Why Satellites Fall from Orbit

With no friction a satellite will orbit forever without engines, but Earth's upper atmosphere provides some friction.

Many years ago I watched a Star Trek episode with a group of science types. The crew was trying to get Enterprise's engines working again, otherwise the Enterprise and her good crew would crash into the planet they were orbiting.

Because we understood how orbits work, we laughed at the writers' lack of physics knowledge. Once something is orbiting a planet or star, it needs no engines to keep it in orbit. Earth doesn't have an engine to orbit the Sun and the Moon does not need engines to orbit Earth. These orbits have been stable for nearly 5 billion years.

Why then do satellites fall from orbit? Lately the possibility of a spy satellite falling from orbit has been in the news. Those of us who are old enough might remember the Skylab falling into the Pacific in the late 1970s. Occasionally other small satellites streak down from orbit.

The key is friction. Orbits remain stable as long as no frictional forces slow the satellite. Rather than ending abruptly; Earth's atmosphere just gradually gets thinner. In a low Earth orbit the atmosphere is very thin but not quite a vacuum. Hence there is a small amount of atmospheric resistance to slow the satellite. As the orbital speed slows, the orbit decays to a closer distance. The atmosphere is thicker, increasing the atmospheric resistance. The orbit decays faster and the satellite streaks down to Earth. Let's hope it doesn't hit a populated area.

If the Star Trek writers had written something into the script about atmospheric friction, we would not have made fun of them.