View source for Astronomy:Satellite

{{Short description|Objects intentionally placed into orbit}}

[[File:Two 3U CubeSats.jpg|thumb|upright=1.5|alt=Two 3U CubeSats|Two CubeSats orbiting around Earth after being deployed from the ISS ''Kibō'' module's Small Satellite Orbital Deployer]]

A '''satellite''' or '''artificial satellite'''{{efn|to distinguish them from natural satellites.}} is an object intentionally placed into [[Physics:Orbit|orbit]] around a [[Astronomy:Astronomical object|celestial body]]. Satellites 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 placed satellites in orbit and formerly useful satellites that are now defunct.

Except for [[Engineering:Passive satellite|passive satellite]]s, most satellites have an [[Engineering:Electricity generation|electricity generation]] system for equipment on board, such as [[Physics:Solar panel|solar panel]]s or [[Physics:Radioisotope thermoelectric generator|radioisotope thermoelectric generator]]s (RTGs). Most satellites also have a method of communication to [[Astronomy:Ground station|ground station]]s, called [[Astronomy:Transponder (satellite communications)|transponders]]. Many satellites use a [[Engineering:Satellite bus|standardized bus]] to save cost and work, the most popular of which is small [[Engineering:CubeSat|CubeSat]]s. Similar satellites can work together as a group, forming [[Engineering:Satellite constellation|constellations]]. Because of the high launch cost to space, satellites are designed to be as lightweight and robust as possible. Most communication satellites are radio [[Engineering:Broadcast relay station|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 [[Engineering:Launch vehicle|launch vehicle]]s, high enough to avoid [[Astronomy:Orbital decay|orbital decay]] by the [[Astronomy:Atmosphere|atmosphere]]. Satellites can then change or maintain the orbit by [[Astronomy:Spacecraft propulsion|propulsion]], usually by [[Engineering:Thrusters (spacecraft)|chemical]] or [[Engineering:Ion thruster|ion thruster]]s. As of 2018, about 90% of satellites orbiting Earth were in [[Astronomy:Low Earth orbit|low Earth orbit]] or [[Astronomy:Geostationary orbit|geostationary orbit]]; geostationary means the satellites stay still in the sky. Some imaging satellites chose a [[Astronomy:Sun-synchronous orbit|Sun-synchronous orbit]] because they can scan the entire globe with similar lighting. As the number of satellites and [[Astronomy:Space debris|space debris]] around Earth increases, the threat of collision has become more severe. A small number of satellites orbit other bodies (such as the [[Astronomy:Lunar orbit|Moon]], [[Astronomy:Areocentric orbit|Mars]], and the [[Astronomy:Heliocentric orbit|Sun]]) or many bodies at once (two for a [[Astronomy:Halo orbit|halo orbit]], three for a [[Lissajous orbit]]).

[[Astronomy:Earth observation satellite|Earth observation satellite]]s gather information for [[Astronomy:Reconnaissance satellite|reconnaissance]], [[Astronomy:Satellite imagery|mapping]], [[Engineering:Weather satellite|monitoring the weather]], ocean, forest, etc. [[Astronomy:Space telescope|Space telescope]]s take advantage of outer space's [[Physics:Ultra-high vacuum|near perfect vacuum]] to observe objects with the entire [[Physics:Electromagnetic spectrum|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 [[Earth:Satellite navigation|satellite navigation]] systems, such as [[Engineering:GPS|GPS]]. [[Engineering:Space probe|Space probe]]s are satellites designed for robotic [[Astronomy:Space exploration|space exploration]] outside of Earth, and space stations are in essence crewed satellites.

The first artificial satellite is launched into the Earth's orbit was the [[Place:Soviet Union|Soviet Union]]'s [[Engineering:Sputnik 1|Sputnik 1]], on 4 October 1957. As of April 2022, there were 5,465 operational satellites in Earth orbit, of which 3,433 belong to the United States (2,992 commercial), 541 belong to China, 172 belong to Russia, and 1,319 belong to other nations.<ref>{{cite web|url=https://www.ucsusa.org/resources/satellite-database |title=UCS Satellite Database |website=Union of Concerned Scientists |date=1 May 2022 }}</ref>

== History ==

=== Early proposals ===
The first published mathematical study of the possibility of an artificial satellite was [[Physics:Newton's cannonball|Newton's cannonball]], a thought experiment by [[Biography:Isaac Newton|Isaac Newton]] to explain the motion of [[Astronomy:Natural satellite|natural satellite]]s, in his ''[[Physics:Philosophiæ Naturalis Principia Mathematica|Philosophiæ Naturalis Principia Mathematica]]'' (1687). The first fictional depiction of a satellite being launched into orbit was a short story by Edward Everett Hale, "The Brick Moon" (1869).<ref>{{cite web
 |title=Rockets in Science Fiction (Late 19th Century)
 |url=http://history.msfc.nasa.gov/rocketry/tl4.html
 |publisher=[[Organization:Marshall Space Flight Center|Marshall Space Flight Center]]
 |url-status=dead
 |archive-url=https://web.archive.org/web/20000901224414/http://history.msfc.nasa.gov/rocketry/tl4.html
 |archive-date=1 September 2000
 |access-date=21 November 2008
}}</ref><ref>{{cite book | title=Science-fiction, the Early Years
 | url=https://archive.org/details/sciencefictionea0000blei
 | url-access=registration
 | first1=Everett Franklin | last1=Bleiler | first2=Richard | last2=Bleiler | page=[https://archive.org/details/sciencefictionea0000blei/page/325 325]
 | isbn=978-0-87338-416-2 | publisher=Kent State University Press | year=1991}}</ref> The idea surfaced again in Jules Verne's ''The Begum's Fortune'' (1879).

