Orbital debris are unevenIy distributed in space; the amount of debris varies significantly with altitude. Maximum debris concentrations can be found in low-Earth orbit at altitudes of 600 km to 1600 km.
Of the 1,540 live satellites in space, 1,300 active satellites are operating at altitudes of 600 km to 1100 km.
Satellites are subject to atmospheric drag from the residual atmosphere present at their operational altitude. This drag slows down a satellite and results in the reduction of its orbital altitude, essentially pulling it downward into denser atmospheres. Eventually the satellite will be pulled into Earth’s atmosphere where it will be consumed by the extreme heat generated by the pressure waves formed in front of the satellite edge as it penetrates Earth’s atmosphere at very high speeds.
The higher a satellite orbits, the less the residual atmosphere and, therefore, less the atmospheric drag. Consequently, satellites flying in lower orbits are pulled back into Earth’s atmosphere faster than higher flying satellites.
It is estimated that satellites without propulsion located below an altitude of 600 km will return into Earth’s atmosphere within 25 years. Above this altitude satellites have to be actively slowed down to fall back into Earth’s atmosphere in a manoeuvre called Post Mission Disposal (or PMD). To perform this operation a satellite has to have a functioning propulsion system and sufficient propellant for the operation. Orbits below an altitude of 600 km are sometimes called self-cleaning orbits.