Satellite

In the context of spaceflight, a satellite is an artificial object which has been intentionally placed into orbit. Such objects are sometimes called artificial satellites to distinguish them from natural satellites such as the Moon.

The world's first artificial satellite, the Sputnik 1, was launched by the Soviet Union in 1957. Since then, thousands of satellites have been launched into orbit around the Earth. Some satellites, notably space stations, have been launched in parts and assembled in orbit. Artificial satellites originate from more than 50 countries and have used the satellite launching capabilities of ten nations.space debris. A few space probes have been placed into orbit around other bodies and become artificial satellites to the Moon, Mercury, Venus, Mars,Jupiter, Saturn, Vesta, Eros, and the Sun.

A few hundred satellites are currently operational, whereas thousands of unused satellites and satellite fragments orbit the Earth as

Satellites are used for a large number of purposes. Common types include military and civilian Earth observation satellites, communications satellites, navigation satellites, weather satellites, and research satellites. Space stations and human spacecraft in orbit are also satellites. Satellite orbits vary greatly, depending on the purpose of the satellite, and are classified in a number of ways. Well-known (overlapping) classes include low Earth orbit, polar orbit, and geostationary orbit.

About 6,600 satellites have been launched. The latest estimates are that 3,600 remain in orbit.^[1] Of those, about 1000 are operational;^[2][3] the rest have lived out their useful lives and are part of the space debris. Approximately 500 operational satellites are in low-Earth orbit, 50 are in medium-Earth orbit (at 20,000 km), the rest are in geostationary orbit (at 36,000 km).^[4]

Satellites are propelled by rockets to their orbits. Usually the launch vehicle itself is a rocket lifting off from a launch pad on land. In a minority of cases satellites are launched at sea (from a submarine or a mobile maritime platform) or aboard a plane (see air launch to orbit).

Satellites are usually semi-independent computer-controlled systems. Satellite subsystems attend many tasks, such as power generation, thermal control, telemetry,attitude control and orbit control.

PLUTO

Pluto, minor-planet designation 134340 Pluto, is the largest object in the Kuiper belt, and the tenth-most-massive body observed directly orbiting the Sun. It is the second-most-massive known dwarf planet, after Eris. Like other Kuiper-belt objects, Pluto is composed primarily of rock and ice^[14] and is relatively small, approximately one-sixth the mass of the Moon and one-third its volume. It has an eccentric and highly inclined orbit that takes it from 30 to 49 AU (4.4–7.4 billion km) from the Sun. This causes Pluto to periodically come closer to the Sun than Neptune. As of 2013, it is 32.6 AU from the Sun.

Discovered in 1930, Pluto was originally classified as the ninth planet from the Sun. However, its status as a major planet fell into question following further study of it and the outer Solar System over the ensuing 75 years. Starting in 1977 with discovery of minor planet 2060 Chiron, numerous icy objects similar to Pluto with eccentric orbits were found.^[15] The most notable of these was the scattered disc object Eris—discovered in 2005, which is 27% more massive than Pluto.^[16] The understanding that Pluto is only one of several large icy bodies in the outer Solar System prompted the International Astronomical Union (IAU) to formally define what it means to be a "planet" in 2006. This definition excluded Pluto and reclassified it as a member of the new "dwarf planet" category (and specifically as a plutoid).^[17] A number of scientists hold that Pluto should have remained classified as a planet, and that other dwarf planets should be added to the roster of planets along with Pluto.^[18][19]

Pluto has five known moons: Charon (the largest, with a diameter just over half that of Pluto), Nix, Hydra, Kerberos, and Styx.^[5] Pluto and Charon are sometimes described as abinary system because the barycenter of their orbits does not lie within either body.^[20]However, the IAU has yet to formalise a definition for binary dwarf planets, and as such Charon is officially classified as a moon of Pluto.^[21]

In 2015, the Pluto system is due to be visited by spacecraft for the first time. The New Horizons probe will perform a flyby during which it will attempt to take detailed measurements and images of the plutoid and its moons.