What is a satellite? A satellite is an object that orbits a larger body in space. The Moon is Earth’s natural satellite, and Earth is a satellite of the Sun. In everyday use, however, ‘satellite’ often means a human-made spacecraft placed in orbit to provide services or collect information. Artificial satellites support communications, navigation, weather forecasting, Earth observation, scientific research and space exploration.
Satellites are not all parked in the same orbit or used for the same purpose. Some travel relatively close to Earth and pass overhead several times a day; others orbit at high altitude and appear nearly fixed above one region. The mission determines the orbit, instruments, power system, communications equipment and operating life.
Quick notes: A satellite remains in orbit because its sideways velocity and gravity combine to create continuous free-fall around the planet. Low Earth orbit (LEO) is used for many imaging and broadband constellations. Medium Earth orbit (MEO) is used by navigation systems such as GPS. Geostationary orbit (GEO) lies above the equator at an altitude of about 35,786 km and matches Earth’s rotation period. Natural and artificial satellites are both satellites.
Natural satellites versus artificial satellites
A natural satellite forms through natural astronomical processes and orbits a planet or other body. Earth’s Moon is a familiar example. Artificial satellites are objects built by people and launched into space to perform a task. The task may be observing Earth, relaying radio signals, measuring the space environment, supporting navigation or exploring another world.
The International Space Station is an artificial satellite of Earth, even though it is crewed and often described as a space station. A spacecraft travelling between planets may not be in orbit around Earth for much of its journey, but once it enters orbit around another body it acts as an artificial satellite of that body.
How does a satellite stay in orbit?
A satellite is moving sideways while Earth’s gravity continually changes the direction of its motion. It is therefore in continuous free fall around Earth. If its sideways speed were too low for its altitude, it could follow a path that intersects the atmosphere or surface. If its speed and direction were sufficiently high, it could move into a different orbit or escape Earth’s gravitational influence.
Orbit is not the absence of gravity. Gravity is essential to the orbit. The satellite keeps missing the ground because it travels forward as it falls. The exact orbit depends on the satellite’s speed, direction, altitude and the gravity field, as well as perturbations from Earth’s shape, the Moon, the Sun and atmospheric drag at lower altitudes.
The main types of Earth orbit
Low Earth orbit (LEO)
LEO is relatively close to Earth, commonly below about 2,000 km altitude. Satellites here travel quickly and circle Earth in roughly an hour and a half to a couple of hours depending on altitude. Many Earth-imaging spacecraft, crewed missions and broadband constellations use LEO. A satellite in LEO does not remain above one location; ground users may need a moving network of satellites or handovers between them.
Medium Earth orbit (MEO)
MEO lies between LEO and GEO. Navigation satellites such as those used by GPS operate in medium Earth orbit. Because these satellites are higher, each can cover a broader region and their orbital periods are longer than those in LEO. A navigation receiver uses signals from multiple satellites and timing information to estimate its position.
Geostationary orbit (GEO)
A geostationary satellite orbits above Earth’s equator at the same angular speed as Earth’s rotation, in the same direction. It therefore appears to remain over nearly the same point on the equator. This is useful for communications, weather monitoring and broadcast services because ground antennas can point toward a stable position in the sky.
GEO is high above Earth, so signals travel a long distance and incur noticeable propagation delay. It is also not ideal for every high-latitude region because the satellite appears low on the horizon or may be blocked by terrain.
Polar and sun-synchronous orbits
A polar orbit takes a satellite over or near the poles. As Earth rotates beneath the orbit, the satellite can observe different strips of the planet on successive passes. Sun-synchronous orbits are designed so that the satellite passes over a location at roughly the same local solar time, helping scientists compare observations under similar lighting conditions.
What are satellites used for?
- Communications: relay television, radio, internet, telephone and data signals across large areas, including remote regions and oceans.
- Navigation and timing: provide positioning, navigation and precise timing signals used by phones, transport, mapping, surveying and infrastructure.
- Weather forecasting: observe clouds, storms, water vapour, temperature patterns and other atmospheric features.
- Earth observation: measure land cover, vegetation, fires, floods, oceans, ice, atmospheric gases and environmental change.
- Science and astronomy: observe the Sun, planets, stars and distant objects from vantage points that avoid some atmospheric interference.
- Defence and security: support communications, navigation, surveillance and other government missions under applicable laws and controls.
How a communications satellite works
A ground station or user terminal sends a radio signal upward to the satellite. The satellite’s communications payload receives the signal, processes or frequency-converts it and transmits it back toward another ground station or service area. The path from Earth to the satellite is called an uplink; the return path is a downlink. Some satellites also communicate with other satellites through crosslinks.
