What Is a Satellite?

A satellite is a spacecraft placed in orbit around a celestial body to perform scientific, commercial, or military functions using its vantage point above the atmosphere. Artificial satellites are designed and launched by humans, as distinguished from natural satellites such as the Moon. Since the launch of Sputnik 1 by the Soviet Union in October 1957, satellites have become foundational infrastructure for global communications, Earth observation, navigation, weather forecasting, and scientific research. The field sits at the intersection of aerospace engineering, electrical engineering, and orbital mechanics.

Satellites range from shoebox-sized CubeSats weighing less than one kilogram to large geostationary communication platforms exceeding six tonnes. All share a common functional architecture: a structural bus housing the power, attitude control, telemetry, and propulsion subsystems, and one or more mission payloads that carry out the spacecraft's intended function. The operating environment in space subjects satellites to vacuum, extreme thermal cycling, ionizing radiation, and micrometeorite flux, demanding materials and designs very different from terrestrial electronics.

Orbital Regimes

Satellites are classified by their orbital altitude and inclination, each regime offering distinct coverage and latency characteristics. Low Earth orbit (LEO) extends from roughly 200 to 2,000 kilometers altitude; satellites here complete an orbit every 90 to 120 minutes, are visible from any ground station for only minutes per pass, but offer low round-trip signal latency of roughly 5 to 20 milliseconds. Medium Earth orbit (MEO), at 2,000 to 35,786 kilometers, is used primarily by navigation constellation satellites such as GPS (at 20,200 km), Galileo, and GLONASS. Geostationary Earth orbit (GEO) at approximately 35,786 kilometers places a satellite above the equator with an orbital period matching Earth's rotation, giving it the appearance of a fixed point in the sky from the ground; this makes GEO ideal for broadcast and wide-area communication, though at the cost of roughly 240-millisecond one-way propagation delay. The IEEE LEO Satellites and Systems (SatS) initiative focuses technical community development on LEO mega-constellations, which deploy hundreds to thousands of satellites to provide global broadband coverage.

Spacecraft Subsystems

Every satellite depends on a core set of engineering subsystems regardless of its mission. The electrical power system generates electricity from solar panels and stores it in batteries to survive orbital eclipse periods. The attitude determination and control system (ADCS) uses star trackers, sun sensors, magnetometers, reaction wheels, and thrusters to orient the spacecraft so that antennas point toward the ground and solar panels face the sun. The command and data handling (C&DH) subsystem runs the onboard computer, manages data storage, and executes commands uploaded from ground stations. The communications subsystem provides the telemetry, tracking, and control (TT&C) link as well as the mission downlink. Thermal control relies on passive techniques, coatings, multilayer insulation blankets, and active heaters to maintain electronics within operating temperature ranges across the orbital cycle. These subsystems interact closely with the payload and are described in depth in the NASA Jet Propulsion Laboratory space mission design resources.

Mission Payloads

The payload defines the satellite's purpose. Communication satellites carry transponders that receive, amplify, and retransmit signals between ground terminals; modern high-throughput satellites (HTS) use frequency reuse across many narrow spot beams to achieve hundreds of gigabits per second of aggregate throughput. Earth observation satellites carry optical cameras, synthetic aperture radar (SAR), or multispectral and hyperspectral imagers for land cover mapping, agricultural monitoring, and disaster response. Scientific satellites host instruments ranging from gamma-ray detectors to magnetometers and lidar systems. Research published in IEEE Xplore on LEO satellite constellations for remote sensing illustrates how onboard AI processing now enables real-time scene analysis without waiting for ground downlink.

Applications

Satellites have applications in a range of fields, including:

• Global telecommunications including broadband internet, telephony, and television broadcasting
• GPS and GNSS navigation for aviation, maritime, automotive, and personal positioning
• Earth observation for agriculture, forestry, urban planning, and climate monitoring
• Weather forecasting using geostationary and polar-orbiting meteorological satellites
• Military reconnaissance, signals intelligence, and missile early warning
• Scientific research including space physics, astronomy, and geodesy