Orbits of Satellites
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A satellite is anything in an orbit around another object, such as, the Moon is the natural satellite of the Earth. Man-made satellites are launched into one of four possible orbits: (1) an almost circular low-Earth orbit is approximately 250 kilometers above Earth's surface; (2) 800 kilometers high are polar orbits; (3) high elliptical orbit is much lower in altitude at its perigee, or closest approach to Earth, than at its apogee, or most distant point from Earth; and, (4) the geostationary orbit is approximately 36,000 kilometers above the equator.
The first space satellite was the Soviet Sputnik 1 launched on October 4, 1957, while Explorer 1 was the first United States satellite launched on February 1, 1958. Explorer 1 discovered signs of the Van Allen radiation belts. The first instrument to study the climate were aboard the US Explorer 7 when it launched on August 7, 1959, and the US Transit 1B was the world's first navigation satellite launched on April 13, 1960. The US TIROS 1 was launched on April 1, 1960, and not only was this the first weather satellite, but its pictures were returned to Earth for two months. On April 6, 1965, Intelsat's Early Bird was launched carrying the first commercial operated satellite communications equipment.
The measuring of the distant from satellites in orbit to the Earth is called telemetry. These measurements are relayed as radio signals and can hold information allowing operators to calculate the position of the satellite. Not only can operators track the satellite, but they can send command signals to the satellite for changing its position. Ground controllers use the data from telemetry to make certain the satellite is in proper working condition.
A satellite needs to be stable, if it is to do its job. The communication satellite dish needs to always point toward the receiving station. It also needs to point to the right country when sending television signal transmissions. Scientists use two methods to maintain stability: spin and three-axis stabilization. Spinning objects are naturally stable, such as, a spinning top stays stable when it spins fast enough. Designers of early satellites utilized this principle resulting in a spin-stabilized satellite. Most often these have a cylinder shape and will have one revolution every second. An antenna dish needs to always point towards the Earth, and, as a result, it does not spin, but careful consideration is taken to make certain the dish cannot cause the satellite to become unstable. Satellites having small spinning wheels rotating in a way to keep the satellite in sync with the Earth and the Sun are called three-axis stabilization. When the sensors on a satellite detect trouble on any of the cube's three axes, they relay a signal to the wheels causing it to spin slower or faster. The changes will return the satellite to its correct position.
Housekeeping data is the term used when giving information about the condition of a satellite. Ground control is contacted when data says something is not quite right in its space home. Ground controllers can be alerted to the instability of a satellite and send commands to correct the problem. If the problem cannot be correct, a rescue mission may be sent.
Satellites are powered by solar cells arranged into solar panels, or arrays. The solar cells provide electrical power to the satellites allowing for the satellite and its payload to remain in orbit.
1. Couper, Heather and Nigel Henbest. Space Encyclopedia DK Publishing, Inc.: NY 1999
2. Editors. Secrets of the Universe. International Master Publishing: US. 1999