What a satellite is
A satellite is any object that orbits a larger one. The Moon is a natural satellite of Earth. The ones people usually mean are artificial: machines built by people, launched on rockets and placed into orbit. Each one is a collection of fairly ordinary parts: a structure, solar panels and batteries for power, radios for talking to the ground, computers, and a payload such as a camera, antenna or scientific instrument. Many also carry small thrusters to adjust their path.
Why a satellite stays up
A common misconception is that satellites float above gravity. They do not. At the height of the International Space Station, roughly 400 kilometers up, gravity is still strong, only slightly weaker than at the surface. A satellite stays up because it is moving sideways very fast, at roughly 7.8 kilometers per second in low orbits.
Picture throwing a ball horizontally. It curves down and lands. Throw it harder and it lands farther away. Throw it fast enough and, because the Earth’s surface curves away beneath it, it keeps falling without ever reaching the ground. That endless fall is an orbit. A rocket’s main job is less about going up and more about getting the payload moving sideways fast enough.
Higher orbits are slower and take longer to circle the planet. A satellite near 400 kilometers takes about 90 minutes per lap, while one at about 35,786 kilometers takes roughly a day.
The main kinds of orbit
Altitude shapes what a satellite can do, so engineers choose it carefully.
- Low Earth orbit (LEO) is roughly 160 to 2,000 kilometers up. It is close to the ground, which helps with sharp images and fast radio signals. A single satellite sees only a small patch at a time and passes over quickly, so covering a region continuously takes many satellites.
- Medium Earth orbit (MEO) lies between LEO and geostationary height. Navigation systems often use it; GPS satellites orbit at about 20,200 kilometers.
- Geostationary orbit (GEO) is about 35,786 kilometers above the equator. There a satellite circles in step with the Earth’s rotation, so it appears fixed in the sky. A ground dish can point at it and stay put. A single satellite can see about a third of the planet, but the distance adds delay to signals.
- Polar and sun-synchronous orbits are LEO paths that pass near the poles. As the Earth turns underneath, the satellite eventually sees nearly the whole surface, often at the same local solar time on each pass, which keeps lighting consistent for imaging.
How a satellite is built and run
Most satellites are designed around the job of the payload. A communications satellite is mostly antennas and amplifiers, an imaging satellite is a telescope and sensors, and a navigation satellite is largely an extremely accurate clock plus a transmitter. The rest, called the bus, keeps the payload alive: it supplies power, controls temperature, points the satellite in the right direction and handles commands.
Space is a harsh place for electronics. There is no air to carry heat away, so parts can overheat in sunlight and become very cold in shadow. Radiation can disturb or damage circuits. Because repairs are rarely possible, satellites are tested heavily before launch, and critical systems often have backups.
A satellite is also not left alone. Ground stations send commands and receive measurements, and operators monitor the health of the spacecraft. Over time, small forces such as thin air at low altitude, sunlight pressure and the gravity of the Moon and Sun nudge a satellite off course, so it uses small thrusters to hold its position. When the fuel runs low, the useful life is usually near its end.
What satellites do for people on the ground
Communication. Satellites relay television, phone calls, data and emergency messages, especially where cables and towers are impractical, such as at sea, in the air and in remote regions. Modern constellations also offer broadband, a topic covered in more detail under satellite internet. Most traffic between continents still travels through undersea cables, with satellites filling gaps and special cases.
Navigation and timing. Satellite navigation systems broadcast precise time signals that receivers turn into positions. The same timing signals also help synchronize clocks used by financial systems, power grids and mobile networks, often without anyone noticing. Satellite navigation is covered in its own article.
Earth observation. Cameras and sensors watch weather, oceans, forests, farmland and cities, which supports forecasting, disaster response and long-term climate records. Earth observation is a large field of its own.
Science. Instruments above the atmosphere can see wavelengths that never reach the ground, and space telescopes use this advantage to study distant objects.
Security and other uses. Governments also operate satellites for defense and intelligence. Details are mostly not public, so this article does not go into them.
Costs, limits and risks
Satellites are expensive to build and launch, and hard to fix once in orbit. Their lifetimes are limited, typically from a few years to a couple of decades depending on the mission and orbit, because fuel, batteries and electronics wear out. Reusable rockets are one way the industry is trying to reduce launch costs, though how far savings go is still being worked out.
Crowding is a growing concern. Dead satellites, spent rocket stages and fragments add to space debris, which can damage working spacecraft at orbital speeds. Large constellations also raise questions about radio spectrum, astronomy and orderly traffic management.
Satellites also depend on ground systems: control centers, antennas and data processing, including cloud computing for the large volume of images and measurements they produce.
Looking ahead
Launch costs, miniaturized electronics and demand for data are changing the field. Smaller satellites built in larger numbers are now common alongside traditional large ones, and ideas such as asteroid mining and Mars settlement attract attention, although they remain far from routine. Predictions about how fast any of this will develop have often proved too optimistic, so it is wise to treat forecasts as possibilities.
Why it matters
Most people use satellite services every day without noticing: weather forecasts, map directions, card payments timed by satellite clocks, and live broadcasts. Satellites are less a distant space topic than infrastructure, much like roads or power lines, only located overhead. Understanding orbits, and why a satellite’s altitude shapes what it can do, is the key to understanding almost everything else in the field.