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Geocentric orbit
A geocentric orbit involves any object orbiting the Earth, such as the Moon or artificial satellites. In 1997 NASA estimated there were approximately 2,465 artificial satellite payloads orbiting the Earth and 6,216 pieces of space debris as tracked by the Goddard Space Flight Center.^{[1]} Over 16,291 previously launched objects have decayed into the Earth's atmosphere.^{[1]}
Contents
List of terms and concepts
In the spirit of brevity some of the definitions have been altered or truncated to reflect only their usage on this page.
 Altitude
 as used here, the height of an object above the average surface of the Earth's oceans.
 Analemma
 a term in astronomy used to describe the plot of the positions of the Sun on the celestial sphere throughout one year. Closely resembles a figureeight.
 Apogee
 is the farthest point that a satellite or celestial body can go from Earth, at which the orbital velocity will be at its minimum.
 Eccentricity
 a measure of how much an orbit deviates from a perfect circle. Eccentricity is strictly defined for all circular and elliptical orbits, and parabolic and hyperbolic trajectories.
 Equatorial plane
 as used here, an imaginary plane extending from the equator on the Earth to the celestial sphere.
 Escape velocity
 as used here, the minimum velocity an object without propulsion needs to have to move away indefinitely from the Earth. An object at this velocity will enter a parabolic trajectory; above this velocity it will enter a hyperbolic trajectory.
 Impulse
 the integral of a force over the time during which it acts. Measured in (N·sec or lb * sec).
 Inclination
 the angle between a reference plane and another plane or axis. In the sense discussed here the reference plane is the Earth's equatorial plane.
 Orbital characteristics
 the six parameters of the Keplerian elements needed to specify that orbit uniquely.
 Orbital period
 as defined here, time it takes a satellite to make one full orbit about the Earth.
 Perigee
 is the nearest approach point of a satellite or celestial body from Earth, at which the orbital velocity will be at its maximum.
 Sidereal day
 the time it takes for a celestial object to rotate 360°. For the Earth this is: 23 hours, 56 minutes, 4.091 seconds.
 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, 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 160 kilometeres (100 statute miles) to Script error: No such module "convert". above mean sea level. At 160 km, one revolution takes approximately 90 minutes, and the circular orbital speed is Script error: No such module "convert"..
 Medium Earth Orbit (MEO)  Geocentric orbits with altitudes at apogee ranging between Script error: No such module "convert". and that of the geosynchronous orbit at Script error: No such module "convert"..
 Geosynchronous Orbit (GEO)  Geocentric circular orbit with an altitude of Script error: No such module "convert".. The period of the orbit equals one sidereal day, coinciding with the rotation period of the Earth. The speed is approximately Script error: No such module "convert"..
 High Earth Orbit (HEO)  Geocentric orbits with altitudes at apogee higher than that of the geosynchronous orbit. A special case of high Earth orbit is the highly elliptical orbit, where altitude at perigee is less than Script error: No such module "convert"..^{[2]}
Inclination classifications
 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 degrees.
 Polar sun synchronous orbit  A nearly polar orbit that passes the equator at the same local time on every pass. Useful for imagetaking satellites because shadows will be the same on every pass.
Eccentricity classifications
 Circular orbit  An orbit that has an eccentricity of 0 and whose path traces a circle.
 Elliptic orbit  An orbit with an eccentricity greater than 0 and less than 1 whose orbit traces the path of an ellipse.
 Hohmann transfer orbit  An orbital maneuver that moves a spacecraft from one circular orbit to another using two engine impulses. This maneuver was named after Walter Hohmann.
 Geosynchronous transfer orbit  A geocentricelliptic orbit where the perigee is at the altitude of a Low Earth Orbit (LEO) and the apogee at the altitude of a geosynchronous orbit.
 Highly elliptical orbit (HEO)  Geocentric orbit with apogee above 35,786 km and low perigee (about 1,000 km) that result in long dwell times near apogee.
 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 Earth.
 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 Earth.
 Hyperbolic trajectory  An "orbit" with eccentricity greater than 1. The object's velocity at perigee reaches some value in excess of the escape velocity, therefore it will escape the gravitational pull of the Earth and continue to travel infinitely with a velocity (relative to Earth) decelerating to some finite value, known as the hyperbolic excess velocity.
