[InclinedPlane]

The influence of the atmosphere on orbiting objects is enormously dependent on altitude. Manned spacecraft usually orbit at extremely low altitude because they tend to be heavy and the tradeoff of regular reboosts is mostly worth it (manned space stations need regular resupply anyway). However, whereas the orbital lifetime of the ISS may be measured in mere years even just a few hundred km higher increases lifetime to centuries and beyond that to millenia.

Here's an excellent infographic on orbital debris: http://media2.s-nbcnews.com/i/MSNBC/Components/ArtAndPhoto-F...

You can see that most of the debris below 700 km has naturally been culled by aerodynamic drag, but above that altitude there is still a large amount of debris, most of which has an expected natural lifetime of many centuries.