The orbit of a satellite is commonly defined through its so called Keplerian parameters. These parameters represent the trajectory that the satellite will follow if no-perturbation are acting on it. A series of forces act on the satellite to perturb it away from the nominal orbit. We can classify these perturbations, or variations in the orbital elements, based on how they affect the Keplerian elements. The actual orbit of a satellite will result from a combination of these perturbations. Periodic maneouvers are needed to bring the orbit back to nominal conditions. The lifetime of a satellite is defined as the time interval that it takes to decay from its initial altitude to an altitude causing the satellite reentry down to the atmosphere. Therefore lifetime of a satellite should not be confused with the time during which the satellite will provide useful information (this operational phase, in general, is designed to last 5 - 7 years). In fact, all satellite terminating operational phases in orbits passing through the LEO region should be de-orbited or, where appropriate, manoeuvred to an orbit with suitably-reduced lifetime, that is, should be left in an orbit where drag and other perturbations will limit lifetime. The actual duration of the satellite in orbit will depend from the intensity of the perturbations which will affect its orbit. In case of satellite on GEO orbit, at the end of the operational phases they will be located on a disposal orbit, that is an orbit which do not cross the protected region. The protected region is the altitude region ranging from GEO - 200 km to GEO + 200 km and inclination region between -15 deg and +15 deg. Satellites in low Earth orbit, with perigee altitudes below 1000 km, are predominantly subject to atmospheric drag. This force very slowly tends to circularise and reduce the altitude of the orbit. The rate of 'decay' of the orbit becomes very rapid at altitudes less than 200 km, and by the time the satellite is down to 180 km it will only have a few hours to live before it makes a fiery re-entry down to the Earth.
Illustrate the factors limiting lifetime of satellites on their originally planned orbits
Planned