What happens to the old satellites and other debris that is left in space? Why is it even acceptable to leave our trash / garbage out there?
Once something is in space, there's only a few ways it'll ever come back down.
- It rockets down to re-enter Earth
- It is in the upper part of the atmosphere, which slowly will bring it down.
- Sunlight pushes it into an orbit where it intersects the atmosphere, or even leaves Earth orbit.
- It hits another object, changing it's velocity.
Once debris is up there, it's hard to get down. Spacecraft are required to put themselves into either a graveyard orbit, or an orbit where the spacecraft will naturally return to Earth shortly thereafter. However, some spacecraft fail, others were built before this was a known problem, and overall, it's expensive to ensure that it happens.