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A satellite or artificial satellite is an object intentionally placed into
orbit In celestial mechanics, an orbit is the curved trajectory of an object such as the trajectory of a planet around a star, or of a natural satellite around a planet, or of an artificial satellite around an object or position in space such as ...
in
outer space Outer space, commonly shortened to space, is the expanse that exists beyond Earth and its atmosphere and between celestial bodies. Outer space is not completely empty—it is a near-perfect vacuum containing a low density of particles, pred ...
. Except for passive satellites, most satellites have an
electricity generation Electricity generation is the process of generating electric power from sources of primary energy. For utilities in the electric power industry, it is the stage prior to its delivery ( transmission, distribution, etc.) to end users or its s ...
system for equipment on board, such as solar panels or
radioisotope thermoelectric generator A radioisotope thermoelectric generator (RTG, RITEG), sometimes referred to as a radioisotope power system (RPS), is a type of nuclear battery that uses an array of thermocouples to convert the heat released by the decay of a suitable radioacti ...
s (RTGs). Most satellites also have a method of communication to ground stations, called
transponders In telecommunications, a transponder is a device that, upon receiving a signal, emits a different signal in response. The term is a blend of ''transmitter'' and ''responder''. In air navigation or radio frequency identification, a flight tran ...
. Many satellites use a standardized bus to save cost and work, the most popular of which is small
CubeSat A CubeSat is a class of miniaturized satellite based around a form factor consisting of cubes. CubeSats have a mass of no more than per unit, and often use commercial off-the-shelf (COTS) components for their electronics and structure. CubeSats ...
s. Similar satellites can work together as a group, forming
constellations A constellation is an area on the celestial sphere in which a group of visible stars forms a perceived pattern or outline, typically representing an animal, mythological subject, or inanimate object. The origins of the earliest constellation ...
. Because of the high
launch cost Space launch market competition is the manifestation of market forces in the launch service provider business. In particular it is the trend of competitive dynamics among payload transport capabilities at diverse prices having a greater influe ...
to space, 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. Satellites are placed from the surface to orbit by
launch vehicle A launch vehicle or carrier rocket is a rocket designed to carry a payload ( spacecraft or satellites) from the Earth's surface to outer space. Most launch vehicles operate from a launch pads, supported by a launch control center and ...
s, high enough to avoid orbital decay by the atmosphere. Satellites can then change or maintain the orbit by
propulsion Propulsion is the generation of force by any combination of pushing or pulling to modify the translational motion of an object, which is typically a rigid body (or an articulated rigid body) but may also concern a fluid. The term is derived f ...
, usually by
chemical A chemical substance is a form of matter having constant chemical composition and characteristic properties. Some references add that chemical substance cannot be separated into its constituent elements by physical separation methods, i.e., w ...
or
ion thruster An ion thruster, ion drive, or ion engine is a form of electric propulsion used for spacecraft propulsion. It creates thrust by accelerating ions using electricity. An ion thruster ionizes a neutral gas by extracting some electrons out of ...
s. In 2018, about 90% of satellites orbiting Earth are in
low Earth orbit A low Earth orbit (LEO) is an orbit around Earth with a period of 128 minutes or less (making at least 11.25 orbits per day) and an eccentricity less than 0.25. Most of the artificial objects in outer space are in LEO, with an altitude never mor ...
