Satellite, Artificial

Satellite, Artificial, a man-made object that orbits the earth, the moon, the sun, or any other celestial body. On October 4, 1957, the Soviet Union's Sputnik I became the first man-made object to be placed in orbitan event that ushered in the space age.

The advent of the space age had a profound influence on education in the United States. The science content of high school curricula was modernized, and advanced study in engineering and the sciences received new emphasis. Later, there was a widening exploration into the methods and principles of teaching, to enable students to cope with the rapidly changing and expanding technological aspects of society. Politically, the ability to orbit ever heavier and more complex satellites immediately became a matter of prestige among nations, particularly the United States and the Soviet Union. The concepts of international law were extended to space, and many political questions were raised. Some of these questions, such as the banning of nuclear weapons in space, were settled, but other issues, such as the development of weapons to destroy satellites in space, remain unresolved.

Satellites have important applications in communications and meteorology. Satellites are also important in the study of some of the most challenging problems of pure science, such as the origin of the earth and, indeed, of the whole universe.

The developments of space science have also provided society with direct benefits in the form of what has been called technological falloutthat is, inventions and processes that were originally developed for the space program (or in an unsuccessful attempt to solve a problem for the space program) and later became useful in other applications. For example, metallized plastic film developed for an early artificial satellite called Echo later found use as a material for camping equipment, food packaging, and winter clothing. One unique application is as a lightweight emergency blanket that can protect a person against freezing temperatures, yet when folded is small enough to fit in a shirt pocket.



Satellites are launched into space by various types of rockets, called launch vehicles. A single launch vehicle can be used to place two or more satellites into orbit at one time. Some launch vehicles are expendablethey are designed to be used only once. The space shuttle, a type of manned spacecraft, serves as a reusable launch vehicle. A few satellites have been launched from high-flying airplanes.

A major consideration in the choice of a launch vehicle for a particular flight, or mission, is the amount of thrust generated by the vehicle. The greater the payload to be lifted and the higher the orbit, the greater the thrust must be. (The payload consists of the satellite, its protective coverings, and any other hardware, such as separation devices, and sometimes a rocket engine that boosts the satellite into its proper orbit.)

The launch vehicle may be a single-stage rocket (having one main set of rocket engines) with or without auxiliary boosters, or it may have two or three stages stacked one on top of another.

Preparation for a mission begins months in advance of the anticipated launch date. The launch vehicle must be assembled and its various partsincluding pumps, fuel tanks, engines, and steering mechanismstested. The satellite is checked to determine, as completely as possible, if it will function as planned in space. In this check the satellite is subjected to, among other things, intense vibration (simulating rocket flight), near vacuum, and extremes of heat and cold.

In the hours prior to a launch, many preparations and checks must be made. Launch vehicles that use liquid propellants must be fueled during this time. The sequence of events leading up to the launch is referred to as the countdown.

For a few seconds after the rocket engines of the launch vehicle are ignited, the vehicle is held down by restraints to permit the engines' thrust to build. The restraints are then released and lift-off occurs. The launch vehicle rises slowly at first, but rapidly gains speed as it gains altitude.

With a multistage launch vehicle, only the engines of the first stage are ignited for lift-off. After its fuel is exhausted, the first stage is jettisoned and the second stage is fired. Each stage is fired and jettisoned in turn. The space shuttle is a single-stage launch vehicle with three liquid-fuel rocket engines and two solid-fuel rocket boosters. The liquid-fuel engines are supplied with fuel from a large external tank. The boosters and the liquid-fuel tank are jettisoned before the shuttle reaches orbit.

A launch vehicle travels straight up only for the first few seconds of its flight. To enter earth orbit, the satellite must be traveling parallel or nearly parallel to the earth's surface at the end of powered flight. Shortly after lift-off, therefore, the launch vehicle begins to tilt over, entering a long curved path that will bring it to orbital altitude in a nearly horizontal position.

In addition to carrying a satellite to a given altitude, a launch vehicle must impart to the satellite enough horizontal speed to keep it in orbit. The speed required depends on the satellite's altitude; the lower the satellite, the faster it must travel to stay in orbit. For example, a satellite must travel at 17,253 miles per hour (27,766 km/h) to stay in a circular orbit at an altitude of 200 miles (322 km) and it must travel 15,802 miles per hour (25,431 km/h) to stay in a circular orbit at an altitude of 1,000 miles (1,609 km). In the lower orbit the satellite will circle the earth in 90.96 minutes; in the higher orbit, in 118.41 minutes.

