Geocentric orbit
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2008-07-13T01:56:30Z
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{{unreferenced|date=September 2006}}
A '''geocentric orbit''' is an orbit of any object [[orbital revolution|orbiting]] the [[Earth]], such as the [[Moon]] or [[artificial satellite]]s. Currently there are approximately 2465 [[artificial satellite]]s [[orbiting]] the Earth and 6216 pieces of [[space debris]] as tracked by the [[Goddard Space Flight Center]]. Over 16,291 previously launched objects have [[Atmospheric reentry|decayed]] into the Earth's [[atmosphere]].
==List of terms and concepts==
{{dablink|The following words may have more than one definition or other Non-Earth specific definition(s).<br>In the spirit of brevity some of the definitions have been altered or [[truncated]] to reflect only their usage on this page.
}}
;[[Analemma]]: a term in [[astronomy]] used to describe the [[chart|plot]] of the positions of the [[Sun]] on the [[celestial sphere]] throughout one year. Closely resembles a figure-eight.
;[[Altitude]]: as used here, the [[height]] of an object above the [[average]] surface of the [[Earth]]'s [[ocean]]s.
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;[[Apogee]]:
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;[[Eccentricity (mathematics)|Eccentricity]]: a measure of how much an [[orbit]] [[deviates]] from a perfect [[circle]]. [[Eccentricity]] is strictly defined for all [[circular orbit|circular]], [[elliptical orbit|elliptic]], [[parabolic orbit|parabolic]] and [[hyperbolic orbit]]s.
;[[Equatorial plane]]: as used here, an [[imaginary]] [[Plane (mathematics)|plane]] extending from the equator on the [[Earth]] to the [[celestial sphere]].
;[[orbital parameters|Orbital characteristics]]: the six [[parameter]]s of the [[Keplerian elements]] needed to [[specify]] that [[orbit]] uniquely.
;[[Escape velocity]]: as used here, the minimum [[velocity]] an object without [[Spacecraft propulsion|propulsion]] needs to have to move away [[indefinitely]] from the [[earth]]. An object with such a [[velocity]] will enter an [[escape orbit]].
;[[Impulse]]: the product of a [[force]] and the [[time]] during which it acts. Measured in ([[kilogram|kg]] [[m/s]] or [[newton|N]]·[[second|s]]).
;[[Inclination]]: the [[angle]] between a [[reference plane]] and another [[Plane (mathematics)|plane]] or [[Coordinate axis|axis]]. In the sense discussed here the [[reference plane]] is the [[Earth]]'s [[equatorial plane]].
;[[Orbital period]]: as defined here, time it takes a [[satellite]] to make one full [[orbit]] about the [[Earth]].
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;[[Perigee]]:
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;[[Sidereal day]]: the time it takes for a [[celestial object]] to rotate 360°. For the [[Earth]] this is: 23 [[hour]]s, 56 [[minute]]s, 4.091 [[second]]s.
;[[Solar time]]: as used here, the local time as measured by a [[sundial]].
;[[Velocity]]: an object's [[speed]] in a particular [[direction]]. Since [[velocity]] is defined as a [[Vector (spatial)|vector]], both [[speed]] and [[direction]] are required to define it.
==Geocentric orbit types==
The following is a list of different geocentric orbit classifications.
===Altitude classifications===
:'''[[Low Earth Orbit]] (LEO)''' - Geocentric orbits ranging in altitude from 0 - 2000 km (0 - 1240 miles); one revolution takes 90 minutes, the speed is 8 kilometers per second.
:'''[[Medium Earth Orbit]] (MEO)''' - Geocentric orbits ranging in altitude from 2000 km - to just below [[geosynchronous orbit]] at 35,786 km (22,240 miles). Also known as an [[intermediate circular orbit]].
:'''[[Highly Elliptic Orbit]] (HEO)''' - Geocentric orbit whose eccentricity is close to 1.
:'''[[Geosynchronous Orbit]] (GEO)''' - Geocentric orbit with an altitude of 35,786 km (22,236 statute miles) above mean sea level. The period of the orbit coincides with the rotation period of the earth: 24 hours; the speed is 3 km/s.
