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(Wikipedia) - Satellite This article is about artificial satellites. For natural satellites, also known as moons, see Natural satellite. For other uses, see Satellite (disambiguation).
This article is outdated. Please update this article to reflect recent events or newly available information. (December 2013)
Play mediaNASA''s Earth-observing fleet as of June 2012.An animation depicting the orbits of GPS satellites in medium Earth orbit.A full-size model of the Earth observation satellite ERS 2

In the context of spaceflight, a satellite is an artificial object which has been intentionally placed into orbit. Such objects are sometimes called artificial satellites to distinguish them from natural satellites such as the Moon.

The world''s first artificial satellite, the Sputnik 1, was launched by the Soviet Union in 1957. Since then, thousands of satellites have been launched into orbit around the Earth. Some satellites, notably space stations, have been launched in parts and assembled in orbit. Artificial satellites originate from more than 40 countries and have used the satellite launching capabilities of ten nations. A few hundred satellites are currently operational, whereas thousands of unused satellites and satellite fragments orbit the Earth as space debris. A few space probes have been placed into orbit around other bodies and become artificial satellites to the Moon, Mercury, Venus, Mars, Jupiter, Saturn, Vesta, Eros, and the Sun.

Satellites are used for a large number of purposes. Common types include military and civilian Earth observation satellites, communications satellites, navigation satellites, weather satellites, and research satellites. Space stations and human spacecraft in orbit are also satellites. Satellite orbits vary greatly, depending on the purpose of the satellite, and are classified in a number of ways. Well-known (overlapping) classes include low Earth orbit, polar orbit, and geostationary orbit.

About 6,600 satellites have been launched. The latest estimates are that 3,600 remain in orbit. Of those, about 1,000 are operational; the rest have lived out their useful lives and are part of the space debris. Approximately 500 operational satellites are in low-Earth orbit, 50 are in medium-Earth orbit (at 20,000 km), the rest are in geostationary orbit (at 36,000 km).

Satellites are propelled by rockets to their orbits. Usually the launch vehicle itself is a rocket lifting off from a launch pad on land. In a minority of cases satellites are launched at sea (from a submarine or a mobile maritime platform) or aboard a plane (see air launch to orbit).

Satellites are usually semi-independent computer-controlled systems. Satellite subsystems attend many tasks, such as power generation, thermal control, telemetry, attitude control and orbit control.


History Early conceptions

"Newton''s cannonball", presented as a "thought experiment" in A Treatise of the System of the World, was the first published mathematical study of the possibility of an artificial satellite.

The first fictional depiction of a satellite being launched into orbit is a short story by Edward Everett Hale, The Brick Moon. The story is serialized in The Atlantic Monthly, starting in 1869. The idea surfaces again in Jules Verne''s The Begum''s Fortune (1879).

Konstantin Tsiolkovsky

In 1903, Konstantin Tsiolkovsky (1857–1935) published Exploring Space Using Jet Propulsion Devices (in Russian: Исследование мировых пространств реактивными приборами), which is the first academic treatise on the use of rocketry to launch spacecraft. He calculated the orbital speed required for a minimal orbit around the Earth at 8 km/s, and that a multi-stage rocket fuelled by liquid propellants could be used to achieve this. He proposed the use of liquid hydrogen and liquid oxygen, though other combinations can be used.

In 1928, Slovenian Herman Potočnik (1892–1929) published his sole book, The Problem of Space Travel — The Rocket Motor (German: Das Problem der Befahrung des Weltraums — der Raketen-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 a 1945 Wireless World article, the English science fiction writer Arthur C. Clarke (1917–2008) described in detail the possible use of communications satellites for mass communications. 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 US military studied the idea of what was referred to as the earth satellite vehicle when Secretary of Defense James Forrestal made a public announcement on December 29, 1948, that his office was coordinating that project between the various services.

History of artificial satellitesSputnik 1: The first artificial satellite to orbit Earth.

The first artificial satellite was Sputnik 1, launched by the Soviet Union on October 4, 1957, and initiating the Soviet Sputnik program, with Sergei Korolev as chief designer (there is a crater on the lunar far side which bears his name). This in turn triggered the Space Race between the Soviet Union and the United States.

Sputnik 1 helped to identify the density of high atmospheric layers through measurement of its orbital change and provided data on radio-signal distribution in the ionosphere. The unanticipated announcement of Sputnik 1''s success precipitated the Sputnik crisis in the United States and ignited the so-called Space Race within the Cold War.

Sputnik 2 was launched on November 3, 1957 and carried the first living passenger into orbit, a dog named Laika.