The space shuttle is designed to orbit the earth at a relatively low altitudetypically about 200 miles above the earth. Satellites carried into orbit by the space shuttle can simply be released into space from the shuttle's cargo bay. These satellites continue in the same general orbit and can be recovered by later shuttle missions to service them or return them to earth. A satellite intended for an orbit higher than that of the shuttle is equipped with a small single-stage or multistage rocket of its own. The satellite is ejected from the cargo bay by a spring mechanism. The satellite's rocket is then fired to propel the satellite into its proper orbit.

Virtually all United States satellites are launched from either Cape Canaveral on the eastern coast of Florida or from Vandenberg Air Force Base on the southern coast of California. The Cape Canaveral launch site is used for launching satellites into orbits that circle the earth above or nearly above the Equator. They are launched eastward, across the Atlantic Ocean. Launching a satellite eastward takes advantage of the speed imparted to it by the earth's rotation. At the latitude of the cape, this speed is 900 miles per hour (1,448 km/h). The Vandenberg launch site is used for launching satellites southward into polar or near-polar orbits. Most United States military satellites are launched from Vandenberg.

The Commonwealth of Independent States also has two major launch sites. Most of the commonwealth's satellites with general west-to-east orbits are launched from a site in Kazakhstan east of the Aral Sea. Most satellites placed in polar or near-polar orbits are launched from a site in Russia south of Archangel, a city near the White Sea.

Other launch sites include one built by France near Kourou, French Guiana, and a Japanese launch site on Tanega Island in southern Japan.



Once a satellite enters coasting flight in orbit, it behaves according to the same rules that govern natural satellites. The low point of an elliptical earth orbit is called perigee, the high point is apogee. In a lunar orbit, the low point is perilune, the high point is apolune. In a solar orbit, the corresponding points are perihelion and aphelion.

Earth Orbit

In general, satellites in an earth orbit with perigee of less than 200 miles stay in orbit for a month or less, many just for a day or so, because of the retarding effect of air present at that altitude. Satellites with perigees of more than 200 miles can remain in orbit for months or years. Various types of orbits are chosen for different missions. Some are circular; others are highly elongated.

To achieve a perfectly circular orbit directly, the satellite must be traveling exactly parallel to the earth's surface as it enters coasting flight. The speed must exactly match the circular orbital speed for the altitude of the satellite. A small deviation in either speed or direction will cause the satellite to enter an elliptical orbit; the greater the deviation, the more elongated the ellipse.

To make an orbit more circular, rocket engineseither in the final stage of the launch vehicle or built into the satellite itselfare fired to accelerate the satellite along a tangent to the orbit at apogee. This action brings the perigee to a higher altitude on the satellite's next pass. A circular orbit is achieved when the perigee is brought up to the altitude of the orbit's apogee.

To elongate an orbit, the rockets are fired when the satellite is at perigee; the apogee is raised without affecting the perigee, causing the satellite to travel in a more elongated ellipse. A circular orbit can be changed to an elongated one by accelerating the satellite at any point along the orbit; this point becomes the perigee.

If the satellite is accelerated to more than 6.95 miles per second (11.18 km/s), or about 25,000 miles per hour (40,200 km/hthe escape velocity of the earth), it will leave orbit around the earth and travel into space, never to return.

A satellite in a circular orbit at an altitude of approximately 22,240 miles (35,790 km) completes one orbit in the same time that the earth completes one rotation. Such a satellite is called a geosynchronous satellite. A geosynchronous satellite in orbit directly over the Equator appears to remain motionless in the sky and is referred to as a geostationary satellite.

Satellites designed to obtain images of cloud patterns or of the earth's surface are often placed in a polar or near-polar orbit. In this type of orbit the satellites can pass above virtually all areas of the earth. A polar or near-polar orbit can be chosen in which the satellite passes over any given area of the earth at the same local time every day. The advantage of such an orbit, called a sun-synchronous orbit, is that the shadows on the earth and the illumination of the clouds in all the photographs that the satellite obtains of a particular region are more or less constant.

