Geocentric orbit

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

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.

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, 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).

Specific Impulse: is defined as the ratio of thrust produced and the mass flow rate into the rocket engine. Its unit of measurement is (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.

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 160–2,000 km (100–1,240 miles); one revolution takes 90 minutes, the speed is 8 kilometers per second.

Medium Earth Orbit (MEO) - Geocentric orbits ranging in altitude from 2,000 km to just below geosynchronous orbit at 35,786 km (22,240 miles). Also known as an intermediate circular orbit.

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.

High Earth Orbit (HEO) - Geocentric orbit higher than 35,786 km (22,236 statute miles)

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 image taking 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.

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.

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 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 its 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.

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 (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.

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.

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.

Tangential velocities at altitude

Orbit	Center-to-center distance	Altitude above the Earth's surface	Speed	Orbital period	Specific orbital energy
Earth's own rotation at surface (for comparison— not an orbit)	6,378 km	0 km	465.1 m/s (1,674 km/h or 1,040 mph)	23 h 56 min 4.09 sec	−62.6 MJ/kg
Orbiting at Earth's surface (equator) theoretical	6,378 km	0 km	7.9 km/s (28,440 km/h or 17,672 mph)	1 h 24 min 18 sec	−31.2 MJ/kg
Low Earth orbit	6,600–8,400 km	200–2,000 km	Circular orbit: 7.7–6.9 km/s (27,772–24,840 km/h or 17,224–15,435 mph) respectively Elliptic orbit: 10.07–8.7 km/s respectively	1 h 29 min – 2 h 8 min	−29.8 MJ/kg
Molniya orbit	6,900–46,300 km	500–39,900 km	1.5–10.0 km/s (5,400–36,000 km/h or 3,335–22,370 mph) respectively	11 h 58 min	−4.7 MJ/kg
Geostationary	42,000 km	35,786 km	3.1 km/s (11,600 km/h or 6,935 mph)	23 h 56 min 4.09 sec	−4.6 MJ/kg
Orbit of the Moon	363,000–406,000 km	357,000–399,000 km	0.97–1.08 km/s (3,492–3,888 km/h or 2,170–2,416 mph) respectively	27.27 days	−0.5 MJ/kg

The lower axis gives orbital speeds of some orbits

References

"Satellite Situation Report". NASA Goddard Space Flight Center. 2000-02-01. Retrieved 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