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Image:ERS 2.jpg
An Earth observation satellite, ERS 2

A satellite is any object that orbits another object (which is known as its primary). Satellites can be manmade or may be naturally occurring such as moons, comets, asteroids, planets, stars, and even galaxies.

All masses that are part of our solar system, including the Earth and Sun, are satellites of either a planet, the Sun, or the galactic center of the Milky Way.


[edit] Definition

Look up satellite in Wiktionary, the free dictionary.

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.[citation needed] 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.

MILSTAR:A communication satellite

What seems the first fictional depiction of an artificial satellite launched into Earth orbit seems to be in Jules Verne's The Begum's Millions (1879). In this book, however, this is a completely unintentional result of the book's villain building an enormous artillery piece in order to destroy his enemies, and imparting to the shell a greater velocity than intended.

In 1903 Konstantin Tsiolkovsky (1857–1935) published Исследование мировых пространств реактивными приборами (The Exploration of Cosmic Space by Means of Reaction Devices), which was arguably the first academic treatise on rocketry. He calculated the escape velocity from Earth into orbit at 8 km/second and that a multi-stage rocket fueled by liquid oxygen and liquid hydrogen would be required. During his lifetime he published over 500 works on space travel and related subjects, including science fiction novels. Among his works are designs for rockets with steering thrusters, multi-stage boosters, space stations, airlocks for exiting a spaceship into the vacuum of space, and closed cycle biological systems to provide food and oxygen for space colonies. He also delved into theories of heavier-than-air flying machines, independently working through many of the same calculations that the Wright brothers were performing at about the same time.

In 1928 Herman Potočnik (1898–1929) published his sole book, Das Problem der Befahrung des Weltraums - der Raketen-motor (The Problem of Space Travel - The Rocket Motor), a plan for a breakthrough into space and a permanent human presence there. He conceived of a space station in detail and calculated its geostationary orbit. He described the use of orbiting spacecraft for detailed peaceful and military observation of the ground and described how the special conditions of space could be useful for scientific experiments. The book described geostationary satellites (first put forward by Tsiolkovsky) and discussed communication between them and the ground using radio, but fell short of the idea of using satellites for mass broadcasting and as telecommunications relays.

In 1945 the English science fiction writer Arthur C. Clarke (b. 1917) conceived of the possibility for mass artificial communication satellites in his Wireless World article.<ref>facsimile at</ref> Clarke examined the logistics of satellite launch, possible orbits and other aspects of the creation of a network of world-circling satellites, pointing to the benefits of high-speed global communications. He also suggested that three geostationary satellites would provide coverage over the entire planet.

The first artificial satellite was Sputnik 1 launched by Soviet Union on 4 October 1957.

In May, 1946, Project RAND released the Preliminary Design of an Experimental World-Circling Spaceship, which 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…"

The space age began in 1946, as scientists began using captured German V-2 rockets to make measurements in the upper atmosphere.<ref>Hess, Wilmot (1968). The Radiation Belt and Magnetosphere.</ref> 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 United States 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 Defence 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. On October 4, 1957 Sputnik 1 was launched into orbit, which triggered the Space Race between the two already adversarial nations.

The largest artificial satellite currently orbiting the Earth is the International Space Station.

[edit] Types

[edit] Orbit types

Low Polar Orbit
Geostationary Orbit

[edit] Centric Classifications

Galacto-centric Orbit - An orbit about the center of a galaxy. Earth's sun follows this type of orbit about the galactic center of the Milky Way.
Heliocentric Orbit - An orbit around the Sun. In our Solar System, all planets, comets, and asteroids are in such orbits, as are many artificial satellites and pieces of space debris. Moons by contrast are not in a heliocentric orbit but rather orbit their parent planet.
Geocentric Orbit - An orbit around the planet Earth, such as the Moon or artificial satellites. Currently there are approximately 2465 artificial satellites orbiting the Earth.
Areocentric Orbit - An orbit around the planet Mars, such as moons or artificial satellites.

[edit] Altitude Classifications

Low Earth Orbit (LEO) - Geocentric orbits ranging in altitude from 0 - 2,000 km (0 - 1,240 miles)
Medium Earth Orbit (MEO) - Geocentric orbits ranging in altitude from 2,000 km (1,240 miles) - to just below geosynchronous orbit at 35,786 km (22,240 miles). Also known as an intermediate circular orbit.
High Earth Orbit (HEO) - Geocentric orbits above the altitude of geosynchronous orbit 35,786 km (22,240 miles).