Lunar or Planetary Orbits

Orbits around the moon and planets other than earth are difficult to achieve. Among the factors that must be accurately determined are the positions of the earth and the target body, the distance between them, and their relative motions. The strength of the gravitational attraction of the target body must be taken into account because it determines the orbital speeds around that body. A spacecraft approaching a body at a speed greater than the body's escape velocity will pass by without becoming a satellite unless sufficiently slowed at the proper time.

Solar Orbit

Some satellites have been placed in solar orbit to study solar phenomena. Others, such as probes sent to acquire data about Venus and Mars, have become solar satellites after passing near their target planets. The probes sent past Jupiter and Saturn, however, are not solar satellites; they have achieved a speed great enough to escape the gravitational pull of the sun.



Most satellites have an electrical system, a communications system, and a computer system. Some have a control system and a life-support system as well.

Electrical System

The active life of a satellite is determined by how long its electric power lasts because all of the other systems require electricity. Most satellites have a combination of storage batteries and solar cells to provide electricity. The satellite draws its power from the rechargeable storage batteries. The solar cells are photoelectric devices that convert sunlight into electric current, which is used to recharge the storage batteries.

Communications System

This system includes equipment to receive command signals from the earth, to emit radio signals used in tracking the satellite, and to transmit data from instruments carried by the satellite. The radio transmission of instrument readings is called telemetry. The telemetry conveys information not only from scientific instruments but also from instruments that measure voltages, temperatures, vibrations, and other indicators of how the satellite and its equipment are operating. Some types of satellites require additional communications equipment. For example, satellites that obtain images of the earth require equipment to convert optical images into electrical signals and transmit these signals to the earth as radio waves.

Computer System

On-board computers in satellites provide the commands by which the satellite carries out its mission. The computers are programmed prior to lift-off. Updated information is usually fed into them later through radio communication with ground-based computers. In addition, the computers may store data acquired during experiments, for later transmission to earth.

Control System

Some satellites can be maneuvered while in space. Common maneuvers include changing orbits and holding the satellite steady (keeping it from rolling or tumbling) with respect to a reference body such as the earth or the sun. The maneuvers are made with small computer-controlled rocket motors that can be turned on and off as needed. Included in the control system are the rocket motors themselves, fuel and oxidizer tanks, and pumps.

Some satellites are set spinning to keep them from tumbling; the axis of rotation tends to remain pointing in one direction. A spin-stabilized satellite's antennas are despunthat is, they are mounted on a motor-driven platform that keeps them directed toward the earth.



Tracking a satellite in space is a major part of any mission. Tracking involves determining the position of a satellite optically or by radio, either continuously or at regular intervals, and is necessary for maintaining radio communications with the satellite.

The tracking of satellites that no longer transmit radio signals can be used (regardless of the satellites' original purpose) to study the earth's gravity and the upper atmosphere. Variations in the strength of gravity from place to place and variations in the density of the atmosphere at very high altitudes cause changes in the orbits of satellites. When such orbital changes are observed, useful data can be acquired about the conditions that produced them.

During lunar or planetary flights, tracking determines the exact path of a spacecraft and the need for any corrections in flight.

The United States has several tracking networks, each for a different purpose. The networks include the Tracking and Data Relay Satellite System (TDRSS), the Deep Space Network, and the Space Surveillance Network (SSN). The first two networks are operated by the National Aeronautics and Space Administration (NASA); the third, by the U.S. Space Command, a joint military command.

The Tracking and Data Relay Satellite System

permits continuous tracking of and communication with spacecraft in relatively low orbits virtually anywhere around the earth. The system uses geostationary satellites called Tracking and Data Relay Satellites to provide a radio link between the spacecraft and a ground facility in New Mexico.

The Deep Space Network

is a radio link with spacecraft that travel beyond earth orbit. There are three main stations, set up around the earth so that contact can be maintained as the earth rotates. The three stations are located about 120 degrees of longitude apart at Goldstone, California; Madrid, Spain; and Canberra, Australia. These stations have large movable antennas that can receive information from the spacecraft and transmit instructions to start or stop experiments and make course corrections at great distances.

The Space Surveillance Network

is designed for air defense. It consists of a worldwide network of stations to track, identify, and catalogue all objects orbiting the earth. The information is relayed to the Space Surveillance Center of the U.S. Space Command. The center supplies the information to the North American Aerospace Defense Command (NORAD). Satellites in low orbits are usually tracked by radar. Satellites beyond radar range are tracked by specially designed telescopes that record images photographically on film or electronically by means of light-sensitive electronic devices.



