The physics of satellites orbiting Earth involves external perturbations that can affect their orbits. For satellites in low Earth orbit below 2000 km, atmospheric drag is the primary force and causes orbital decay over time through collisions with gas molecules. Outside of low Earth orbit, other forces like thermal drag and gravity perturbations still influence satellite orbits but do not necessarily decrease altitude. Atmospheric drag is the largest source of error in modeling low Earth orbiting satellite trajectories due to its unpredictable nature.