[edit] Inclination Classifications

Inclined Orbit - An orbit whose inclination in refrence to the equatorial plane is not 0.
Polar Orbit - An orbit that passes above or nearly above both poles of the planet on each revolution. Therefore it has an inclination of (or very close to) 90 degrees.
Polar Sun-synchronous Orbit - A nearly polar orbit that passes the equator at the same local time on every pass. Useful for image taking satellites because shadows will be the same on every pass.

[edit] Eccentricity Classifications

Circular Orbit - An orbit that has an eccentricity of 0 and whose path traces a circle.
Hohmann transfer orbit - An orbital maneuver that moves a spacecraft from one circular orbit to another using two engine impulses. This maneuver was named after Walter Hohmann.
Elliptic Orbit - An orbit with an eccentricity greater than 0 and less than 1 whose orbit traces the path of an ellipse.
Geosynchronous Transfer Orbit - An elliptic orbit where the perigee is at the altitude of a Low Earth Orbit (LEO) and the apogee at the altitude of a geosynchronous orbit.
Geostationary Transfer Orbit - An elliptic orbit where the perigee is at the altitude of a Low Earth Orbit (LEO) and the apogee at the altitude of a geostationary orbit.
Molniya Orbit - A highly elliptic orbit with inclination of 63.4° and orbital period of ½ of a sidereal day (roughly 12 hours). Such a satellite spends most of its time over a designated area of the planet.
Tundra Orbit - A highly elliptic orbit with inclination of 63.4° and orbital period of one sidereal day (roughly 24 hours). Such a satellite spends most of its time over a designated area of the planet.
Hyperbolic orbit - An orbit with the eccentricity greater than 1. Such an orbit also has a velocity in excess of the escape velocity and as such, will escape the gravataional pull of the planet and continue to travel infinitely.
Parabolic Orbit - An orbit with the eccentricity equal to 1. Such an orbit also has a velocity equal to the escape velocity and therefore will escape the gravatational pull of the planet and travel until its velocity relative to the planet is 0. If the speed of such an orbit is increased it will become a hyperbolic orbit.
Escape Orbit (EO) - A high-speed parabolic orbit where the object has escape velocity and is moving away from the planet.
Capture Orbit - A high-speed parabolic orbit where the object has escape velocity and is moving toward the planet.

[edit] Synchronous Classifications

Synchronous Orbit - An orbit where the satellite has an orbital period equal to the average rotational period (earth's is: 23 hours, 56 minutes, 4.091 seconds) of the body being orbited and in the same direction of rotation as that body. To a ground observer such a satellite would trace an analemma (figure 8) in the sky.
Semi-Synchronous Orbit (SSO) - An orbit with an altitude of approximately 20,200 km (12544.2 miles ) and an orbital period of approximately 12 hours
Geosynchronous Orbit (GEO) - Orbits with an altitude of approximately 35,786 km (22,240 miles). Such a satellite would trace an analemma (figure 8) in the sky.
Geostationary orbit (GSO): A geosynchronous orbit with an inclination of zero. To an observer on the ground this satellite would appear as a fixed point in the sky.
Clarke Orbit - Another name for a geostationary orbit. Named after the writer Arthur C. Clarke.
Supersynchronous orbit - A disposal / storage orbit above GSO/GEO. Satellites will drift west. Also a synonym for Disposal Orbit.
Subsynchronous orbit - A drift orbit close to but below GSO/GEO. Satellites will drift east.
Graveyard Orbit - An orbit a few hundred kilometers above geosynchronous that satellites are moved into at the end of their operation.
Disposal Orbit - A synonym for graveyard orbit.
Junk Orbit - A synonym for graveyard orbit.
Areosynchronous Orbit - A synchronous orbit around the planet Mars with an orbital period equal in length to Mars' sidereal day, 24.6229 hours.
Areostationary Orbit (ASO) - A circular areosynchronous orbit on the equatorial plane and about 17,000 km(10557 miles) above the surface. To an observer on the ground this satellite would appear as a fixed point in the sky.
Heliosynchronous Orbit - An heliocentric orbit about the Sun where the satellite's orbital period matches the Sun's period of rotation. These orbits occur at a radius of 24.360 Gm (0.1628 AU) around the Sun, a little less than half of the orbital radius of Mercury.

