All of the known planets in the Solar System have natural satellites, bodies in orbit round them. The Earth has one natural satellite, which we call the Moon, and many Man-made, or artificial, satellites. Today many people other than astronomers often refer to a planet's natural satellites as its moons and reserve the word satellite for artificial satellites in orbit round the Earth.
The orbit of a satellite may be elliptical or circular, at any height above the surface of the planet, and the plane of its orbit may be at any angle to the equatorial plane of the planet. The orbit of the Moon round the Earth is almost circular, at a distance of about 400 000 km from the Earth, and at a slight angle to the Equator.
The orbit of the Moon and the other planets and their natural satellites are discussed at greater length in other science pages of this web site - to see the index to the science pages please click here 
The rest of this page is concerned only with the Earth's artificial satellites.
The Earth's atmosphere stretches thousands of kilometres into space, although of course it gets thinner as you go higher. All satellites are slowed down by air friction, and will eventually fall back to Earth - most burn up harmlessly as they do so. A satellite in a very high orbit will last much longer than one in a low orbit, but the higher the orbit the greater the energy needed to put it into orbit.
The higher the orbit of a satellite, that is, the further it is from the Earth, the longer it takes to orbit the Earth. The time it takes to orbit the Earth is called its period. A satellite in a circular orbit at a height of 800 km has a period of about a hundred minutes, and the Moon, at a height of about 400 000 km, has a period of about twenty eight days.
A satellite in an equatorial orbit, that is, going round the Earth in a circle above the Equator, at a height of about 40 000 km will have a period of twenty four hours, so will go round the Earth at exactly the speed of the Earth's rotation. This means it will always stay above exactly the same point on the equator.This orbit is called a geosynchronous orbit (from the Greek for at the same rate as the Earth) or sometimes, but incorrectly, a geostationary orbit. A satellite in geosynchronous orbit is however 40 000 km from the Earth and so you need a special very directional hi-gain aerial (usually a dish or a special aerial called a YAGI) to send signals to it or receive signals from it, but once you have aligned your dish or YAGI to point to the satellite you never have to align it again.
The footprint of a satellite depends upon its height. A satellite in a high orbit can see, and be seen from, a much larger area of the Earth than one in a low orbit, and what is true of light waves is also true of radio waves: a satellite which is out of line of sight is also out of radio contact.
A satellite in a geosynchronous orbit will see almost the whole of one hemisphere, but always the same hemisphere, whereas a satellite in any other orbit will pass over different parts of the Earth's surface. A satellite in a polar orbit, that is, where the satellite always passes over both the North Pole and the South Pole every orbit, will in the course of a few orbits pass directly over every part of the Earth's surface. The International Space Station (ISS) is in an orbit at a height of about 400km and has a period of about ninety minutes. The angle of its orbit takes it over 85% of the Earth's surface and 95% of its population. There are many web sites devoted to the ISS; for one giving a general overview please click here
Geosynchronous orbits are used for tv satellites as once the satellite dish has been correctly aligned on the satellite it will remain aligned - of course if you want to receive signals from more than one satellite you must have more than one dish, or a motorised dish. They are also used for some weather satellites because you can take still pictures of a part of the surface of the Earth at regular intervals and transmit them back to Earth and store them, and then play them back at a higher speed to produce an animated sequence, as we see on the weather forecast on tv.
A satellite in a low orbit will see a much smaller part of the Earth's surface but from much closer, and so photographs will show much more detail, but of course only a satellite in geosynchronous orbit takes all its pictures from the same point in the sky, so only pictures from a geosynchronous satellite can be animated. Satellites used for making maps or carrying out geological surveys are usually in low orbits.
The satellite is visible, and of course we can only receive radio signals from it, as it passes from A to B - in any other part of its orbit it will be below the horizon. The lower the orbit of the satellite the shorter this distance, and also, because lower satellites have shorter periods, the faster the satellite appears to be moving across the sky. So a satellite in an orbit at a height of 800 km is visible for up to fifteen minutes whereas one in an orbit at a height of 400 km is visible for less than four. Of course these times are shortened if the satellite does not pass directly overhead. The rotation of the Earth on its axis also affects the time of visibility.
If you want to be able to see a satellite or receive signals from it you must know when it will be passing over. There are computer programs which enable you to calculate this information, and there are also web sites giving the times of visibility for many satellites. To find the times of visibility of the ISS you can click here
To see a picture of the position of the ISS superimposed on a globe, updated every minute, please click here
Both of these sites are operated by NASA.
All satellites get the energy they need to drive their cameras, on-board computers, transmitters and other electronic equipment from solar panels. Satellites in low orbits are often in the Earth's shadow (that is, the Earth is between them and the Sun) so need to have large rechargeable batteries on board; a satellite in geosynchronous orbit is so far from the Earth that the only time it is in the Earth's shadow is for a few hours a day for a few days near the time of the equinoxes (March 21st and September 21st), so it only carries small rechargeable batteries. Near the equinoxes therefore it will be very short of power. To save power, weather satellites transmit far fewer pictures than usual at the times of the equinoxes, and so at these times you may not see animated pictures on tv weather forecasts. Satellite tv pictures may also be of poorer quality at this time.
A satellite in orbit round the Earth is subject to the Earth's gravity, which is the force keeping it in its orbit. But it is also subject to the gravity of the Sun and Moon, even the gravity of the asteroids and other planets, to atmospheric drag, and to impact from particles of dust and other space debris. All of these tend to disturb its orbit or set it spinning, so that for example its solar panels no longer point at the Sun. Tiny on-board rockets are needed to correct for these disturbances. These rockets are controlled by either on-board computers or radio signals from an Earth station.
At some stage the satellite will come to the end of its life: it may be damaged by meteorite impacts so its solar panels or other equipment no longer work properly, its task may be taken over by another satellite, for example analogue tv satellites are replaced by digital ones, or it may just run out of fuel for its control rockets. Once it is no longer being controlled its orbit will begin to change, and almost all changes to its orbit will tend to make it more elliptical - elliptical orbits are discussed in the next section. An elliptical orbit will take it nearer the Earth for a part of each orbit, and the increased atmospheric drag will gradually cause it to be pulled back to Earth, and eventually it will burn up in the Earth's atmosphere.
This is the fate of all satellites, even Space Stations, except that Space Stations such as the Russian Mir are too big to burn up completely in the Earth's atmosphere. These are brought down in a controlled way, by firing special on-board rockets, so that the pieces fall into the sea or uninhabited parts of the Earth's surface.
Elliptical orbits are often used for observation and reconnaissance (spy) satellites. These are designed to come very near the Earth at the point of interest, for example a place where there is a lot of military activity, to take photographs and record radio transmissions. These are stored on board the satellite and then transmitted back to its base as it flies over it. A satellite in a circular orbit at this height would be subject to so much atmospheric drag that it would have a very short life. To extend its life it is put into an elliptical orbit so that for most of its orbit it is much higher and so experiencing very much less drag.
© Barry Gray September 2001
