Artificial Satellite, any of the objects placed into orbit around the Earth for a variety of scientific and technological purposes. The first artificial satellite, Sputnik 1, was launched by the Soviet Union on October 4, 1957. The first United States satellite, Explorer 1, was launched on January 31, 1958, and was instrumental in the discovery of the Earth's radiation belts. In the years that followed, several thousand satellites were launched, mostly by the United States and the Union of Soviet Socialist Republics (USSR), until 1983, when the European Space Agency began launching from a space centre in French Guiana. On August 27, 1989, for the first time in aerospace history, a privately owned rocket was used to launch a satellite. The rocket, built and launched by a US company, placed a British television broadcasting satellite into geosynchronous orbit.

Satellites are used for both exploration and communication. Exploratory satellites are equipped with instruments to measure the density, temperature, and ionization of the upper atmosphere; cosmic radiation; the numbers and sizes of micrometeoroids; and the strength and direction of the geomagnetic field.

At the beginning of the 21st century there were more than 2,600 artificial satellites in orbit around the Earth, several hundred of which were in active operation. Many of these are communications satellites, used for telephone communication and the transmission of digital data and television images. Weather satellites photograph the Earth regularly in visible and infrared light, and they provide data to weather stations on Earth, thus permitting the forecasting of weather conditions around the world. Navigation satellites allow positions at sea to be determined with an error of as little as 10 m (33ft), and they also aid navigation by locating ice and mapping ocean currents. SARSAT (Search and Rescue Satellite System) monitors distress calls from ships and aircraft by means of a network of three American satellites (NOAA-9, 10, 11) and two that were launched by the former Soviet Union.

Astronomical instruments placed aboard satellites are used to make observations that are impossible to make from Earth because of the absorption of radiation in the atmosphere. A large number of X-ray sources have been discovered, using X-ray detectors and telescopes. Observations of ultraviolet radiation and the detection of gamma rays emitted by celestial objects are also possible. In 1983, with the infrared astronomy satellite IRAS, astronomers made their first detailed observations of the core of our galaxy.

Artificial satellites are powered by solar cells (see Photoelectric Cell), by batteries that often are charged by solar cells, and in many cases by nuclear generators, in which heat produced by the decay of radioisotopes is converted into electricity. Satellites are equipped with radio transmitters for transmitting data from on-board instruments (see Telemetry), with radio receivers and electronic circuits for storing data, and with control equipment such as radar and star-tracking systems.

Satellites are placed into orbit by multi-stage rockets. To reduce launching costs, the US National Aeronautics and Space Administration (NASA) has also developed the space shuttle (see Space Exploration), which can carry satellites in its cargo bay and launch them into orbit. It can also retrieve satellites from their orbits and repair them for re-release or bring them back to Earth for further work—a capability first proved during shuttle missions in 1984.

See also Remote Sensing.