Satellites are commonly classified, according to the purpose they serve, as scientific (or research), applications, or military satellites. These classifications overlap to some extent.

Scientific, or Research, Satellites

contain instruments designed to make observations and gather data for studies in geophysics, astronomy, and other scientific fields. Investigations of the earth's upper atmosphere, the Van Allen radiation belts, and levels of high-energy radiation in space were among the first investigations made by scientific satellites. Many kinds of astronomical observations that cannot be made from the surface of the earth can be made by satellites in orbit above the atmosphere. Of special importance have been satellites designed to detect and map celestial sources of various kinds of electromagnetic radiation, including ultraviolet radiation, infrared radiation, and X rays. Some satellites have been used to conduct biological experiments to determine how the conditions of space affect plants, insects, one-celled organisms, and other living things.

There have been several major series of American scientific satellites, including the Explorer, Orbiting Solar Observatory (OSO), Orbiting Geophysical Observatory (OGO), High Energy Astronomy Observatory (HEAO), and Great Observatories series.

Application Satellites

are designed for such practical purposes as weather forecasting and telecommunications. Weather satellites obtain images of cloud patterns. They also obtain information on sea surface and air temperatures. Weather satellites are especially useful in tracking hurricanes and other large storms. Since the advent of weather satellites, the average number of people killed by hurricanes in the United States each year has declined greatly. Some American weather satellites are geostationary; others circle the earth in polar orbits.

Communications satellites serve as relays for long-range communications. Telephone messages, television programs, and various types of data are transmitted to a communications satellite as high-frequency radio signals. The satellite strengthens the signals and retransmits them to earth, where they can be received over a wide area. Many communications satellites can routinely handle signals for thousands of telephone circuits simultaneously.

A global network of Intelsat satellites is used for international communications. Some communications satellites are intended for communications services within a single country; some for specific users, such as ships at sea.

Many communications satellites are placed in geostationary orbits. In such an orbit, the satellite is available for uninterrupted service and fixed antennas can be used for transmission and reception.

Other applications satellites include navigation satellites and the Landsats. A network of navigation satellites, such as the U.S. Navy's Transit satellites, makes it possible for a ship's navigator to determine the ship's position very accurately anywhere in the world. Landsats carry various types of sensors that provide detailed images of the earth's surface for information about land use and crop production. The images are also useful in locating mineral resources and in determining the extent of snow cover or flood damage.

Military satellites have many uses. Some military satellites, such as military communications and weather satellites, have functions similar to those of civilian satellites Other military satellites are used for reconnaissance. They carry cameras that provide extremely detailed images of the earth's surface, showing even relatively small features and objects. These satellites are useful in monitoring the deployment of military forces and in checking for the construction of new military facilities. Yet other military satellites are designed for detecting nuclear test explosions or for detecting and tracking ballistic missiles.



When Isaac Newton published his law of gravitation in 1687 the basic information necessary to conceive of artificial satellites became available. Space travel became a favorite subject in science fiction, but the subject of artificial satellites was almost ignored until the mid-20th century.

In 1955 the United States and the Soviet Union both announced plans to put scientific satellites into orbit during the International Geophysical Year (IGY), 1957-58.

The first artificial satellite placed in orbit was the Soviet Union's Sputnik I, launched on October 4, 1957. The satellite, a 23-inch (58-cm) sphere weighing 184 pounds (83 kg), remained in orbit for three months; radio contact with it, however, was lost after 21 days.

Sputnik II was launched on November 3, 1957. It weighed 1,121 pounds (508 kg) and carried the first animal into orbita dog named Laika. Radio contact was maintained for a week, during which time much information was acquired about the animal's adjustment to the conditions of space.

The United States IGY effort was Project Vanguard, conducted by the U.S. Navy and designed to put a small sphere into orbit. On December 6, 1957, the first Vanguard launch failed.

While attempts were being made to correct the Vanguard launch system, Explorer I was orbited by the U.S. Army on January 31, 1958, using a Jupiter-C launch vehicle. Explorer I was a cylinder 80 inches (203 cm) long and 6 inches (15 cm) in diameter; it weighed 31 pounds (14 kg). Its orbit had a perigee of 224 miles (360 km) and an apogee of 1,584 miles (2,549 km). Through Explorer I scientists discovered that an area of radiation encircles the earth. Explorer I remained aloft for more than a decade.