 Escape Trajectory  This trajectory must be used to launch an interplanetary probe away from Earth, because the excess over escape velocity is what changes its heliocentric orbit from that of Earth.
 Capture Trajectory  This is the mirror image of the escape trajectory; an object traveling with sufficient speed, not aimed directly at Earth, will move toward it and accelerate. In the absence of a decelerating engine impulse to put it into orbit, it will follow the escape trajectory after periapsis.
 Parabolic trajectory  An "orbit" with eccentricity exactly equal to 1. The object's velocity at perigee equals the escape velocity, therefore it will escape the gravitational pull of the Earth and continue to travel with a velocity (relative to Earth) decelerating to 0. A spacecraft launched from Earth with this velocity would travel some distance away from it, but follow it around the Sun in the same heliocentric orbit. It is possible, but not likely that an object approaching Earth could follow a parabolic capture trajectory, but speed and direction would have to be precise.
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
 Semisynchronous orbit (SSO)  An orbit with an altitude of approximately Script error: No such module "convert". and an orbital period of approximately 12 hours
 Geosynchronous orbit (GEO)  Orbits with an altitude of approximately Script error: No such module "convert".. 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.
 Earth orbital libration points: The libration points for objects orbiting Earth are at 105 degrees west and 75 degrees east. More than 160 satellites are gathered at these two points.^{[3]}
 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  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  A synonym for graveyard orbit.
Special classifications
 Sunsynchronous 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, and weather satellites.
 Moon orbit  The orbital characteristics of Earth's Moon. Average altitude of Script error: No such module "convert"., elliptical–inclined orbit.
Nongeocentric classifications
 Horseshoe orbit  An orbit that appears to a ground observer to be orbiting a planet but is actually in coorbit with it. See asteroids 3753 (Cruithne) and 2002 AA_{29}.
 Exoorbit  A maneuver where a spacecraft approaches the height of orbit but lacks the velocity to sustain it.
 Suborbital spaceflight  A synonym for Exoorbit.
Tangential velocities at altitude
orbit  centertocenter distance 
altitude above the Earth's surface 
speed(relative to Earth's surface)  Orbital period  specific orbital energy 

Standing on Earth's surface at the equator (for comparison  not an orbit)  6,378 km  0 km  465.1 m/s (1,040 mph)  1 day (24h)  −62.6 MJ/kg 
Orbiting at Earth's surface (equator)  6,378 km  0 km  7.9 km/s (17,672 mph)  1 h 24 min 18 sec  −31.2 MJ/kg 
Low Earth orbit  6,600 to 8,400 km  200 to 2,000 km  circular orbit: 6.9 to 7.8 km/s (15,430 mph to 17,450 mph) respectively elliptic orbit: 6.5 to 8.2 km/s respectively 
1 h 29 min to 2 h 8 min  −29.8 MJ/kg 
Molniya orbit  6,900 to 46,300 km  500 to 39,900 km  1.5 to 10.0 km/s (3,335 mph to 22,370 mph) respectively  11 h 58 min  −4.7 MJ/kg 
Geostationary  42,000 km  35,786 km  3.1 km/s (6,935 mph)  23 h 56 min  −4.6 MJ/kg 
Orbit of the Moon  363,000 to 406,000 km  357,000 to 399,000 km  0.97 to 1.08 km/s (2,170 to 2,416 mph) respectively  27.3 days  −0.5 MJ/kg 
See also
 Earth's orbit
 List of orbits
 Astrodynamics
 Celestial sphere
 Heliocentric orbit
 Areosynchronous satellite
 Areostationary satellite
 Escape velocity
 Satellite
 Space station
References
Citations
 ^ ^{a} ^{b} "Satellite Situation Report, 1997". NASA Goddard Space Flight Center. 20000201. Archived from the original on 20060823. Retrieved 20060910.
 ^ Definitions of geocentric orbits from the Goddard Space Flight Center
 ^ OutofControl Satellite Threatens Other Nearby Spacecraft, by Peter B. de Selding, SPACE.com, 5/3/10.
This article includes a list of references, but its sources remain unclear because it has insufficient inline citations. (April 2009) 
Bibliography
 http://www.freemars.org/jeff/speed/index.htm
 http://www.techfaq.com/mediumearthorbit.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
