Editing Geostationary orbit (section)

== Stability ==
A geostationary orbit can be achieved only at an altitude very close to {{convert|35786|km|mi|0|abbr=off}} and directly above the equator. This equates to an orbital speed of {{convert|3.07|km/s|mi/s|2| abbr=off}} and an orbital period of 1,436 minutes, one [[sidereal day]]. This ensures that the satellite will match the Earth's rotational period and has a stationary [[Footprint (satellite)|footprint]] on the ground. All geostationary satellites have to be located on this ring.

A combination of [[Moon|lunar]] gravity, [[Sun|solar]] gravity, and the [[Equatorial bulge#On Earth|flattening of the Earth]] at its poles causes a [[precession]] motion of the orbital plane of any geostationary object, with an [[orbital period]] of about 53 years and an initial inclination gradient of about 0.85° per year, achieving a maximal inclination of 15° after 26.5 years.<ref name=Anderson2015>{{Cite conference|url=http://hanspeterschaub.info/Papers/Anderson2015c.pdf |archive-url=https://ghostarchive.org/archive/20221009/http://hanspeterschaub.info/Papers/Anderson2015c.pdf |archive-date=2022-10-09 |url-status=live|title=Operational Considerations of GEO Debris Synchronization Dynamics|first1=Paul|last1=Anderson|display-authors=etal|conference=66th [[International Astronautical Congress]]|location=Jerusalem, Israel|date=2015|id=IAC-15,A6,7,3,x27478}}</ref><ref name="smad"/>{{rp|156}} To correct for this [[perturbation (astronomy)|perturbation]], regular [[orbital stationkeeping]] maneuvers are necessary, amounting to a [[delta-v]] of approximately 50&nbsp;m/s per year.<ref name="iop">{{cite conference|doi-access=free|title=Plasma propulsion for geostationary satellites fortelecommunication and interplanetary missions |conference=IOP Conference Series: Materials Science and Engineering |first1=M |last1=Dundeck |first2 = F |last2=Doveil |first3=N |last3=Arcis |first4=S |last4=Zurbach |year=2012 |number=29 |doi=10.1088/1757-899X/29/1/012010}}</ref>

A second effect to be taken into account is the longitudinal drift, caused by the asymmetry of the Earth – the equator is slightly elliptical ([[Figure of the Earth#Triaxiality (equatorial eccentricity)|equatorial eccentricity]]).<ref name="smad"/>{{rp|156}} There are two stable equilibrium points sometimes called "gravitational wells"<ref>{{cite book |last1=Jefferson Barker |title=U.S. Military Space Reference Text |date=Mar 2006 |publisher=National Security Space Institute |page=56 |url=https://edocs.nps.edu/2012/December/U.S.%20Military%20Space%20Reference%20Text%20-%202006.pdf}}</ref> (at 75.3°E and 108°W) and two corresponding unstable points (at 165.3°E and 14.7°W). Any geostationary object placed between the equilibrium points would (without any action) be slowly accelerated towards the stable equilibrium position, causing a periodic longitude variation.<ref name=Anderson2015/> The correction of this effect requires [[Orbital station-keeping#Station-keeping in geostationary orbit|station-keeping maneuvers]] with a maximal delta-v of about 2&nbsp;m/s per year, depending on the desired longitude.<ref name="iop"/>

[[Solar wind]] and [[radiation pressure]] also exert small forces on satellites: over time, these cause them to slowly drift away from their prescribed orbits.<ref>{{cite conference|url=http://www.riccardobevilacqua.com/SOLAR%20RADIATION%20PRESSURE%20APPLICATIONS%20ON%20GEOSTATIONARY%20SATELLITES_SUBMISSION.pdf |archive-url=https://ghostarchive.org/archive/20221009/http://www.riccardobevilacqua.com/SOLAR%20RADIATION%20PRESSURE%20APPLICATIONS%20ON%20GEOSTATIONARY%20SATELLITES_SUBMISSION.pdf |archive-date=2022-10-09 |url-status=live |first1=Patrick |last1=Kelly |first2=Richard S.|last2= Erwin|first3=Riccardo |last3=Bevilacqua|first4=Leonel |last4=Mazal |title=Solar radiation pressure applications on geostationary satellites |conference=Proceedings of the 2016 AAS GP & C Conference |publisher=[[American Astronautical Society]] |year=2016}}</ref>

In the absence of servicing missions from the Earth or a renewable propulsion method, the consumption of thruster propellant for station-keeping places a limitation on the lifetime of the satellite. [[Hall-effect thruster]]s, which are currently in use, have the potential to prolong the service life of a satellite by providing high-efficiency [[Electrically powered spacecraft propulsion|electric propulsion]].<ref name="iop"/>