A satellite does not create internet access by itself. The system also needs user terminals, ground gateways, network routing, spectrum rights, backhaul, service providers and power. Weather, frequency band, antenna size, congestion, orbit and network architecture affect performance.
How Earth-observation satellites collect information
Earth-observation satellites carry sensors that measure reflected or emitted energy from land, ocean and atmosphere. Some sensors capture visible images; others measure infrared, microwave or other parts of the electromagnetic spectrum. Different wavelengths reveal different properties, such as vegetation health, surface temperature, moisture, clouds or ocean conditions.
The image seen by the public is often a processed product rather than a simple camera photograph. Scientists calibrate sensor readings, correct geometry, remove some atmospheric effects and combine data into maps or time series. A single satellite image shows conditions at a particular time and resolution; it does not automatically prove why a change occurred.
How satellites support weather and climate science
Weather satellites help meteorologists monitor cloud systems, tropical cyclones, water vapour and atmospheric temperatures. Geostationary satellites provide frequent views of the same broad region, while polar-orbiting satellites collect global measurements as they pass over different latitudes.
Climate science uses long, carefully calibrated records to study changes over time. Satellites contribute measurements of sea level, ice, land cover, clouds, radiation and atmospheric composition. A climate conclusion should normally use multiple data sources and account for changes in instruments, coverage and measurement methods.
What are the main parts of a satellite?
- Payload: the instruments or communications equipment that perform the mission.
- Power system: solar panels, batteries and power electronics supply and manage electricity.
- Communications system: antennas and radio or optical equipment receive commands and send data.
- Attitude and orbit control: sensors, reaction wheels, thrusters and control software help point and position the spacecraft.
- Structure and thermal control: the spacecraft body supports components and helps manage the extreme temperature changes in space.
- On-board computer: handles commands, data, autonomy and spacecraft health monitoring.
What happens when a satellite reaches the end of its life?
Satellites have finite power, fuel, components and operating lifetimes. Some become inactive because their systems fail or they run out of fuel. A responsible mission plans for disposal or passivation where possible. Depending on orbit and design, a satellite may be guided to re-enter the atmosphere, moved to a disposal orbit or otherwise placed in a safer end-of-mission condition.
Inactive spacecraft and fragments can become space debris. Debris increases collision risk, and a collision can create more fragments. Space agencies and operators use tracking, collision-avoidance procedures and debris-mitigation practices, but the orbital environment remains a shared resource that requires cooperation.
Common satellite misconceptions
- ‘Satellites float because there is no gravity.’ Gravity is what keeps them in orbit.
- ‘Every satellite is geostationary.’ Many satellites move across the sky in LEO or other orbits.
- ‘A satellite image is always a normal photograph.’ Many instruments measure wavelengths that human eyes cannot see, and data are processed into products.
- ‘GPS works with one satellite.’ A receiver uses signals from multiple satellites to estimate its position and timing.
- ‘Satellites provide service on their own.’ Ground stations, user equipment, networks and operators are also needed.
Frequently asked questions about satellites
What is a satellite in simple words?
A satellite is an object that orbits a larger body. It may be natural, such as the Moon, or artificial, such as a communications spacecraft.
What is the difference between LEO and GEO?
LEO is close to Earth and satellites move quickly across the sky. GEO is much higher above the equator and a satellite appears fixed over one location.
How do satellites send information to Earth?
They use radio or other communication links to transmit signals to ground stations or user terminals, which then process and route the data.
Do satellites need fuel to stay in orbit?
A satellite does not need continuous thrust to maintain an ideal orbit, but real spacecraft may use fuel for orbit adjustments, collision avoidance, attitude control and end-of-life disposal.
Can satellites see through clouds?
Visible and many infrared sensors are affected by clouds, while some radar and microwave systems can observe through cloud cover under suitable conditions.
Why are there so many satellites?
Different missions require different coverage, revisit times, frequencies and orbital positions. Some services use constellations so that coverage is available more frequently or across more locations.
Continue learning at eduKateSG
For the gravity that keeps satellites in orbit, read What Is Gravity?. For the wider neighbourhood around our planet, continue with What Is the Solar System? and the Remote Sensing and Earth Observation guide.
Sources and further reading
NASA’s What Is a Satellite? explains natural and artificial satellites, major orbits and everyday uses. NASA’s satellite overview describes spacecraft functions and communications. The Canadian Space Agency’s What Is a Satellite? provides a further introduction. Orbital classifications and mission details vary by satellite; current mission information should be checked with the responsible operator.
Continue reading: Browse the World and Knowledge Hub to choose another related guide from the 14 November 2026 reading list.