Humans, since time immemorial, have had the habit of polluting the environment that they live in no matter where they go. And they have not left the outer space junk-free either. All the defunct, disused objects, and satellites, that are no longer valuable to mankind, are trashed in space where they obit around the planet, posing serious risks to passing spacecrafts or the orbital space stations.
There have also been incidents where there debris, reenter the orbit of the earth, travelling even in low orbital ranges at 17 thousand mph, and are instantly incinerated on encountering the earth's atmosphere where by very few of them actually reach the ground level.
