﻿ Geocentric orbit
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# Geocentric orbit

geocentric orbit, geocentric orbit definition
A geocentric orbit or Earth 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 Center1 Over 16,291 previously launched objects have decayed into the Earth's atmosphere1

## Contents

• 1 List of terms and concepts
• 2 Geocentric orbit types
• 21 Altitude classifications
• 22 Inclination classifications
• 23 Eccentricity classifications
• 24 Directional classifications
• 25 Geosynchronous classifications
• 26 Special classifications
• 27 Non-geocentric classifications
• 3 Tangential velocities at altitude
• 5 References

## List of terms and conceptsedit

The following words may have more than one definition or other non-Earth specific definitions
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 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 around 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, 4091 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 typesedit

The following is a list of different geocentric orbit classifications

### Altitude classificationsedit

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 kilometers 100 statute 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 mi2

### Inclination classificationsedit

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 classificationsedit

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 634° 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 634° 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 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 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 classificationsedit

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 classificationsedit

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 points3 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 classificationsedit

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 classificationsedit

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 altitudeedit

orbit Center-to-center
distance
Altitude above
the Earth's surface
Speed Orbital period Specific orbital energy
Standing on Earth's surface at the equator for comparison – not an orbit 6,378 km 0 km 4651 m/s 1,040 mph 1 day 24h −626 MJ/kg
Orbiting at Earth's surface equator 6,378 km 0 km 79 km/s 17,672 mph 1 h 24 min 18 sec −312 MJ/kg
Low Earth orbit 6,600–8,400 km 200–2,000 km circular orbit: 78–69 km/s 17,450–14,430 mph respectively
elliptic orbit: 65–82 km/s respectively
1 h 29 min – 2 h 8 min −298 MJ/kg
Molniya orbit 6,900–46,300 km 500–39,900 km 15–100 km/s 3,335–22,370 mph respectively 11 h 58 min −47 MJ/kg
Geostationary 42,000 km 35,786 km 31 km/s 6,935 mph 23 h 56 min −46 MJ/kg
Orbit of the Moon 363,000–406,000 km 357,000–399,000 km 097–108 km/s 2,170–2,416 mph respectively 273 days −05 MJ/kg

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

## Referencesedit

1. ^ a b "Satellite Situation Report, 1997" NASA Goddard Space Flight Center 2000-02-01 Archived from the original on 2006-08-23 Retrieved 2006-09-10
2. ^ Definitions of geocentric orbits from the Goddard Space Flight Center Archived May 27, 2010, at the Wayback Machine
3. ^ Out-of-Control Satellite Threatens Other Nearby Spacecraft, by Peter B de Selding, SPACEcom, 5/3/10 Archived May 5, 2010, at the Wayback Machine

• http://wwwfreemarsorg/jeff/speed/indexhtm
• http://wwwtech-faqcom/medium-earth-orbitshtml
• http://wwwhqnasagov/office/pao/History/conghand/trajecthtm
• https://webarchiveorg/web/20100221072300/http://wwwspacecom/scienceastronomy/solarsystem/second_moon_991029html
• http://wwwastrouwoca/~wiegert/3753/3753html
• http://wwwastrouwoca/~wiegert/AA29/AA29html

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29.10.2014

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