What is a satellite?
A question of orbit
In the case of a satellite, an orbit is its trajectory in space around the Earth or another planet under the effect of gravity*. Close to earth (orbit" >low orbit) or far away (high orbit), a satellite will have very different functions.
Is an orbit round or not?
Well, that depends! If the satellite is put into an orbit close to earth (below 500 km), it won’t need too much energy to “detach” it from earth’s pull; a launcher* speed of about 8 km per second will do the trick. All of a sudden the speed transmitted and Earth’s continuing pull cancel each other out exactly and the satellite describes an almost perfect circle. But had we wanted to send the satellite further and had the rocket provided greater speed, then the satellite would free itself up a bit from the pull. The orbit wouldn’t be circular anymore, and would become longer on the side opposite the point of projection, turning into an ellipse. This orbit could then be stabilised and made circular again so that the distance between Earth and the satellite would remain constant.
OK, but so what?
Based on their orbits, satellites are used for different things. We might say that their orbit is a bit like their identity card. In practice, if a satellite is very high up, it “sees” a much larger part of Earth, virtually half! That’s what happens with what are called “geostationary” satellites. Positioned 36,000 km from Earth on a geosynchronous* orbit, they turn at the same time as the earth, giving the impression, from earth, that they are not moving. These are perfect satellites for communications. On the other hand, the lower the orbit (between 500 and 1,000 km), the better will be the photos taken by the satellite, which is ideal for “observation” satellites with civilian applications (such as weather forecasting, for example) or “reconnaissance” ones when they are military. With the most powerful ones you can see with a resolution of several centimetres!