or geostationary orbit; geostationary means the satellites stay still at the sky. Some imaging satellites chose a Sun-synchronous orbit because they can scan the entire globe with similar lighting. As the number of satellites and space debris around Earth increases, the collision threat is becoming more severe. A small number of satellites orbit other bodies (such as the
Moon The Moon is Earth's only natural satellite. It is the fifth largest satellite in the Solar System and the largest and most massive relative to its parent planet, with a diameter about one-quarter that of Earth (comparable to the width of ...
,
Mars Mars is the fourth planet from the Sun and the second-smallest planet in the Solar System, only being larger than Mercury. In the English language, Mars is named for the Roman god of war. Mars is a terrestrial planet with a thin at ...
, and the
Sun The Sun is the star at the center of the Solar System. It is a nearly perfect ball of hot plasma, heated to incandescence by nuclear fusion reactions in its core. The Sun radiates this energy mainly as light, ultraviolet, and infrared radi ...
) or many bodies at once (two for a halo orbit, three for a Lissajous orbit). Earth observation satellites gather information for
reconnaissance In military operations, reconnaissance or scouting is the exploration of an area by military forces to obtain information about enemy forces, terrain, and other activities. Examples of reconnaissance include patrolling by troops (skirmisher ...
, mapping, monitoring the weather, ocean, forest, etc.
Space telescope A space telescope or space observatory is a telescope in outer space used to observe astronomical objects. Suggested by Lyman Spitzer in 1946, the first operational telescopes were the American Orbiting Astronomical Observatory, OAO-2 launch ...
s take advantage of outer space's near perfect vacuum to observe objects with the entire
electromagnetic spectrum The electromagnetic spectrum is the range of frequencies (the spectrum) of electromagnetic radiation and their respective wavelengths and photon energies. The electromagnetic spectrum covers electromagnetic waves with frequencies ranging fro ...
. Because satellites can see a large portion of the Earth at once,
communications satellites A communications satellite is an artificial satellite that relays and amplifies radio telecommunication signals via a transponder; it creates a communication channel between a source transmitter and a receiver at different locations on Earth. C ...
can relay information to remote places. The signal delay from satellites and their orbit's predictability are used in
satellite navigation A satellite navigation or satnav system is a system that uses satellites to provide autonomous geo-spatial positioning. It allows satellite navigation devices to determine their location (longitude, latitude, and altitude/elevation) to high pr ...
systems, such as
GPS The Global Positioning System (GPS), originally Navstar GPS, is a satellite-based radionavigation system owned by the United States government and operated by the United States Space Force. It is one of the global navigation satellite sy ...
.
Space probe A space probe is an artificial satellite that travels through space to collect scientific data. A space probe may orbit Earth; approach the Moon; travel through interplanetary space; flyby, orbit, or land or fly on other planetary bodies; or ...
s are satellites designed for robotic space exploration outside of Earth, and space stations are in essence crewed satellites. The first artificial satellite to be launched into the Earth's orbit was the
Soviet Union The Soviet Union,. officially the Union of Soviet Socialist Republics. (USSR),. was a List of former transcontinental countries#Since 1700, transcontinental country that spanned much of Eurasia from 1922 to 1991. A flagship communist state, ...
's Sputnik 1, on 4 October 1957.


