How long does it take for a satellite to fall to Earth?

The purpose of this paper is to introduce students of science and engineering to the orbital lifetimes of satellites in circular low Earth orbits. It is only necessary to know about classical mechanics for this calculation. The orbital decay of satellites is due to the interaction of the satellite with the surrounding gas, atmospheric drag. Perturbative forces arise from solar and lunar gravitational fields, solar radiation pressure and geomagnetic activity, in addition to aerodynamic drag. Our calculation uses a simple model for atmospheric density as a function of altitude. Finally, the result is compared with another calculation using an improved force model.