Satellites experiencing Orbital Decay
Satellites at lower altitudes do not stay in orbit forever and old or faulty spacecraft sometimes fall back to Earth. Low Earth Orbiting (LEO) satellites experience what is called orbital decay, the drag force caused by interaction with the few air molecules that exist at lower altitudes slow the satellite down reducing it's orbital altitude. The lowest altitude above the Earth at which an object in a circular orbit can complete at least one full revolution without propulsion is approximately 160km, if the orbit is not corrected gravity will inevitably pull the Satellite back into the Earths atmosphere where the harsh conditions of reentry are likely to destroy it.