Re: Could I deorbit an object by throwing it at the earth?

This is a very good question.  The short answer is yes, you could de-orbit
a 10-kg object by throwing it at the Earth.  As for how long it would take
to hit the upper atmosphere, that depends on where you define the "upper
atmosphere" to be.  

The atmosphere is often taken to "end" a few hundred kilometers above the
Earth, but really the density just decreases out into space.  Even at the
orbit of the shuttle (around 380-390km), there is still enough atmosphere
that any space junk will eventually fall to Earth on its own, eventually. 
In fact, one could say that the shuttle doesn't really leave the Earth's
atmosphere!

Other scientists have answered similar questions about the atmosphere at 
 http://image.gsfc.nasa.gov/poetry/ask/aearth.html