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| A further form of geosynchronous orbit is obtained by the theoretical ] in which one end of the structure is tethered to the ground, maintaining a longer orbital period than by gravity alone. | A further form of geosynchronous orbit is obtained by the theoretical ] in which one end of the structure is tethered to the ground, maintaining a longer orbital period than by gravity alone. | ||
| ==Derivation of orbital period== | |||
| For the gravity-only circular geosynchronous orbit for Earth: | |||
| <math>r = \sqrt(G m_e / \omega ^2) \approx 42000 km</math> (the distance from the centre of the Earth). | |||
| See ] for the full derivation. | |||
| ==History== | ==History== | ||
Revision as of 03:09, 1 May 2004
A geosynchronous orbit is an orbit that has the same orbital period as the body it orbits (usually the Earth but geosynchronous orbits exist around all moons, planets and suns that have sufficient rotational speed for an orbit to be maintained above the surface).
If a geosynchronous orbit is circular and equatorial then it is also a geostationary orbit, and will maintain the same position relative to the body it rotates around. If one could see a satellite in geostationary orbit it would appear to hover in the same position.
A circular geosynchronous orbit in the plane of the Earth's equator has a radius of approximately 42,164 km (from the center of the Earth), approximately 35,790 km (22,240 statute miles) above mean sea level.
This can be demonstrated analytically by application of the Law of Gravity and the physics of centripetal acceleration. Drawing the free body diagram and using the analysis methods of engineering dynamics and Physics allows the determination of the distance from Earth's center of mass which will satisfy this specified operating condition.
Circular geosynchronous orbits
A perfect stable geostationary orbit is an ideal that can only be approximated. In practice the satellite will drift out of this orbit (caused by forces such as the solar wind, and the gravitational effect of the moon) and thrusters are used to maintain the orbit.
The "Clarke Belt" is the part of space approximately 35,790 km directly above Earth's equator where near-geostationary orbits may be achieved, named after Science fiction writer and scientist Arthur C. Clarke who wrote about this belt in 1945.
Geostationary orbits can only be acheived very close to the ring 35,790 km directly above the equator. All other circular non-active geosynchronous orbits will cross the geostationary orbit and possibly collide with satellites there. In practice this means that all geostationary satellites have to exist on this ring, which poses problems for satellites needing to be decommisioned at the end of their service life (for example when they run out of thrust fuel).
Other geosynchronous orbits
Elliptical orbits can be and are designed for communications satellites that keep the satellite within view of its assigned ground stations or receivers. A satellite in an elliptical geosynchronous orbit will appear to oscillate in the sky from the viewpoint of a ground station, and satellites in highly elliptical orbits must be tracked by steerable ground stations.
Theoretically an active geosynchronous orbit can be maintained if forces other than gravity are also used to maintain the orbit, such as a solar sail. Such a statite can be geosynchronous in an orbit different (higher, lower, more or less elliptical, or some other path) from the conic section orbit formed by a gravitational body. Such devices are still theoretical.
A further form of geosynchronous orbit is obtained by the theoretical space elevator in which one end of the structure is tethered to the ground, maintaining a longer orbital period than by gravity alone.
History
The first communications satellite placed in a geosynchronous orbit was Syncom 2, launched in 1963. Geosynchronous orbits have been in common use ever since including satellite television. Initially geostationary satellites also carried telephone calls but are no longer used for voice communication, partly due to the inherent disconcerting delay in getting information to the satellite and back (it takes light or radio about a quarter of a second to make the round trip.)