Satellite

Artificial satellites are human-made objects placed into orbit around astronomical bodies for scientific, commercial, military, and navigational purposes. Since the launch of the first satellite in 1957, satellites have become indispensable tools in global communication, Earth observation, meteorology, astronomy, and space science. Their versatility and increasing numbers have transformed both technological capability and society’s relationship with space, while also introducing new challenges such as orbital congestion and space debris.

Definition, Design, and Function

An artificial satellite is typically a spacecraft intentionally positioned into orbit around an astronomical body, most commonly Earth. Satellites serve a broad range of civil and military functions, including communication relays, weather monitoring, global navigation, scientific observation, and reconnaissance. Specialised satellites also support early warning systems, signals intelligence, and experimental or defence-related missions. Some objects in orbit, such as final rocket stages and inactive satellites, also function as unintentional satellites once their operational life ends.
Most operational satellites are equipped with a power generation system, usually solar panels or radioisotope thermoelectric generators, to support on-board instruments. They also contain communication systems called transponders, which relay and amplify signals between ground stations and users. Standardised designs known as satellite buses have become increasingly common, allowing multiple satellites to be built on similar platforms. CubeSats and other small satellites exemplify this shift towards modularity and cost efficiency. Satellites may operate individually or as part of satellite constellations, which provide coordinated coverage of the Earth.
To enter orbit, satellites must attain sufficient orbital velocity through launch systems. Once in space, they use propulsion systems including chemical thrusters or ion engines to maintain or adjust their orbits. The choice of orbit depends on the mission. Low Earth orbit and geostationary orbit are the most commonly used; the latter keeps the satellite fixed above a single point on Earth. Sun-synchronous orbits provide consistent lighting conditions for Earth observation missions.
As the number of satellites has increased, so has the risk of collision and the accumulation of space debris. Orbital traffic management and debris mitigation have therefore become vital to long-term sustainability of satellite operations.
Crewed spacecraft that remain in orbit, including space stations, are also classified as artificial satellites. A spacecraft that travels from another location and enters into orbit around a body becomes an orbiter, and thus an artificial satellite. Some satellites orbit the Moon, Mars, or even the Sun, while specialised trajectories such as halo or Lissajous orbits allow satellites to position themselves with respect to gravitational equilibrium points.

Uses and Applications

Artificial satellites support numerous civilian and scientific activities. Earth observation satellites collect data on weather patterns, ocean conditions, forests, and urban development. Reconnaissance satellites provide imagery and intelligence for national security. Space telescopes exploit the low-interference environment of space to observe across the electromagnetic spectrum.
Communication satellites relay data, radio, television broadcasts, and internet services. Navigation satellites such as those in the GPS, Galileo, and other global navigation systems use predictable orbital paths and precise timing signals to determine positions on Earth.

Early Proposals and Theoretical Foundations

The conceptual origin of artificial satellites dates to Isaac Newton’s 1687 cannonball thought experiment, which illustrated the orbital motion of natural satellites. Early fictional and speculative treatments appeared in the nineteenth century, including Edward Everett Hale’s The Brick Moon (1869) and Jules Verne’s The Begum’s Fortune (1879).
By the early twentieth century, a scientific basis for satellites had begun to emerge. Konstantin Tsiolkovsky’s 1903 treatise Exploring Space Using Jet Propulsion Devices calculated minimum orbital velocities and argued for multistage liquid rockets. Hermann Potočnik’s 1928 work The Problem of Space Travel outlined scientific uses of orbiting spacecraft and examined geostationary concepts first noted by Tsiolkovsky. In 1945 Arthur C. Clarke described the use of geostationary satellites for global telecommunications, predicting networks of satellites that could broadcast worldwide.
During the 1940s and early 1950s, numerous studies by the United States Air Force and the RAND Corporation examined the scientific potential of satellites. Proposals considered space-based telescopes, global observation systems, and experimental vehicles to gather atmospheric and environmental data.

First Satellites and the Space Race

The Soviet Union launched the world’s first artificial satellite, Sputnik 1, on 4 October 1957. The satellite provided information on upper atmospheric density through orbital decay patterns and studied radio propagation in the ionosphere. Its success surprised the United States and triggered the Sputnik crisis, intensifying the Cold War competition in space.
Sputnik 2, launched on 3 November 1957, carried the first living passenger, a dog named Laika, marking an important but ethically contentious milestone.
The United States responded by accelerating Project Vanguard while pursuing a parallel effort, Project Orbiter. Explorer 1, launched on 31 January 1958 using a Jupiter-C rocket, became the first American satellite. Data returned from Explorer 1 led to the discovery of the Van Allen radiation belts, a major scientific breakthrough.
In 1960 NASA’s TIROS-1 transmitted the first satellite images of weather systems, initiating modern meteorology. By June 1961 the United States had catalogued 115 satellites in orbit.
Many nations’ first satellites were launched on foreign rockets, including those of Canada, Australia, and Italy. France became the third country to deploy a satellite using its own launch vehicle when the Astraix satellite was launched by a Diamant rocket in 1965.

Later Satellite Development and Standardisation

Early satellites were designed individually, each as a distinct engineering project. As satellite technology advanced, manufacturers adopted standardised satellite buses to simplify production and reduce costs. The HS-333 geostationary communications satellite, launched in 1972, is recognised as the first major standardised bus. Standardisation facilitated the mass production of satellites and supported the emergence of large-scale satellite constellations in later decades.
Today, thousands of satellites operate in Earth orbit. As of 31 December 2022, there were 6,718 operational satellites, with the United States owning the largest share. Their functions span telecommunications, global navigation, scientific discovery, defence, and commercial services.