The first successful Vanguard, weighing 3 pounds (1.36 kg), was launched in March, 1958. The Vanguard program, however, continued to have trouble; of 11 attempts to launch Vanguards, only 3 were successful.

During the rest of the 1950's, satellite launching remained a highly difficult endeavor. In 1958 and 1959, the United States attempted to put up 36 satellites, of which only 18 reached orbit. During the same period, the Soviet Union successfully launched three satellites.

With the 1960's came rapid improvement in launch techniques. The number of launches rapidly increased, as did the number of countries taking part in satellite programs. Reliability of launch systems so improved that by 1964 more than 90 per cent of the satellites launched achieved orbit. The number of launchings varied from year to year. In 1962, for example, the United States attempted to launch 58 satellites, of which 50 achieved orbit. During the same year, the Soviet Union announced 20 launchings; 3 of them failed to achieve orbit.

The first nations other than the Soviet Union and the United States to orbit satellites were Great Britain, Canada, Italy, France, Australia, Japan, and China. Of these, only France, Japan, and China orbited their first satellites completely on their own; the others used American launch vehicles.

On April 26, 1962, Ariel I, a satellite launched as part of a joint program by the United States and Great Britain, was placed in orbit to make X-ray studies in the ionosphere. In September of the same year, Alouette I, the first Canadian satellite, was placed in orbit at about 600 miles (965 km) altitude, also for ionospheric research.

Italy joined the nations having satellites in orbit on December 15, 1964, with a 254-pound (115-kg) satellite, called San Marco I, launched from Wallops Island. On November 26, 1965, France placed a 92-pound (42-kg) satellite, called AI, into an orbit with perigee of 328 miles (528 km) and apogee of 1,099 miles (1,769 km). Australia, in November of 1967, launched its first satellite, WRESAT I, from facilities near Woomera, Australia, using a modified Redstone rocket. Japan's first successful launching was in February, 1970, and China's in April.

In 1960, the United States began programs of applications satellites. Tiros 1, the first weather satellite, went into orbit on April 1, and Echo 1, the first communications satellite, orbited on August 12. Echo 1 was a 100-foot (30-m) balloon made of a thin plastic film coated with aluminum. Unlike the communications satellites in use today, Echo was a passive communications satellitethat is, it simply reflected the radio signals transmitted to it.

Also in 1960, Transit 1B, the U.S. Navy's first navigation satellite, was placed in orbit. A second navigation satellite, Transit 2A, was one of two satellites placed in orbit with a single Thor-Able-Star rocket on June 22 the first multiple launch. By 1965, multiple launches were common in both the American and the Soviet space programs, and in that year 61 satellites were orbited using 18 launch vehicles.

Telstar I, an American Telephone and Telegraph communications satellite, relayed the first live television broadcasts between Europe and the United States in 1962. Satellite transmission of regular commercial television became possible about three years later, when Early Bird, a satellite owned by the Communications Satellite Corporation, was placed in geosynchronous orbit over the Atlantic Ocean. Early Bird was the first geosynchronous communications satellite.

Artificial satellites have been in orbit around the sun since the beginning of 1959, when the first Soviet lunar probe passed the moon and continued in orbit around the sun. However, it was not until spring of 1966 that the first satellite was placed in lunar orbit. Luna 10, the first Soviet lunar satellite, was launched on March 31, 1966, followed by the American Lunar Orbiter 1, in August of the same year.

More than 700 satellites were placed in orbit in the first decade of space exploration, and by the 1970's satellite launchings had become commonplace. In the 1970's two major new types of satellite were launched space stations (the Soviet Salyut 1 in 1971 and the American Skylab in 1973) and satellites designed to provide data about the earth's surface and resources (beginning with the American Landsat series in 1972). Beginning in the 1980's American space shuttles were used to launch satellites from orbit. They were also used as a base from which to repair satellites in orbit and as a vehicle to return satellites to earth.

In the 1990's, important new satellite applications included the use of Global Positioning System satellites for civilian navigation and a variety of uses for communications, such as Direct Broadcast Satellite television and broadband Internet connections. In 2000, the International Space Station received its first crew.