Geocentric orbit

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Geocentric orbit refers to the orbit of any object orbiting the Earth, such as the Moon or artificial satellites. Currently there are approximately 2465 artificial satellites orbiting the earth and 6216 pieces of space debris as tracked by the Goddard Space Flight Center. Over 16,291 previously launched objects have decayed into to the earth's atmosphere.

Contents

[edit] List of Terms and Concepts

The following words may have more than one definition or other non-Earth specific definition(s).
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 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 oceans.
Eccentricity
a measure of how much an orbit deviates from a perfect circle. Eccentricity is strictly defined for all circular, elliptic, parabolic and hyperbolic orbits.
Equatorial plane
as used here, an imaginary plane extending from the equator on the Earth to the celestial sphere.
Orbital characteristics
the six parameters of the Keplerian elements needed to specify that orbit uniquely.
Escape velocity
as used here, the minimum velocity an object without 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 (kg m/s or N·s).
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 period
as defined here, time it takes a satellite to make one full orbit about the Earth.
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.

[edit] Geocentric Orbit Types

The following is a list of different geocentric orbit classifications.

[edit] Altitude Classifications

Low Earth Orbit (LEO) - Geocentric orbits ranging in altitude from 0 - 2000 km (0 - 1240 miles)
Medium Earth Orbit (MEO) - Geocentric orbits ranging in altitude from 2000 km - to just below geosynchronous orbit at 35786 km (22,240 miles). Also known as an intermediate circular orbit.
Highly Elliptical Orbit (HEO) - Geocentric orbits with a high apogee and low perigee that result in long dwell times near perigee.
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 rate of the earth.

[edit] Inclination Classifications

Inclined Orbit - An orbit whose inclination in refrence 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 image taking satllites because shadows will be the same on every pass.

[edit] Eccentricity Classifications

Circular Orbit - An orbit that has an 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 impulses. This maneuver was named after Walter Hohmann.
Elliptic Orbit - An orbit with an eccentricity greater than 0 and less than 1 whose orbit traces the path of an ellipse.
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.
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.
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 - 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 to the planet is 0. If the speed of such an orbit is increased it will become a hyperbolic orbit.
Escape Orbit (EO) - A high-speed parabolic orbit where the object has escape velocity and is moving away from the planet.
Capture Orbit - A high-speed parabolic orbit where the object has escape velocity and is moving toward the planet.

[edit] Geosynchronous Classifications

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 - 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.

[edit] Special Classifications

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, and weather satellites.
Moon Orbit - The orbital characteristics of Earth's Moon. Average altitude of 384,403 kilometres (238,857 mi), elliptical-inclined orbit.

[edit] Non-geocentric Classifications

Horseshoe Orbit - An orbit that appears to a ground observer to be orbiting a planet but is actually in co-orbit with it. See asteroids 3753 (Cruithne) and 2002 AA29.
Exo-orbit - A maneuver where a spacecraft approaches the height of orbit but lacks the velocity to sustain it.
Sub-Orbital Spaceflight - A synonym for Exo-orbit.


[edit] See also

[edit] Sources

Satellite Situation Report. NASA Goddard Space Flight Center (2000-02-01). Retrieved on 2006-09-10.

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