If there is so much space junk floating around the earth, how come no spacecrafts returning to earth are hit?
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Space is big. About 20,000 objects are large enough to be tracked. If you would distribute 20,000 objects uniformly over the surface of Earth then you get one every 25500 km^(2), that’s about the size of Wales or Vermont. What is the chance that you randomly run into e.g. one particular car parked anywhere in these places? To make things worse (or better, for spacecraft), in space things can pass each other at a different altitude. Imagine we replace Vermont with a massive parking garage, taking up the same space but having 200,000 floors. Somewhere in that parking garage is a single car. You are unlikely to find that car even if you actively search for it your whole life. Spacecraft do the opposite, of course, if they would get close to one of these objects they do maneuvers to avoid them.
It’s estimated that there are up to a million objects larger than 1 cm, with only 1 in 50 of them tracked. These could still be catastrophic on impact, so let’s fill the parking garage with 50 cars. Would you find one now? It’s still very unlikely.
There are even more smaller objects, but impacts of them are rarely catastrophic. They might damage some individual components. The ISS has a few holes in its thin solar panels, but that’s not an issue for the station overall. Spacecraft that carry people are shielded against these smaller impacts.
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