What Are Satellites?

Satellites are objects that orbit a larger body under gravitational force. In engineering and telecommunications, the term nearly always refers to artificial satellites: spacecraft placed in orbit by rockets or other launch vehicles to perform missions such as communications relay, Earth observation, navigation, weather monitoring, and scientific research. An artificial satellite differs from a crewed spacecraft in that it operates autonomously under remote control, though the distinction blurs for crewed stations that also carry instruments. The first artificial satellite, Sputnik 1, was launched by the Soviet Union on October 4, 1957, marking the beginning of the space age and the engineering discipline of satellite design.

More than 10,000 active and defunct artificial satellites orbit Earth as of 2025, a number that has grown sharply with the deployment of large commercial constellations in low Earth orbit. Satellites are now foundational infrastructure for global communications, navigation, weather forecasting, and environmental monitoring.

Orbit Classification

The orbit of a satellite determines virtually every aspect of its mission: the area it can see, how long it remains in view of a given ground station, and the radiation and thermal environment it must survive. Geostationary orbit (GEO) at 35,786 km altitude is used for communications and weather satellites because a satellite there matches Earth's rotation and appears fixed in the sky to ground-based antennas. Low Earth orbit (LEO), below 2,000 km, is used for Earth observation, crewed stations, and the new generation of broadband constellations, offering shorter propagation delays and higher spatial resolution from imaging sensors. Medium Earth orbit (MEO), between roughly 2,000 and 35,000 km, is the home of global navigation satellite systems such as GPS and Galileo. The ESA technical overview of orbit types describes sun-synchronous, highly elliptical, and transfer orbits used for specific mission requirements beyond these three main categories.

Spacecraft Bus and Payload Design

Every satellite consists of two major parts: the bus and the payload. The bus provides the supporting infrastructure: structural panels that withstand launch loads, solar arrays and batteries that supply power throughout the mission, a thermal control system of heaters and radiators that keeps components within operating temperature ranges, attitude determination and control systems that orient the satellite using reaction wheels and thrusters, and a propulsion system for orbit raising and station-keeping. The payload carries out the actual mission: transponders for communications, cameras and spectrometers for Earth observation, signal generators and atomic clocks for navigation, or scientific instruments for space physics. NASA's small satellite technology documentation covers how standardized bus platforms, from the 1-unit CubeSat to 500-kg smallsat buses, have lowered development costs and enabled new classes of commercial and scientific operators to access orbit.

Deployment and Operational Lifecycle

A satellite's life begins with launch vehicle integration and culminates in controlled deorbit or graveyard-orbit disposal. After launch, the satellite undergoes an early orbit phase during which controllers verify all subsystems, deploy solar arrays and antennas, and perform orbit-raising maneuvers. On-orbit testing confirms that the payload meets specification before the satellite enters commercial or operational service. Stations in GEO are maintained within a tight longitude box using north-south and east-west station-keeping maneuvers that consume most of the on-board propellant. When propellant runs low, the satellite is maneuvered to a graveyard orbit above the geostationary arc to prevent it from interfering with operational satellites. LEO satellites at altitudes below about 600 km naturally deorbit due to atmospheric drag within a few years, while those in higher LEO orbits must perform active deorbit maneuvers to comply with the ITU's orbital debris mitigation guidelines.

Applications

Satellites have applications in a wide range of fields, including:

• Global and regional telecommunications, broadband internet, and broadcast television
• Earth observation for climate monitoring, agriculture, disaster management, and mapping
• Navigation and precise timing through GNSS constellations
• Meteorology and severe weather forecasting from geostationary and polar platforms
• Military surveillance, early warning, and secure communications
• Scientific research including astronomy, planetary science, and geodesy