Geocentric orbit 568972 225318974 2008-07-13T01:56:30Z 76.173.76.40 /* Altitude classifications */ {{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. <!-- Need help. Don't know how to define this succinctly ;[[Apogee]]: --> ;[[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]]. <!-- Need help. Don't know how to define this succinctly ;[[Perigee]]: --> ;[[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]]. <!-- '''[[Retrograde orbit]]''' - '''[[Decaying orbit]]''' - See [[orbital decay]]. --> {{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 {{orbits}} [[Category:Astrodynamics]] [[Category:Earth orbits|*]] [[Category:Satellites]] [[de:Erdbahn]] [[fr:Orbite terrestre]] [[id:Orbit bumi]] [[it:Orbita geocentrica]] [[lv:Ģeocentriskā orbīta]]