In 1903, [[Biography:Konstantin Tsiolkovsky|Konstantin Tsiolkovsky]] (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 [[Astronomy:Orbital speed|orbital speed]] required for a minimal orbit, and that a multi-stage rocket fueled by liquid [[Physics:Propellant|propellant]]s 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 [[Astronomy:Geostationary|geostationary]] satellites (first put forward by [[Biography:Konstantin Tsiolkovsky|Konstantin 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.<ref>{{Cite web |date=2 September 2016 |title=Introduction to satellite |url=http://www.sasmac.cn/portal/html/fc4f335929b0df0d0129b0e348f90003/_content/10_07/09/1278668367217.html |url-status=live |archive-url=https://web.archive.org/web/20160916064537/http://www.sasmac.cn/portal/html/fc4f335929b0df0d0129b0e348f90003/_content/10_07/09/1278668367217.html |archive-date=16 September 2016 |access-date=25 May 2022 |website=www.sasmac.cn}}</ref>

In a 1945 ''Wireless World'' article, the English science fiction writer [[Biography:Arthur C. Clarke|Arthur C. Clarke]] described in detail the possible use of [[Astronomy:Communications satellite|communications satellite]]s for mass communications. He suggested that three geostationary satellites would provide coverage over the entire planet.<ref name=":0">{{Cite book |last=Pratt |first=Timothy |url= |title=Satellite Communications |last2=Allnutt |first2=Jeremy E. |year=2019 |publisher=John Wiley & Sons Ltd |isbn=978-1-119-48217-8 |edition=3rd |oclc=1098222848}}</ref>{{Rp|pages=1–2}}

In May 1946, the {{wipe|United States Air Force}}'s Project RAND released the [[Engineering:Preliminary Design of an Experimental World-Circling Spaceship|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."<ref>{{cite journal | url=https://www.rand.org/pubs/special_memoranda/SM11827/ | title=Preliminary Design of an Experimental World-Circling Spaceship | journal=RAND | date=July 1946 | access-date=6 March 2008 | archive-date=15 June 2010 | archive-url=https://web.archive.org/web/20100615024409/http://rand.org/pubs/special_memoranda/SM11827/ | url-status=live }}</ref> 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.<ref>{{cite book | title=Venture into Space: Early Years of Goddard Space Flight Center
 | first=Alfred | last=Rosenthal | publisher=NASA | year=1968 | page=15}}</ref>

In 1946, American theoretical astrophysicist [[Biography:Lyman Spitzer|Lyman Spitzer]] proposed an orbiting [[Astronomy:Space telescope|space telescope]].<ref>{{cite web|title=Hubble Essentials: About Lyman Spitzer, Jr.|publisher=Hubble Site|url=http://hubblesite.org/the_telescope/hubble_essentials/lyman_spitzer.php|access-date=16 October 2020|archive-date=9 October 2018|archive-url=https://web.archive.org/web/20181009202238/http://hubblesite.org/the_telescope/hubble_essentials/lyman_spitzer.php|url-status=live}}</ref>

In February 1954, Project RAND released "Scientific Uses for a Satellite Vehicle", by R. R. Carhart.<ref>R. R. Carhart, Scientific Uses for a Satellite Vehicle, Project RAND Research Memorandum. (Rand Corporation, Santa Monica) 12 February 1954.</ref> 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.<ref>2. H. K. Kallmann and W. W. Kellogg, Scientific Use of an Artificial Satellite, Project RAND Research Memorandum. (Rand Corporation, Santa Monica) 8 June 1955.</ref>

=== First satellites ===
[[File:Sputnik asm.jpg|alt=Steel ball with 4 antennas|thumb|Replica of the Sputnik 1]]
In the context of activities planned for the [[Earth:International Geophysical Year|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 [[Astronomy:Project Vanguard|Project Vanguard]]. On 31 July, the Soviet Union announced its intention to launch a satellite by the fall of 1957.

The first artificial satellite was [[Engineering:Sputnik 1|Sputnik 1]], launched by the [[Place:Soviet Union|Soviet Union]] on 4 October 1957 under the Sputnik program, with [[Biography:Sergei Korolev|Sergei Korolev]] as chief designer. 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 [[Astronomy:Ionosphere|ionosphere]]. The unanticipated announcement of Sputnik 1's success precipitated the [[Engineering:Sputnik crisis|Sputnik crisis]] in the United States and ignited the so-called Space Race within the [[History:Cold War|Cold War]].

[[Engineering:Sputnik 2|Sputnik 2]] was launched on 3 November 1957 and carried the first living passenger into orbit, a dog named [[Engineering:Laika|Laika]].<ref name="nasa_animals">{{cite web | title=A Brief History of Animals in Space | url=https://history.nasa.gov/animals.html | first1=Tara | last1=Gray | first2=Steve | last2=Garber | publisher=[[Organization:NASA|NASA]] | date=2 August 2004 | access-date=12 July 2017 | archive-date=11 October 2004 | archive-url=https://web.archive.org/web/20041011053912/https://history.nasa.gov/animals.html | url-status=live }}</ref>