In May, 1946, Project RAND had 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 United States had been considering launching orbital satellites since 1945 under the Bureau of Aeronautics of the United States Navy. The United States 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."

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.

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 Army and Navy were working on Project Orbiter, two competing programs: the army''s which involved using a Jupiter C rocket, and the civilian/Navy Vanguard Rocket, to launch a satellite. At first, they failed: initial preference was given to the Vanguard program, whose first attempt at orbiting a satellite resulted in the explosion of the launch vehicle on national television. But finally, three months after Sputnik 2, the project succeeded; Explorer 1 became the United States'' first artificial satellite on January 31, 1958.

In June 1961, three-and-a-half years after the launch of Sputnik 1, the Air Force used resources of the United States Space Surveillance Network to catalog 115 Earth-orbiting satellites.

Early satellites were constructed as "one-off" designs. With growth in geosynchronous (GEO) satellite communication, multiple satellites began to be built on single model platforms called satellite buses. The first standardized satellite bus design was the HS-333 GEO commsat, launched in 1972.

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

1U CubeSat ESTCube-1, developed mainly by the students from the University of Tartu, carries out a tether deployment experiment on the low Earth orbit.Space Surveillance Network Main article: United States Space Surveillance Network

The United States Space Surveillance Network (SSN), a division of The United States Strategic Command, has been tracking objects in Earth''s orbit since 1957 when the Soviets opened the space age with the launch of Sputnik I. Since then, the SSN has tracked more than 26,000 objects. The SSN currently tracks more than 8,000 man-made orbiting objects. The rest have re-entered Earth''s atmosphere and disintegrated, or survived re-entry and impacted the Earth. The SSN tracks objects that are 10 centimeters in diameter or larger; those now orbiting Earth range from satellites weighing several tons to pieces of spent rocket bodies weighing only 10 pounds. About seven percent are operational satellites (i.e. ~560 satellites), the rest are space debris. The United States Strategic Command is primarily interested in the active satellites, but also tracks space debris which upon reentry might otherwise be mistaken for incoming missiles.

A search of the NSSDC Master Catalog at the end of October 2010 listed 6,578 satellites launched into orbit since 1957, the latest being Chang''e 2, on 1 October 2010.

Non-military satellite services

There are three basic categories of non-military satellite services:

Fixed satellite services

Fixed satellite services handle hundreds of billions of voice, data, and video transmission tasks across all countries and continents between certain points on the Earth''s surface.

Mobile satellite systems

Mobile satellite systems help connect remote regions, vehicles, ships, people and aircraft to other parts of the world and/or other mobile or stationary communications units, in addition to serving as navigation systems.

Scientific research satellites (commercial and noncommercial)

Scientific research satellites provide meteorological information, land survey data (e.g. remote sensing), Amateur (HAM) Radio, and other different scientific research applications such as earth science, marine science, and atmospheric research.

TypesMILSTAR: A communication satelliteInternational Space Station as seen from Space Orbit types Main article: List of orbitsVarious earth orbits to scale; cyan represents low earth orbit, yellow represents medium earth orbit, the black dashed line represents geosynchronous orbit, the green dash-dot line the orbit of Global Positioning System (GPS) satellites, and the red dotted line the orbit of the International Space Station (ISS).

The first satellite, Sputnik 1, was put into orbit around Earth and was therefore in geocentric orbit. By far this is the most common type of orbit with approximately 2,456 artificial satellites orbiting the Earth. Geocentric orbits may be further classified by their altitude, inclination and eccentricity.

The commonly used altitude classifications of geocentric orbit are Low Earth orbit (LEO), Medium Earth orbit (MEO) and High Earth orbit (HEO). Low Earth orbit is any orbit below 2,000 km. Medium Earth orbit is any orbit between 2,000 km-35,786 km. High Earth orbit is any orbit higher than 35,786 km.

Centric classifications

The general structure of a satellite is that it is connected to the earth stations that are present on the ground and connected through terrestrial links.

Altitude classifications Orbital Altitudes of several significant satellites of earth.Inclination classifications Eccentricity classifications Synchronous classifications Special classifications Pseudo-orbit classifications Satellite subsystems

The satellite''s functional versatility is imbedded within its technical components and its operations characteristics. Looking at the "anatomy" of a typical satellite, one discovers two modules. Note that some novel architectural concepts such as Fractionated Spacecraft somewhat upset this taxonomy.

Spacecraft bus or service module

This bus module consist of the following subsystems:

The structural subsystem provides the mechanical base structure with adequate stiffness to withstand stress and vibrations experienced during launch, maintain structural integrity and stability while on station in orbit, and shields the satellite from extreme temperature changes and micro-meteorite damage.

