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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.

Definition of a 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 of the two objects is inside the primary object.

In popular usage, the term satellite normally refers to an artificial satellite, which would be 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. In general word usage, "natural satellite" is the term used to refer to moons.

Artificial satellites

The first artificial satellite

It was the English science fiction writer Arthur C. Clarke who conceived the possibility of artificial communication satellites in 1945 (facsimile at * ) 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 3 geostationary satellites would provide coverage over the entire planet. The first artificial satellite was Sputnik 1 launched by Soviet Union on October 4, 1957.

Early history of the US satellite program

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. 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 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.

Killer Satellites / Anti-Satellite Weapons 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 US 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

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).

Biosatellites are satellites designed to carry living organisms, generally for scientific experimentation.

Orbit types

Many times satellites are characterized by their orbit. Although a satellite may orbit at almost any altitude, satellites are commonly categorized by their altitude:

The following orbits are special orbits that are also used to categorize satellites:

Satellites can also orbit libration points.

Countries with satellite launch capability

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.

First launch by country
Country Year of first launch First satellite
1957 Sputnik 1
1958 Explorer 1
1965 Astérix
1970 Osumi
1970 Dong Fang Hong I
1971 Prospero X-3
1979 Rohini-1
1979 CAT 1
1988 Ofeq 1
Both North Korea and Iraq have claimed orbital launches but these are unconfirmed. Norway has launched domestic and international satellites from its space center on Andøya. As of 2006, only eight countries have independently launched satellites into orbit on their own indigenously developed launch vehicles - in chronological order: USSR, USA, France, 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. 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 one 'minor' space power: 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. First italian satellite was San Marco 1, launched from Wallops Island (USA) on 15 December, 1964 on a scout launcher. Italy used modified Scout launcher and never developed his own launcher: italian satellites use Soyuz or Ariane (ESA) launchers.

Heraldry

The (artificial, though this is not stated in the blazon) satellite appears as a charge in the arms of Arthur Maxwell House.* 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.

See also

Reference

External links

Facsimile of Arthur C. Clarke's 1945 paper on communication satellites

Satellites | Spacecraft | Spaceflight | Unmanned vehicles

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This article is licensed under the GNU Free Documentation License. It uses material from the "Satellite".

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