History


Early proposals

The first published mathematical study of the possibility of an artificial satellite was
Newton's cannonball Newton's cannonball was a thought experiment Isaac Newton used to hypothesize that the force of gravity was universal, and it was the key force for planetary motion. It appeared in his posthumously published 1728 work ''De mundi systemate'' (als ...
, a thought experiment by
Isaac Newton Sir Isaac Newton (25 December 1642 – 20 March 1726/27) was an English mathematician, physicist, astronomer, alchemist, theologian, and author (described in his time as a " natural philosopher"), widely recognised as one of the grea ...
to explain the motion of
natural satellite A natural satellite is, in the most common usage, an astronomical body that orbits a planet, dwarf planet, or small Solar System body (or sometimes another natural satellite). Natural satellites are often colloquially referred to as ''moons'' ...
s, in his '' Philosophiæ Naturalis Principia Mathematica'' (1687). The first fictional depiction of a satellite being launched into orbit was a
short story A short story is a piece of prose fiction that typically can be read in one sitting and focuses on a self-contained incident or series of linked incidents, with the intent of evoking a single effect or mood. The short story is one of the oldest ...
by
Edward Everett Hale Edward Everett Hale (April 3, 1822 – June 10, 1909) was an American author, historian, and Unitarian minister, best known for his writings such as " The Man Without a Country", published in '' Atlantic Monthly'', in support of the Union ...
, " The Brick Moon" (1869). The idea surfaced again in Jules Verne's '' The Begum's Fortune'' (1879). In 1903,
Konstantin Tsiolkovsky Konstantin Eduardovich Tsiolkovsky (russian: Константи́н Эдуа́рдович Циолко́вский , , p=kənstɐnʲˈtʲin ɪdʊˈardəvʲɪtɕ tsɨɐlˈkofskʲɪj , a=Ru-Konstantin Tsiolkovsky.oga; – 19 September 1935) ...
(1857–1935) published ''Exploring Space Using Jet Propulsion Devices'', which is the first academic treatise on the use of rocketry to launch spacecraft. He calculated the
orbital speed In gravitationally bound systems, the orbital speed of an astronomical body or object (e.g. planet, moon, artificial satellite, spacecraft, or star) is the speed at which it orbits around either the barycenter or, if one body is much more mas ...
required for a minimal orbit, and that a
multi-stage rocket A multistage rocket or step rocket is a launch vehicle that uses two or more rocket ''stages'', each of which contains its own engines and propellant. A ''tandem'' or ''serial'' stage is mounted on top of another stage; a ''parallel'' stage is ...
fueled by liquid propellants 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 Konstantin Eduardovich Tsiolkovsky (russian: Константи́н Эдуа́рдович Циолко́вский , , p=kənstɐnʲˈtʲin ɪdʊˈardəvʲɪtɕ tsɨɐlˈkofskʲɪj , a=Ru-Konstantin Tsiolkovsky.oga; – 19 September 1935) ...
) 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 ''Electronics World'' (''Wireless World'', founded in 1913, and in September 1984 renamed ''Electronics & Wireless World'') is a technical magazine in electronics and RF engineering aimed at professional design engineers. It is produced monthly in ...
'' article, the English science fiction writer Arthur C. Clarke described in detail the possible use of
communications satellite A communications satellite is an artificial satellite that relays and amplifies radio telecommunication signals via a transponder; it creates a communication channel between a source transmitter and a receiver at different locations on Earth ...
s for mass communications. He suggested that three geostationary satellites would provide coverage over the entire planet. In May 1946, the
United States Air Force The United States Air Force (USAF) is the Aerial warfare, air military branch, service branch of the United States Armed Forces, and is one of the eight uniformed services of the United States. Originally created on 1 August 1907, as a part ...
's
Project RAND The RAND Corporation (from the phrase "research and development") is an American nonprofit global policy think tank created in 1948 by Douglas Aircraft Company to offer research and analysis to the United States Armed Forces. It is financed ...
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 The Bureau of Aeronautics (BuAer) was the U.S. Navy's material-support organization for naval aviation from 1921 to 1959. The bureau had "cognizance" (''i.e.'', responsibility) for the design, procurement, and support of naval aircraft and relate ...
of the
United States Navy The United States Navy (USN) is the maritime service branch of the United States Armed Forces and one of the eight uniformed services of the United States. It is the largest and most powerful navy in the world, with the estimated tonnage ...
. 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 Lyman Spitzer Jr. (June 26, 1914 – March 31, 1997) was an American theoretical physicist, astronomer and mountaineer. As a scientist, he carried out research into star formation, plasma physics, and in 1946, conceived the idea of telesco ...
proposed an orbiting
space telescope A space telescope or space observatory is a telescope in outer space used to observe astronomical objects. Suggested by Lyman Spitzer in 1946, the first operational telescopes were the American Orbiting Astronomical Observatory, OAO-2 launch ...
. In February 1954, Project RAND released "Scientific Uses for a Satellite Vehicle", by R. R. Carhart. 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.