The telemetry subsystem monitors the on-board equipment operations, transmits equipment operation data to the earth control station, and receives the earth control station''s commands to perform equipment operation adjustments.

The power subsystem consists of solar panels to convert solar energy into electrical power, regulation and distribution functions, and batteries that store power and supply the satellite when it passes into the Earth''s shadow. Nuclear power sources (Radioisotope thermoelectric generator have also been used in several successful satellite programs including the Nimbus program (1964–1978).

The thermal control subsystem helps protect electronic equipment from extreme temperatures due to intense sunlight or the lack of sun exposure on different sides of the satellite''s body (e.g. Optical Solar Reflector)

Main articles: Attitude control and Spacecraft propulsion

The attitude and orbit control subsystem consists of sensors to measure vehicle orientation; control laws embedded in the flight software; and actuators (reaction wheels, thrusters) to apply the torques and forces needed to re-orient the vehicle to a desired attitude, keep the satellite in the correct orbital position and keep antennas positioning in the right directions.

Communication payload

The second major module is the communication payload, which is made up of transponders. A transponder is capable of :

End of life

When satellites reach the end of their mission, satellite operators have the option of de-orbiting the satellite, leaving the satellite in its current orbit or moving the satellite to a graveyard orbit. Historically, due to budgetary constraints at the beginning of satellite missions, satellites were rarely designed to be de-orbited. One example of this practice is the satellite Vanguard 1. Launched in 1958, Vanguard 1, the 4th manmade satellite put in Geocentric orbit, was still in orbit as of August 2009.

Instead of being de-orbited, most satellites are either left in their current orbit or moved to a graveyard orbit. As of 2002, the FCC requires all geostationary satellites to commit to moving to a graveyard orbit at the end of their operational life prior to launch. In cases of uncontrolled de-orbiting, the major variable is the solar flux, and the minor variables the components and form factors of the satellite itself, and the gravitational perturbations generated by the Sun and the Moon (as well as those exercised by large mountain ranges, whether above or below sea level). The nominal breakup altitude due to aerodynamic forces and temperatures is 78 km, with a range between 72 and 84 km. Solar panels, however, are destroyed before any other component at altitudes between 90 and 95 km.

Launch-capable countries Main article: Timeline of first orbital launches by nationality

This list includes countries with an independent capability of states to place satellites in orbit, including production of the necessary launch vehicle. Note: many more countries have the capability to design and build satellites 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. The list include multi-national state organization ESA but does not include private consortiums.

First launch by country Order Country Date of first launch Rocket Satellite
1  Soviet Union 4 October 1957 Sputnik-PS Sputnik 1
2  United States 1 February 1958 Juno I Explorer 1
3  France 26 November 1965 Diamant-A Astérix
4  Japan 11 February 1970 Lambda-4S Ōsumi
5  China 24 April 1970 Long March 1 Dong Fang Hong I
6  United Kingdom 28 October 1971 Black Arrow Prospero
7  India 18 July 1980 SLV Rohini D1
8  Israel 19 September 1988 Shavit Ofeq 1
-  Russia 21 January 1992 Soyuz-U Kosmos 2175
-  Ukraine 13 July 1992 Tsyklon-3 Strela
9  Iran 2 February 2009 Safir-1 Omid
10  North Korea 12 December 2012 Unha-3 Kwangmyŏngsŏng-3 Unit 2
Attempted first launches
This section requires expansion. (May 2012)
Other notes Launch capable private entities

A few other private companies are capable of sub-orbital launches.

