Satellite

For moons, see Natural satellite. For other uses, see Satellite (disambiguation).

A satellite, or artificial satellite, is an object, usually a spacecraft, that is placed into orbit around a celestial body. Satellites have many uses. They relay messages, predict the weather, and help people find their way (GPS). They send broadcasting signals, support science, and take pictures of Earth. Militaries use them to watch from far away, for early warning, and to listen for signals. They could also be used to deliver weapons. Some satellites are the last rocket parts that lift other satellites into orbit. Some old satellites keep circling Earth long after they stop working.

Most satellites have a way to make electricity for their equipment, such as solar panels or radioisotope thermoelectric generators (RTGs). Except for passive satellites, they also have transponders that let them talk to ground stations. Many satellites use a standardized bus to save money and effort. The most popular are small CubeSats. Satellites that work together as a group form constellations. Launching things into space is very expensive, so most satellites are built to be as light and strong as possible.

To become satellites, spacecraft must speed up or slow down to reach orbital velocities. They must also be high enough to avoid orbital decay. This happens when drag from the atmosphere slowly pulls them down. They must also stay above their Roche limit. Satellites are carried from the ground into space by launch systems. They can change or keep their orbit using propulsion. About 90% of the satellites orbiting Earth are in low Earth orbit or geostationary orbit. In geostationary orbit, satellites seem to stay in one place in the sky. As more satellites and space debris fill the space around Earth, the risk of crashes has grown.

The first artificial satellite launched into Earth's orbit was Sputnik 1 from the Soviet Union, on October 4, 1957. As of June 28, 2025, there are 12,952 satellites in Earth's orbit. Of these, 8,530 belong to the United States, 1,559 to Russia, and 908 to China.

History

See also: Timeline of first artificial satellites by country

In 1687, Isaac Newton used a thought experiment called Newton's cannonball in his book Philosophiæ Naturalis Principia Mathematica to explain how natural satellites move. This was the first published mathematical study of an artificial satellite. In 1869, Edward Everett Hale wrote the first story about a satellite launched into orbit, "The Brick Moon". Jules Verne used the idea again in The Begum's Fortune (1879).

In 1903, Konstantin Tsiolkovsky wrote the first academic book about using rockets to launch spacecraft. He calculated the orbital speed needed for a low orbit. He also suggested that a multi-stage rocket fueled by liquid propellants could reach that speed.

In 1928, Herman Potočnik described spacecraft that could orbit and watch the ground, for peaceful or military use. The idea of geostationary satellites was first put forward by Konstantin Tsiolkovsky.

In 1945, Arthur C. Clarke wrote in Wireless World about using communications satellites to send information to many people. In 1946, the United States Air Force's Project RAND released a report called Preliminary Design of an Experimental World-Circling Spaceship. In the same year, Lyman Spitzer proposed an orbiting space telescope.

The first artificial satellite was Sputnik 1. The Soviet Union launched it on 4 October 1957 as part of the Sputnik program, and Sergei Korolev was the chief designer. Its success caused the Sputnik crisis in the United States and started the Space Race during the Cold War.

Sputnik 2 carried a dog named Laika into orbit on 3 November 1957. There was no way for the dog to return. Explorer 1 became the first artificial satellite of the United States on 31 January 1958. France became the sixth country to have an artificial satellite when Astérix was launched from Hammaguir, Algeria, on 26 November 1965.

Early satellites were each built with their own designs. Later satellites were often built on shared single model platforms called satellite buses. The first standard bus was the HS-333 geosynchronous (GEO) communication satellite, launched in 1972. Today, FreeFlyer is a commercial software program used to plan, design, and run satellite missions.

After the late 2010s, companies building large satellite internet constellations began to plan how to bring down older satellites that reached the end of life. This was part of the regulatory process for getting a launch license.

The largest artificial satellite ever built is the International Space Station. By the early 2000s, CubeSats and microsats were being launched more often to the lower altitudes of low Earth orbit (LEO). Satellites were increasingly designed to burn up in the air. For example, SpaceX Starlink satellites are designed to burn up completely when they come back into the atmosphere.

