In short, a Satellite is an object that orbits round another object in space. The object being orbited could be a star, a galaxy, a moon or a planet. Likewise, the orbiting object can be the same, a star, a galaxy, a moon or planet. A planet is a satellite of a star unless of course, it is a Rogue Planet in which case it is not a satellite as it orbits nothing. Again, a star that is free from the bounds of a galaxy is a rogue star and not a satellite. There are two types, artificial and natural.
In the case of natural, they are for example but limited to :- a star, a planet, a Moon and even a galaxy itself. the milky way galaxy has a number of satellite galaxies, an example of one is the Canis Major Dwarf galaxy.
A star's orbit is round the centre of a galaxy such as the milky way. Our star, the Sun orbits round the centre of the Milky Way at a speed of 220km/s. This means a Galactic Year, the time it takes for the Sun to complete one full orbit of the galaxy is 225 Million Years. Stars will rotate round what is normally a super-massive black hole. A planet orbits a star and a moon orbits round a planet.
This page deals with artificial satellite, the objects that we put into space. Satellites can travel round the planet or be put in a geo-stationary position so they are fixed over one location all the time. The International Space Station is an artificial satellite but its not one we usually refer to as being a satellite but a space station.
The purpose of the satellite is whatever the creators designed it for. A satellite could help with communication, navigation, Earth monitoring or spying as they are high in the atmosphere so they can't be shot down.
There is no treaty to say who can or can't put a satellite up in space. It is getting extremely crowded up there as more and more satellites are put into orbit. When a satellite has finished it usefulness, it may re-enter the atmosphere and hopefully burn up but is not always the case.
The first satellite put into space was Sputnik 1, launched by the Russians on October 4th 1957. It was the first time that humans had put anything into space. It was a small satellite, about the size of a beach ball (58cm/22.8 inches in diameter). It kicked off the Space Age which would see Russia win more firsts
The Americans would eventually win the race to put a man (Neil Armstrong) on the moon. Putting people into space are subjects for other pages, this page is about non-human efforts. Below is a picture of what Sputnik 1 looked like.
The Americans launched their first satellite, Explorer 1 on January 31st 1958. Which by that time, Russia had followed up Sputnik 1 with Sputnik 2. Sputnik 2 contained Laika the dog to test the affects of space travel on life. The dog never made it back home, it died in re-entry.
Britain developed a series of rockets and satellites but only one, Prospero was successfully launched into space using a British space rocket. The Government at the time, in the sixties decided to withdraw from the project on cost grounds and work with its American and European partners. Britain still builds satellites but they don't have the means to get them into space on their own. Satellites are predominantly but not exclusively built and designed in the U.K. by EADS Astrium in Guildford and Stevenage mainly. Also based in the UK is Inmarsat, a company that runs and controls communication satellite orbiting the globe enabling communication where no other means of communication is possible especially in the case of disasters.
Some satellites will orbit the Earth freely but some satellites especially GPS (Geo-Positioning Satellites) will appear fixed in space. GPS satellites are used by satellite navigation equipment in your cars for example, the equipment receive signals from more that one satellite to calculate the users position. According to Einstein, time is different up in space and satellites needs to be aware of this otherwise they'll be a drift of 11 kilometers a day. 4 On Earth, a day is 24 hours but in space, satellites need to account for Sidereel Days which is 23hrs, 56 mins and 4 secs. The author Arthur C. Clarke first proposed geostationary objects in space and geostationary orbits are in fact known as the Clarke Orbit in his honour. The first geosynchronous satellite was Syncom 2 sent into orbit by National Aeronautics and Space Administration (N.A.S.A.) on 26th July 1963 but it wasn't perfect as the antenna need moving. NASA followed up Syncom 2 with aptly named Syncom 3 which was geostationary the following year August 19th, 1964. 6 Syncom 2 orbit was slightly inclined, not level with the equator so to speak. 5
The gravitational pull of the Moon and the Sun can cause changes in the orbit inclination, to rectify this, the satellites will have a propulsion system to correct any inclination. 7. This correction is known as the North-South Station keeping. The Clarke Orbit is above the Equator as there is only one equator, it can be rather crowded. To add to this, the geostationary satellite are at a certain distance from the Earth, about 35,786 kilometers. 8
The most well known of this type of satellites is the Hubble space telescope launched on April 24th 1990. They stay in orbit round the Earth and beam back pictures of deep space.
The picture below is the Hubble successor, the James Webb Telescope, named after the second N.A.S.A. Administrator. The James Webb will be located further out from the Earth's orbit than the Hubble was. If something goes wrong with any piece of equipment which happened with Hubble, they won't be able to fix it. The reason for the location of the James Webb is so that its not affected by the Earth's atmosphere. The expected launch of the satellite hasn't been finalised yet because it keeps being delayed by project changes.
Communication satellites refer to satellites used for telephone (Inc. Mobile) and/or television both (terrestrial and satellite). Communication satellites tend to be geo-stationary around the equator. Being in the Equator, they tend to travel at the same speed as the Earth. They are high in orbit, about 35,900 feet or 22,300 miles. The asteroid Apophis is predicted to fly between high orbit satellites and Earth but there is no risk of collision with Earth. Due to the large number of satellites out there communication and otherwise, there is a possible risk of it hitting a satellite.
Satellites have been used to determine the weather. They beam back images of what the current state of the weather is in a given location. Future direction of clouds are done on the ground with powerful supercomputers. The first weather satellite launched was TIROS-1 launched by N.A.S.A. on April 1st, 1960.