[edit] Special Classifications

Sun-synchronous Orbit - An orbit which combines altitude and inclination in such a way that the satellite passes over any given point of the planets's surface at the same local solar time. Such an orbit can place a satellite in constant sunlight and is useful for imaging, spy, and weather satellites.
Moon Orbit - The orbital characteristics of earth's moon. Average altitude of 384,403 kilometres (238,857 mi), elliptical-inclined orbit.

[edit] Pseudo-Orbit Classifications

Horseshoe Orbit - An orbit that appears to a ground observer to be orbiting a certain planet but is actually in co-orbit with the planet. See asteroids 3753 (Cruithne) and 2002 AA29.
Exo-orbit - A maneuver where a spacecraft approaches the height of orbit but lacks the velocity to sustain it.
Orbital Spaceflight - A synonym for Exo-orbit.
Lunar transfer orbit (LTO) -
Prograde Orbit - An orbit with an inclination of less than 90°. Or rather, an orbit that is in the same direction as the rotation of the primary.
Retrograde orbit - An orbit with an inclination of more than 90°. Or rather, an orbit counter to the direction of rotation of the planet. Almost no satellites are launched into retrograde orbit because the quantity of fuel required to launch them is much greater than for a prograde orbit. This is because when the rocket starts out on the ground, it already has an eastward component of velocity equal to the rotational velocity of the planet at its launch latitude.

Satellites can also orbit Lagrangian Points.

[edit] Launch capable countries

This list includes countries with an independent capability to place satellites in orbit, including production of the necessary launch vehicle. Note: many more countries have the capability to design and build satellites — which relatively speaking, does not require much economic, scientific and industrial capacity — but are unable to launch them, instead relying on foreign launch services. This list does not consider those numerous countries, but only lists those capable of launching satellites indigenously, and the date this capability was first demonstrated. Does not include consortium satellites or multi-national satellites.

First launch by country
Country Year of first launch First satellite Payloads in orbit in 2006[1]
Image:Flag of the Soviet Union.svg USSR 1957 Sputnik 1 1390
Image:Flag of the United States.svg United States 1958 Explorer 1 999
Image:Flag of Australia.svg Australia 1964 Blue Streak 9
Image:Flag of France.svg France 1965 Astérix 43
Image:Flag of Japan (bordered).svg Japan 1970 Osumi 102
Image:Flag of the People's Republic of China.svg China 1970 Dong Fang Hong I 53
Image:Flag of the United Kingdom.svg United Kingdom 1971 Prospero X-3 23[citation needed]
Image:Flag of India.svg India 1979 Rohini-1 31
Image:Flag of Israel (bordered).svg Israel 1988 Ofeq 1 6
Image:Flag of North Korea.svg North Korea 1998 Kwangmyongsong Unknown

Both North Korea and Iraq have claimed orbital launches but these are unconfirmed. As of 2006, only ten countries have independently launched satellites into orbit on their own indigenously developed launch vehicles - in chronological order: USSR, USA, Canada, France, Australia, Japan, China, UK, India and Israel. It should be noted that while Kazakhstan did launch their satellite independently, it was built by the Russians, and the rocket was not independently designed. While Canada was the third country into Space, it has since retired its launch capabilities, and depends upon USA, Russia or the EU for its launches. Australia's launch project may have involved some input from the United Kingdom, although they are still considered an independent launch nation (see Australian Space Research Institute. The launch capabilities of the United Kingdom and France now fall under the European Union, and the launch capabilities of the Soviet Union fall under Russia, reducing the number of political entities with active satellite launch capabilities to seven - six 'major' space powers: USA, Russia, China, India, EU, Japan, and Israel.

Several other countries such as South Korea, Pakistan, Iran and Brazil are in the early stages of developing their own small-scale launch capabilities, and seek to become 'minor' space powers - others may have the scientific and industrial capability, but not the economic or political will. The first Italian satellite was San Marco 1, launched from Wallops Island (USA) on 15 December, 1964 on a scout launcher. Italy used a modified Scout launcher and never developed her own launcher: Italian satellites use Soyuz or Ariane (ESA) launchers.

[edit] Heraldry

The (artificial, though this is not stated in the blazon) satellite appears as a charge in the arms of Arthur Maxwell House.<ref></ref> This is in addition to numerous appearances of the natural satellite the moon, and the moons of the planets Jupiter and Saturn (with those planets) in the arms of Pierre-Simon LaPlace.

[edit] See also

A model satellite in a museum

[edit] References

<references />

[edit] External links

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