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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 6,600 satellites from more than 40 countries have been launched. According to a 2013 estimate, 3,600 remained in orbit.[1] Of those, about 1,000 were operational;[2] while the rest have lived out their useful lives and become space debris. 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).[3] 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,[4] a comet and the Sun.
Satellites are used for many purposes. 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.

History[edit]

Early conceptions[edit]

"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.[5][6] The idea surfaced again in Jules Verne's The Begum's Fortune (1879).
Konstantin Tsiolkovsky
In 1903, Konstantin Tsiolkovsky (1857–1935) published Exploring Space Using Jet Propulsion Devices (in Russian: Исследование мировых пространств реактивными приборами), which is the first academic treatise on the use of rocketry to launch spacecraft. He calculated the orbital speed required for a minimal orbit, and that a multi-stage rocketfuelled 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.
Animation depicting the orbits of GPS satellites in medium Earth orbit.
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.[7] 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.[8]

Artificial satellites[edit]

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.[9]
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."[10] 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."[11] In February 1954 Project RAND released "Scientific Uses for a Satellite Vehicle," written by R.R. Carhart.[12] 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.[13]
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.[14]
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.[15]
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.

Space Surveillance Network[edit]

Main article: United States Space Surveillance Network
The United States Space Surveillance Network (SSN), a division of the United States Strategic Command, has been tracking objects in Earth's orbit since 1957 when the Soviet Union opened the Space Age with the launch of Sputnik I. Since then, the SSN has tracked more than 26,000 objects. The SSN currently tracks more than 8,000 man-made orbiting objects. The rest have re-entered Earth's atmosphere and disintegrated, or survived re-entry and impacted the Earth. The SSN tracks objects that are 10 centimeters in diameter or larger; those now orbiting Earth range from satellites weighing several tons to pieces of spent rocket bodies weighing only 10 pounds. About seven percent are operational satellites (i.e. ~560 satellites), the rest are space debris.[16] The United States Strategic Command is primarily interested in the active satellites, but also tracks space debris which upon reentry might otherwise be mistaken for incoming missiles.

Non-military satellite services[edit]

There are three basic categories of non-military satellite services:[17]

Fixed satellite services[edit]

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

Mobile satellite systems[edit]

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

Scientific research satellites (commercial and noncommercial)[edit]

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

Types[edit]

International Space Station
  • Space stations are artificial orbital structures that are designed for human beings to live on in outer space. A space station is distinguished from other crewed spacecraft by its lack of major propulsion or landing facilities. Space stations are designed for medium-term living in orbit, for periods of weeks, months, or even years.
  • Tether satellites are satellites which are connected to another satellite by a thin cable called a tether.
  • Weather satellites are primarily used to monitor Earth's weather and climate.[19]

Orbit types[edit]

Main article: List of orbits
Various earth orbits to scale; cyan represents low earth orbit, yellow represents medium earth orbit, the black dashed line represents geosynchronous orbit, the green dash-dot line the orbit of Global Positioning System (GPS) satellites, and the red dotted line the orbit of the International Space Station (ISS).
The first satellite, Sputnik 1, was put into orbit around Earth and was therefore in geocentric orbit. By far this is the most common type of orbit with approximately 1,459[20] artificial satellites orbiting the Earth. Geocentric orbits may be further classified by their altitude, inclination and eccentricity.
The commonly used altitude classifications of geocentric orbit are Low Earth orbit (LEO), Medium Earth orbit (MEO) and High Earth orbit (HEO). Low Earth orbit is any orbit below 2,000 km. Medium Earth orbit is any orbit between 2,000 and 35,786 km. High Earth orbit is any orbit higher than 35,786 km.

