A satellite is any object that orbits another object (which is known as its primary). All masses that are part of the solar system, including the Earth, are satellites either of the Sun, or satellites of those objects, such as the Moon.
It is not always a simple matter to decide which is the 'satellite' in a pair of bodies. Because all objects exert 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 of the two objects is inside the primary object.
In popular usage, the term 'satellite' normally refers to an artificial satellite (a man-made object that orbits the Earth or another body). However, scientists may also use the term to refer to natural satellites, or moons.
This article is primarily concerned with artificial satellites. See natural satellite for information on moons.
In May, 1946, the Preliminary Design of an Experimental World-Circling Spaceship stated, "A satellite vehicle with appropriate instrumentation can be expected to be one of the most potent scientific tools of the Twentieth Century. The achievement of a satellite craft would produce repercussions comparable to the explosion of the atomic bomb..." (see: Project RAND)
The space age began in 1946, as scientists began using captured German V-2 rockets to make measurements in the upper atmosphere. Before this period, scientists used balloons that went up to 30 km and radio waves to study the ionosphere. From 1946 to 1952, upper-atmosphere research was conducted using V-2s and Aerobee rockets. This allowed measurements of atmospheric pressure, density, and temperature up to 200 km. (see also: magnetosphere, Van Allen radiation belt)
The U.S. had been considering launching orbital satellites since 1945 under the Bureau of Aeronautics of the United States Navy. The Air Force's Project RAND eventually released the above report, but did not believe that the satellite was a potential military weapon; rather they considered it to be a tool for science, politics, and propaganda. In 1954, the Secretary of Defense stated, "I know of no American satellite program."
Following pressure by the American Rocket Society, the National Science Foundation, and the International Geophysical Year, military interest picked up and in early 1955 the Air Force and Navy were working on Project Orbiter, which involved using a Jupiter C rocket to launch a small satellite called Explorer 1 on January 31, 1958.
On July 29, 1955, the White House announced that the U.S. intended to launch satellites by the spring of 1958. This became known as Project Vanguard. On July 31, the Soviets announced that they intended to launch a satellite by the fall of 1957 and on October 4, 1957 Sputnik I was launched into orbit, which triggered the Space Race between the two nations.
The largest artificial satellite currently orbiting the earth is the International Space Station.
Types of satellites
Astronomical satellites are satellites used for observation of distant planets, galaxies, and other outer space objects.
Communications satellites are artificial satellites stationed in space for the purposes of telecommunications using radio at microwave frequencies. Most communications satellites use geosynchronous orbits or near-geostationary orbits, although some recent systems use low Earth-orbiting satellites.
Earth observation satellites are satellites specifically designed to observe Earth from orbit, similar to reconnaissance satellites but intended for non-military uses such as environmental monitoring, meteorology, map making etc. (See especially Earth Observing System.)
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.
Solar power satellites are proposed satellites built in high Earth orbit that use microwave power transmission to beam solar power to very large antenna 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 the weather and/or climate of the Earth.
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).
Many times satellites are characterized by their orbit. Although a satellite may orbit at almost any height, satellites are commonly categorized by their altitude:
- Low Earth Orbit (LEO: 200 - 1200km above the Earth's surface)
- Medium Earth Orbit (ICO or MEO: 1200 - 35286 km)
- Geosynchronous Orbit (GEO: 35786 km above Earth's surface)
- Geostationary Orbit (GSO: zero inclination geosynchronous orbit)
- High Earth Orbit (HEO: above 35786 km)
The following orbits are special orbits that are also used to categorize satellites:
- Molniya orbits
- Heliosynchronous or sun-synchronous orbit
- Polar orbit
- LTO lunar transfer orbit
- Hohmann transfer orbit For this particular orbit type, it is more common to identify the satellite as a spacecraft.
- Supersynchronous orbit or drift orbit - orbit above GEO. Satellites will drift in a westerly direction.
- (GEO + 235 km + (1000 × CR × A/m) km)
- where CR is the solar pressure radiation coefficient (typically between 1.2 and 1.5) and A/m is the aspect area [m2] to dry mass [kg] ratio
- (GEO + 235 km + (1000 × CR × A/m) km)
- Subsynchronous orbit or drift orbit - orbits close to but below GEO. Used for satellites undergoing station changes in an eastern direction.
Satellites can also orbit libration points.
This list includes counties with an independent capability to place satellites in orbit, including production of the necessary launch vehicle. Many more countries have built satellites that were launched with the aid of others. The French and British capabilities are now subsumed by the European Union under the European Space Agency.
|Country||Year of first launch||First satellite|
|Russia (formerly the Soviet Union)||1957||"Sputnik 1"|
|United States||1958||"Explorer 1"|
|China||1970||"Dong Fang Hong I"|
|United Kingdom||1971||"Prospero X-3"|
|European Union||1979||"Ariane 1"|
In 1998, North Korea claimed to have launched a satellite, but this was never confirmed, and widely believed to be a cover for the test launch of the Taepodong-1 missile over Japan (See Kwangmyongsong).