frozen orbit]], which reduces the need for [[orbital station-keeping|stationkeeping]].<ref name="scs">{{cite book|url=https://books.google.com/books?id=PEsmLaDXzvsC&q=tundra&pg=PT110 |section=2.2.1.2 Tundra Orbits |isbn=978-1-119-96509-1 |title=Satellite Communications Systems: Systems, Techniques and Technology|last1=Maral |first1=Gerard |last2=Bousquet |first2=Michel |date=2011-08-24|publisher=John Wiley & Sons }}</ref> At least two satellites are needed to provide continuous coverage over an area.<ref name="jenkin">{{cite conference|title=Tundra Disposal Orbit Study |surname1=Jenkin |given1=A.B. |surname2=McVey |given2=J.P. |surname3=Wilson |given3=J.R. |surname4=Sorge |given4=M.E. |date=2017 |publisher=ESA Space Debris Office|conference=7th European Conference on Space Debris|url=https://conference.sdo.esoc.esa.int/proceedings/sdc7/paper/328|access-date=2017-10-02|archive-url=https://web.archive.org/web/20171002121240/https://conference.sdo.esoc.esa.int/proceedings/sdc7/paper/328|archive-date=2017-10-02|url-status=dead}}</ref> It was used by the [[Sirius XM Satellite Radio]] to improve signal strength in the northern US and Canada.<ref name="Sirius Launch">{{cite web
  | title=Sirius Rising: Proton-M Ready to Launch Digital Radio Satellite Into Orbit
  | url=http://www.americaspace.com/2013/10/18/sirius-rising-proton-m-ready-to-launch-digital-radio-satellite-into-orbit/
  | website=AmericaSpace
  | access-date=8 July 2017
  | date=2013-10-18
  | archive-url=https://web.archive.org/web/20170628043459/http://www.americaspace.com/2013/10/18/sirius-rising-proton-m-ready-to-launch-digital-radio-satellite-into-orbit/
  | archive-date=28 June 2017
  | url-status=live
  }}</ref>

====Quasi-zenith orbit====
The [[Quasi-Zenith Satellite System]] (QZSS) is a four-satellite system that operates in a geosynchronous orbit at an inclination of 42° and a 0.075 eccentricity.<ref>{{citation |title=Interface Specifications for QZSS |version=version 1.7 |url=http://qz-vision.jaxa.jp/USE/is-qzss/index_e.html |date=2016-07-14 |author=Japan Aerospace Exploration Agency |pages=7–8 |url-status=dead |archive-url=https://web.archive.org/web/20130406032030/http://qz-vision.jaxa.jp/USE/is-qzss/index_e.html |archive-date=2013-04-06}}</ref> Each satellite dwells over [[Japan]], allowing signals to reach receivers in [[urban canyons]] then passes quickly over Australia.<ref>{{cite web |url=http://qzss.go.jp/en/technical/technology/orbit.html |title=Quasi-Zenith Satellite Orbit (QZO) |access-date=2018-03-10 |archive-url=https://web.archive.org/web/20180309194252/http://qzss.go.jp/en/technical/technology/orbit.html |archive-date=2018-03-09 |url-status=live}}</ref>

==Launch==
{{see also|Geostationary transfer orbit}}
{{Multiple image
| perrow            = 1
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| image1            = Animation of EchoStar XVII trajectory.gif
| image2            = Animation of EchoStar XVII trajectory Equatorial view.gif
| footer            = An example of a transition from Geostationary Transfer Orbit (GTO) to Geosynchronous Orbit (GSO):<br />{{legend2|magenta|[[EchoStar XVII]]}}{{·}}{{legend2|RoyalBlue|[[Earth]]}}.
}}

Geosynchronous satellites are launched to the east into a prograde orbit that matches the rotation rate of the equator. The smallest inclination that a satellite can be launched into is that of the launch site's latitude, so launching the satellite from close to the equator limits the amount of [[Orbital inclination change|inclination change]] needed later.<ref name="conf"/> Additionally, launching from close to the equator allows the speed of the Earth's rotation to give the satellite a boost. A launch site should have water or deserts to the east, so any failed rockets do not fall on a populated area.<ref>{{cite web|url=https://www.eumetsat.int/website/home/Satellites/LaunchesandOrbits/LaunchingSatellites/index.html|title=Launching Satellites|website=[[EUMETSAT]]|access-date=January 26, 2020|archive-date=December 21, 2019|archive-url=https://web.archive.org/web/20191221164243/https://www.eumetsat.int/website/home/Satellites/LaunchesandOrbits/LaunchingSatellites/index.html|url-status=dead}}</ref>