Centric classifications[edit]

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

Altitude classifications[edit]

  • Low Earth orbit (LEO): Geocentric orbits ranging in altitude from 180 km - 2,000 km (1,200 mi)
  • Medium Earth orbit (MEO): Geocentric orbits ranging in altitude from 2,000 km (1,200 mi) - 35,786 km (22,236 mi). Also known as an intermediate circular orbit.
  • Geosynchronous orbit (GEO): Geocentric circular orbit with an altitude of 35,786 kilometres (22,236 mi). The period of the orbit equals one sidereal day, coinciding with the rotation period of the Earth. The speed is approximately 3,000 metres per second (9,800 ft/s).
  • High Earth orbit (HEO): Geocentric orbits above the altitude of geosynchronous orbit 35,786 km (22,236 mi).
Orbital Altitudes of several significant satellites of earth.

Inclination classifications[edit]

  • Inclined orbit: An orbit whose inclination in reference to the equatorial plane is not zero degrees.
    • Polar orbit: An orbit that passes above or nearly above both poles of the planet on each revolution. Therefore, it has an inclination of (or very close to) 90 degrees.
    • Polar sun synchronous orbit: A nearly polar orbit that passes the equator at the same local time on every pass. Useful for image taking satellites because shadows will be nearly the same on every pass.

Eccentricity classifications[edit]

  • Circular orbit: An orbit that has an eccentricity of 0 and whose path traces a circle.
    • Hohmann transfer orbit: An orbit that moves a spacecraft from one approximately circular orbit, usually the orbit of a planet, to another, using two engine impulses. The perihelion of the transfer orbit is at the same distance from the Sun as the radius of one planet's orbit, and the aphelion is at the other. The two rocket burns change the spacecraft's path from one circular orbit to the transfer orbit, and later to the other circular orbit. This maneuver was named after Walter Hohmann.
  • Elliptic orbit: An orbit with an eccentricity greater than 0 and less than 1 whose orbit traces the path of an ellipse.
    • Geosynchronous transfer orbit: An elliptic orbit where the perigee is at the altitude of a Low Earth orbit (LEO) and the apogee at the altitude of a geosynchronous orbit.
    • Geostationary transfer orbit: An elliptic orbit where the perigee is at the altitude of a Low Earth orbit (LEO) and the apogee at the altitude of a geostationary orbit.
    • Molniya orbit: A highly elliptic orbit with inclination of 63.4° and orbital period of half of a sidereal day (roughly 12 hours). Such a satellite spends most of its time over two designated areas of the planet (specifically Russia and the United States).
    • Tundra orbit: A highly elliptic orbit with inclination of 63.4° and orbital period of one sidereal day (roughly 24 hours). Such a satellite spends most of its time over a single designated area of the planet.

Synchronous classifications[edit]

  • Synchronous orbit: An orbit where the satellite has an orbital period equal to the average rotational period (earth's is: 23 hours, 56 minutes, 4.091 seconds) of the body being orbited and in the same direction of rotation as that body. To a ground observer such a satellite would trace an analemma (figure 8) in the sky.
  • Semi-synchronous orbit (SSO): An orbit with an altitude of approximately 20,200 km (12,600 mi) and an orbital period equal to one-half of the average rotational period (Earth's is approximately 12 hours) of the body being orbited
  • Geosynchronous orbit (GSO): Orbits with an altitude of approximately 35,786 km (22,236 mi). Such a satellite would trace an analemma (figure 8) in the sky.
  • Areosynchronous orbit: A synchronous orbit around the planet Mars with an orbital period equal in length to Mars' sidereal day, 24.6229 hours.
  • Areostationary orbit (ASO): A circular areosynchronous orbit on the equatorial plane and about 17000 km (10557 miles) above the surface. To an observer on the ground this satellite would appear as a fixed point in the sky.
  • Heliosynchronous orbit: A heliocentric orbit about the Sun where the satellite's orbital period matches the Sun's period of rotation. These orbits occur at a radius of 24,360 Gm (0.1628 AU) around the Sun, a little less than half of the orbital radius of Mercury.

