A satellite is any object that orbits a larger object in space. Earth is a satellite of the sun, and the moon is a satellite of Earth. But when most people say “satellite,” they mean the thousands of machines humans have launched into orbit to handle everything from GPS navigation to weather forecasting. More than 7,560 of these artificial satellites are currently operating in orbit around Earth, and the number grows every year.
Natural Satellites in Our Solar System
Every moon in the solar system is a natural satellite. The International Astronomical Union lists 146 moons orbiting the planets, not counting those awaiting official recognition or the smaller moons circling dwarf planets and asteroids. Jupiter alone has dozens, including Ganymede, the largest moon in the solar system at roughly 5,260 kilometers across. That makes it bigger than the planet Mercury. Saturn’s moon Titan, the second largest, is the only moon known to have a thick atmosphere.
Earth’s own moon is the natural satellite most of us think about first, and it follows the same basic physics as every artificial satellite we’ve ever launched: it stays in orbit because its forward speed perfectly balances the gravitational pull trying to drag it inward.
How Satellites Stay in Orbit
A satellite doesn’t float in space. It’s constantly falling toward Earth but moving forward fast enough that the planet’s surface curves away beneath it at the same rate. The result is a continuous loop around the planet. A satellite closer to Earth feels a stronger gravitational pull and needs to travel faster to avoid being dragged down. One farther away can move more slowly and still maintain its orbit.
In low Earth orbit, satellites travel at roughly 7.8 kilometers per second, completing a full lap around the planet in about 90 minutes. At geostationary altitude, nearly 36,000 kilometers up, the required speed drops to about 3 kilometers per second, and one orbit takes almost exactly 24 hours. That speed match with Earth’s rotation is what makes certain satellites appear to hover over a single spot on the ground.
Three Main Orbital Zones
Satellites occupy different altitudes depending on their job, and those altitudes fall into three broad categories.
Low Earth orbit (LEO) ranges from about 180 kilometers up to 2,000 kilometers. The lower boundary exists because any closer and atmospheric drag would pull a satellite down quickly. LEO is where you’ll find the International Space Station, Earth-imaging satellites, and the large constellations that provide broadband internet. The closeness to Earth’s surface allows for high-resolution photography and lower communication delays, but a single satellite in LEO can only “see” a small slice of the planet at any moment. That’s why companies like SpaceX deploy hundreds or thousands of satellites in coordinated networks to maintain continuous coverage.
Medium Earth orbit (MEO) sits above LEO and below geostationary altitude. Navigation systems live here: the U.S. GPS constellation, Europe’s Galileo system, and Russia’s GLONASS all orbit at medium altitudes. This positioning gives each satellite a wider view of Earth’s surface while keeping signal travel times short enough for precise location measurements.
Geostationary orbit (GEO) is a narrow ring 35,786 kilometers above the equator. Because a satellite here matches Earth’s rotation exactly, a ground antenna can point at one fixed spot in the sky and stay connected around the clock. This makes GEO the go-to altitude for telecommunications satellites and weather satellites that need to continuously monitor one region to track developing storms.
What’s Inside an Artificial Satellite
Every satellite, whether it’s the size of a shoebox or a school bus, has two fundamental parts: a bus and a payload. The bus is the satellite’s body and support system. It contains the computer processors that run the spacecraft, the power supply (usually solar panels that unfold after launch), thermal controls to manage extreme temperature swings, and the communications antennas that link the satellite to the ground. The payload is the part that actually does the mission, whether that’s a camera, a radar instrument, a signal relay, or a scientific sensor.
Thermal management is one of the trickier engineering challenges. A satellite can swing from blistering sunlight to deep shadow every 90 minutes in low orbit. Heat pipes, louvers, and specialized coatings keep internal components within a safe temperature range. Attitude control systems, essentially small thrusters or spinning wheels, keep the satellite pointed in the right direction so its antennas face Earth and its solar panels face the sun.
How Satellites Talk to the Ground
Satellites communicate with Earth through a network of ground stations, each equipped with large directional antennas. When a satellite passes over a station, the antenna tracks its position across the sky. The satellite sends data down as radio waves. The ground station’s antenna collects those waves, converts them to electrical signals, and feeds them through a radio that translates the signal into usable digital data.
Commands travel the opposite direction. A mission operations center sends instructions to the ground station, which modulates the data onto a radio signal, amplifies it, and beams it up through the antenna. The satellite’s onboard software unpacks the commands and executes them. For satellites in low orbit, which are only visible to any single ground station for a few minutes per pass, operators rely on a global network of stations to maintain contact.
What Satellites Do for Daily Life
GPS is probably the most visible example. GPS receivers are now embedded in nearly every cell phone, and high-precision versions guide farming equipment, construction machinery, surveying tools, and snow removal fleets. Multiple satellite navigation systems orbit Earth simultaneously, giving users worldwide access to precise location data.
Weather forecasting depends heavily on satellites in geostationary orbit, which can watch a storm system develop in real time, and polar-orbiting satellites in LEO that scan the entire planet twice a day. Communications satellites relay television broadcasts, phone calls, and internet connections, particularly to remote areas where ground-based infrastructure is impractical. Earth observation satellites track deforestation, ice melt, crop health, and urban expansion. Scientific satellites study everything from cosmic radiation to the gravitational field of the planet itself.
The First Artificial Satellite
The space age began on October 4, 1957, when the Soviet Union launched Sputnik I. It was about the size of a beach ball, 58 centimeters in diameter, and weighed just 83.6 kilograms. It orbited Earth on an elliptical path, completing one lap roughly every 98 minutes, and transmitted a simple radio pulse that anyone with a shortwave receiver could pick up. Sputnik worked for only a few weeks before its batteries died, but it triggered the space race and, within a decade, led to the satellite infrastructure the modern world now depends on.
Space Debris and Satellite Retirement
With thousands of active satellites and many more defunct ones still circling the planet, orbital debris is a growing concern. Regulations now require that satellites in low Earth orbit be removed within a set timeframe after their mission ends. The traditional guideline allowed up to 25 years, but in 2022 the FCC shortened that window to just 5 years for commercially licensed satellites below 2,000 kilometers.
Most satellites are retired through atmospheric reentry. They use remaining fuel to lower their orbit until drag pulls them in, and they burn up on the way down. For any satellite too large to fully disintegrate, operators must design a reentry trajectory that ensures debris lands in unpopulated ocean areas. Satellites in geostationary orbit, where atmospheric reentry isn’t practical, are instead boosted into a “graveyard orbit” a few hundred kilometers above the active zone, clearing the way for working spacecraft below.