This article includes a list of references, related reading or external links, but its sources remain unclear because it lacks inline citations. Please improve this article by introducing more precise citations. (April 2009)

A geocentric orbit involves any object orbiting the Earth, such as the Moon or artificial satellites. Currently there are approximately 2,465 artificial satellites orbiting the Earth and 6,216 pieces of space debris as tracked by the Goddard Space Flight Center. Over 16,291 previously launched objects have decayed into the Earth's atmosphere.

List of terms and concepts [link]

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 figure-eight.

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 [link]

The following is a list of different geocentric orbit classifications.

Altitude classifications [link]

Low (cyan) and Medium (yellow) Earth orbit regions to scale. The black dashed line is the geosynchronous orbit. The green dashed line is the 20,230 km orbit used for GPS satellites.

Low Earth Orbit (LEO) - Geocentric orbits ranging in altitude from 160 kilometeres (100 statue miles) to 2,000 kilometres (1,200 mi) above mean sea level. At 160 km, one revolution takes approximately 90 minutes, and the circular orbital speed is 8,000 metres per second (26,000 ft/s).

Medium Earth Orbit (MEO) - Geocentric orbits with altitudes at apogee ranging between 2,000 kilometres (1,200 mi) and that of the geosynchronous orbit at 35,786 kilometres (22,236 mi).

Geosynchronous Orbit (GEO) - Geocentric circular orbit with an altitude of 35,786 kilometres (22,236 mi). The period of the orbit equals one sidereal day, coinciding with the rotation period of the Earth. The speed is approximately 3,000 metres per second (9,800 ft/s).

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 2,000 kilometres (1,200 mi).^[1]

Inclination classifications [link]

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 image-taking satellites because shadows will be the same on every pass.

Eccentricity classifications [link]

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 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.

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 [link]

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 [link]

Semi-synchronous orbit (SSO) - An orbit with an altitude of approximately 20,200 km (12,600 mi) and an orbital period of approximately 12 hours

Geosynchronous orbit (GEO) - Orbits with an altitude of approximately 35,786 km (22,236 mi). 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.^[2]

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 [link]

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.

Non-geocentric classifications [link]

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] Earth orbits

See also [link]

Spaceflight portal

• Earth's orbit
• List of orbits
• Astrodynamics
• Celestial sphere
• Heliocentric orbit
• Areosynchronous satellite
• Areostationary satellite
• Escape velocity
• Satellite
• Space station

References [link]

• "Satellite Situation Report". NASA Goddard Space Flight Center. 2000-02-01. Archived from the original on 2006-08-23. https://web.archive.org/web/20060823011134/https://liftoff.msfc.nasa.gov/academy/rocket_sci/satellites/ssr.html. Retrieved 2006-09-10. 
• https://www.freemars.org/jeff/speed/index.htm
• https://www.tech-faq.com/medium-earth-orbit.shtml
• https://www.hq.nasa.gov/office/pao/History/conghand/traject.htm
• https://www.space.com/scienceastronomy/solarsystem/second_moon_991029.html
• https://www.astro.uwo.ca/~wiegert/3753/3753.html
• https://www.astro.uwo.ca/~wiegert/AA29/AA29.html