What is satellite ?

It is not always a simple matter to decide which is the "satellite" in a pair of bodies. Because all objects with mass are affected by gravity, the motion of the primary object is also affected by the satellite. If two objects are sufficiently similar in mass, they are generally referred to as a binary system rather than a primary object and satellite; an extreme example is the 'double asteroid' 90 Antiope. The general criterion for an object to be a satellite is that the center of mass (known as the Barycenter) of the two objects is inside the primary object. Another, perhaps better known, example is the dwarf planet Pluto and its companion, Charon. While many consider Charon to be a satellite of Pluto, this can be debated, as their masses are similar to the point where their barycenter is not within either object. Accordingly, some consider Pluto and Charon to be the two members of a double planet system.

Types


Anti-Satellite weapons, sometimes called "Killer satellites" are satellites designed to destroy "enemy" satellites, other orbital weapons and targets. Some are armed with kinetic rounds, while others use energy and/or particle weapons to destroy satellites, ICBMs, MIRVs. Both the U.S. and the USSR had these satellites. Links discussing "Killer satellites", ASATS (Anti-Satellite satellite) include USSR Tests ASAT weapon and ASAT Test. See also IMINT
Astronomical satellites are satellites used for observation of distant planets, galaxies, and other outer space objects.
Biosatellites are satellites designed to carry living organisms, generally for scientific experimentation.
Communications satellites are an artificial satellite stationed in space for the purposes of telecommunications. Modern communications satellites typically use geosynchronous orbits, Molniya orbits or low Earth orbits.
Miniaturized satellites are satellites of unusually low weights and small sizes. New classifications are used to categorize these satellites: minisatellite (500–200 kg), microsatellite (below 200 kg), nanosatellite (below 10 kg).
Navigation satellites are satellites which use radio time signals transmitted to enable mobile receivers on the ground to determine their exact location. The relatively clear line of sight between the satellites and receivers on the ground, combined with ever-improving electronics, allows satellite navigation systems to measure location to accuracies on the order of a few metres in real time.
Reconnaissance satellites are Earth observation satellite or communications satellite deployed for military or intelligence applications. Little is known about the full power of these satellites, as governments who operate them usually keep information pertaining to their reconnaissance satellites classified.
Earth observation satellites are satellites intended for non-military uses such as environmental monitoring, meteorology, map making etc. (See especially Earth Observing System.)
Solar power satellites are proposed satellites built in high Earth orbit that use microwave power transmission to beam solar power to very large antennae on Earth where it can be used in place of conventional power sources.
Space stations are man-made structures that are designed for human beings to live on in outer space. A space station is distinguished from other manned spacecraft by its lack of major propulsion or landing facilities — instead, other vehicles are used as transport to and from the station. Space stations are designed for medium-term living in orbit, for periods of weeks, months, or even years.
Weather satellites are satellites that primarily are used to monitor Earth's weather and climate.

Satellites can be used for astronomy, war purposes, detecting living things or geology.
Satellites are actually very useful. They are any object that orbits another object (which is known as its primary). Satellites can be man-made or may be naturally occurring such as moons, comets, asteroids, planets, stars, and even galaxies.