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A satellite or an artificial satellite is an object, typically a spacecraft, placed into orbit around a celestial body. They have a variety of uses, including communication relay, weather forecasting, navigation (GPS), broadcasting, scientific research, and Earth observation. Additional military uses are reconnaissance, early warning, signals intelligence and, potentially, weapon delivery. Other satellites include the final rocket stages that place satellites in orbit and formerly useful satellites that later become defunct.
Except for passive satellites, most satellites have an electricity generation system for equipment on board, such as or radioisotope thermoelectric generators (RTGs). Most satellites also have a method of communication to , called transponders. Many satellites use a Satellite bus to save cost and work, the most popular of which are small . Similar satellites can work together as groups, forming constellations. Because of the high launch cost to space, most satellites are designed to be as lightweight and robust as possible. Most communication satellites are radio relay stations in orbit and carry dozens of transponders, each with a bandwidth of tens of megahertz.
Spaceships become satellites by accelerating or decelerating to reach orbital velocities, occupying an orbit high enough to avoid orbital decay due to atmospheric drag in the presence of an atmosphere and above their Roche limit. Satellites are spacecraft launched from the surface into space by . Satellites can then change or maintain their orbit by propulsion, usually by chemical or . As of 2018, about 90% of the satellites orbiting the Earth are in low Earth orbit or geostationary orbit; geostationary means the satellites stay still in the sky (relative to a fixed point on the ground). Some imaging satellites choose a Sun-synchronous orbit because they can scan the entire globe with similar lighting. As the number of satellites and amount of space debris around Earth increases, the threat of collision has become more severe. An orbiter is a spacecraft that is designed to perform an orbital insertion, entering orbit around an astronomical body from another, and as such becoming an artificial satellite. A small number of satellites orbit other bodies (such as the Lunar orbit, Mars, and the Sun) or many bodies at once (two for a halo orbit, three for a Lissajous orbit).
Earth observation satellites gather information for reconnaissance, mapping, monitoring the weather, ocean, forest, etc. take advantage of outer space's near perfect vacuum to observe objects with the entire electromagnetic spectrum. Because satellites can see a large portion of the Earth at once, communications satellites can relay information to remote places. The signal delay from satellites and their orbit's predictability are used in satellite navigation systems, such as GPS. Crewed spacecrafts which are in orbit or remain in orbit, like , are artificial satellites as well.
The first artificial satellite launched into the Earth's orbit was the Soviet Union's Sputnik 1, on October 4, 1957. As of December 31, 2022, there are 6,718 operational satellites in the Earth's orbit, of which 4,529 belong to the United States (3,996 commercial), 590 belong to China, 174 belong to Russia, and 1,425 belong to other nations.
In 1903, Konstantin Tsiolkovsky (1857–1935) published Exploring Space Using Jet Propulsion Devices, which was the first academic treatise on the use of rocketry to launch spacecraft. He calculated the orbital speed required for a minimal orbit, and inferred that a multi-stage rocket fueled by liquid could achieve this.
Herman Potočnik explored the idea of using orbiting spacecraft for detailed peaceful and military observation of the ground in his 1928 book, The Problem of Space Travel. He described how the special conditions of space could be useful for scientific experiments. The book described geostationary satellites (first put forward by Konstantin Tsiolkovsky) and discussed the communication between them and the ground using radio, but fell short with the idea of using satellites for mass broadcasting and as telecommunications relays.
In a 1945 Wireless World article, English science fiction writer Arthur C. Clarke described in detail the possible use of communications satellites for mass communications. He suggested that three geostationary satellites would provide coverage over the entire planet. (2025). 9781119482178, John Wiley & Sons Ltd. ISBN 9781119482178
In May 1946, the United States Air Force's 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 United States had been considering launching orbital satellites since 1945 under the Bureau of Aeronautics of the United States Navy. Project RAND eventually released the report, but considered the satellite to be a tool for science, politics, and propaganda, rather than a potential military weapon.
In 1946, American theoretical astrophysicist Lyman Spitzer proposed an orbiting space telescope.
In February 1954, Project RAND released "Scientific Uses for a Satellite Vehicle", by R. R. Carhart.R. R. Carhart, Scientific Uses for a Satellite Vehicle, Project RAND Research Memorandum. (Rand Corporation, Santa Monica) 12 February 1954. This expanded on potential scientific uses for satellite vehicles and was followed in June 1955 with "The Scientific Use of an Artificial Satellite", by H. K. Kallmann and W. W. Kellogg.H. K. Kallmann and W. W. Kellogg, Scientific Use of an Artificial Satellite, Project RAND Research Memorandum. (Rand Corporation, Santa Monica, California) 8 June 1955.
