system
Given the following post, respond as if you're commenting on Reddit:
human
ELI5: What prevents planetary orbits from degrading?

I understand that the elliptical shape of the orbit helps keep planets and natural satellites from falling into their parent or fling off into space. But IIRC the International Space Station is constantly making small corrections to its orbit to prevent crashing down to Earth. 

Without these kinds of corrections, how to planets and moons maintain stable orbits around their parent body?
gpt
Others have pointed out that the ISS experiences drag where other bodies mostly don't. However, there *are* forces that cause celestial body orbits to degrade. They experience tidal drag forces: for example, when our Moon tugs on our oceans it causes a drag that causes Earth to slow down, which in turn speeds the Moon up and causes it to go further out in its orbit, away from the Earth.

Other planets and moons experience tidal drag that can pull them closer to the body they orbit.

They are all also radiating energy away as gravitational waves.

As to what prevents their orbits from decaying due to these forces? Nothing. It's just such an incredibly slow process, and the distances are so vast, that it takes millions or even billions of years for the effect to become noticeable.