===Inclination classifications===
:'''[[Inclined orbit|Inclined Orbit]]''' - An orbit whose [[inclination]] in reference to the [[equatorial plane]] is not 0.
:::'''[[Polar Orbit]]''' - A satellite that passes above or nearly above both poles of the [[planet]] on each revolution. Therefore it has an [[inclination]] of (or very close to) 90 [[degree (angle)|degree]]s.
:::'''[[Polar sun synchronous orbit|Polar Sun-synchronous Orbit]]''' - A nearly [[polar orbit]] that passes the [[equator]] at the same local time on every pass. Useful for [[image]] taking satellites because [[shadow]]s will be the same on every pass.
===Eccentricity classifications===
:'''[[Circular Orbit]]''' - An [[orbit]] that has an [[Orbital eccentricity|eccentricity]] of 0 and whose path traces a [[circle]].
::'''[[Hohmann transfer orbit]]''' - An orbital maneuver that moves a [[spacecraft]] from one [[circular orbit]] to another using two engine [[impulse]]s. This maneuver was named after [[Walter Hohmann]].
:'''[[Elliptic orbit|Elliptic Orbit]]''' - An [[orbit]] with an [[Orbital eccentricity|eccentricity]] greater than 0 and less than 1 whose orbit traces the path of an [[ellipse]].
::'''[[Geosynchronous transfer orbit|Geosynchronous Transfer Orbit]]''' - A geocentric-[[elliptic orbit]] where the [[perigee]] is at the [[altitude]] of a [[Low Earth Orbit]] (LEO) and the [[apogee]] at the [[altitude]] of a [[geosynchronous orbit]].
::'''[[Geosynchronous transfer orbit|Geostationary Transfer Orbit]]''' - A geocentric-[[elliptic orbit]] where the [[perigee]] is at the [[altitude]] of a [[Low Earth Orbit]] (LEO) and the [[apogee]] at the [[altitude]] of a [[geostationary orbit]].
::'''[[Highly Elliptical Orbit]] (HEO)''' - Geocentric orbit with apogee above 35,786 km and low perigee (about 1000 km) that result in long dwell times near apogee.
:::'''[[Molniya orbit|Molniya Orbit]]''' - A [[highly elliptical orbit]] with [[inclination]] of 63.4° and [[orbital period]] of ½ of a [[sidereal day]] (roughly 12 hours). Such a satellite spends most of its time over a designated area of the [[planet]].
:::'''[[Tundra orbit|Tundra Orbit]]''' - A [[highly elliptical orbit]] with [[inclination]] of 63.4° and [[orbital period]] of one [[sidereal day]] (roughly 24 hours). Such a satellite spends most of its time over a designated area of the [[planet]].
:'''[[Hyperbolic orbit]]''' - An [[orbit]] with the eccentricity greater than 1. Such an orbit also has a [[velocity]] in excess of the [[escape velocity]] and as such, will escape the gravataional pull of the [[planet]] and continue to travel [[infinitely]].
:'''[[Parabolic Orbit]]''' - An [[orbit]] with the eccentricity equal to 1. Such an orbit also has a [[velocity]] equal to the [[escape velocity]] and therefore will escape the gravatational pull of the [[planet]] and travel until it's [[velocity]] [[Relative motion#Relative motion|relative]] to the [[planet]] is 0. If the speed of such an orbit is increased it will become a [[hyperbolic orbit]].
::'''[[Escape orbit|Escape Orbit]] (EO)''' - A high-speed [[parabolic orbit]] where the object has [[escape velocity]] and is moving away from the [[planet]].
::'''[[Capture orbit|Capture Orbit]]''' - A high-speed [[parabolic orbit]] where the object has [[escape velocity]] and is moving toward the [[planet]].
===Directional classifications===
:'''[[Prograde orbit]]''' - an orbit in which the projection of the object onto the equatorial plane revolves about the Earth in the same direction as the rotation of the Earth.
:'''[[Retrograde orbit]]''' - an orbit in which the projection of the object onto the equatorial plane revolves about the Earth in the direction opposite that of the rotation of the Earth.