First satellites

In the context of activities planned for the International Geophysical Year (1957–1958), the
White House The White House is the official residence and workplace of the president of the United States. It is located at 1600 Pennsylvania Avenue NW in Washington, D.C., and has been the residence of every U.S. president since John Adams in ...
announced on 29 July 1955 that the U.S. intended to launch satellites by the spring of 1958. This became known as
Project Vanguard Project Vanguard was a program managed by the United States Navy Naval Research Laboratory (NRL), which intended to launch the first artificial satellite into low Earth orbit using a Vanguard rocket. as the launch vehicle from Cape Canaveral ...
. On 31 July, the Soviet Union announced its intention to launch a satellite by the fall of 1957. The first artificial satellite was Sputnik 1, launched by the
Soviet Union The Soviet Union,. officially the Union of Soviet Socialist Republics. (USSR),. was a List of former transcontinental countries#Since 1700, transcontinental country that spanned much of Eurasia from 1922 to 1991. A flagship communist state, ...
on 4 October 1957 under the
Sputnik program Sputnik (Спутник, Russian for "satellite"NOTE: The Russian word "sputnik" can have many meanings: "satellite", "travelling companion", "fellow traveller", etc. However, in astronomy, it means only "satellite".) is a spacecraft launched ...
, with
Sergei Korolev Sergei Pavlovich Korolev (russian: Сергей Павлович Королёв, Sergey Pavlovich Korolyov, sʲɪrˈɡʲej ˈpavləvʲɪtɕ kərɐˈlʲɵf, Ru-Sergei Pavlovich Korolev.ogg; ukr, Сергій Павлович Корольов, ...
as chief designer. Sputnik 1 helped to identify the density of high
atmospheric layers The atmosphere of Earth is the layer of gases, known collectively as air, retained by Earth's gravity that surrounds the planet and forms its planetary atmosphere. The atmosphere of Earth protects life on Earth by creating pressure allowing for ...
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 Sputnik 2 (, russian: Спутник-2, ''Satellite 2''), or Prosteyshiy Sputnik 2 (PS-2, russian: Простейший Спутник 2, italic=yes, ''Simplest Satellite 2'') was the second spacecraft launched into Earth orbit, on 3 November 195 ...
was launched on 3 November 1957 and carried the first living passenger into orbit, a dog named
Laika Laika (russian: link=no, Лайка; – 3 November 1957) was a Soviet space dog who was one of the first animals in space and the first to orbit the Earth. A stray mongrel from the streets of Moscow, she flew aboard the Sputnik 2 space ...
. In early 1955, after pressure by the
American Rocket Society The American Rocket Society (ARS) began its existence on , under the name of the American Interplanetary Society. It was founded by science fiction writers G. Edward Pendray, David Lasser, Laurence Manning, Nathan Schachner, and others. Pendra ...
, the
National Science Foundation The National Science Foundation (NSF) is an independent agency of the United States government that supports fundamental research and education in all the non-medical fields of science and engineering. Its medical counterpart is the National ...
, and the International Geophysical Year, the Army and Navy were working on Project Orbiter with two competing programs. The army used the Jupiter C rocket, while the civilian–Navy program used the
Vanguard rocket The Vanguard rocket was intended to be the first launch vehicle the United States would use to place a satellite into orbit. Instead, the Sputnik crisis caused by the surprise launch of Sputnik 1 led the U.S., after the failure of Vanguard TV-3 ...
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 TIROS-1 (or TIROS-A) was the first full-scale weather satellite (the Vanguard 2 satellite was the first experimental/prototype weather satellite), the first of a series of Television Infrared Observation Satellites placed in low Earth orbit. P ...
spacecraft, launched on April 1, 1960, as part of NASA's
Television Infrared Observation Satellite TIROS, or Television InfraRed Observation Satellite, is a series of early weather satellites launched by the United States, beginning with TIROS-1 in 1960. TIROS was the first satellite that was capable of remote sensing of the Earth, enabli ...
(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 The United States Space Surveillance Network (SSN) detects, tracks, catalogs and identifies artificial objects orbiting Earth, e.g. active/inactive satellites, spent rocket bodies, or fragmentation debris. The system is the responsibility of Uni ...
cataloged 115 Earth-orbiting satellites. Early satellites were built to unique designs. With advancements in technology, multiple satellites began to be built on single model platforms called
satellite bus A satellite bus (or spacecraft bus) is the main body and structural component of a satellite or spacecraft, in which the payload and all scientific instruments are held. Bus-derived satellites are opposed to specially produced satellites. Bus- ...
es. The first standardized satellite bus design was the HS-333
geosynchronous A geosynchronous orbit (sometimes abbreviated GSO) is an Earth-centered orbit with an orbital period that matches Earth's rotation on its axis, 23 hours, 56 minutes, and 4 seconds (one sidereal day). The synchronization of rotation and orbital ...
(GEO)
communication satellite A communications satellite is an artificial satellite that relays and amplifies radio telecommunication signals via a transponder; it creates a communication channel between a source transmitter and a receiver at different locations on Earth. C ...
launched in 1972. Begun in 1997, FreeFlyer is a commercial off-the-shelf software application for satellite mission analysis, design, and operations.


