Ioonos

Two philosophies of travel…

There are two reasons to want to drive a satellite. The first is to want to keep it in its orbit, the second is to take it into its final orbit. In the first case, you have to counteract the tendency of the satellite to get further away from its orbit. There are several causes for this phenomenon: the residue of friction from the Earth’s atmosphere, solar wind, the irregularities of the Earth, which is not a perfect sphere and other reasons. By activating its motors for short bursts you can get it back into its “right lane”.

Reaching the final orbit

This is the second case. It applies especially to geostationary* satellites, very high up, at 36,000 km. A launcher* cannot manage in one go to send them so far in a perfectly circular way. The launcher first takes the satellite on a first orbit, called “the transfer”. When “releasing” the satellite at an altitude of 600 km, the launcher provides it with enough speed to achieve an elliptical orbit, which is where the distance to the centre of the Earth varies, with one very distant point (the “apogee”, in this case at around 35,700 km) and a very close point (the “perigee*”). The highest point, where the atmosphere’s resistance is weakest, is used to provide, with the satellite’s motors, the extra speed needed to go into the final orbit. End of the line, all change!

Drive or watch, you have to choose

But the satellites’ fuel* reserves are limited. So you have to reduce as much as possible corrections to the trajectory to extend their life. The satellite’s real motor remains the gravitational pull around Earth. And if another satellite on the same orbit starts getting in the way, it is best to brake to drop a bit and carry on the orbit lower down, where the higher, angular velocity lets it catch up the lag before going back up to its normal orbit…