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A geosynchronous orbit is a gravitational path curved around a planet or moon with a time period identical to the rotation of that planet or moon. Specifically, the orbital period is considered to be matched with the sidereal day, the time it takes for a planet or moon to make a full rotation. In the case of Earth, this is roughly 23 hours and 56 minutes. Usually, geosynchronous orbit refers to a the synchronized rotation of an object around the Earth, commonly a satellite or space vehicle.
Any object with a geosynchronous orbit holds a ground track of a single point on the Earth's surface. A ground track is the position on the surface of the Earth below the satellite. This point tracks around the world in the form of a figure-eight, returning to the exact same place each day.
Telecommunications satellites, as well as other types of satellites, maintain a geosynchronous orbit known as the Clarke orbit. This is essentially a stationary orbit located at an altitude of 22,236 miles (35,786 km) above sea level. An object in Clarke orbit would appear to maintain the same position above the planet at all times. This concept was proposed by author Arthur C. Clarke specifically for communications satellites, as a way to ensure relative stability of point to point communications. All satellites in orbit at this altitude are part of the Clarke Belt.
One challenge with keeping objects in geosynchronous orbits is the fact that satellites drift from this orbit. Factors such as solar winds, radiation pressure, and effects from the gravitational fields of the Moon, Sun and Earth itself can cause drifting. To offset this effect, satellites are fitted with thrusters that keep the object in orbit. This process is known as station-keeping.
Certain additional geosynchronous orbits outside of the Clarke Belt exist to deal with position changes and disposal of satellites. Supersynchronous orbit, located above normal geosynchronous orbit, is used to store or dispose of satellites or spacecraft that reach their operational end. Also known as graveyard orbit, this is designed to limit the possibility of collisions with usable craft and maintains a westerly directional path. Likewise, subsynchronous orbit is located beneath geosynchronous orbit and is generally used for objects undergoing changes in location. These objects maintain an easterly directional path.
The first satellite to be placed in geostationary orbit was Syncom 3, launched aboard a Delta D launch vehicle on 19 August 1964 from Cape Canaveral. It was used to broadcast the 1964 Summer Olympics from Tokyo to the United States. By the early 21st century, thousands of satellites from 50 countries had been launched into orbit, although only a few hundred are operational at any given time.
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