Later development

While Canada was the third country to build a satellite which was launched into space, it was launched aboard an American rocket from an American spaceport. The same goes for Australia, who launched first satellite involved a donated U.S. Redstone rocket and American support staff as well as a joint launch facility with the
United Kingdom The United Kingdom of Great Britain and Northern Ireland, commonly known as the United Kingdom (UK) or Britain, is a country in Europe, off the north-western coast of the European mainland, continental mainland. It comprises England, Scotlan ...
. The first Italian satellite San Marco 1 launched on 15 December 1964 on a U.S.
Scout rocket The Scout family of rockets were American launch vehicles designed to place small satellites into orbit around the Earth. The Scout multistage rocket was the first orbital launch vehicle to be entirely composed of solid fuel stages. It was also t ...
from
Wallops Island Wallops Island is a island in Accomack County, Virginia, part of the Virginia Barrier Islands that stretch along the eastern seaboard of the United States of America. It is just south of Chincoteague Island, a popular tourist destination. W ...
(Virginia, United States) with an Italian launch team trained by
NASA The National Aeronautics and Space Administration (NASA ) is an independent agencies of the United States government, independent agency of the US federal government responsible for the civil List of government space agencies, space program ...
. By similar occasions, almost all further first national satellites was launched by foreign rockets. After the late 2010s, and especially after the advent and operational fielding of large
satellite internet constellation A satellite internet constellation is a constellation of artificial satellites providing satellite internet service. In particular, the term has come to refer to a new generation of very large constellations (sometimes referred to as a megaco ...
s—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 reach 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 The International Space Station (ISS) is the largest modular space station currently in low Earth orbit. It is a multinational collaborative project involving five participating space agencies: NASA (United States), Roscosmos (Russia), JAXA ( ...
. By the early 2000s, and particularly after the advent of
CubeSat A CubeSat is a class of miniaturized satellite based around a form factor consisting of cubes. CubeSats have a mass of no more than per unit, and often use commercial off-the-shelf (COTS) components for their electronics and structure. CubeSats ...
s and increased launches of microsats—frequently launched to the lower altitudes of
low Earth orbit A low Earth orbit (LEO) is an orbit around Earth with a period of 128 minutes or less (making at least 11.25 orbits per day) and an eccentricity less than 0.25. Most of the artificial objects in outer space are in LEO, with an altitude never mor ...
(LEO)—satellites began to more frequently be designed to demise, or breakup and burnup entirely in the atmosphere. For example, SpaceX
Starlink Starlink is a satellite internet constellation operated by SpaceX, providing satellite Internet access coverage to 45 countries. It also aims for global mobile phone service after 2023. SpaceX started launching Starlink satellites in 2019. As ...
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 atmospheric reentry at end of life, or in the event of an early satellite failure.