In early 1955, after pressure by the [[Organization:American Rocket Society|American Rocket Society]], the [[Organization:National Science Foundation|National Science Foundation]], and the International Geophysical Year, the Army and Navy were working on [[Engineering:Project Orbiter|Project Orbiter]] with two competing programs. The army used the Jupiter C rocket, while the civilian–Navy program used the [[Engineering:Vanguard (rocket)|Vanguard rocket]] to launch a satellite. [[Astronomy:Explorer 1|Explorer 1]] became the United States' first artificial satellite, on 31 January 1958.<ref>{{cite news|title=50th anniversary of first U.S. satellite launch celebrated |url=http://www.sfgate.com/cgi-bin/article.cgi?f=/n/a/2008/01/30/state/n151715S68.DTL |agency=Associated Press |first=Alicia |last=Chang |work=San Francisco Chronicle |date=30 January 2008 |url-status=dead |archive-date=1 February 2008 |archive-url=https://web.archive.org/web/20080201193510/http://www.sfgate.com/cgi-bin/article.cgi?f=%2Fn%2Fa%2F2008%2F01%2F30%2Fstate%2Fn151715S68.DTL}}</ref> The information sent back from its radiation detector led to the discovery of the Earth's [[Astronomy:Van Allen radiation belt|Van Allen radiation belt]]s.<ref name="Allen">{{cite web |title=James A. Van Allen |url=http://www.nmspacemuseum.org/halloffame/detail.php?id=86 |url-status=live |archive-url=https://web.archive.org/web/20180515112204/http://www.nmspacemuseum.org/halloffame/detail.php?id=86 |archive-date=15 May 2018 |access-date=14 May 2018 |website=nmspacemuseum.org |publisher=New Mexico Museum of Space History}}</ref> The [[Engineering:TIROS-1|TIROS-1]] spacecraft, launched on April 1, 1960, as part of NASA's [[Engineering:Television Infrared Observation Satellite|Television Infrared Observation Satellite]] (TIROS) program, sent back the first television footage of weather patterns to be taken from space.<ref name="Tatem">{{cite journal |last1=Tatem |first1=Andrew J. |last2=Goetz |first2=Scott J. |last3=Hay |first3=Simon I. |date=2008 |title=Fifty Years of Earth-observation Satellites |journal=American Scientist |volume=96 |issue=5 |pages=390–398 |doi=10.1511/2008.74.390 |pmc=2690060 |pmid=19498953}}</ref>

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.<ref>{{cite web|first1=David S. F. |last1=Portree |first2=Joseph P. Jr. |last2=Loftus |url=http://ston.jsc.nasa.gov/collections/TRS/_techrep/TP-1999-208856.pdf |title=Orbital Debris: A Chronology |page=18 |work=Lyndon B. Johnson Space Center |year=1999 |url-status=dead |archive-url=https://web.archive.org/web/20000901071135/http://ston.jsc.nasa.gov/collections/TRS/_techrep/TP-1999-208856.pdf |archive-date=1 September 2000 |access-date=21 November 2008}}</ref>

Early satellites were built to unique designs. With advancements in technology, multiple satellites began to be built on [[Astronomy:Comparison of satellite buses|single model platforms]] called [[Engineering:Satellite bus|satellite bus]]es. The first standardized satellite bus design was the [[Engineering:HS-333|HS-333]] [[Astronomy:Geosynchronous orbit|geosynchronous]] (GEO) communication satellite launched in 1972. Begun in 1997, [[Software:FreeFlyer|FreeFlyer]] is a commercial off-the-shelf software application for satellite mission analysis, design, and operations.

=== Later development ===
[[File:Space capabilities - launch and satellite.png|thumb|upright=1.35|
{{legend|#007F00|[[Engineering:Timeline of first orbital launches by country|Orbital launch]] and satellite operation}}
{{legend|#00FF00|Satellite operation, launched by foreign supplier}}
{{legend|#FF8040|Satellite in development}}
{{legend|#FF0000|Orbital launch project at advanced stage or indigenous [[Physics:Ballistic missile|ballistic missile]]s deployed}}]]While Canada was the third country to build a satellite which was launched into space,<ref>{{cite book |last=Burleson |first=Daphne |title=Space Programs Outside the United States |publisher=McFarland & Company |year=2005 |isbn=978-0-7864-1852-7 |page=43}}</ref> it was launched aboard an {{wipe|United States}} rocket from an American spaceport. The same goes for Australia, who launched first satellite involved a donated U.S. [[Engineering:PGM-11 Redstone|Redstone]] rocket and American support staff as well as a joint launch facility with the United Kingdom.<ref>{{cite book |author=Mike Gruntman |title=Blazing the Trail |publisher=[[Organization:American Institute of Aeronautics and Astronautics|American Institute of Aeronautics and Astronautics]] |year=2004 |isbn=978-1-56347-705-8 |page=426 |author-link=Mike Gruntman}}</ref> The first Italian satellite [[Engineering:San Marco 1|San Marco 1]] launched on 15 December 1964 on a U.S. Scout rocket from Wallops Island (Virginia, United States) with an Italian launch team trained by [[Organization:NASA|NASA]].<ref>{{cite book |last=Harvey |first=Brian |title=Europe's Space Programme |publisher=[[Company:Springer Science+Business Media|Springer Science+Business Media]] |year=2003 |isbn=978-1-85233-722-3 |page=114}}</ref> 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 [[Engineering:Satellite internet constellation|satellite internet constellation]]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 (product)|end of life]], as a part of the regulatory process of obtaining a launch license.{{citation needed|date=June 2022}} The largest artificial satellite ever is the [[Astronomy:International Space Station|International Space Station]].<ref>{{Cite book |last1=Welch |first1=Rosanne |url=https://books.google.com/books?id=aWGHDwAAQBAJ&q=largest+artificial+satellite&pg=RA2-PA126 |title=Technical Innovation in American History: An Encyclopedia of Science and Technology [3 volumes] |last2=Lamphier |first2=Peg A. |year=2019|publisher=ABC-CLIO |isbn=978-1-61069-094-2 |page=126 |language=en |access-date=4 October 2020 |archive-url=https://web.archive.org/web/20210214004816/https://books.google.com/books?id=aWGHDwAAQBAJ&q=largest+artificial+satellite&pg=RA2-PA126 |archive-date=14 February 2021 |url-status=live}}</ref>

