A spacecraft enters orbit when its centripetalacceleration due to gravity is less than or equal to the centrifugal acceleration due to the horizontal component of its velocity. For a low Earth orbit, this velocity is about 7.8 km/s (28,100 km/h; 17,400 mph);[2] by contrast, the fastest crewed airplane speed ever achieved (excluding speeds achieved by deorbiting spacecraft) was 2.2 km/s (7,900 km/h; 4,900 mph) in 1967 by the North American X-15.[3] The energy required to reach Earth orbital velocity at an altitude of 600 km (370 mi) is about 36 MJ/kg, which is six times the energy needed merely to climb to the corresponding altitude.[4]
Spacecraft with a perigee below about 2,000 km (1,200 mi) are subject to drag from the Earth's atmosphere,[5] which decreases the orbital altitude. The rate of orbital decay depends on the satellite's cross-sectional area and mass, as well as variations in the air density of the upper atmosphere. Below about 300 km (190 mi), decay becomes more rapid with lifetimes measured in days. Once a satellite descends to 180 km (110 mi), it has only hours before it vaporizes in the atmosphere.[6] The escape velocity required to pull free of Earth's gravitational field altogether and move into interplanetary space is about 11.2 km/s (40,300 km/h; 25,100 mph).[7]
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.
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.
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.
an object's speed in a particular direction. Since velocity is defined as a vector, both speed and direction are required to define it.
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.
Geocentric orbits ranging in altitude from 160 km (100 mi) to 2,000 km (1,200 mi) above mean sea level. At 160 km, one revolution takes approximately 90 minutes, and the circular orbital speed is 8 km/s (26,000 ft/s).
Geocentric circular orbit with an altitude of 35,786 km (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 km/s (9,800 ft/s).
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 km (1,200 mi).[9]
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.
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.
An orbital maneuver that moves a spacecraft from one circular orbit to another using two engine impulses. This maneuver was named after Walter Hohmann.
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.
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.
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.
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.[10]
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.
^Hill, James V. H. (April 1999), , Space Future, archived from the original on 2012-03-19, retrieved 2012-03-18.
^Shiner, Linda (November 1, 2007), X-15 Walkaround, Air & Space Magazine, retrieved 2009-06-19.
^Dimotakis, P.; et al. (October 1999), , The Mitre Corporation, pp. 1–39, archived from the original on 2017-08-29, retrieved 2012-01-21.
^Ghosh, S. N. (2000), Atmospheric Science and Environment, Allied Publishers, pp. 47–48, ISBN978-8177640434
^Kennewell, John; McDonald, Andrew (2011), Satellite Lifetimes and Solar Activity, Commonwealth of Australia Bureau of Weather, Space Weather Branch, from the original on 2011-12-28, retrieved 2011-12-31.
^Williams, David R. (November 17, 2010), "Earth Fact Sheet", Lunar & Planetary Science, NASA, from the original on October 30, 2010, retrieved 2012-05-10.
^McDowell, Jonathan (24 May 1998). "Jonathan's Space Report". Transatmospheric orbit (TAO): orbital flight with perigee less than 80 km but more than zero. Potentially used by aerobraking missions and transatmospheric vehicles, also in some temporary phases of orbital flight (e.g. STS pre OMS-2, some failures when no apogee restart)
^Definitions of geocentric orbits from the Goddard Space Flight Center May 27, 2010, at the Wayback Machine
^Out-of-Control Satellite Threatens Other Nearby Spacecraft, by Peter B. de Selding, SPACE.com, 5/3/10. May 5, 2010, at the Wayback Machine
Near-Earth asteroid 3753 Cruithne – Earth's curious companion
Earth coorbital asteroid 2002 AA29
April 11, 2024
geocentric, orbit, this, article, needs, additional, citations, verification, please, help, improve, this, article, adding, citations, reliable, sources, unsourced, material, challenged, removed, find, sources, news, newspapers, books, scholar, jstor, december. This article needs additional citations for verification Please help improve this article by adding citations to reliable sources Unsourced material may be challenged and removed Find sources Geocentric orbit news newspapers books scholar JSTOR December 2015 Learn how and when to remove this template message A geocentric orbit Earth centered orbit or Earth orbit involves any object orbiting Earth such as the Moon or artificial satellites In 1997 NASA estimated there were approximately 2 465 artificial satellite payloads orbiting Earth and 6 216 pieces of space debris as tracked by the Goddard Space Flight Center 1 More than 16 291 objects previously launched have undergone orbital decay and entered Earth s atmosphere 1 A spacecraft enters orbit when its centripetal acceleration due to gravity is less than or equal to the centrifugal acceleration due to the horizontal component of its velocity For a low Earth orbit this velocity is about 7 8 km s 28 100 km h 17 400 mph 2 by contrast the fastest crewed airplane speed ever achieved excluding speeds achieved by deorbiting spacecraft was 2 2 km s 7 900 km h 4 900 mph in 1967 by the North American X 15 3 The energy required to reach Earth orbital velocity at an altitude of 600 km 370 mi is about 36 MJ kg which is six times the energy needed merely to climb to the corresponding altitude 4 Spacecraft with a perigee below about 2 000 km 1 200 mi are subject to drag from the Earth s atmosphere 5 which decreases the orbital altitude The rate of orbital decay depends on the satellite s