NASA's Earth-observing fleet as of June 2012.
A full-size model of the Earth observation satellite ERS 2

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

In 1957 the Soviet Union launched the world's first artificial satellite, Sputnik 1. Since then, about 8,100 satellites from more than 40 countries have been launched. According to a 2018 estimate, some 4,900 remain in orbit, of those about 1,900 were operational; while the rest have lived out their useful lives and become space debris.[1] Approximately 500 operational satellites are in low-Earth orbit, 50 are in medium-Earth orbit (at 20,000 km), and the rest are in geostationary orbit (at 36,000 km).[2] A few large satellites have been launched in parts and assembled in orbit. Over a dozen space probes have been placed into orbit around other bodies and become artificial satellites to the Moon, Mercury, Venus, Mars, Jupiter, Saturn, a few asteroids,[3] a comet and the Sun.

Satellites are used for many purposes. Among several other applications, they can be used to make star maps and maps of planetary surfaces, and also take pictures of planets they are launched into. Common types include military and civilian Earth observation satellites, communications satellites, navigation satellites, weather satellites, and space telescopes. Space stations and human spacecraft in orbit are also satellites. Satellite orbits vary greatly, depending on the purpose of the satellite, and are classified in a number of ways. Well-known (overlapping) classes include low Earth orbit, polar orbit, and geostationary orbit.

A launch vehicle is a rocket that places a satellite into orbit. Usually, it lifts off from a launch pad on land. Some are launched at sea from a submarine or a mobile maritime platform, or aboard a plane (see air launch to orbit).

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


Early conceptions

Konstantin Tsiolkovsky
A 1949 issue of Popular Science depicts the idea of an "artificial moon"
Animation depicting the orbits of GPS satellites in medium Earth orbit.

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

The first fictional depiction of a satellite being launched into orbit was a short story by Edward Everett Hale, The Brick Moon.[4][5] The idea surfaced again in Jules Verne's The Begum's Fortune (1879).

In 1903, Konstantin Tsiolkovsky (1857–1935) published Exploring Space Using Jet Propulsion Devices (in Russian: Исследование мировых пространств реактивными приборами), which is the first academic treatise on the use of rocketry to launch spacecraft. He calculated the orbital speed required for a minimal orbit, and that a multi-stage rocket fuelled by liquid propellants could achieve this.

In 1928, Herman Potočnik (1892–1929) published his sole book, The Problem of Space Travel — The Rocket Motor (German: Das Problem der Befahrung des Weltraums — der Raketen-Motor). He described the use of orbiting spacecraft for observation of the ground and described how the special conditions of space could be useful for scientific experiments.

In a 1945 Wireless World article, the English science fiction writer Arthur C. Clarke (1917–2008) described in detail the possible use of communications satellites for mass communications.[6] He suggested that three geostationary satellites would provide coverage over the entire planet.

The US military studied the idea of what was referred to as the "earth satellite vehicle" when Secretary of Defense James Forrestal made a public announcement on 29 December 1948, that his office was coordinating that project between the various services.[7]

Artificial satellites

Sputnik 1: The first artificial satellite to orbit Earth.

The first artificial satellite was Sputnik 1, launched by the Soviet Union on 4 October 1957, and initiating the Soviet Sputnik program, with Sergei Korolev as chief designer. This in turn triggered the Space Race between the Soviet Union and the United States.

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

Sputnik 2 was launched on 3 November 1957 and carried the first living passenger into orbit, a dog named Laika.[8]

In May, 1946, Project RAND had released the Preliminary Design of an Experimental World-Circling Spaceship, which stated, "A satellite vehicle with appropriate instrumentation can be expected to be one of the most potent scientific tools of the Twentieth Century."[9] The United States had been considering launching orbital satellites since 1945 under the Bureau of Aeronautics of the United States Navy. The United States Air Force's Project RAND eventually released the report, but considered the satellite to be a tool for science, politics, and propaganda, rather than a potential military weapon. In 1954, the Secretary of Defense stated, "I know of no American satellite program."[10] In February 1954 Project RAND released "Scientific Uses for a Satellite Vehicle," written by R.R. Carhart.[11] 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.[12]

In the context of activities planned for the International Geophysical Year (1957–58), 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 Soviets announced that they intended to launch a satellite by the fall of 1957.

