C w kingori
Lister
On 4 October 1957 the Soviet Union
launched the world's fi rst artif i cial satellite,
Sputnik 1. Since then, about 8,900
satellites from more than 40 countries
have been launched. According to a 2018estimate, 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
signif i cantly leads the way with 859
satellites, China is second with 250, and
Russia third with 146. These are then
followed by Japan (72), India (55) and the
UK (52).[1] A few large space stations have
been launched in parts and assembled inorbit. Over a dozen space probes have
been placed into orbit around other bodie
and become artif i cial satellites of th
Moon, Mercury, Venus, Mars, Jupiter
Saturn, a few asteroids,[2] a comet and th
Sun
Satellites are used for many purposes
Among several other applications, the
can be used to make star maps and map
of planetary surfaces, and also tak
pictures of planets they are launched into
Common types include military and civilia
Earth observation satellites
communications satellites, navigatio
satellites, weather satellites, and spaces
telescopes. Space stations and human
spacecraft in orbit are also satellites.
Satellites can operate by themselves or as
part of a larger system, a satellite
formation or satellite constellation.
Satellite orbits vary greatly, depending on
the purpose of the satellite, and are
classif i ed in a number of ways. Well-
known (overlapping) classes include low
Earth orbit, polar orbit, and geostationary
orbit.
A launch vehicle is a rocket that places a
satellite into orbit. Usually, it lifts off from
a launch pad on land. Some 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, scientif i c
instrumentation, communication,