Most [[launch vehicle]]s place geosynchronous satellites directly into a [[geosynchronous transfer orbit]] (GTO), an elliptical orbit with an [[apsis|apogee]] at GSO height and a low [[apsis|perigee]]. On-board satellite propulsion is then used to raise the perigee, circularise and reach GSO.<ref name="conf">{{cite conference|url=https://www.researchgate.net/publication/282014319 |conference=20th International Symposium on Space Flight Dynamics|first1=Nicholas |last1=Farber |first2=Andrea |last2=Aresini |first3=Pascal |last3=Wauthier |first4=Philippe |last4=Francken|date=September 2007 |title=A general approach to the geostationary transfer orbit mission recovery |page=2}}</ref><ref>{{cite web|url=http://www.planetary.org/blogs/jason-davis/20140116-how-to-get-a-satellite-to-gto.html |title=How to get a satellite to geostationary orbit |first=Jason |last=Davis|date=January 17, 2014|access-date=October 2, 2019 |publisher=The Planetary Society}}</ref>

Once in a viable geostationary orbit, spacecraft can change their longitudinal position by adjusting their semi-major axis such that the new period is shorter or longer than a sidereal day, in order to effect an apparent "drift" Eastward or Westward, respectively.  Once at the desired longitude, the spacecraft's period is restored to geosynchronous.<ref name="satsig-repo-gso">{{cite web |title=Repositioning geostationary satellites |url=https://www.satsig.net/orbit-research/geo-orbit-repositioning.htm |website=Satellite Signals |access-date=23 May 2023 |archive-url=https://web.archive.org/web/20221127202135/https://www.satsig.net/orbit-research/geo-orbit-repositioning.htm |archive-date=27 November 2022 |date=22 February 2022 |url-status=live}}</ref>

==Proposed orbits==

===Statite proposal===
A [[statite]] is a hypothetical satellite that uses [[Radiation pressure#Solar radiation pressure|radiation pressure]] from the Sun against a [[solar sail]] to modify its orbit.<ref name="st"/>

It would hold its location over the dark side of the Earth at a latitude of approximately 30 degrees. It would return to the same spot in the sky every 24 hours from an Earth-based viewer's perspective, so be functionally similar to a geosynchronous orbit.<ref name="st">{{cite patent |country=US |number=5183225 |status=patent |title=Statite: Spacecraft That Utilizes Sight Pressure and Method of Use |pubdate=February 2, 1993 |pridate=1989-01-09 |inventor-surname=Forward |inventor-given=Robert}}</ref><ref>{{cite magazine |magazine=New Scientist |date=March 9, 1991 |title=Science: Polar 'satellite' could revolutionise communications |issue=1759|url=https://www.newscientist.com/article/mg12917594-000-science-polar-satellite-could-revolutionisecommunications/ |access-date=October 2, 2019}}</ref>

===Space elevator===
A further form of geosynchronous orbit is the theoretical [[space elevator]]. If a mass orbiting above the geostationary belt is tethered to the earth’s surface, and the mass is accelerated to maintain an orbital period equal to one sidereal day, then since the orbit now requires more downward force than is supplied by gravity alone.  The tether will become tensioned by the extra centripetal force required, and this tension force is available to hoist objects up the tether structure.<ref>{{cite web |url=http://www.niac.usra.edu/files/studies/final_report/521Edwards.pdf |archive-url=https://ghostarchive.org/archive/20221009/http://www.niac.usra.edu/files/studies/final_report/521Edwards.pdf |archive-date=2022-10-09 |url-status=live |title=The Space Elevator NIAC Phase II Final Report |date=1 March 2003 |first=Bradley C. |last=Edwards |page=26 |publisher=[[NASA Institute for Advanced Concepts]]}}</ref>

== Retired satellites ==
[[File:Debris-GEO1280.jpg|thumb|alt=Earth from space, surrounded by small white dots|A computer-generated image of space debris. Two debris fields are shown: around geosynchronous space and low Earth orbit.]]
Geosynchronous satellites require some [[Orbital station-keeping|station-keeping]] in order to remain in position, and once they run out of thruster fuel and are no longer useful they are moved into a higher [[graveyard orbit]]. It is not feasible to deorbit geosynchronous satellites, for to do so would take far more fuel than would be used by slightly elevating the orbit; and atmospheric drag is negligible, giving GSOs lifetimes of thousands of years.<ref>{{cite web |url=https://www.nasa.gov/news/debris_faq.html |title=Frequently Asked Questions: Orbital Debris |publisher=NASA |date=September 2, 2011 |access-date=February 9, 2020 |archive-date=March 23, 2020 |archive-url=https://web.archive.org/web/20200323170238/https://www.nasa.gov/news/debris_faq.html |url-status=dead}}</ref>