The most secret of all satellites are the ones used by the Military and Intelligence Agencies such as M.I.6. and the C.I.A.. Those organisations use the technology to monitor what other countries are doing, including taking aerial photos of areas of land which would otherwise would not be possible to do so by plane. With satellites, you cannot easily shoot them down and because there is no one inside, they can run 24x7 and no one can get killed or captured. Before spy satellites were used, pilots used to fly in planes at very high altitude to take photos of places of interest. In 1960, a U2 spy-plane piloted by Gary Powers was shot down over Russia and held in prison to be released two years later in a prisoner exchange.
Satellites are held in position by gravity whether they are orbiting the Earth or another planet. Satellites orbiting the Sun are held in place by gravitational pull of both the Sun and the Earth. If it wasn't for the pull of both objects, the satellite could quite easily fly into the Sun and burn up. The James Webb Space Telescope (JWST) will use the gravitational pull of Earth to ensure that it completes an orbit of the Sun in line with the Earth. The JWST is going to be orbiting at a distance further away than any other telescopic satellite has done so before so repairs aren't going to be easy to make. A docking facility has been added to the design to ensure that repairs by a robot or a future Earth crew is possible. 3
Some satellites return to Earth, some are planned and will not be of danger as their descent is controlled. As all satellites are monitored presumably, if one does becomes dangerously out of control, Governments have a duty to warn their populace of the risk which has happened on numerous occasions. Most of the time, the satellites if they've not burnt up in the atmosphere will fall into the oceans or in a non-populated area such as a mountain range.
An example of a satellite that has returned back to Earth is the Gravity field and steady-state Ocean Circulation Explorer satellite (GOCE) which returned when its energy supply ran out. 9 Satellites aren't powered by nuclear power for the very reason that they might return to Earth. Those satellite that are sent into space powered by nuclear power are ones that orbit other planets or are space probes such as new horizons mission to pluto because they travel so far that solar energy would not provide enough power for them to work.
Space Debris is also known as Space Junk. With the number of satellites increasing and their relative short life spans, there are a lot of junk satellites just up in the clouds doing nothing. Some satellites fall back down to Earth, some are burnt up in re-entry whereas others fall into the Ocean. There's not been a case where the satellite has fallen onto land causing damage to property or death. It could only be a case of when not if such a thing happens. They are stationed at different distances from the atmosphere so don't tend to cross paths. However, in February 2009, two communication satellites, one Russian, one American did smash into one another 1 causing satellites to break up and increase a number of space debris into space. Space debris that is large enough can be monitored and active satellites can be moved out of the way. However there are some pieces that are too small that can't be, such as specs of paint and it is hoped that they don't come into contact with satellites but just burn up on re-entry. Its not possible to throw up a net and grab the items, the items are travelling at very high speeds and at those speeds, can cause a lot of damage including to the net.
In January 2007, China intentionally destroyed one its defunct weather satellites 2 which at the time was cause of about a third of all space debris. It caused alarm and anger by other nations as nations had agreed not to militarise space. Some people feared it would start a new Space Arms race.
It isn't just satellites that need to be aware of space debris but also astronauts and the International Space Station. The ISS has to re-orbit itself every so often to avoid junk. The problems of space debris was brought to the cinema in the film Gravity starring George Clooney and Sandra Bullock when a satellite is destroyed and a debris cloud orbits the planet destroying or damaging everything in its path.
In the eighties when the Cold War was still taking active, the U.S. Government at that time led by Ronald Reagan launched a project that would be known as Star Wars. It had nothing to do with the series of films by George Lucas. Instead, the plan was to put a nuclear missile early warning system in space that would be able to shoot incoming missiles. The Official name for the project was Strategic Defense Initiative. Much more detail on this subject can be found at the dedicated Cold War site. The below video, five minutes in length will give a brief explanation of what SDI-StarWars was about.
Whenever a nuclear device explodes, the shockwave will affect electronic equipment in the surrounding area. This was discovered when nations started developing nuclear weapons in the early days. There have been experiments by both the United States and the Russian Federation on weapons containing nuclear devices exploding in the high atmosphere and also possibly with satellites that will explode when given the signal to. One of the draw backs of using high altitude weaponry like this is that you can't prevent damage to non-targets. After a brief flurry of experimentation between the late fifties and early sixties, weaponising of space was banned under the Outer Space Treaty of 1967. The treaty prevented nuclear weapons and other such weapons from being placed in space but didn't ban weapons in space hence why Reagan did the Star Wars project.
Electro-Magnetic Pulse satellites have been the stuff of fiction and have featured in numerous films such as the James Bond film Goldeneye, where a rogue MI6 agent Alec Trevelyan gains controls of the Goldeneye satellite. Goldeneye is the code name for two first-strike satellites orbiting the Earth in space. The premise is that the nuclear explosion would destroy all electrical equipment below. As to whether those nuclear satellites still exist is open to debate.
As well as Electro-Magnetic Pulse satellites, there have other forms of weapons attached to spy satellites such as the laser satellites such as the one that SPECTRE obtains in Diamonds are Forever.
Every so often, the Sun ejects a massive amount of solar plasma through solar flares which can come towards Earth. People on Earth are shielded by the plasma by the Magnetic Field. Astronauts need to be inside the ISS or space vehicle when a solar storm hits them otherwise it can be serious to their health. When a solar storm is discovered, the satellite operators instruct their satellites to "take cover", this usually means shutting down temporarily and protecting any sensitive parts.
In the 1996 movie, Independence Day, the aliens used our satellites against us. In order to communicate with other craft scattered around the planet without line of sight, the aliens used the satellites to bounce signals off to communicate with one another. The aliens couldn't use the technology directly, instead the signals bounced off the outside of the satellites to reach their targets. The aliens wouldn't instruct the satellites to change angle or give other instruction. The aliens could move the satellites after all they had small craft in addition to the larger craft to move the satellites where they wanted. There are so many satellites that if the order came, only spy satellite would possibly have a self destruct mechanism to fall into enemy hands. I say might because most likely not.