Components


Orbit and altitude control

Most satellites use chemical or
ion propulsion An ion thruster, ion drive, or ion engine is a form of electric propulsion used for spacecraft propulsion. It creates thrust by accelerating ions using electricity. An ion thruster ionizes a neutral gas by extracting some electrons out of ...
to adjust or maintain their orbit, coupled with reaction wheels to control its three axis of rotation or attitude. Satellites close to Earth are affected the most by variations in the Earth's magnetic, gravitational field and the Sun's
radiation pressure Radiation pressure is the mechanical pressure exerted upon any surface due to the exchange of momentum between the object and the electromagnetic field. This includes the momentum of light or electromagnetic radiation of any wavelength that is a ...
; satellites that are further away are affected more by other bodies' gravitational field by the Moon and the Sun. Without orbit and orientation control, satellites in orbit will not be able to communicate with ground stations on Earth. Chemical thrusters on satellites usually use
monopropellant Monopropellants are propellants consisting of chemicals that release energy through exothermic chemical decomposition. The molecular bond energy of the monopropellant is released usually through use of a catalyst. This can be contrasted with bipro ...
(one-part) or
bipropellant The highest specific impulse chemical rockets use liquid propellants (liquid-propellant rockets). They can consist of a single chemical (a monopropellant) or a mix of two chemicals, called bipropellants. Bipropellants can further be divided into ...
(two-parts) that are
hypergolic A hypergolic propellant is a rocket propellant combination used in a rocket engine, whose components spontaneously ignite when they come into contact with each other. The two propellant components usually consist of a fuel and an oxidizer. T ...
. Hypergolic means able to combust spontaneously in contact to each other or to a
catalyst Catalysis () is the process of increasing the rate of a chemical reaction by adding a substance known as a catalyst (). Catalysts are not consumed in the reaction and remain unchanged after it. If the reaction is rapid and the catalyst recyc ...
. The most commonly used propellant mixtures on satellites are hydrazine-based monopropellant or
monomethylhydrazine Monomethylhydrazine (mono-methyl hydrazine, MMH) is a highly toxic, volatile hydrazine derivative with the chemical formula . It is used as a rocket propellant in bipropellant rocket engines because it is hypergolic with various oxidizers such as ...
dinitrogen tetroxide bipropellant. Ion thrusters on satellites usually are
Hall-effect thruster In spacecraft propulsion, a Hall-effect thruster (HET) is a type of ion thruster in which the propellant is accelerated by an electric field. Hall-effect thrusters (based on the discovery by Edwin Hall) are sometimes referred to as Hall thruste ...
s, which generate thrust by accelerating
positive ions An ion () is an atom or molecule with a net electrical charge. The charge of an electron is considered to be negative by convention and this charge is equal and opposite to the charge of a proton, which is considered to be positive by convent ...
through a negatively-charged grid. Ion propulsion is more efficient propellant-wise than chemical propulsion but its thrust is very small (around ), thus requires a longer burn time. The thrusters usually use
xenon Xenon is a chemical element with the symbol Xe and atomic number 54. It is a dense, colorless, odorless noble gas found in Earth's atmosphere in trace amounts. Although generally unreactive, it can undergo a few chemical reactions such as the ...
because it is inert, can be easily
ionized Ionization, or Ionisation is the process by which an atom or a molecule acquires a negative or positive charge by gaining or losing electrons, often in conjunction with other chemical changes. The resulting electrically charged atom or molecule ...
, has a high
atomic mass The atomic mass (''m''a or ''m'') is the mass of an atom. Although the SI unit of mass is the kilogram (symbol: kg), atomic mass is often expressed in the non-SI unit dalton (symbol: Da) – equivalently, unified atomic mass unit (u). 1&nb ...
and storable as a high-pressure liquid.


Power

Most satellites use solar panels to generate power, and a few in deep space with limited sunlight use
radioisotope thermoelectric generator A radioisotope thermoelectric generator (RTG, RITEG), sometimes referred to as a radioisotope power system (RPS), is a type of nuclear battery that uses an array of thermocouples to convert the heat released by the decay of a suitable radioacti ...
s. Slip rings attach solar panels to the satellite; the slip rings can rotate to be perpendicular with the sunlight and generate the most power. All satellites with a solar panel must also have batteries, because sunlight is blocked inside the launch vehicle and at night. The most common types of batteries for satellites are
lithium-ion A lithium-ion or Li-ion battery is a type of rechargeable battery which uses the reversible reduction of lithium ions to store energy. It is the predominant battery type used in portable consumer electronics and electric vehicles. It also se ...
, and in the past nickel–hydrogen.


