In the context of spaceflight, a satellite is an object that has been intentionally placed into orbit. These objects are called artificial satellites to distinguish them from natural satellites such as Earth's Moon.
On 4 October 1957 the Soviet Union launched the world's first artificial satellite, Sputnik 1. Since then, about 8,900 satellites from more than 40 countries have been launched. According to a 2018 estimate, some 5,000 remain in orbit. Of those, about 1,900 were operational, while the rest have exceeded their useful lives and become space debris. Approximately 63% of operational satellites are in low Earth orbit, 6% are in medium-Earth orbit (at 20,000 km), 29% are in geostationary orbit (at 36,000 km) and the remaining 2% are in various elliptical orbits. In terms of countries with the most satellites, the USA has the most with 859 satellites, China is second with 250, and Russia third with 146. These are then followed by India (118), Japan (72) and the UK (52).
A few large space stations, including the International Space Station, 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 of the Moon, Mercury, Venus, Mars, Jupiter, Saturn, a few asteroids, a comet and the Sun.
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.
Satellites are usually semi-independent computer-controlled systems. Satellite subsystems attend many tasks, such as power generation, thermal control, telemetry, attitude control, scientific instrumentation, communication, etc.
In 1903, 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 orbital speed required for a minimal orbit, and that a multi-stage rocket fueled by liquid propellants could achieve this.
In 1928, Herman Poto?nik (1892-1929) published his sole book, The Problem of Space Travel - The Rocket 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 February 1954 Project RAND released "Scientific Uses for a Satellite Vehicle," written 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.
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.
Satellites can be tracked from Earth stations and also from other satellites.
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-artificial 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. 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.
There are three basic categories of (non-military) satellite services:
Fixed satellite services
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
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)
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.
Astronomical satellites are satellites used for observation of distant planets, galaxies, and other outer space objects.
Navigational satellites are satellites which 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.
Killer satellites are satellites that are designed to destroy enemy warheads, satellites, and other space assets.
Crewed spacecraft (spaceships) are large satellites able to put humans into (and beyond) an orbit, and return them to Earth. (The Lunar Module of the U.S. Apollo program was an exception, in that it did not have the capability of returning human occupants to Earth.) Spacecraft including spaceplanes of reusable systems have major propulsion or landing facilities. They can be used as transport to and from the orbital stations.
Recovery satellites are satellites that provide a recovery of reconnaissance, biological, space-production and other payloads from orbit to Earth.
Space-based solar power satellites are proposed satellites that would collect energy from sunlight and transmit it for use on Earth or other places.
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.
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. This is the most common type of orbit by far, with approximately 2,787 active 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.
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).
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.
Molniya orbit: A highly eccentric 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 (usually Russia and the United States).
Tundra orbit: A highly eccentric 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 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.
Geostationary orbit (GEO): A geosynchronous orbit with an inclination of zero. 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.
Sun-synchronous orbit: An orbit which combines altitude and inclination in such a way that the satellite passes over any given point of the planets' surface at the same local solar time. Such an orbit can place a satellite in constant sunlight and is useful for imaging, spy, and weather satellites.
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.
The satellite's functional versatility is embedded within its technical components and its operations characteristics. Looking at the "anatomy" of a typical satellite, one discovers two modules. Note that some novel architectural concepts such as Fractionated spacecraft somewhat upset this taxonomy.
Spacecraft bus or service module
The bus module consists of the following subsystems:
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.
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.
The power subsystem may consist 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).
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
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 the desired attitude, keep the satellite in the correct orbital position, and keep antennas pointed in the right directions.
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
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 artificial satellite to be put in Geocentric orbit, was still in orbit as of March 2015[update], as well as the upper stage of its launch rocket.
Instead of being de-orbited, most satellites are either left in their current orbit or moved to a graveyard orbit. 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. 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.
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 does not include the European Space Agency, a multi-national state organization, nor private consortiums.
The United States tried in 1957 to launch the first satellite using its own launcher before successfully completing a launch in 1958.