By the early 2000s, and particularly after the advent of [[Engineering:CubeSat|CubeSat]]s and increased launches of [[Engineering:Microsatellite (spaceflight)|microsats]]—frequently launched to the lower altitudes of [[Astronomy:Low Earth orbit|low Earth orbit]] (LEO)—satellites began to more frequently be designed to demise, or breakup and burnup entirely in the atmosphere.<ref>{{cite journal |last1=Slejko |first1=EA |last2=Gregorio |first2=A |last3=Lughi |first3=V |year=2021 |title=Material selection for a CubeSat structural bus complying with debris mitigation |url=https://www.sciencedirect.com/science/article/abs/pii/S0273117720308383 |url-status=live |journal=Advances in Space Research |volume=67 |issue=5 |pages=1468–1476 |bibcode=2021AdSpR..67.1468S |doi=10.1016/j.asr.2020.11.037 |archive-url=https://web.archive.org/web/20220603044215/https://www.sciencedirect.com/science/article/abs/pii/S0273117720308383 |archive-date=3 June 2022 |access-date=3 June 2022 |s2cid=233841294}}</ref>
For example, [[Company:SpaceX|SpaceX]] [[Engineering:Starlink|Starlink]] satellites, the first large satellite internet constellation to exceed 1000 active satellites on orbit in ~2020, are designed<!-- these are the v1.0 sats; the v0.9 sats had at least one component that was not fully demiseable --> to be 100% demisable and burn up completely on atmospheric reentry at end of life, or in the event of an early satellite failure.<ref name="tasia202104">{{cite news |last1=Garrity |first1=John |last2=Husar |first2=Arndt |date=April 2021 |title=Digital Connectivity and Low Earth Orbit Satellite Constellations: Opportunities for Asia and the Pacific |work=think-asia.org |url=https://think-asia.org/handle/11540/13626 |url-status=live |access-date=3 June 2022 |archive-url=https://web.archive.org/web/20220721104450/https://think-asia.org/handle/11540/13626 |archive-date=21 July 2022}}</ref>

==Components==
=== Orbit and altitude control ===
{{further|Astronomy:Spacecraft propulsion}}
[[File:Ion Engine Test Firing - GPN-2000-000482.jpg|thumb|Firing of ''[[Astronomy:Deep Space 1|Deep Space 1]]''<nowiki/>'s ion thruster]]

Most satellites use chemical or ion propulsion to [[Astronomy:Orbital maneuver|adjust]] or [[Astronomy:Orbital station-keeping|maintain their orbit]],<ref name=":0" />{{Rp|pages=|page=78}} coupled with [[Engineering:Reaction wheel|reaction wheel]]s to control its [[Physics:Aircraft principal axes|three axis of rotation]] or attitude. Satellites close to Earth are affected the most by variations in the [[Earth:Earth's magnetic field|Earth's magnetic]], [[Physics:Gravity of Earth|gravitational field]] and the Sun's [[Astronomy:Radiation pressure|radiation pressure]]; satellites that are further away are affected more by other bodies' gravitational field by the Moon and the Sun. Satellites utilize ultra-white reflective coatings to prevent damage from UV radiation.<ref>{{cite web|url=https://www.americanelements.com/space-industry.html|title=American Elements Assists NASA in Development of Ultra White Coating for Satellite Applications|publisher=American Elements|access-date=2023-04-06}}</ref> Without orbit and orientation control, satellites in orbit will not be able to communicate with [[Astronomy:Ground station|ground station]]s on Earth.<ref name=":0" />{{Rp|pages=75–76}}

Chemical thrusters on satellites usually use [[Chemistry:Monopropellant|monopropellant]] (one-part) or bipropellant (two-parts) that are [[Chemistry:Hypergolic propellant|hypergolic]]. Hypergolic means able to combust spontaneously in contact to each other or to a [[Chemistry:Catalysis|catalyst]]. The most commonly used propellant mixtures on satellites are [[Chemistry:Hydrazine|hydrazine]]-based monopropellant or [[Chemistry:Monomethylhydrazine|monomethylhydrazine]]–[[Chemistry:Dinitrogen tetroxide|dinitrogen tetroxide]] bipropellant. Ion thrusters on satellites usually are [[Physics:Hall-effect thruster|Hall-effect thruster]]s, 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 {{Cvt|0.5|N|disp=or|sigfig=1}}), thus requires a longer burn time. The thrusters usually use [[Chemistry:Xenon|xenon]] because it is [[Physics:Inert gas|inert]], can be easily ionized, has a high [[Physics:Atomic mass|atomic mass]] and storable as a high-pressure liquid.<ref name=":0" />{{Rp|pages=78–79}}

=== Power ===
{{Main|Engineering:Batteries in space|Physics:Nuclear power in space|Physics:Solar panels on spacecraft}}
[[File:ISS-54 ELC-1, main solar arrays and radiators seen from the Cupola.jpg|alt=see caption|thumb|The International Space Station's black solar panels on the left and white radiators on the right]]
Most satellites use [[Physics:Solar panels on spacecraft|solar panels]] to generate power, and a few in deep space with limited sunlight use [[Physics:Radioisotope thermoelectric generator|radioisotope thermoelectric generator]]s. [[Engineering:Slip ring|Slip ring]]s 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 [[Engineering:Batteries in space|batteries]], because sunlight is blocked inside the launch vehicle and at night. The most common types of batteries for satellites are [[Engineering:Lithium-ion battery|lithium-ion]], and in the past nickel–hydrogen.<ref name=":0" />{{Rp|pages=88–89}}