Communications

*


Applications


Earth observation

Earth observation satellites is designed to monitor and survey the Earth, called
remote sensing Remote sensing is the acquisition of information about an object or phenomenon without making physical contact with the object, in contrast to in situ or on-site observation. The term is applied especially to acquiring information about Eart ...
. Most Earth observation satellites are placed in low Earth orbit for a high data resolution, though some are placed in a geostationary orbit for an uninterrupted coverage. Some satellites are placed in a Sun-synchronous orbit to have consistent lighting and obtain a total view of the Earth. Depending on the satellites' functions, they might have a normal camera,
radar Radar is a detection system that uses radio waves to determine the distance ('' ranging''), angle, and radial velocity of objects relative to the site. It can be used to detect aircraft, ships, spacecraft, guided missiles, motor vehicles, we ...
, lidar,
photometer A photometer is an instrument that measures the strength of electromagnetic radiation in the range from ultraviolet to infrared and including the visible spectrum. Most photometers convert light into an electric current using a photoresistor, ...
, or atmospheric instruments. Earth observation satellite's data is most used in
archaeology Archaeology or archeology is the scientific study of human activity through the recovery and analysis of material culture. The archaeological record consists of artifacts, architecture, biofacts or ecofacts, sites, and cultural landsca ...
,
cartography Cartography (; from grc, χάρτης , "papyrus, sheet of paper, map"; and , "write") is the study and practice of making and using maps. Combining science, aesthetics and technique, cartography builds on the premise that reality (or an i ...
,
environmental monitoring Environmental monitoring describes the processes and activities that need to take place to characterize and monitor the quality of the environment. Environmental monitoring is used in the preparation of environmental impact assessments, as well a ...
,
meteorology Meteorology is a branch of the atmospheric sciences (which include atmospheric chemistry and physics) with a major focus on weather forecasting. The study of meteorology dates back millennia, though significant progress in meteorology did no ...
, and
reconnaissance In military operations, reconnaissance or scouting is the exploration of an area by military forces to obtain information about enemy forces, terrain, and other activities. Examples of reconnaissance include patrolling by troops (skirmisher ...
applications. As of 2021, there are over 950 Earth observation satellites, with the largest number of satellites operated Planet Labs.
Weather satellite A weather satellite or meteorological satellite is a type of Earth observation satellite that is primarily used to monitor the weather and climate of the Earth. Satellites can be polar orbiting (covering the entire Earth asynchronously), or ...
s monitor
cloud In meteorology, a cloud is an aerosol consisting of a visible mass of miniature liquid droplets, frozen crystals, or other particles suspended in the atmosphere of a planetary body or similar space. Water or various other chemicals may ...
s, city lights,
fire Fire is the rapid oxidation of a material (the fuel) in the exothermic chemical process of combustion, releasing heat, light, and various reaction products. At a certain point in the combustion reaction, called the ignition point, flames a ...
s, effects of
pollution Pollution is the introduction of contaminants into the natural environment that cause adverse change. Pollution can take the form of any substance (solid, liquid, or gas) or energy (such as radioactivity, heat, sound, or light). Pollutants, the ...
,
aurora An aurora (plural: auroras or aurorae), also commonly known as the polar lights, is a natural light display in Earth's sky, predominantly seen in high-latitude regions (around the Arctic and Antarctic). Auroras display dynamic patterns of bri ...
s, sand and dust storms,
snow Snow comprises individual ice crystals that grow while suspended in the atmosphere—usually within clouds—and then fall, accumulating on the ground where they undergo further changes. It consists of frozen crystalline water throughout ...
cover,
ice Ice is water frozen into a solid state, typically forming at or below temperatures of 0 degrees Celsius or Depending on the presence of impurities such as particles of soil or bubbles of air, it can appear transparent or a more or less opaqu ...
mapping, boundaries of
ocean current An ocean current is a continuous, directed movement of sea water generated by a number of forces acting upon the water, including wind, the Coriolis effect, breaking waves, cabbeling, and temperature and salinity differences. Depth contours ...
s,
energy In physics, energy (from Ancient Greek: ἐνέργεια, ''enérgeia'', “activity”) is the quantitative property that is transferred to a body or to a physical system, recognizable in the performance of work and in the form of hea ...
flows, etc. Environmental monitoring satellites can detect changes in the Earth's
vegetation Vegetation is an assemblage of plant species and the ground cover they provide. It is a general term, without specific reference to particular taxa, life forms, structure, spatial extent, or any other specific botanical or geographic characte ...
, 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.


Communication


Navigation

Navigational satellites are satellites that 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 meters in real time


Telescope

Astronomical satellites are satellites used for observation of distant planets, galaxies, and other outer space objects.


Experimental

Tether satellite Space tethers are long cables which can be used for propulsion, momentum exchange, stabilization and attitude control, or maintaining the relative positions of the components of a large dispersed satellite/spacecraft sensor system. Depending on t ...
s are satellites that are connected to another satellite by a thin cable called a
tether A tether is a cord, fixture, or flexible attachment that characteristically anchors something movable to something fixed; it also maybe used to connect two movable objects, such as an item being towed by its tow. Applications for tethers includ ...
. Recovery satellites are satellites that provide a recovery of reconnaissance, biological, space-production and other payloads from orbit to Earth. Biosatellites are satellites designed to carry living organisms, generally for scientific experimentation. Space-based solar power satellites are proposed satellites that would collect energy from sunlight and transmit it for use on Earth or other places.


