I kind of understand that it’s like falling but falling past Earth but not really. Does it require power (in the case of artificial satellites, obviously the moon doesn’t have power) or does orbit last basically forever. If two things in orbit collide will they fall? If Deimos and Phobos somehow crash into each other will they just fall to Mars?
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You’re falling down, but you’re just moving sideways fast enough that you miss the planet, it’s curving away from you as fast as you’re accelerating towards it. Imagine shooting a bullet perfectly sideways with no air, gravity pulls it down, but the curvature of the earth gives it extra height. Go fast enough and the planet can’t pull you down fast enough to make you hit the ground.
Orbits don’t last forever, if nothing else the gravitational waves will sap the energy required to maintain it. However that’s not going to happen on timescales we need to worry about. The moon is also gaining orbital energy from tidal forces, slowing Earth’s spin in the process.
What does matter for low orbits is the atmosphere. There’s not really a hard cutoff point where there’s no more atmosphere. Stuff in lower orbits will hit enough air molecules to deorbit in a few years. Stuff out at geostationary orbit will be there for a VERY long time.
If two objects collide they may or may not stay in orbit, it depends on what their velocity ends up after the collision. If they’re moving in the same direction around the planet, the objects or debris can stay in orbit. if they’re moving in opposite directions there’s a good chance the bulk of the objects won’t have enough energy to stay in orbit, though some fragments might.
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