===Communications===
{{Main|Astronomy:Transponder (satellite communications)}}

== Applications ==

=== Earth observation ===
{{Main|Astronomy:Earth observation satellite}}
[[File:Earth Radiation Budget Satellite.jpg|thumb|Deployment of the [[Engineering:Earth Radiation Budget Satellite|Earth Radiation Budget Satellite]] on STS-41-G, collecting data on Earth's weather and climate]]
Earth observation satellites is designed to monitor and survey the Earth, called [[Earth:Remote sensing|remote sensing]]. Most Earth observation satellites are placed in low Earth orbit for a high data resolution, though some are placed in a [[Astronomy:Geostationary orbit|geostationary orbit]] for an uninterrupted coverage. Some satellites are placed in a [[Astronomy:Sun-synchronous orbit|Sun-synchronous orbit]] to have consistent lighting and obtain a total view of the Earth. Depending on the satellites' functions, they might have a [[Astronomy:Satellite imagery|normal camera]], [[Astronomy:Space-based radar|radar]], [[Engineering:Lidar|lidar]], [[Physics:Photometer|photometer]], or atmospheric instruments. Earth observation satellite's data is most used in [[Earth:Archaeology|archaeology]], [[Earth:Cartography|cartography]], environmental monitoring, [[Physics:Meteorology|meteorology]], and reconnaissance applications.{{Citation needed|date=July 2022}} As of 2021, there are over 950 Earth observation satellites, with the largest number of satellites operated [[Company:Planet Labs|Planet Labs]].<ref>{{cite web |date=18 August 2021 |title=How many Earth observation satellites are orbiting the planet in 2021? |url=https://www.pixalytics.com/eo-sats-2021/ |url-status=live |archive-url=https://web.archive.org/web/20220721104450/https://www.pixalytics.com/eo-sats-2021/ |archive-date=21 July 2022 |access-date=25 May 2022}}</ref>

[[Engineering:Weather satellite|Weather satellite]]s monitor [[Earth:Cloud|cloud]]s, [[Engineering:Street light|city lights]], [[Astronomy:Fire|fire]]s, effects of pollution, auroras, [[Earth:Dust storm|sand and dust storms]], [[Physics:Snow|snow]] cover, [[Earth:Ice|ice]] mapping, boundaries of [[Earth:Ocean current|ocean current]]s, [[Physics:Energy|energy]] flows, etc. Environmental monitoring satellites can detect changes in the Earth's [[Biology:Vegetation|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.<ref>NASA, [http://earthobservatory.nasa.gov/Drought/ Drought.] {{webarchive|url=https://web.archive.org/web/20080819121047/http://earthobservatory.nasa.gov/Drought/|date=19 August 2008}} Retrieved on 4 July 2008 {{PD-notice}}</ref> Anthropogenic emissions can be monitored by evaluating data of tropospheric NO<sub>2</sub> and SO<sub>2</sub>.{{Citation needed|date=July 2022}}

=== Communication ===
{{Main|Astronomy:Communications satellite}}
{{Excerpt|Communications satellite|files=no|hat=no}}

=== Navigation ===
{{Main|Earth:Satellite 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 ===
{{Main|Astronomy:Space telescope}}

Astronomical satellites are satellites used for observation of distant planets, galaxies, and other outer space objects.
[[File:Hubble 01.jpg|thumb|The Hubble Space Telescope]]