Japan tried four times in 1966-1969 to launch a satellite with its own launcher before successfully completing a launch in 1970.
China tried in 1969 to launch the first satellite using its own launcher before successfully completing a launch in 1970.
India, after launching its first national satellite using a foreign launcher in 1975, tried in 1979 to launch the first satellite using its own launcher before succeeding in 1980.
Iraq have claimed an orbital launch of a warhead in 1989, but this claim was later disproved.
Brazil, after launching its first national satellite using a foreign launcher in 1985, tried to launch a satellite using its own VLS 1 launcher three times in 1997, 1999, and 2003, but all attempts were unsuccessful.
North Korea claimed a launch of Kwangmy?ngs?ng-1 and Kwangmy?ngs?ng-2 satellites in 1998 and 2009, but U.S., Russian and other officials and weapons experts later reported that the rockets failed to send a satellite into orbit, if that was the goal. The United States, Japan and South Korea believe this was actually a ballistic missile test, which was a claim also made after North Korea's 1998 satellite launch, and later rejected. The first (April 2012) launch of Kwangmy?ngs?ng-3 was unsuccessful, a fact publicly recognized by the DPRK. However, the December 2012 launch of the "second version" of Kwangmy?ngs?ng-3 was successful, putting the DPRK's first confirmed satellite into orbit.
South Korea (Korea Aerospace Research Institute), after launching their first national satellite by foreign launcher in 1992, unsuccessfully tried to launch its own launcher, the KSLV (Naro)-1, (created with the assistance of Russia) in 2009 and 2010 until success was achieved in 2013 by Naro-3.
The First European multi-national state organization ELDO tried to make the orbital launches at Europa I and Europa II rockets in 1968-1970 and 1971 but stopped operation after failures.
^Russia and Ukraine were parts of the Soviet Union and thus inherited their launch capability without the need to develop it indigenously. Through the Soviet Union they are also on the number one position in this list of accomplishments.
Only twelve, countries from the list below (USSR, USA, France, Japan, China, UK, India, Russia, Ukraine, Israel, Iran and North Korea) and one regional organization (the European Space Agency, ESA) have independently launched satellites on their own indigenously developed launch vehicles.
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 first Italian satellite 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 NASA. By similar occasions, almost all further first national satellites was launched by foreign rockets.
Attempted first satellites
United States tried unsuccessfully to launch its first satellite in 1957; they were successful in 1958.
China tried unsuccessfully to launch its first satellite in 1969; they were successful in 1970.
Chile tried unsuccessfully in 1995 to launch its first satellite FASat-Alfa by foreign rocket; in 1998 they were successful.+
North Korea has tried in 1998, 2009, 2012 to launch satellites, first successful launch on 12 December 2012.
Libya since 1996 developed its own national Libsat satellite project with the goal of providing telecommunication and remote sensing services that was postponed after the fall of Gaddafi.
Belarus tried unsuccessfully in 2006 to launch its first satellite BelKA by foreign rocket.+
+-note: Both Chile and Belarus used Russian companies as principal contractors to build their satellites, they used Russian-Ukrainian manufactured rockets and launched either from Russia or Kazakhstan.
Planned first satellites
Afghanistan announced in April 2012 that it is planning to launch its first communications satellite to the orbital slot it has been awarded. The satellite Afghansat 1 was expected to be obtained by a Eutelsat commercial company in 2014.
Armenia in 2012 founded Armcosmos company and announced an intention to have the first telecommunication satellite ArmSat. The investments estimates as $250 million and country selecting the contractor for building within 4 years the satellite amongst Russia, China and Canada
Cambodia's Royal Group plans to purchase for $250-350 million and launch in the beginning of 2013 the telecommunication satellite.
Myanmar plans to purchase for $200 million their own telecommunication satellite.
Nicaragua ordered for $254 million at November 2013 in China the first telecommunication satellite Nicasat-1 (to be built at DFH-4 satellite bus platform by CAST and CGWIC), that planning to launch in China at 2016.
Serbia's first satellite Tesla-1 was designed, developed and assembled by nongovernmental organisations in 2009 but still remains unlaunched.
