A
satellite
is an object that
orbits
another object. In
space
, satellites may be natural, or artificial. The
moon
is a
natural satellite
that orbits the
Earth
. Most artificial satellites also orbit the Earth, but some orbit other planets, or the
Sun
or Moon. Satellites are used for many purposes. There are
weather
satellites,
communications satellites
,
navigation satellites
,
reconnaissance
satellites,
astronomy
satellites.
The world's first artificial satellite, the
Sputnik
1, was launched by the
Soviet Union
on October 4,1957. This surprised the world, and the United States quickly worked to launch their own satellite, starting the
space race
. Sputnik 2 was launched on November 3, 1957 and carried the first living passenger into orbit, a dog named
Laika
. The United States launched their first satellite, called
Explorer 1
on January 31, 1958.
[1]
The
UK
launched its first satellite in 1962.
[2]
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 come from more than 50 countries and have used the satellite launching capabilities of ten nations. A few hundred satellites are currently working, but thousands of unused satellites and satellite fragments orbit the Earth as
space debris
. The largest satellite is the
International Space Station
, which was put together by several different countries (including the organizations of
NASA
,
ESA
,
JAXA
and
RKA
). It usually has a crew of six
astronauts
or
cosmonauts
living on board. It is permanently occupied, but the crew changes. The
Hubble Space Telescope
has been repaired and updated by astronauts in space several times.
There are also artificial satellites orbiting something other than the Earth. The
Mars Reconnaissance Orbiter
is one of those orbiting
Mars
.
Cassini-Huygens
orbited Saturn.
Venus Express
, run by the ESA, orbited
Venus
.
[3]
Two
GRAIL
satellites orbited the moon until December 2012.
[4]
Several satellites have orbited the Sun for years.
[5]
Artificial satellites have several main uses:
Most of the artificial satellites are in a
low Earth orbit
(LEO) or a
geostationary orbit
. To stay in orbit, the satellite's sideways speed must balance the force of
gravity
. Satellites in low orbit are often less than one thousand kilometers above the ground. Close to the Earth, in LEO, the satellites must move faster to stay in orbit. Low orbits work well for satellites that take pictures of the Earth. Many do jobs that call for high
orbital inclination
(they swing above and below the equator), so they can communicate, or look at other areas. It is easier to put a satellite in low Earth orbit, but the satellite appears to move when viewed from Earth. This means a satellite dish (a type of
antenna
) must be always moving in order to send or receive communications with that satellite.
Medium orbit works well for
GPS
satellites - receivers on Earth use the satellite's changing position and precise time (and a type of antenna that does not have to be pointed) to find where on Earth the receiver is. But constantly changing positions does not work for satellite TV and other types of satellites that send and receive a lot of information. Those need to be in geostationary orbit.
A satellite in a geostationary orbit moves around the Earth as fast as the Earth spins, so from the ground it looks like it is
stationary
(not moving). To move this way, the satellite must be straight above the
equator
, and 35,786 kilometers (22,236 miles) above the ground.