First satellites of countries Main article: Timeline of first satellites by country First satellites of countries including launched indigenously or by help of other Country Year of first launch First satellite Payloads in orbit as of Jan 2013
 Soviet Union ( Russia) 1957 (1992) Sputnik 1 (Kosmos 2175) 1457
 United States 1958 Explorer 1 1110
 United Kingdom 1962 Ariel 1 0030
 Canada 1962 Alouette 1 0034
 Italy 1964 San Marco 1 0022
 France 1965 Astérix 0057
 Australia 1967 WRESAT 0012
 Germany 1969 Azur 0042
 Japan 1970 Ōsumi 0134
 China 1970 Dong Fang Hong I 0140
 Netherlands 1974 ANS 0004
 Spain 1974 Intasat 0009
 India 1975 Aryabhata 0054
 Indonesia 1976 Palapa A1 0012
 Czechoslovakia 1978 Magion 1 0004
 Bulgaria 1981 Intercosmos Bulgaria 1300 0001
 Saudi Arabia 1985 Arabsat-1A 0012
 Brazil 1985 Brasilsat A1 0013
 Mexico 1985 Morelos 1 0007
 Sweden 1986 Viking 0011
 Israel 1988 Ofeq 1 00011
 Luxembourg 1988 Astra 1A 005
 Argentina 1990 Lusat 009
 Pakistan 1990 Badr-1 0003
 South Korea 1992 Kitsat A 0011
 Portugal 1993 PoSAT-1 0001
 Thailand 1993 Thaicom 1 0007
 Turkey 1994 Turksat 1B 0008
 Ukraine 1995 Sich-1 0006
 Malaysia 1996 MEASAT 0006
 Norway 1997 Thor 2 0003
 Philippines 1997 Mabuhay 1 0002
 Egypt 1998 Nilesat 101 0004
 Chile 1998 FASat-Bravo 0002
 Singapore 1998 ST-1 0003
 Taiwan 1999 ROCSAT-1 0008
 Denmark 1999 Ørsted 0004
 South Africa 1999 SUNSAT 0002
 United Arab Emirates 2000 Thuraya 1 0006
 Morocco 2001 Maroc-Tubsat 0001
 Algeria 2002 Alsat 1 0001
 Greece 2003 Hellas Sat 2 0002
 Cyprus 2003 Hellas Sat 2 0002
 Nigeria 2003 Nigeriasat 1 0004
 Iran 2005 Sina-1 0001
 Kazakhstan 2006 KazSat 1 0002
 Colombia 2007 Libertad 1 0001
 Mauritius 2007 Rascom-QAF 1 0002
 Vietnam 2008 Vinasat-1 0003
 Venezuela 2008 Venesat-1 0002
  Switzerland 2009 SwissCube-1 0002
 Poland 2012 PW-Sat 00002
 Hungary 2012 MaSat-1 0001
 Romania 2012 Goliat 0001
 Belarus 2012 BKA (BelKA-2) N/A
 North Korea 2012 Kwangmyŏngsŏng-3 Unit 2 1
 Azerbaijan 2013 Azerspace 1
 Austria 2013 TUGSAT-1/UniBRITE 2
 Ecuador 2013 NEE-01 Pegaso 1
 Estonia 2013 ESTCube-1 1
 Jersey 2013 O3b-1,-2,-3,-4 4
 Qatar 2013 Es''hailSat1 1
 Peru 2013 PUCPSAT-1 1
 Bolivia 2013 TKSat-1 1
 Lithuania 2014 LituanicaSAT-1 and LitSat-1 2
  orbital launch and satellite operation   satellite operation, launched by foreign supplier   satellite in development   orbital launch project at advanced stage or indigenous ballistic missiles deployed

While Canada was the third country to build a satellite which was launched into space, it was launched aboard a U.S. rocket from a U.S. spaceport. The same goes for Australia, who launched on board a donated Redstone rocket. The first Italian-launched was San Marco 1, launched on 15 December 1964 on a U.S. Scout rocket from Wallops Island (VA, USA) with an Italian Launch Team trained by NASA. Australia''s launch project (WRESAT) involved a donated U.S. missile and U. S. support staff as well as a joint launch facility with the United Kingdom.

Attempted first satellite
This section requires expansion. (May 2012)

†-note: Both Chile and Belarus used Russian companies as principal contractors to build their satellites, they used Russian-Ukrainian manufactured rockets and launched either from Russia or Kazakhstan.

Planned first satellites
This section requires expansion. (June 2011)
Attacks on satellites For more details on this topic, see Anti-satellite weapon.

In recent times satellites have been hacked by militant organizations to broadcast propaganda and to pilfer classified information from military communication networks.

For testing purposes, satellites in low earth orbit have been destroyed by ballistic missiles launched from earth. Russia, the United States and China have demonstrated the ability to eliminate satellites. In 2007 the Chinese military shot down an aging weather satellite, followed by the US Navy shooting down a defunct spy satellite in February 2008.

Jamming See also: Satellite Jamming in Iran

Due to the low received signal strength of satellite transmissions, they are prone to jamming by land-based transmitters. Such jamming is limited to the geographical area within the transmitter''s range. GPS satellites are potential targets for jamming, but satellite phone and television signals have also been subjected to jamming.

Also, it is trivial to transmit a carrier radio signal to a geostationary satellite and thus interfere with the legitimate uses of the satellite''s transponder. It is common for Earth stations to transmit at the wrong time or on the wrong frequency in commercial satellite space, and dual-illuminate the transponder, rendering the frequency unusable. Satellite operators now have sophisticated monitoring that enables them to pinpoint the source of any carrier and manage the transponder space effectively.

Satellite services

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