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In the context of spaceflight, a satellite is an object that has been intentionally placed into orbit. These objects are called artificial satellites to distinguish them from natural satellites such as Earth's Moon. On 4 October 1957 the Soviet Union launched the world's first artificial satellite, Sputnik 1. Since then, about 8,900 satellites from more than 40 countries have been launched. According to a 2018 estimate, some 5,000 remain in orbit. Of those about 1,900 were operational, while the rest have lived out their useful lives and become space debris. Approximately 63% of operational satellites are in low Earth orbit, 6% are in medium-Earth orbit (at 20,000 km), 29% are in geostationary orbit (at 36,000 km) and the remaining 2% are in elliptic orbit. In terms of countries with the most satellites the USA significantly leads the way with 859 satellites, China is second with 250, and Russia third with 146. These are then followed by India (118), Japan (72) and the UK (5...

The first published mathematical study of the possibility of an artificial satellite was Newton's cannonball, a thought experiment in A Treatise of the System of the World by Isaac Newton (1687). The first fictional depiction of a satellite being launched into orbit was a short story by Edward Everett Hale, The Brick Moon . The idea surfaced again in Jules Verne's The Begum's Fortune (1879). In 1903, Konstantin Tsiolkovsky (1857–1935) published Exploring Space Using Jet Propulsion Devices , which is the first academic treatise on the use of rocketry to launch spacecraft. He calculated the orbital speed required for a minimal orbit, and that a multi-stage rocket fueled by liquid propellants could achieve this. In 1928, Herman Potočnik (1892–1929) published his sole book, The Problem of Space Travel – The Rocket Motor . He described the use of orbiting spacecraft for observation of the ground and described how the special conditions of space could be useful for scientific ex...

Satellites can be tracked from Earth stations and also from other satellites. Space Surveillance Network edit The United States Space Surveillance Network (SSN), a division of the United States Strategic Command, has been tracking objects in Earth's orbit since 1957 when the Soviet Union opened the Space Age with the launch of Sputnik I. Since then, the SSN has tracked more than 26,000 objects. The SSN currently tracks more than 8,000-artificial orbiting objects. The rest have re-entered Earth's atmosphere and disintegrated, or survived re-entry and impacted the Earth. The SSN tracks objects that are 10 centimeters in diameter or larger; those now orbiting Earth range from satellites weighing several tons to pieces of spent rocket bodies weighing only 10 pounds. About seven percent are operational satellites (i.e. ~560 satellites), the rest are space debris. The United States Strategic Command is primarily interested in the active satellites, but also tracks space debris which ...

There are three basic categories of (non-military) satellite services: Fixed satellite services edit Fixed satellite services handle hundreds of billions of voice, data, and video transmission tasks across all countries and continents between certain points on the Earth's surface. Mobile satellite systems edit Mobile satellite systems help connect remote regions, vehicles, ships, people and aircraft to other parts of the world and/or other mobile or stationary communications units, in addition to serving as navigation systems. Scientific research satellites (commercial and noncommercial) edit Scientific research satellites provide meteorological information, land survey data (e.g. remote sensing), Amateur (HAM) Radio, and other different scientific research applications such as earth science, marine science, and atmospheric research.

Astronomical satellites are satellites used for observation of distant planets, galaxies, and other outer space objects. Biosatellites are satellites designed to carry living organisms, generally for scientific experimentation. Communication satellites are satellites stationed in space for the purpose of telecommunications. Modern communications satellites typically use geosynchronous orbits, Molniya orbits or Low Earth orbits. Earth observation satellites are satellites intended for non-military uses such as environmental monitoring, meteorology, map making etc. (See especially Earth Observing System.) Navigational satellites are satellites which use radio time signals transmitted to enable mobile receivers on the ground to determine their exact location. The relatively clear line of sight between the satellites and receivers on the ground, combined with ever-improving electronics, allows satellite navigation systems to measure location to accuracies on the order of a few mete...

The first satellite, Sputnik 1, was put into orbit around Earth and was therefore in geocentric orbit. This is the most common type of orbit by far, with approximately 2,787 active artificial satellites orbiting the Earth. Geocentric orbits may be further classified by their altitude, inclination and eccentricity. The commonly used altitude classifications of geocentric orbit are Low Earth orbit (LEO), Medium Earth orbit (MEO) and High Earth orbit (HEO). Low Earth orbit is any orbit below 2,000 km. Medium Earth orbit is any orbit between 2,000 and 35,786 km. High Earth orbit is any orbit higher than 35,786 km. Centric classifications edit Galactocentric orbit : An orbit around the centre of a galaxy. The Sun follows this type of orbit about the galactic centre of the Milky Way. Geocentric orbit : An orbit around the planet Earth, such as the Moon or artificial satellites. Currently there are over 2,787 active artificial satellites orbiting the Earth. Heliocentric orbit : An orbit aro...