Over the years, many countries have had satellites in orbit. These include Algeria, Argentina, Australia, Austria, Brazil, Canada, Chile, China, Denmark, Egypt, Finland, France, Germany, India, Indonesia, Iran, Israel, Italy, Japan, Kazakhstan, South Korea, Malaysia, Mexico, the Netherlands, Norway, Pakistan, Poland, Russia, Saudi Arabia, South Africa, Spain, Switzerland, Thailand, Turkey, Ukraine, the United Kingdom and the United States. Japan's space agency (JAXA) and NASA plan to send a wooden satellite called LingoSat into orbit in the summer of 2024.

Components

Orbit and altitude control

Further information: Spacecraft propulsion

"Altitude control" redirects here; not to be confused with Attitude control.

Most satellites use chemical or ion propulsion to adjust or maintain their orbit. They also use reaction wheels to control how they point in space, which is called their attitude. Satellites close to Earth are affected most by Earth's magnetic and gravitational field, and by the push of sunlight from the Sun's radiation pressure. Satellites that are farther away are pulled more by the Moon and the Sun. Without control of their orbit and direction, satellites cannot talk to ground stations on Earth.

Power

Main articles: Batteries in space, Nuclear power in space, and Solar panels on spacecraft

Most satellites use solar panels to make power. A few that travel into deep space, where there is little sunlight, use radioisotope thermoelectric generators. Slip rings connect the solar panels to the satellite and let them turn to face the sunlight so they can make the most power. Satellites with solar panels also need batteries, because sunlight is blocked inside the rocket that launches them and at night. The most common batteries for satellites are lithium-ion. In the past, they used nickel–hydrogen.

Applications

Main article: Earth observation satellite

Main article: Communications satellite

Main article: Reconnaissance satellite

Main article: Space telescope

Main articles: Space weapon, Anti-satellite weapon, and Early warning satellite

Earth observation satellites watch and study the Earth. This work is called remote sensing. Most are placed in low Earth orbit to collect detailed information. Some are placed in a geostationary orbit, which lets them keep watching the same area all the time. Some are placed in a Sun-synchronous orbit, which gives steady light so they can see the whole Earth. Depending on their job, they may carry a normal camera, radar, lidar, photometer, or tools that study the air. Their information is often used for archaeology, cartography, environmental monitoring, meteorology, and reconnaissance. As of 2021, there were over 950 Earth observation satellites. The largest number are run by Planet Labs.

Weather satellites watch clouds, city lights, fires, pollution, auroras, sand and dust storms, snow cover, ice maps, the edges of ocean currents, and energy flows. Environmental monitoring satellites can notice changes in the Earth's vegetation, air, seas, ocean color, and ice. Watching plants change over time can show when a area is becoming very dry. They can also detect gases in the lower air that come from human activity.

Satellites used for military or intelligence work are sometimes called spy satellites. They can help with early missile warning, detecting nuclear explosions, and taking pictures or radar images of places on Earth.

Navigation satellites send out radio time signals. These signals help receivers on the ground find their exact location. Because satellites and ground receivers have a fairly clear view of each other, these systems can measure location to within a few meters in real time.

Astronomical satellites are used to observe distant planets, galaxies, and other objects in outer space. Tether satellites are connected to another satellite by a thin cable called a tether. Recovery satellites bring equipment and experiments from orbit back to Earth. Biosatellites carry living things, usually for science experiments. Space-based solar power satellites are an idea for collecting energy from sunlight and sending it for use on Earth or elsewhere.

Since the mid-2000s, some groups have hacked satellites to spread messages and steal secret information. Some satellites in low Earth orbit have also been destroyed by missiles launched from Earth, as tests. Russia, the United States, China, and India have all shown they can destroy satellites. In 2007, the Chinese military destroyed an old weather satellite. In February 2008, the US Navy destroyed a defunct spy satellite. On 18 November 2015, after two failed tries, Russia successfully tested an anti-satellite missile called Nudol. On 27 March 2019, India destroyed a test satellite at 300 km altitude in 3 minutes. This made India the fourth country able to destroy satellites in orbit.