The retirement process is becoming increasingly regulated and satellites must have a 90% chance of moving over 200&nbsp;km above the geostationary belt at end of life.<ref>{{cite web |url=https://phys.org/news/2017-04-satellites-die.html |title=Where old satellites go to die |website=phys.org |date=April 3, 2017 |author=EUMETSAT |author-link=EUMETSAT}}</ref>

=== Space debris ===
{{main|Space debris#Characterization}}
Space debris in geosynchronous orbits typically has a lower collision speed than at LEO since most GSO satellites orbit in the same plane, altitude and speed; however, the presence of satellites in [[eccentric orbit]]s allows for collisions at up to 4&nbsp;km/s. Although a collision is comparatively unlikely, GSO satellites have a limited ability to avoid any debris.<ref>{{cite web |url=https://physicsworld.com/a/space-debris-threat-to-geosynchronous-satellites-has-been-drastically-underestimated/ |title=Space debris threat to geosynchronous satellites has been drastically underestimated |date=December 12, 2017 |website=Physics World |first=Marric |last=Stephens}}</ref>

Debris less than 10&nbsp;cm in diameter cannot be seen from the Earth, making it difficult to assess their prevalence.<ref name="telk1">{{cite web |url=https://spacenews.com/exoanalytic-video-shows-telkom-1-satellite-erupting-debris/ |title=ExoAnalytic video shows Telkom-1 satellite erupting debris |date=August 30, 2017 |website=SpaceNews.com |first=Caleb |last=Henry}}</ref>

Despite efforts to reduce risk, spacecraft collisions have occurred. The [[European Space Agency]] telecom satellite [[Olympus-1]] was struck by a [[meteoroid]] on August 11, 1993, and eventually moved to a [[graveyard orbit]],<ref name="The Olympus failure">{{cite press release |title=N° 40–1993: OLYMPUS: End of mission |url=https://www.esa.int/Newsroom/Press_Releases/OLYMPUS_End_of_mission |publisher=[[ESA]] |access-date=23 May 2023 |archive-url=https://web.archive.org/web/20221031211545/https://www.esa.int/Newsroom/Press_Releases/OLYMPUS_End_of_mission |archive-date=31 October 2022 |date=26 August 1993 |url-status=live |id=40–1993}}</ref> and in 2006 the Russian [[Express (satellite)|Express-AM11]] communications satellite was struck by an unknown object and rendered inoperable,<ref name=srdc20060419>{{cite web |url=http://www.spaceref.com/news/viewsr.html?pid=20320 |archive-url=https://archive.today/20130104185122/http://www.spaceref.com/news/viewsr.html?pid=20320 |url-status=dead |archive-date=January 4, 2013 |title=Notification for Express-AM11 satellite users in connection with the spacecraft failure |publisher=Russian Satellite Communications Company |date=April 19, 2006 |via=Spaceref}}</ref> although its engineers had enough contact time with the satellite to send it into a graveyard orbit. In 2017 both [[AMC-9]] and [[Telkom-1]] broke apart from an unknown cause.<ref>{{cite web |url=https://spacenews.com/op-ed-do-we-care-about-orbital-debris-at-all/|title=Do we care about orbital debris at all? |first=James E. |last=Dunstan |date=January 30, 2018 |website=SpaceNews.com}}</ref><ref name="telk1"/><ref>{{cite web|url=http://spaceflight101.com/amc-9-satellite-anomaly-orbit-change/|title=AMC 9 Satellite Anomaly associated with Energetic Event & sudden Orbit Change – Spaceflight101|date=June 20, 2017|website=spaceflight101.com|access-date=January 27, 2020|archive-date=December 26, 2019|archive-url=https://web.archive.org/web/20191226011502/http://spaceflight101.com/amc-9-satellite-anomaly-orbit-change/|url-status=dead}}</ref>

==Properties==
[[File:Geosynchronous no geostationary orbit.gif|thumb|The orbit of a geosynchronous satellite at an inclination, from the perspective of an off-Earth observer ([[Earth-centered inertial|ECI]]) and of an observer rotating around the Earth at its spin rate ([[ECEF]]).]]