Special classifications[edit]

Pseudo-orbit classifications[edit]

  • Horseshoe orbit: An orbit that appears to a ground observer to be orbiting a certain planet but is actually in co-orbit with the planet. See asteroids 3753(Cruithne) and 2002 AA29.
  • Exo-orbit: A maneuver where a spacecraft approaches the height of orbit but lacks the velocity to sustain it.
  • Lunar transfer orbit (LTO)
  • Prograde orbit: An orbit with an inclination of less than 90°. Or rather, an orbit that is in the same direction as the rotation of the primary.
  • Retrograde orbit: An orbit with an inclination of more than 90°. Or rather, an orbit counter to the direction of rotation of the planet. Apart from those in sun-synchronous orbit, few satellites are launched into retrograde orbit because the quantity of fuel required to launch them is much greater than for a prograde orbit. This is because when the rocket starts out on the ground, it already has an eastward component of velocity equal to the rotational velocity of the planet at its launch latitude.
  • Halo orbit and Lissajous orbit: Orbits "around" Lagrangian points.

Satellite subsystems[edit]

The satellite's functional versatility is imbedded within its technical components and its operations characteristics. Looking at the "anatomy" of a typical satellite, one discovers two modules.[17] Note that some novel architectural concepts such as Fractionated spacecraft somewhat upset this taxonomy.

Spacecraft bus or service module[edit]

The bus module consists of the following subsystems:

Structural subsystem[edit]

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

Telemetry subsystem[edit]

The telemetry subsystem (aka Command and Data Handling, C&DH) monitors the on-board equipment operations, transmits equipment operation data to the earth control station, and receives the earth control station's commands to perform equipment operation adjustments.

Power subsystem[edit]

The power subsystem consists of solar panels to convert solar energy into electrical power, regulation and distribution functions, and batteries that store power and supply the satellite when it passes into the Earth's shadow. Nuclear power sources (Radioisotope thermoelectric generator) have also been used in several successful satellite programs including the Nimbus program (1964–1978).[22]

Thermal control subsystem[edit]

Main article: Spacecraft thermal control
The thermal control subsystem helps protect electronic equipment from extreme temperatures due to intense sunlight or the lack of sun exposure on different sides of the satellite's body (e.g. optical solar reflector)

Attitude and orbit control subsystem[edit]

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

Communication payload[edit]

The second major module is the communication payload, which is made up of transponders. A transponder is capable of :
  • Receiving uplinked radio signals from earth satellite transmission stations (antennas).
  • Amplifying received radio signals
  • Sorting the input signals and directing the output signals through input/output signal multiplexers to the proper downlink antennas for retransmission to earth satellite receiving stations (antennas).

End of life[edit]

When satellites reach the end of their mission (this normally occurs within 3 or 4 years after launch), satellite operators have the option of de-orbiting the satellite, leaving the satellite in its current orbit or moving the satellite to a graveyard orbit. Historically, due to budgetary constraints at the beginning of satellite missions, satellites were rarely designed to be de-orbited. One example of this practice is the satellite Vanguard 1. Launched in 1958, Vanguard 1, the 4th manmade satellite put in Geocentric orbit, was still in orbit as of March 2015, as well as the upper stage of its launch rocket.[23][24]
Instead of being de-orbited, most satellites are either left in their current orbit or moved to a graveyard orbit.[25] As of 2002, the FCC requires all geostationary satellites to commit to moving to a graveyard orbit at the end of their operational life prior to launch.[26] In cases of uncontrolled de-orbiting, the major variable is the solar flux, and the minor variables the components and form factors of the satellite itself, and the gravitational perturbations generated by the Sun and the Moon (as well as those exercised by large mountain ranges, whether above or below sea level). The nominal breakup altitude due to aerodynamic forces and temperatures is 78 km, with a range between 72 and 84 km. Solar panels, however, are destroyed before any other component at altitudes between 90 and 95 km.[27]

Launch-capable countries[edit]

Main article: Timeline of first orbital launches by nationality
This list includes countries with an independent capability to place satellites in orbit, including production of the necessary launch vehicle. Note: many more countries have the capability to design and build satellites but are unable to launch them, instead relying on foreign launch services. This list does not consider those numerous countries, but only lists those capable of launching satellites indigenously, and the date this capability was first demonstrated. The list includes the European Space Agency, a multi-national state organization, but does not include private consortiums.
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