Orbital Decay

Orbital decay is the process of prolonged reduction in the altitude of a satellite's orbit.

This can be due to drag produced by an atmosphere due to frequent collisions between the satellite and surrounding air molecules. The drag experienced by the object is larger in the case of increased solar activity, because it heats and expands the upper atmosphere. For larger bodies, tidal effects can cause orbital decay, and for even larger ones gravitational radiation can have an effect.