Weapon

Since the mid-2000s, 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 Russia (, , ), or the Russian Federation, is a transcontinental country spanning Eastern Europe and Northern Asia. It is the largest country in the world, with its internationally recognised territory covering , and encompassing one-eig ...
,
United States The United States of America (U.S.A. or USA), commonly known as the United States (U.S. or US) or America, is a country primarily located in North America. It consists of 50 states, a federal district, five major unincorporated territori ...
, China and
India India, officially the Republic of India (Hindi: ), is a country in South Asia. It is the seventh-largest country by area, the second-most populous country, and the most populous democracy in the world. Bounded by the Indian Ocean on the so ...
have demonstrated the ability to eliminate satellites. In 2007, the
Chinese Chinese can refer to: * Something related to China * Chinese people, people of Chinese nationality, citizenship, and/or ethnicity **''Zhonghua minzu'', the supra-ethnic concept of the Chinese nation ** List of ethnic groups in China, people of ...
military shot down an aging weather satellite, followed by the
US Navy The United States Navy (USN) is the maritime service branch of the United States Armed Forces and one of the eight uniformed services of the United States. It is the largest and most powerful navy in the world, with the estimated tonnage ...
shooting down a defunct spy satellite in February 2008. On 18 November 2015, after two failed attempts, Russia successfully carried out a flight test of an anti-satellite missile known as '' Nudol''. On 27 March 2019, India shot down a live test satellite at 300 km altitude in 3 minutes.
India India, officially the Republic of India (Hindi: ), is a country in South Asia. It is the seventh-largest country by area, the second-most populous country, and the most populous democracy in the world. Bounded by the Indian Ocean on the so ...
became the fourth country to have the capability to destroy live satellites.


Pollution and interference

Issues like space debris, radio and
light pollution Light pollution is the presence of unwanted, inappropriate, or excessive use of artificial Visible spectrum, lighting. In a descriptive sense, the term ''light pollution'' refers to the effects of any poorly implemented lighting, during the day ...
are increasing in magnitude and at the same time lack progress in national or international regulation. Space debris poses dangers to spacecraft (including satellites) in or crossing geocentric orbits and have the potential to drive a
Kessler syndrome The Kessler syndrome (also called the Kessler effect, collisional cascading, or ablation cascade), proposed by NASA scientist Donald J. Kessler in 1978, is a scenario in which the density of objects in low Earth orbit (LEO) due to space pollutio ...
which could potentially curtail humanity from conducting space endeavors in the future by making such nearly impossible. With the increase in numbers of
satellite constellation A satellite constellation is a group of artificial satellites working together as a system. Unlike a single satellite, a constellation can provide permanent global or near-global coverage, such that at any time everywhere on Earth at least one s ...
s, like SpaceX
Starlink Starlink is a satellite internet constellation operated by SpaceX, providing satellite Internet access coverage to 45 countries. It also aims for global mobile phone service after 2023. SpaceX started launching Starlink satellites in 2019. As ...
, the astronomical community, such as the
IAU The International Astronomical Union (IAU; french: link=yes, Union astronomique internationale, UAI) is a nongovernmental organisation with the objective of advancing astronomy in all aspects, including promoting astronomical research, outreach ...
, 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. Some notable satellite failures that polluted and dispersed radioactive materials are Kosmos 954,
Kosmos 1402 Kosmos 1402 (russian: Космос 1402) was a Soviet spy satellite that malfunctioned, resulting in the uncontrolled re-entry of its nuclear reactor and its radioactive uranium fuel. Kosmos 1402 was launched on August 30, 1982, and re-entered the ...
and the Transit 5-BN-3. Generally liability has been covered by the Liability Convention. Using wood as an alternative material has been posited in order to reduce pollution and debris from satellites that reenter the atmosphere. 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 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 that enables them to pinpoint the source of any carrier and manage the transponder space effectively.


References


External links

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