Sri Lanka has a goal to construct two satellites beside of rent the national SupremeSAT payload in Chinese satellites. Sri Lankan Telecommunications Regulatory Commission has signed an agreement with Surrey Satellite Technology Ltd to get relevant help and resources. Launch into Earth orbit would be done by a foreign provider.
Syrian Space Research Center developing CubeSat-like small first national satellite since 2008.
Tunisia is developing its first satellite, ERPSat01. Consisting of a CubeSat of 1 kg mass, it will be developed by the Sfax School of Engineering. ERPSat satellite is planned to be launched into orbit in 2013.
Uzbekistan's State Space Research Agency (UzbekCosmos) announced in 2001 about intention of launch in 2002 first remote sensing satellite. Later in 2004 was stated that two satellites (remote sensing and telecommunication) will be built by Russia for $60-70 million each
Attacks on satellites
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, United States, China and India have demonstrated the ability to eliminate satellites. In 2007 the Chinese military shot down an aging weather satellite, followed by the US Navy shooting down a defunct spy satellite in February 2008. On 27 March 2019 India shot down a live test satellite at 300 km altitude in 3 minutes. India became the fourth country to have the capability to destroy live satellites.
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.
During the last five decades, space agencies have sent thousands of space crafts, space capsules, or satellites to the universe. In fact, weather forecasters make predictions on the weather and natural calamities based on observations from these satellites.
The National Aeronautics and Space Administration (NASA) requested the National Academies to publish a report entitled, Earth Observations from Space; The First 50 Years of Scientific Achievements in 2008. It described how the capability to view the whole globe simultaneously from satellite observations revolutionized studies about the planet Earth. This development brought about a new age of combined Earth sciences. The National Academies report concluded that continuing Earth observations from the galaxy are necessary to resolve scientific and social challenges in the future.
The NASA introduced an Earth Observing System (EOS) composed of several satellites, science component, and data system described as the Earth Observing System Data and Information System (EOSDIS). It disseminates numerous science data products as well as services designed for interdisciplinary education. EOSDIS data can be accessed online and accessed through File Transfer Protocol (FTP) and Hyper Text Transfer Protocol Secure (HTTPS). Scientists and researchers perform EOSDIS science operations within a distributed platform of multiple interconnected nodes or Science Investigator-led Processing Systems (SIPS) and discipline-specific Distributed Active Archive Centers (DACCs).
The European Space Agency have been operating Earth Observation satellites since the launch of Meteosat 1 in November 1977. ESA currently has plans to launch a satellite equipped with an artificial intelligence (AI) processor that will allow the spacecraft to make decisions on images to capture and data to transmit to the Earth. BrainSat will use the Intel Myriad X vision processing unit (VPU). The launching will be scheduled in 2019. ESA director for Earth Observation Programs Josef Aschbacher made the announcement during the PhiWeek in November 2018. This is the five-day meet that focused on the future of Earth observation. The conference was held at the ESA Center for Earth Observation in Frascati, Italy. ESA also launched the PhiLab, referring to the future-focused team that works to harness the potentials of AI and other disruptive innovations. Meanwhile, the ESA also announced that it expects to commence the qualification flight of the Space Rider space plane in 2021. This will come after several demonstration missions. Space Rider is the sequel of the Agency's Intermediate Experimental vehicle (IXV) which was launched in 2015. It has the capacity payload of 800 kilograms for orbital missions that will last a maximum of two months.
Pollution and regulation
Generally liability has been covered by the Liability Convention.
Issues like space debris, radio and light pollution are increasing in magnitude and at the same time lack progress in national or international regulation.
With future increase in numbers of satellite constellations, like SpaceXStarlink, it is feared especially by the astronomical community, such as the IAU, that orbital pollution will increase 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. Some notable satellite failures that polluted and dispersed radioactive materials are Kosmos 954, Kosmos 1402 and the Transit 5-BN-3. Using wood as an alternative material has been posited in order to reduce pollution and debris from satellites that reenter the atmosphere.