In the context of activities planned for the International Geophysical Year (1957–1958), the White House announced on 29 July 1955 that the U.S. intended to launch satellites by the spring of 1958. This became known as Project Vanguard. On 31 July, the Soviet Union announced its intention to launch a satellite by the fall of 1957.
Sputnik 2 was launched on 3 November 1957 and carried the first living passenger into orbit, a dog named Laika. The dog was sent without possibility of return.
In early 1955, after being pressured by the American Rocket Society, the National Science Foundation, and the International Geophysical Year, the Army and Navy worked on Project Orbiter with two competing programs. The army used the Jupiter-C IRBM, while the civilian–Navy program used the Vanguard rocket to launch a satellite. Explorer 1 became the United States' first artificial satellite, on 31 January 1958. The information sent back from its radiation detector led to the discovery of the Earth's Van Allen radiation belts. The TIROS-1 spacecraft, launched on April 1, 1960, as part of NASA's Television Infrared Observation Satellite (TIROS) program, sent back the first television footage of weather patterns to be taken from space.
In June 1961, three and a half years after the launch of Sputnik 1, the United States Space Surveillance Network cataloged 115 Earth-orbiting satellites.
While Canada was the third country to build a satellite which was launched into space, it was launched aboard an United States rocket from an American spaceport. The same goes for Australia, whose launch of the first satellite involved a donated U.S. Redstone rocket and American support staff as well as a joint launch facility with the United Kingdom.Mike Gruntman (2025). 9781563477058, American Institute of Aeronautics and Astronautics. ISBN 9781563477058 The first Italian satellite San Marco 1 was launched on 15 December 1964 on a U.S. Scout rocket from Wallops Island (Virginia, United States) with an Italian launch team trained by NASA. In similar occasions, almost all further first national satellites were launched by foreign rockets.
France was the third country to launch a satellite on its own rocket. On 26 November 1965, the Astérix or A-1 (initially conceptualized as FR.2 or FR-2), was put into orbit by a Diamant A rocket launched from the CIEES site at Hammaguir, Algeria. With Astérix, France became the sixth country to have an artificial satellite.
After the late 2010s, and especially after the advent and operational fielding of large satellite internet constellations—where on-orbit active satellites more than doubled over a period of five years—the companies building the constellations began to propose regular planned deorbiting of the older satellites that reached the end of life, as a part of the regulatory process of obtaining a launch license. The largest artificial satellite ever is the International Space Station.
By the early 2000s, and particularly after the advent of and increased launches of microsats—frequently launched to the lower altitudes of low Earth orbit (LEO)—satellites began to more frequently be designed to get destroyed, or breakup and burnup entirely in the atmosphere. For example, SpaceX Starlink satellites, the first large satellite internet constellation to exceed 1000 active satellites on orbit in 2020, are designed to be 100% demisable and burn up completely on their atmospheric reentry at the end of their life, or in the event of an early satellite failure.
In different periods, many countries, such as Algeria, Argentina, Australia, Austria, Brazil, Canada, Chile, China, Denmark, Egypt, Finland, France, Germany, India, Iran, Israel, Italy, Japan, Kazakhstan, South Korea, Malaysia, Mexico, the Netherlands, Norway, Pakistan, Poland, Russia, Saudi Arabia, South Africa, Spain, Switzerland, Thailand, Turkey, Ukraine, the United Kingdom and the United States, had some satellites in orbit.
Japan's space agency (JAXA) and NASA plan to send a wooden satellite prototype called LingoSat into orbit in the summer of 2024. They have been working on this project for few years and sent first wood samples to the space in 2021 to test the material's resilience to space conditions.
Chemical thrusters on satellites usually use monopropellant (one-part) or bipropellant (two-parts) that are hypergolic. Hypergolic means able to combust spontaneously when in contact with each other or to a Catalysis. The most commonly used propellant mixtures on satellites are hydrazine-based monopropellants or monomethylhydrazine–dinitrogen tetroxide bipropellants. Ion thrusters on satellites usually are Hall-effect thrusters, which generate thrust by accelerating positive ions through a negatively-charged grid. Ion propulsion is more efficient propellant-wise than chemical propulsion but its thrust is very small (around ), and thus requires a longer burn time. The thrusters usually use xenon because it is Inert gas, can be easily ionized, has a high atomic mass and storable as a high-pressure liquid.
Weather satellites monitor , Street light, , effects of pollution, auroral light, Dust storm, snow cover, ice mapping, boundaries of , energy flows, etc. Environmental monitoring satellites can detect changes in the Earth's vegetation, atmospheric trace gas content, sea state, ocean color, and ice fields. By monitoring vegetation changes over time, droughts can be monitored by comparing the current vegetation state to its long term average. Anthropogenic emissions can be monitored by evaluating data of tropospheric NO2 and SO2.