Environmental impact

The environmental impact of satellites is not yet well understood. People once thought they were harmless because only a few were launched. Now that launches are happening much more often, the topic is getting more attention. The main concerns are the materials used to make satellites and the pollution released at different stages of their life.

It is hard to measure how many resources satellites and launch vehicles use, because that information is often kept private. Aluminium is a popular metal for satellites because it is light and cheap. It makes up about 40% of a satellite's mass. Mining and refining it harm the environment, and it is one of the most carbon-intensive metals. Satellites also need rare materials such as lithium, gold, and gallium. Some of these are hard to mine safely or are in short supply. Launch vehicles need even more raw materials. Their booster stages usually fall into the ocean after their fuel runs out, and they are not normally recovered. Two empty boosters for the Ariane 5, made mostly of steel, each weighed about 38 tons.

Rocket launches release pollution into every layer of the atmosphere. The pollution is especially high above the tropopause, where it can stay for a long time. This pollution includes black carbon, CO₂, nitrogen oxides (NO_x), aluminium, and water vapour. The exact mix depends on the rocket's design and fuel. Rockets release only a small share of green house gases, about 0.01%. Aviation makes up 2-3% of global greenhouse gas emissions each year. Scientists are only beginning to study rocket emissions in the stratosphere, and their effects may be more serious than those lower down. The stratosphere contains the ozone layer, which rocket pollution can damage through ozone depletion. Some substances called radicals can have large effects, even in tiny amounts.

Satellites in LEO also release pollution as they orbit. Orbital decay caused by air drag means satellites sometimes need to be moved back into position. Most use hydrazine for this, which releases ammonia, hydrogen, and nitrogen into the upper atmosphere. Their outer materials, such as Kapton, Teflon, and Mylar, also break down and release gases like CO₂ and CO.

When satellites reach the end of their lives, they are either moved to a graveyard orbit or allowed to fall back to Earth. This helps reduce space debris. Some satellites have failed while falling out of orbit, and their radioactive materials were spread out. Examples include Kosmos 954, Kosmos 1402, and the Transit 5-BN-3.

Some concerns also involve the albedo of the Earth, and the possibility of accidental geoengineering of its climate. Wood has been suggested as an alternative material that could reduce pollution from satellites that re-enter the atmosphere.

Interference

Space debris can harm spacecraft, including satellites, that travel around Earth. It could also start a Kessler syndrome, which might make it harder for people to explore space in the future. As satellite constellations, such as SpaceX Starlink, grow larger, the IAU says that pollution in orbit is increasing a lot. A 2020 workshop report found that large groups of satellites can greatly affect some astronomy research. It listed six ways to reduce this harm. The IAU is setting up a center, called CPS, to coordinate these efforts.

Satellite signals are weak, so they are easy to jam using transmitters on the ground. This jamming only affects the area within the transmitter's range. GPS, satellite phone, and television signals have all been jammed. Someone can also send a radio signal to a geostationary satellite to interfere with its use. Stations on Earth sometimes send signals at the wrong time or on the wrong frequency, which can make that frequency unusable. Satellite operators now have advanced tools to find the source of these signals and manage how they are used.

Regulation

Some problems are getting bigger. These include space debris, radio interference, and light pollution. Rules to manage them, either within countries or between countries, are not being made very quickly.

Liability

Responsibility for satellites is usually covered by the Liability Convention.

Operation

Satellites are used in many different ways, and new uses keep appearing. Running a satellite takes money, the skill to build and launch it, and a ground segment on the ground to work with it.

List

See also: List of heaviest spacecraft

This section gives a list of some heavy satellites that orbit Earth. It does not include space stations with people on board or rockets used for launches. It also leaves out satellites that are not meant to stay in orbit around Earth, such as missions that travel farther into space. Satellites at the Sun-Earth Lagrange point 2 are included.