A satellite is an object that orbits another object. In space, satellites may be made by man, or they may be natural. The moon is a natural satellite that orbits the Earth. Most man-made satellites also orbit the Earth, but some orbit other planets, such as Saturn, Venus or Mars, or the moon.
The idea of a man-made satellite has been around for a long time. When Isaac Newton was thinking about gravity, he came up with the thought experiment called Newton's cannonball. He wondered what would happen if a cannonball was shot from a tall mountain. If fired at just the right speed (and ignoring the friction of air), he realized it would orbit the Earth. Later, Jules Verne wrote about a satellite in 1879 in a book called Begum's Fortune.
In 1903, Konstantin Tsiolkovsky wrote Means of Reaction Devices (in Russian: Исследование мировых пространств реактивными приборами), which was the first serious study on how to use rockets to launch spacecraft. He calculated the speed needed to reach orbit around the Earth (at 8 km/s). He also wrote that a multi-stage rocket, using liquid fuel could reach that speed. He recommended liquid hydrogen and liquid oxygen, though other fuels could be used. He was correct on all of these points.
The English science fiction writer Arthur C. Clarke is given the credit of coming up with the idea of the communication satellite in 1945. He described in detail the possible use of satellites for mass communication, how to launch satellites, what orbits they could use, and the benefits of having a network of world-circling 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. The UK launched its first satellite in 1962.
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 in space from sections made 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 crews change. The Hubble Space Telescope has been repaired and updated by astronauts in space several times.
There are also man-made satellites orbiting something other than the Earth. The Mars Reconnaissance Orbiter is orbiting Mars. Cassini-Huygens is orbiting Saturn. Venus Express, run by the ESA, is orbiting Venus. Two GRAIL satellites orbited the moon until December 2012. There are plans to launch a satellite in 2017 called the Solar Orbiter (SolO) that will orbit the sun.
Man-made satellites have several main uses:
- Scientific Investigation
- Earth Observation - including weather forecasting and tracking storms and pollution
- Communications - including satellite television and telephone calls
- Navigation - including Global Positioning System (GPS)
- Military - including spy photography and communications (nuclear weapons are not allowed in space)
Most of the man-made 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. Closer 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. 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. This 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. Satellites in low Earth orbit are often less than one thousand kilometers above the ground. They move much faster. Many are in tilted orbits (they swing above and below the equator), so they can communicate, or see what is happening in other areas, depending on what they are used for.
- "Arthur C. Clarke". NY Times. http://www.nytimes.com/top/reference/timestopics/people/c/arthur_c_clarke/. Retrieved 1 April 2013.
- "Dawn of Space Age". NASA. http://history.nasa.gov/sputnik/. Retrieved 1 April 2013.
- "Solar Orbiter". BBC. http://www.bbc.co.uk/news/science-environment-17743190. Retrieved 1 April 2013.
- "Venus Express satellite". European Space Agency. http://sci.esa.int/science-e/www/area/index.cfm?fareaid=64. Retrieved 1 April 2013.
- "GRAIL satellites to be crashed into the moon". SpaceFlightNow. http://spaceflightnow.com/news/n1212/13grail/#.UVkR2Y5qLOE. Retrieved 1 April 2013.
- Satellite Tracker - Find out what satellites can be seen going by in the sky in your area (including the ISS).
- NASA's main space missions