cross sectional area and mass as well as variations in the air density of the upper atmosphere Below about 300 km 190 mi decay becomes more rapid with lifetimes measured in days Once a satellite descends to 180 km 110 mi it has only hours before it vaporizes in the atmosphere 6 The escape velocity required to pull free of Earth s gravitational field altogether and move into interplanetary space is about 11 2 km s 40 300 km h 25 100 mph 7 Contents 1 List of terms and concepts 2 Types 2 1 Altitude classifications 2 2 Inclination classifications 2 3 Eccentricity classifications 2 4 Directional classifications 2 5 Geosynchronous classifications 2 6 Special classifications 2 7 Non geocentric classifications 3 Tangential velocities at altitude 4 See also 5 References 6 External linksList of terms and concepts editThe 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 Altitude as used here the height of an object above the average surface of the Earth s oceans mean sea level 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 arc an imaginary arc in the sky as seen from any given location on the surface of the Earth 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 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 Types editThe following is a list of different geocentric orbit classifications Altitude classifications edit nbsp 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 Transatmospheric orbit TAO Geocentric orbits with altitudes at apogee higher than 100 km 62 mi and perigee that intersects with the defined atmosphere 8 Low Earth orbit LEO Geocentric orbits ranging in altitude from 160 km 100 mi to 2 000 km 1 200 mi above mean sea level At 160 km one revolution takes approximately 90 minutes and the circular orbital speed is 8 km s 26 000 ft s Medium Earth orbit MEO Geocentric orbits with altitudes at apogee ranging between 2 000 km 1 200 mi and that of the geosynchronous orbit at 35 786 km 22 236 mi Geosynchronous orbit GEO Geocentric circular orbit with an altitude of 35 786 km 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 km s 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 km 1 200 mi 9 Inclination classifications edit 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 dd Eccentricity classifications edit 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 GTO 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 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 dd dd 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 dd 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 classifications edit 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 edit 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 10 dd 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 disposal orbit junk orbit An orbit a few hundred kilometers above geosynchronous that satellites are moved into at the end of their operation dd Special classifications edit 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 edit 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 Sub orbital flight A launch where a spacecraft approaches the height of orbit but lacks the velocity to sustain it Tangential velocities at altitude editOrbit Center to centerdistance Altitude abovethe Earth s surface Speed Orbital period Specific orbital energyEarth 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 kgOrbiting 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 kgLow 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 respectivelyElliptic orbit 10 07 8 7 km s respectively 1 h 29 min 2 h 8 min 29 8 MJ kgMolniya 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 kgGeostationary 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 kgOrbit 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 nbsp The lower axis gives orbital speeds of some orbitsSee also edit nbsp Spaceflight portalEarth s orbit List of orbits Orbital mechanics Celestial sphere Heliocentric orbit Areosynchronous orbit Areostationary orbit Escape velocity Satellite Satellite watching Space stationReferences edit 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 Hill James V H April 1999 Getting to Low Earth Orbit Space Future archived from the original on 2012 03 19 retrieved 2012 03 18 Shiner Linda November 1 2007 X 15 Walkaround Air amp Space Magazine retrieved 2009 06 19 Dimotakis P et al October 1999 100 lbs to Low Earth Orbit LEO Small Payload Launch Options The Mitre Corporation pp 1 39 archived from the original on 2017 08 29 retrieved 2012 01 21 Ghosh S N 2000 Atmospheric Science and Environment Allied Publishers pp 47 48 ISBN 978 8177640434 Kennewell John McDonald Andrew 2011 Satellite Lifetimes and Solar Activity Commonwealth of Australia Bureau of Weather Space Weather Branch archived from the original on 2011 12 28 retrieved 2011 12 31 Williams David R November 17 2010 Earth Fact Sheet Lunar amp Planetary Science NASA archived from the original on October 30 2010 retrieved 2012 05 10 McDowell Jonathan 24 May 1998 Jonathan s Space Report Transatmospheric orbit TAO orbital flight with perigee less than 80 km but more than zero Potentially used by aerobraking missions and transatmospheric vehicles also in some temporary phases of orbital flight e g STS pre OMS 2 some failures when no apogee restart Definitions of geocentric orbits from the Goddard Space Flight Center Archived May 27 2010 at the Wayback Machine Out of Control Satellite Threatens Other Nearby Spacecraft by Peter B de Selding SPACE com 5 3 10 Archived May 5 2010 at the Wayback MachineExternal links editOrbital speed Medium Earth Orbit NASA gov Archived 2015 05 04 at the Wayback Machine More Moons Around Earth Its Not So Loony archived 21 February 2010 Near Earth asteroid 3753 Cruithne Earth s curious companion Earth coorbital asteroid 2002 AA29 Retrieved from https en wikipedia org w index php title Geocentric orbit amp oldid 1211167034, wikipedia, wiki, book, books, library,