A geosynchronous orbit has the following properties:

* Period: 1436 minutes (one [[sidereal day]])
* [[Semi-major and semi-minor axes#Astronomy|Semi-major axis]]: 42,164&nbsp;km<ref name="smad"/>{{rp|121}}

===Period===
All geosynchronous orbits have an orbital period equal to exactly one sidereal day.<ref>{{cite book |editor-first1=Vladimir|editor-last1=Chobotov |year=1996 |title=Orbital Mechanics |publisher=AIAA Education Series |page=304|edition=2nd|location=Washington, DC|isbn=9781563471797|oclc=807084516}}</ref> This means that the satellite will return to the same point above the Earth's surface every (sidereal) day, regardless of other orbital properties.<ref>
{{cite book |title=Fundamentals of Astrodynamics and Applications |last=Vallado |first=David A. |year=2007 |publisher=Microcosm Press |location=Hawthorne, CA  |pages=31|oclc=263448232}}
</ref><ref name="smad"/>{{rp|121}} This orbital period, T, is directly related to the semi-major axis of the orbit through the formula:

: <math>T = 2\pi\sqrt{a^3 \over \mu}</math>

where:
: {{mvar|a}} is the length of the orbit's semi-major axis
: <math>\mu</math> is the [[standard gravitational parameter]] of the central body<ref name="smad"/>{{rp|137}}

===Inclination===
A geosynchronous orbit can have any inclination.

Satellites commonly have an inclination of zero, ensuring that the orbit remains over the equator at all times, making it stationary with respect to latitude from the point of view of a ground observer (and in the [[ECEF]] reference frame).<ref name="smad"/>{{rp|122}}

Another popular inclinations is 63.4° for a Tundra orbit, which ensures that the orbit's [[argument of perigee]] does not change over time.<ref name="scs"/>

===Ground track===

In the special case of a geostationary orbit, the [[ground track]] of a satellite is a single point on the [[equator]]. In the general case of a geosynchronous orbit with a non-zero [[inclination]] or [[Eccentricity (orbit)|eccentricity]], the ground track is a more or less distorted figure-eight, returning to the same places once per sidereal day.<ref name="smad"/>{{rp|122}}

==See also==
{{Portal|Spaceflight}}
* [[Geostationary orbit]]
* [[Geosynchronous satellite]]
* [[Graveyard orbit]]
* [[High Earth orbit]]
* [[List of orbits]]
* [[List of satellites in geosynchronous orbit]]
* [[Low Earth orbit]]
* [[Medium Earth orbit]]
* [[Molniya orbit]]
* [[Subsynchronous orbit]]
* [[Supersynchronous orbit]]
* [[Synchronous orbit]]

== References ==
{{Reflist
|refs =
<ref name = NASA2001>{{cite web
 |url          = http://www.nasa.gov/audience/forstudents/5-8/features/orbit_feature_5-8.html
 |title        = What is orbit?
 |date         = October 25, 2001
 |publisher    = [[NASA]]
 |quote        = Satellites that seem to be attached to some location on Earth are in Geosynchronous Earth Orbit (GEO)...Satellites headed for GEO first go to an elliptical orbit with an apogee about 23,000 miles. Firing the rocket engines at apogee then makes the orbit round. Geosynchronous orbits are also called geostationary.
 |access-date  = 2013-03-10
 |archive-date = April 6, 2013
 |archive-url  = https://web.archive.org/web/20130406021840/http://www.nasa.gov/audience/forstudents/5-8/features/orbit_feature_5-8.html
 |url-status   = dead
}}</ref>
}}

== External links ==
* [https://web.archive.org/web/20131019171701/http://www.zarya.info/Diaries/Launches/geo-loc.php Satellites currently in Geosynchronous Orbit, list updated daily]
* [https://web.archive.org/web/20050821080635/http://science.nasa.gov/realtime/rocket_sci/satellites/geo-high.html Science@NASA – Geosynchronous Orbit]
* [http://www2.jpl.nasa.gov/basics/bsf5-1.php NASA – Planetary Orbits]
* [https://web.archive.org/web/20041104195134/http://www.sciencepresse.qc.ca/clafleur/Scfam-geostationary.html Science Presse data on Geosynchronous Orbits (including historical data and launch statistics)]
* [https://web.archive.org/web/20120204054322/http://www.braeunig.us/space/orbmech.htm Orbital Mechanics] (Rocket and Space Technology)
* {{APOD |date=11 April 2012 |title=Time lapse of Geostationary Satellites Beyond the Alps}}
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[[Category:Earth orbits]]
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