I have to write something about this soon, based on my AAA paper "The gravity of archaeology".
Richard Cathcart is not only an excellent space junk poet but has some interesting insights on this topic (in his 1979 publication The Developing Artificial Geography of the Solar System, Public Administration Series P-206, Illinois). He makes the point that the lithosphere is currently as impenetrable to humans as space used to be, and that the upper limit of the biosphere is where the International Space Station now orbits.
He also notes that the Earth is "eroding", in a sense, as material is injected into orbit. But it is also aggrading as far huger quantities of cosmically derived material fall to earth every day. This interchange of material between what we call earth and space is a good illustration of the artificiality of these boundaries, as Nigel Clark (2005) also argues in Ex-orbitant Globality (Theory, Culture and Society 22(5):165-185).