===Geosynchronous classifications===
:'''[[Semi-synchronous orbit|Semi-Synchronous Orbit]] (SSO)''' - An [[orbit]] with an [[altitude]] of approximately 20,200 km (12544.2 miles) and an [[orbital period]] of approximately 12 hours
:'''[[Geosynchronous Orbit]] (GEO)''' - Orbits with an altitude of approximately 35,786 km (22,240 miles). Such a satellite would trace an [[analemma]] (figure 8) in the sky.
:::'''[[Geostationary orbit]] (GSO)''': A [[geosynchronous orbit]] with an [[inclination]] of zero. To an observer on the ground this satellite would appear as a fixed point in the sky.
:::'''[[Clarke Orbit]]''' - Another name for a [[geostationary orbit]]. Named after the writer [[Arthur C. Clarke]].
:::'''[[Supersynchronous orbit]]''' - A disposal / storage orbit above GSO/GEO. Satellites will drift west.
:::'''[[Subsynchronous orbit]]''' - A drift orbit close to but below GSO/GEO. Satellites will drift east.
:::'''[[Graveyard orbit|Graveyard Orbit]]''' - An orbit a few hundred kilometers above [[geosynchronous]] that satellites are moved into at the end of their operation.
::::'''[[Disposal Orbit]]''' - A [[synonym]] for [[graveyard orbit]].
::::'''[[junk orbit|Junk Orbit]]''' - A [[synonym]] for [[graveyard orbit]].
===Special classifications===
:'''[[Sun-synchronous orbit|Sun-synchronous Orbit]]''' - An orbit which combines [[altitude]] and [[inclination]] in such a way that the [[satellite]] passes over any given point of the [[planet]]'s surface at the same local [[solar time]]. Such an orbit can place a satellite in constant sunlight and is useful for [[imaging]], [[spy satellite|spy]], and [[weather satellite]]s.
:'''[[Moon orbit|Moon Orbit]]''' - The [[Orbital parameters|orbital characteristics]] of [[Earth]]'s [[Moon]]. Average [[altitude]] of 384,403 kilometres (238,857 mi), [[elliptical orbit|elliptical]]-[[inclined orbit|inclined]] orbit.
===Non-geocentric classifications===
:'''[[Horseshoe orbit|Horseshoe Orbit]]''' - An [[orbit]] that appears to a ground observer to be orbiting a [[planet]] but is actually in [[Co-orbital satellite|co-orbit]] with it. See asteroids [[3753 Cruithne|3753]] (Cruithne) and [[2002 AA29|2002 AA<sub>29</sub>]].
:'''[[Exo-orbit]]''' - A maneuver where a [[spacecraft]] approaches the height of [[orbit]] but lacks the [[velocity]] to sustain it.
::'''[[Sub-Orbital spaceflight|Sub-Orbital Spaceflight]]''' - A [[synonym]] for [[Exo-orbit]].
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'''[[Retrograde orbit]]''' -
'''[[Decaying orbit]]''' - See [[orbital decay]].
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{{earth orbits}}
==See also==
*[[List of orbits]]
*[[Astronomy]]
*[[Astrodynamics]]
*[[Celestial sphere]]
*[[Orbit]]
*[[Heliocentric orbit]]
*[[Areosynchronous satellite]]
*[[Areostationary satellite]]
*[[Escape velocity]]
*[[Satellite]]
==References==
*{{cite web | title=Satellite Situation Report | publisher=NASA Goddard Space Flight Center | date=[[2000-02-01]] | url=http://liftoff.msfc.nasa.gov/academy/rocket_sci/satellites/ssr.html | accessdate=2006-09-10}}
*http://www.freemars.org/jeff/speed/index.htm
*http://www.tech-faq.com/medium-earth-orbit.shtml
*http://www.hq.nasa.gov/office/pao/History/conghand/traject.htm
*http://www.space.com/scienceastronomy/solarsystem/second_moon_991029.html
*http://www.astro.uwo.ca/~wiegert/3753/3753.html
*http://www.astro.uwo.ca/~wiegert/AA29/AA29.html
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[[Category:Astrodynamics]]
[[Category:Earth orbits|*]]
[[Category:Satellites]]
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