A satellite is any object that orbits another object in space. The Moon is a satellite of Earth, Earth is a satellite of the Sun, and the thousands of machines circling our planet right now are satellites too. The word covers both natural bodies held in orbit by gravity and the artificial machines we launch to handle everything from GPS navigation to weather forecasting.
Natural vs. Artificial Satellites
Natural satellites are moons, planets, and other celestial bodies kept in orbit by gravity alone. Earth orbits the Sun, the Moon orbits Earth, and Jupiter has at least 95 known moons orbiting it. No engineering required.
Artificial satellites are machines built and launched by humans. The first one, Sputnik 1, reached orbit on October 4, 1957. It was about the size of a beach ball (58 centimeters across), weighed just 83.6 kilograms, and completed one trip around Earth every 98 minutes. Today, thousands of active artificial satellites orbit the planet, doing work that ranges from broadcasting television signals to monitoring hurricanes.
How Satellites Stay in Orbit
A satellite stays in orbit by moving fast enough sideways that, as gravity pulls it downward, the curve of Earth drops away beneath it at the same rate. It’s constantly falling toward Earth but never hitting it. The closer a satellite is to Earth, the stronger gravity’s pull, so the faster it needs to travel to avoid being dragged down.
Satellites in low Earth orbit (LEO), typically below about 2,000 kilometers up, need speeds up to 28,000 kilometers per hour (17,500 mph). At that pace, they circle the planet roughly every 90 minutes. The International Space Station flies in this zone.
Medium Earth orbit (MEO) sits between roughly 2,000 and 35,786 kilometers. Satellites here travel at about 11,200 kilometers per hour and circle the globe roughly every 12 hours. GPS satellites operate in this range.
Geostationary orbit (GEO) is a special altitude, about 35,786 kilometers up, where a satellite’s orbital period matches Earth’s rotation. From the ground, a geostationary satellite appears to hover over the same spot permanently. This makes GEO ideal for communications and weather satellites that need to watch one region continuously.
What Satellites Are Made Of
Every artificial satellite has two main sections: the bus and the payload. The bus is the body of the spacecraft. It handles all the housekeeping: generating power (usually with solar panels), controlling temperature, communicating with ground stations, and orienting the satellite so its instruments point the right direction. The payload is whatever the satellite was actually built to do, whether that’s a camera, a radar antenna, a communications relay, or a scientific instrument.
The structure and frame typically account for about 20% of a satellite’s dry mass. Power systems, including solar panels, batteries, and wiring, take up another 19% or so. The rest is divided among computers for processing commands and data, systems for pointing and stabilizing the satellite, thermal insulation to handle temperature swings of hundreds of degrees between sunlight and shadow, and sometimes a small propulsion system for adjusting orbit.
What Satellites Do
Navigation
The GPS system most people use daily relies on a constellation of 31 satellites in medium Earth orbit. Your phone or car’s GPS receiver picks up signals from at least four of these satellites simultaneously. Each signal carries a precise timestamp from an onboard atomic clock. By measuring how long each signal took to arrive, the receiver calculates its distance from each satellite, then uses those distances to pinpoint your latitude, longitude, and altitude. Three satellites are enough for a position fix in theory, but a fourth eliminates the need for your device to carry its own atomic clock, which makes consumer GPS practical.
Earth Observation
Hundreds of satellites watch Earth’s surface, atmosphere, and oceans. Some carry optical cameras that work like extremely high-altitude photography. Others use synthetic aperture radar (SAR), which bounces microwave signals off the ground and reads the reflections. SAR works at night, through clouds, and in any weather, and it can detect changes on Earth’s surface down to very small scales. This data feeds weather forecasts, crop monitoring, disaster response, climate tracking, and military intelligence.
Communications
Communication satellites relay signals between points on Earth that can’t connect directly because of distance or terrain. A TV broadcast, a ship’s internet connection in the middle of the Pacific, or a phone call from a remote area all may travel through a satellite. Geostationary satellites have dominated this role for decades because a single satellite can see about a third of Earth’s surface from its fixed position. More recently, large constellations of smaller satellites in low Earth orbit are providing broadband internet with lower signal delay, since the satellites are much closer to the ground.
Science
Some satellites look outward instead of down. The Hubble Space Telescope, orbiting about 540 kilometers above Earth, observes distant galaxies, nebulae, and stars without the blurring effects of the atmosphere. Other science satellites study the Sun, measure Earth’s magnetic field, or detect gravitational waves.
How Satellites Reach Orbit
Satellites ride to space inside the nose cone (fairing) of a rocket. During launch, the rocket’s stages fire in sequence, each one accelerating the payload and then dropping away when its fuel is spent. Once the final stage reaches the target orbit, the satellite separates.
Separation systems vary by satellite size. Large satellites often use a circular clamping ring: one ring stays attached to the rocket, the other stays with the satellite, and springs push them apart when the clamp releases. Smaller satellites called CubeSats slide out of box-shaped dispensers on rails or tabs, deployed by a spring once a door opens. Some newer systems skip explosive bolts entirely, resulting in a gentler release with less vibration and no debris. Commercial “orbital tugs” are also emerging. These are small spacecraft that ride along on a rocket, then use their own engines to carry hosted satellites to different orbits for deployment.
Space Debris and Orbital Safety
With thousands of satellites launched over the decades, plus spent rocket stages and fragments from collisions, Earth orbit is getting crowded. The U.S. Space Surveillance Network tracks objects in low Earth orbit down to about 10 centimeters across. In higher geostationary orbit, tracking sensitivity drops, so only objects roughly 70 centimeters or larger are reliably cataloged. Below those thresholds, NASA tracks and characterizes smaller debris using other methods.
Even a small piece of debris is dangerous at orbital speeds. When an object approaches the International Space Station within a box-shaped zone of 4 by 10 by 4 kilometers, mission controllers evaluate the collision risk. If the probability exceeds about 1 in 10,000, the station may perform a collision-avoidance maneuver. For uncrewed satellites, the threshold is less cautious, around 1 in 1,000.
The concern is a runaway chain reaction sometimes called Kessler Syndrome: one collision creates debris that causes more collisions, generating an expanding cloud of fragments that makes certain orbits unusable. To slow this down, international guidelines established a “25-year rule” requiring that spacecraft components be removed from low Earth orbit within 25 years of a mission ending, either by deorbiting into the atmosphere to burn up or by boosting to a disposal orbit. As the number of satellites in orbit grows, particularly from large commercial constellations, managing debris is becoming one of the most pressing challenges in spaceflight.