Their uses include early missile warning, nuclear explosion detection, electronic reconnaissance, and optical or radar imaging surveillance.
Rocket emissions in the stratosphere and their effects are only beginning to be studied and it is likely that the impacts will be more critical than emissions in the troposphere. The stratosphere includes the ozone layer and pollutants emitted from rockets can contribute to ozone depletion in a number of ways. Radicals such as NOx, HOx, and ClOx deplete stratospheric O3 through intermolecular reactions and can have huge impacts in trace amounts. However, it is currently understood that launch rates would need to increase by ten times to match the impact of regulated ozone-depleting substances. Whilst emissions of water vapour are largely deemed as inert, H2O is the source gas for HOx and can also contribute to ozone loss through the formation of ice particles. Black carbon particles emitted by rockets can absorb solar radiation in the stratosphere and cause warming in the surrounding air which can then impact the circulatory dynamics of the stratosphere. Both warming and changes in circulation can then cause depletion of the ozone layer.
Some notable satellite failures that polluted and dispersed radioactive materials are Kosmos 954, Kosmos 1402 and the Transit 5-BN-3.
When in a controlled manner satellites reach the end of life they are intentionally deorbited or moved to a graveyard orbit further away from Earth in order to reduce space debris. Physical collection or removal is not economical or even currently possible. Moving satellites out to a graveyard orbit is also unsustainable because they remain there for hundreds of years. It will lead to the further pollution of space and future issues with space debris. When satellites deorbit much of it is destroyed during re-entry into the atmosphere due to the heat. This introduces more material and pollutants into the atmosphere. There have been concerns expressed about the potential damage to the ozone layer and the possibility of increasing the earth's albedo, reducing warming but also resulting in accidental geoengineering of the earth's climate. After deorbiting 70% of satellites end up in the ocean and are rarely recovered.
With increase in the number of satellite constellations, like SpaceX Starlink, the astronomical community, such as the IAU, report that orbital pollution is getting increased significantly. A report from the SATCON1 workshop in 2020 concluded that the effects of large satellite constellations can severely affect some astronomical research efforts and lists six ways to mitigate harm to astronomy. The IAU is establishing a center (CPS) to coordinate or aggregate measures to mitigate such detrimental effects.
Also, it is very easy 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 tools and methods that enable them to pinpoint the source of any carrier and manage the transponder space effectively.
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The operation capabilities and use have very much diversified and is broadening increasingly.
Satellite operation needs not only access to financial, manufacturing and launch capabilities, but also a ground segment infrastructure.
| Polyus | Prototype Soviet orbital weapons platform | LEO | Lost | 1987 | ||
| KH-11 | p.199-200 | Electro-optical reconnaissance satellite | SSO | In service | 1976– (current version: 2005–) | |
| Proton satellite | Space research satellite | LEO | Deorbited | 1965–1969 | ||
| Compton Gamma Ray Observatory | Space observatory | LEO | Deorbited | 1991–2000 | ||
| Lacrosse | ||||||
| Radar imaging reconnaissance satellite | SSO | Retired Lacrosse 5 still in orbit | 1988–2005 | |||
| Hubble Space Telescope | Space observatory | LEO | In service | 1990– | ||
| Jupiter-3 (EchoStar) | Communications satellite | GEO | In service | 2023– | ||
| Envisat | ESA | Earth observing satellite Kessler syndrome threat | LEO | In orbit, inoperable | 2002–2012 | |
| Shijian | Communication Technology Test Satellite | GEO | In service | 2019– | ||
| Telstar 19V | Communications satellite | GEO | In service | 2018– | ||
| TerreStar-1 | Communications satellite | GEO | In service | 2009– | ||
| EchoStar XXI | Communications satellite | GEO | In service | 2017– | ||
| UARS | Earth science | LEO | Deorbited 2011 | 1991–2005 | ||
| James Webb Space Telescope | Space observatory | Sun-Earth L2 | In service | 2021– | ||
| Chandra X-ray Obs. | Space observatory | HEO | In service | 1999– | ||
| GSAT-11 | Heaviest Indian communications satellite | GEO | In service | 2018– | ||
| Terra | Earth observing satellite | SSO | In service | 1999– | ||
| GSAT-24 | 4,181 kg (9,218 lb) | Indian Communication Satellite | GEO | In service | 2022- | |
| GPS IIIA | Current GPS satellite series | MEO | In service | 2018– | ||
| Spektr-R | Space observatory | HEO | In service | 2011– | ||
| Herschel | ESA | Space observatory | Sun-Earth L2 | Retired | 2009–2013 | |
| Astrosat | Space observatory from India | LEO | In service | 2015– | ||
| Malligyong-1 | Heaviest North Korean reconnaissance satellite, 21 Nov 2023 launch | SSO | In service | 2023– |
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