Following pressure by the American Rocket Society, the National Science Foundation, and the International Geophysical Year, military interest picked up and in early 1955 the Army and Navy were working on Project Orbiter, two competing programs: the army's which involved using a Jupiter C rocket, and the civilian/Navy Vanguard Rocket, to launch a satellite. At first, they failed: initial preference was given to the Vanguard program, whose first attempt at orbiting a satellite resulted in the explosion of the launch vehicle on national television. But finally, three months after Sputnik 2, the project succeeded; Explorer 1 became the United States' first artificial satellite on 31 January 1958.[13]

In June 1961, three-and-a-half years after the launch of Sputnik 1, the Air Force used resources of the United States Space Surveillance Network to catalog 115 Earth-orbiting satellites.[14]

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

Currently the largest artificial satellite ever is the International Space Station.

1U CubeSat ESTCube-1, developed mainly by the students from the University of Tartu, carries out a tether deployment experiment in low Earth orbit.
Other Languages
Afrikaans: Satelliet
العربية: قمر اصطناعي
azərbaycanca: Süni peyk
تۆرکجه: مصنوعی قمر
беларуская (тарашкевіца)‎: Штучны спадарожнік Зямлі
भोजपुरी: उपग्रह
brezhoneg: Loarell
čeština: Umělá družice
Cymraeg: Lloeren
dansk: Satellit
ދިވެހިބަސް: ސެޓެލައިޓް
فارسی: ماهواره
føroyskt: Fylgisveinur
Frysk: Satellyt
furlan: Satelit
Gàidhlig: Saideal fuadain
贛語: 衛星
한국어: 인공위성
հայերեն: Արբանյակ
हिन्दी: उपग्रह
hrvatski: Umjetni satelit
Bahasa Indonesia: Satelit
íslenska: Gervitungl
עברית: לוויין
Basa Jawa: Satelit
Kiswahili: Satelaiti
Lëtzebuergesch: Satellit (Raumfaart)
Limburgs: Kónsmaon
magyar: Műhold
मैथिली: उपग्रह
македонски: Вештачки сателит
Māori: Āmiorangi
मराठी: उपग्रह
Bahasa Melayu: Satelit
မြန်မာဘာသာ: ဂြိုဟ်တု
Nederlands: Kunstmaan
नेपाली: उपग्रह
日本語: 人工衛星
Nordfriisk: Satellit
norsk nynorsk: Satellitt
Novial: Satelite
ଓଡ଼ିଆ: ଉପଗ୍ରହ
ਪੰਜਾਬੀ: ਉਪਗ੍ਰਹਿ
پښتو: سپوږمکۍ
Patois: Satilait
Plattdüütsch: Satellit (Ruumfohrt)
русиньскый: Штучный сателит
Scots: Satellite
Simple English: Satellite (artificial)
slovenčina: Umelá družica
slovenščina: Satelit
Soomaaliga: Dayax gacmeed
српски / srpski: Вештачки сателит
srpskohrvatski / српскохрватски: Umjetni satelit
Basa Sunda: Satelit
svenska: Satellit
Tagalog: Buntabay
татарча/tatarça: Cirneñ yasalma iärçene
తెలుగు: ఉపగ్రహము
тоҷикӣ: Моҳвора
Türkçe: Yapay uydu
українська: Штучний супутник
ئۇيغۇرچە / Uyghurche: سۈنئىي ھەمراھ
Tiếng Việt: Vệ tinh
Winaray: Satelayt
吴语: 人造卫星
ייִדיש: סאטעליט
粵語: 人造衞星
中文: 人造衛星