If there is so much space junk floating around the earth, how come no spacecrafts returning to earth are hit?
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Most space junk is tracked. If a astronaut drops a wrench while doing a space walk, it’s someone in NASA’s job to plot that wrench’s orbit until it reenters the earth’s atmosphere (if ever). If these orbits are projected to cause a collision with another satellite, they can usually use what fuel they have left to move out of the way to avoid hitting it.
That said, satellites don’t have infinite fuel. Eventually they’ll lose the ability to avoid predicted collisions. Usually before this happens there is a plan in place to use what fuel it has left to crash into a vacant part of the Earth. That doesn’t always work out as plan, and older satellites didn’t even make such plans.
On top of this not every single thing floating around the earth is tracked. Some objects are just too small and numerous, or NASA just doesn’t know of it’s existence yet. It’s not really practical for NASA or other space agencies to notice and track a piece of paint that flaked off a satellite. [Yet even such a small thing can cause real damage](https://www.theverge.com/2016/5/12/11664668/iss-window-chip-space-debris-tim-peake) without wind resistance to slow it down.
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