=== Experimental ===
Tether satellites are satellites that are connected to another satellite by a thin cable called a [[Engineering:Tether|tether]]. [[Engineering:Recovery satellite|Recovery satellite]]s are satellites that provide a recovery of reconnaissance, biological, space-production and other payloads from orbit to Earth. [[Engineering:Biosatellite|Biosatellite]]s are satellites designed to carry living organisms, generally for scientific experimentation. [[Physics:Space-based solar power|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 ===
{{Main|Social:Space weapon|Engineering:Anti-satellite weapon}}
Since the mid-2000s, satellites have been hacked by militant organizations to broadcast propaganda and to pilfer classified information from military communication networks.<ref>{{cite web |last=Morrill |first=Dan |title=Hack a Satellite while it is in orbit |url=http://blogs.ittoolbox.com/security/dmorrill/archives/hack-a-satellite-while-it-is-in-orbit-15690 |url-status=dead |archive-url=https://web.archive.org/web/20080320084037/http://blogs.ittoolbox.com/security/dmorrill/archives/hack-a-satellite-while-it-is-in-orbit-15690 |archive-date=20 March 2008 |access-date=25 March 2008 |publisher=ITtoolbox}}</ref><ref>{{cite news |date=22 November 2004 |title=AsiaSat accuses Falungong of hacking satellite signals |agency=Press Trust of India |url=http://www.accessmylibrary.com/coms2/summary_0286-5205866_ITM |access-date=19 May 2008 |archive-date=19 July 2012 |archive-url=https://archive.today/20120719005250/http://www.accessmylibrary.com/coms2/summary_0286-14557996_ITM |url-status=live }}</ref> For testing purposes, satellites in low earth orbit have been destroyed by ballistic missiles launched from earth. {{wipe|Russia}}, {{wipe|United States}}, {{wipe|China}} and {{wipe|India}} have demonstrated the ability to eliminate satellites.<ref name="asat">{{cite news |last1=Broad |first1=William J. |last2=Sanger |first2=David E. |date=18 January 2007 |title=China Tests Anti-Satellite Weapon, Unnerving U.S. |work={{wipe|The New York Times}} |url=https://www.nytimes.com/2007/01/18/world/asia/18cnd-china.html |access-date=23 February 2017 |archive-date=17 April 2017 |archive-url=https://web.archive.org/web/20170417190320/http://www.nytimes.com/2007/01/18/world/asia/18cnd-china.html |url-status=live }}</ref> In 2007, the {{wipe|China}} military shot down an aging weather satellite,<ref name="asat" /> followed by the US Navy shooting down a defunct spy satellite in February 2008.<ref>{{cite web |year=2008 |title=Navy Missile Successful as Spy Satellite Is Shot Down |url=http://www.popularmechanics.com/blogs/science_news/4251430.html |access-date=25 March 2008 |work=Popular Mechanics |archive-date=1 April 2008 |archive-url=https://web.archive.org/web/20080401054054/http://www.popularmechanics.com/blogs/science_news/4251430.html |url-status=live }}</ref> On 18 November 2015, after two failed attempts, Russia successfully carried out a flight test of an anti-satellite missile known as ''Nudol''.<ref>{{cite news|url=https://freebeacon.com/national-security/russia-conducts-successful-flight-test-of-anti-satellite-missile/|title=Russia Flight Tests Anti-Satellite Missile|first=Bill|last=Gertz|newspaper=The Washington Free Beacon|date=2 December 2015}}</ref> On 27 March 2019, India shot down a live test satellite at 300&nbsp;km altitude in 3 minutes. {{wipe|India}} became the fourth country to have the capability to destroy live satellites.<ref>{{Cite web |title=India successfully tests anti-satellite weapon: Modi |url=https://www.theweek.in/news/india/2019/03/27/india-test-anti-satellite-weapon-modi.html |access-date=27 March 2019 |website=The Week |language=en |archive-date=12 July 2021 |archive-url=https://web.archive.org/web/20210712020749/https://www.theweek.in/news/india/2019/03/27/india-test-anti-satellite-weapon-modi.html |url-status=live }}</ref><ref>{{Cite web |last=Diplomat |first=Harsh Vasani, The |title=India's Anti-Satellite Weapons |url=https://thediplomat.com/2016/06/indias-anti-satellite-weapons/ |access-date=27 March 2019 |website=The Diplomat |language=en-US |archive-date=1 January 2018 |archive-url=https://web.archive.org/web/20180101131652/https://thediplomat.com/2016/06/indias-anti-satellite-weapons/ |url-status=live }}</ref>

==Pollution and interference==
[[File:The growth of all tracked objects in space over time (space debris and satellites).png|thumb|The growth of all tracked objects in space over time<ref name="10.1038/s41550-022-01655-6">{{cite journal |last1=Lawrence |first1=Andy |last2=Rawls |first2=Meredith L. |last3=Jah |first3=Moriba |last4=Boley |first4=Aaron |last5=Di Vruno |first5=Federico |last6=Garrington |first6=Simon |last7=Kramer |first7=Michael |last8=Lawler |first8=Samantha |last9=Lowenthal |first9=James |last10=McDowell |first10=Jonathan |last11=McCaughrean |first11=Mark |date=April 2022 |title=The case for space environmentalism |journal=Nature Astronomy |language=en |volume=6 |issue=4 |pages=428–435 |arxiv=2204.10025 |bibcode=2022NatAs...6..428L |doi=10.1038/s41550-022-01655-6 |issn=2397-3366 |s2cid=248300127}}</ref>]]
Issues like [[Astronomy:Space debris|space debris]], radio and [[Astronomy:Light pollution|light pollution]] are increasing in magnitude and at the same time lack progress in national or international regulation.<ref name="sp1">{{cite news |last1=Seidler |first1=Christoph |date=22 April 2017 |title=Problem Weltraumschrott: Die kosmische Müllkippe – Wissenschaft |work=Der Spiegel |url=http://www.spiegel.de/wissenschaft/weltall/weltraumschrott-esa-tagung-in-darmstadt-ohne-ergebnisse-a-1144174.html |access-date=22 April 2017 |archive-date=23 April 2017 |archive-url=https://web.archive.org/web/20170423013706/http://www.spiegel.de/wissenschaft/weltall/weltraumschrott-esa-tagung-in-darmstadt-ohne-ergebnisse-a-1144174.html |url-status=live }}</ref><ref name="10.1038/s41550-022-01655-6" /> Space debris poses dangers to spacecraft<ref>{{cite web |last1=Garcia |first1=Mark |date=13 April 2015 |title=Space Debris and Human Spacecraft |url=https://www.nasa.gov/mission_pages/station/news/orbital_debris.html |access-date=22 March 2022 |website=NASA |archive-date=22 March 2022 |archive-url=https://web.archive.org/web/20220322131144/https://www.nasa.gov/mission_pages/station/news/orbital_debris.html |url-status=live }}</ref><ref name="physUT">{{cite web |last1=Williams |first1=Matt |title=What would a sustainable space environment look like? |url=https://phys.org/news/2022-03-sustainable-space-environment.html |access-date=22 March 2022 |website=phys.org |publisher=Universe Today |language=en |archive-date=22 March 2022 |archive-url=https://web.archive.org/web/20220322160515/https://phys.org/news/2022-03-sustainable-space-environment.html |url-status=live }}</ref> (including satellites)<ref name="physUT" /><ref>{{cite news |date=10 March 2022 |title=Chinese official calls for protection of space assets, international coordination mechanisms |work=SpaceNews |url=https://spacenews.com/chinese-official-calls-for-protection-of-space-assets-international-coordination-mechanisms/ |access-date=22 March 2022 |archive-date=21 July 2022 |archive-url=https://web.archive.org/web/20220721104448/https://spacenews.com/chinese-official-calls-for-protection-of-space-assets-international-coordination-mechanisms/ |url-status=live }}</ref> in or crossing geocentric orbits and have the potential to drive a [[Earth:Kessler syndrome|Kessler syndrome]]<ref>{{cite web |title=The Kessler Effect and how to stop it |url=https://www.esa.int/Enabling_Support/Space_Engineering_Technology/The_Kessler_Effect_and_how_to_stop_it |access-date=22 March 2022 |publisher=ESA |language=en |archive-date=22 March 2022 |archive-url=https://web.archive.org/web/20220322160515/https://www.esa.int/Enabling_Support/Space_Engineering_Technology/The_Kessler_Effect_and_how_to_stop_it |url-status=live }}</ref> which could potentially curtail humanity from conducting space endeavors in the future by making such nearly impossible.<ref>{{cite news |last1=Wattles |first1=Jackie |title=Space is becoming too crowded, Rocket Lab CEO warns |work=CNN |url=https://edition.cnn.com/2020/10/07/business/rocket-lab-debris-launch-traffic-scn/index.html |access-date=26 May 2022 |archive-date=26 May 2022 |archive-url=https://web.archive.org/web/20220526100211/https://edition.cnn.com/2020/10/07/business/rocket-lab-debris-launch-traffic-scn/index.html |url-status=live }}</ref><ref>{{cite news |date=16 October 2020 |title=What happens if two bits of space junk actually collide? |language=en |work=The Independent |url=https://www.independent.co.uk/tech/space-junk-collision-satellite-debris-iss-nasa-b1074079.html |access-date=26 May 2022 |archive-date=26 May 2022 |archive-url=https://web.archive.org/web/20220526100210/https://www.independent.co.uk/tech/space-junk-collision-satellite-debris-iss-nasa-b1074079.html |url-status=live }}</ref>

