Satellite

An artificial satellite is an object placed into orbit around a celestial body. Most satellites are spacecraft designed for specific tasks. They serve many vital roles in our modern world. These include communication relays, weather forecasting, and navigation systems like GPS. Some satellites perform scientific research or Earth observation. Others are used for military reconnaissance or early warning. Even large crewed spacecraft, such as the International Space Station, are considered artificial satellites.

To become a satellite, a spacecraft must achieve orbital velocity. This requires accelerating or decelerating to reach a specific speed. The craft must occupy an orbit high enough to avoid orbital decay. This decay is caused by drag from the Earth's atmosphere. The satellite must also stay above its Roche limit. Once in orbit, satellites can change their position using propulsion. They often use chemical thrusters or ion thrusters for this task. They also use reaction wheels to control their rotation or attitude.

Satellites require power to operate their onboard equipment. Most use an electricity generation system. Solar panels are a very common choice for this. Some use radioisotope thermoelectric generators, or RTGs, for power. To talk to Earth, most satellites use transponders. These devices allow communication with ground stations. Communication satellites often act as radio relay stations. A single satellite might carry dozens of transponders. Each of these can have a bandwidth of tens of megahertz.

History shows that the idea of satellites evolved over centuries. Isaac Newton first studied the math in 1687. Later, Konstantin Tsiolkovsky calculated the speed needed for orbit. In 1945, Arthur C. Clarke proposed using geostationary satellites for mass communication. The first real artificial satellite was Sputnik 1. The Soviet Union launched it on October 4, 1957. This event helped scientists measure the density of high atmospheric layers. It also provided data on radio signals in the ionosphere.

Different types of orbits serve different scientific purposes. Many satellites stay in low Earth orbit or geostationary orbit. Geostationary satellites appear to stay still relative to a fixed point on the ground. Some imaging satellites use a Sun-synchronous orbit. This allows them to scan the globe with similar lighting. Other satellites orbit different bodies like the Moon or Mars. Some even follow complex paths called halo or Lissajous orbits. Space telescopes are a special type of satellite. They use the vacuum of space to observe the electromagnetic spectrum.

As technology advanced, satellite design became more efficient. Many engineers now use a standardized satellite bus. This is a common platform that saves time and money. Small CubeSats are a very popular type of bus. The largest artificial satellite is the International Space Station. As the number of satellites grows, space debris becomes a concern. The threat of collisions is increasing in crowded orbits. To help, some companies design satellites to be "demisable." For example, SpaceX Starlink satellites are designed to burn up completely during reentry.

Satellites are essential to global systems and infrastructure. They provide the signals used for satellite navigation. They also allow us to monitor the Earth's forests and oceans. In 1960, the TIROS-1 spacecraft sent the first weather footage from space. As of June 28, 2025, there were 12,952 satellites in Earth's orbit. Of these, 8,530 belong to the United States. Russia has 1,559 satellites, and China has 908. These numbers show how much we rely on orbital technology.