With the increase in numbers of [[Engineering:Satellite constellation|satellite constellation]]s, like [[Company:SpaceX|SpaceX]] [[Engineering:Starlink|Starlink]], the astronomical community, such as the [[Organization:International Astronomical Union|IAU]], report that orbital pollution is getting increased significantly.<ref name="iau">{{cite web |title=IAU's statement on satellite constellations |url=https://www.iau.org/news/announcements/detail/ann19035/ |access-date=3 June 2019 |website=International Astronomical Union |archive-date=27 May 2020 |archive-url=https://web.archive.org/web/20200527073625/https://www.iau.org/news/announcements/detail/ann19035/ |url-status=live }}</ref><ref>{{cite web |date=14 June 2019 |title=Light pollution from satellites will get worse. But how much? |url=http://www.astronomy.com/news/2019/06/light-pollution-from-satellites-will-get-worse-but-how-much |website=astronomy.com |access-date=7 November 2019 |archive-date=28 April 2021 |archive-url=https://web.archive.org/web/20210428172415/https://astronomy.com/news/2019/06/light-pollution-from-satellites-will-get-worse-but-how-much |url-status=live }}</ref><ref>{{cite journal |last1=Hainaut |first1=Olivier R. |last2=Williams |first2=Andrew P. |date=1 April 2020 |title=Impact of satellite constellations on astronomical observations with ESO telescopes in the visible and infrared domains |url=https://www.aanda.org/articles/aa/full_html/2020/04/aa37501-20/aa37501-20.html |journal=Astronomy & Astrophysics |language=en |volume=636 |page=A121 |arxiv=2003.01992 |bibcode=2020A&A...636A.121H |doi=10.1051/0004-6361/202037501 |issn=0004-6361 |doi-access=free |accessdate=22 November 2020 |archive-date=19 December 2020 |archive-url=https://web.archive.org/web/20201219192027/https://www.aanda.org/articles/aa/full_html/2020/04/aa37501-20/aa37501-20.html |url-status=live }}</ref><ref>{{cite journal |last1=Mróz |first1=Przemek |last2=Otarola |first2=Angel |last3=Prince |first3=Thomas A. |last4=Dekany |first4=Richard |last5=Duev |first5=Dmitry A. |last6=Graham |first6=Matthew J. |last7=Groom |first7=Steven L. |last8=Masci |first8=Frank J. |last9=Medford |first9=Michael S. |date=1 January 2022 |title=Impact of the SpaceX Starlink Satellites on the Zwicky Transient Facility Survey Observations |journal=The Astrophysical Journal Letters |language=en |volume=924 |issue=2 |pages=L30 |arxiv=2201.05343 |bibcode=2022ApJ...924L..30M |doi=10.3847/2041-8213/ac470a |issn=2041-8205 |s2cid=245986575}}</ref><ref>{{cite web |title=Impacts of Large Satellite Constellations on Astronomy: Live Updates {{!}} American Astronomical Society |url=https://aas.org/posts/advocacy/2021/08/impacts-large-satellite-constellations-astronomy-live-updates |access-date=22 March 2022 |work=American Astronomical Society |language=en |archive-date=19 April 2022 |archive-url=https://web.archive.org/web/20220419152431/https://aas.org/posts/advocacy/2021/08/impacts-large-satellite-constellations-astronomy-live-updates |url-status=live }}</ref> 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.<ref>{{cite news |last1=Zhang |first1=Emily |title=SpaceX's Dark Satellites Are Still Too Bright for Astronomers |language=en |work=Scientific American |url=https://www.scientificamerican.com/article/spacexs-dark-satellites-are-still-too-bright-for-astronomers/ |access-date=16 September 2020 |archive-date=2 January 2021 |archive-url=https://web.archive.org/web/20210102065559/https://www.scientificamerican.com/article/spacexs-dark-satellites-are-still-too-bright-for-astronomers/ |url-status=live }}</ref><ref>{{cite news |title=Report Offers Roadmap to Mitigate Effects of Large Satellite Constellations on Astronomy {{!}} American Astronomical Society |work=aas.org |url=https://aas.org/press/report-offers-roadmap-mitigate-effects-large-satellite-constellations-astronomy |access-date=16 September 2020 |archive-date=24 September 2020 |archive-url=https://web.archive.org/web/20200924025446/https://aas.org/press/report-offers-roadmap-mitigate-effects-large-satellite-constellations-astronomy |url-status=live }}</ref> The IAU is establishing<!--until 1 April 2022--> a center (CPS) to coordinate or aggregate measures to mitigate such detrimental effects.<ref>{{cite news |date=4 February 2022 |title=Astronomers stand up to satellite mega-constellations |work=BBC News |url=https://www.bbc.com/news/science-environment-60262100 |access-date=10 March 2022 |archive-date=10 March 2022 |archive-url=https://web.archive.org/web/20220310041256/https://www.bbc.com/news/science-environment-60262100 |url-status=live }}</ref><ref>{{cite web |title=Protection of the Dark and Quiet Sky from Satellite Constellation Interference |url=https://www.mpifr-bonn.mpg.de/announcements/2022/1 |access-date=10 March 2022 |publisher=Max Planck Institute for Radio Astronomy, Bonn |language=en |archive-date=15 March 2022 |archive-url=https://web.archive.org/web/20220315090741/https://www.mpifr-bonn.mpg.de/announcements/2022/1 |url-status=live }}</ref><ref>{{cite web |title=International Astronomical Union {{!}} IAU |url=https://www.iau.org/science/scientific_bodies/centres/CPS/ |access-date=10 March 2022 |website=www.iau.org |archive-date=13 March 2022 |archive-url=https://web.archive.org/web/20220313084016/https://www.iau.org/science/scientific_bodies/centres/CPS/ |url-status=live }}</ref>

Some notable satellite failures that polluted and dispersed radioactive materials are [[Engineering:Kosmos 954|Kosmos 954]], [[Engineering:Kosmos 1402|Kosmos 1402]] and the [[Engineering:List of nuclear power systems in space|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.<ref>{{cite news |last=Harper |first=Justin |date=29 December 2020 |title=Japan developing wooden satellites to cut space junk |work=bbc.co.uk |url=https://www.bbc.co.uk/news/business-55463366 |access-date=29 December 2020 |archive-date=29 December 2020 |archive-url=https://web.archive.org/web/20201229014751/https://www.bbc.co.uk/news/business-55463366 |url-status=live }}</ref>

Due to the low received signal strength of satellite transmissions, they are prone to [[Radio jamming|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,<ref>{{cite web |last=Singer |first=Jeremy |year=2003 |title=U.S.-Led Forces Destroy GPS Jamming Systems in Iraq |url=http://www.space.com/news/gps_iraq_030325.html |url-status=dead |archive-url=https://web.archive.org/web/20080526204956/http://www.space.com/news/gps_iraq_030325.html |archive-date=26 May 2008 |access-date=25 March 2008 |publisher=Space.com}}</ref><ref>{{cite web |last=Brewin |first=Bob |year=2003 |title=Homemade GPS jammers raise concerns |url=http://www.computerworld.com/securitytopics/security/story/0,10801,77702,00.html |url-status=dead |archive-url=https://web.archive.org/web/20080422164053/http://computerworld.com/securitytopics/security/story/0%2C10801%2C77702%2C00.html |archive-date=22 April 2008 |access-date=25 March 2008 |work=Computerworld}}</ref> but satellite phone and television signals have also been subjected to jamming.<ref>{{cite web |year=2008 |title=Iran government jamming exile satellite TV |url=http://www.iranfocus.com/modules/news/article.php?storyid=2852 |access-date=25 March 2008 |work=Iran Focus |archive-date=14 January 2006 |archive-url=https://web.archive.org/web/20060114173408/http://www.iranfocus.com/modules/news/article.php?storyid=2852 |url-status=live }}</ref><ref>{{cite web |last=Selding |first=Peter de |year=2007 |title=Libya Pinpointed as Source of Months-Long Satellite Jamming in 2006 |url=http://www.space.com/spacenews/businessmonday_070409.html |url-status=live |archive-url=https://web.archive.org/web/20080429193429/http://www.space.com/spacenews/businessmonday_070409.html |archive-date=29 April 2008 |publisher=Space.com}}</ref>

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. {{Citation needed|date=September 2011}}

==Notes==

{{notelist}}

== References ==
{{Reflist}}

==External links==
* {{curlie|Business/Telecommunications/Services/Wireless/Satellite/|Satellite}}
* [https://directory.eoportal.org/web/eoportal/satellite-missions EO Portal directory] {{Webarchive|url=https://web.archive.org/web/20130923090413/https://directory.eoportal.org/web/eoportal/satellite-missions |date=23 September 2013 }}

{{Space-based meteorological observation}}
{{Spaceflight}}

{{DEFAULTSORT:Earth Observation Satellite}}
[[Category:Earth observation satellites| ]]
[[Category:Satellites by type]]
[[Category:Satellite imagery]]
[[Category:Satellites| ]]

{{Sourceattribution|Satellite|1}}