|MadSci Network: Physics|
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
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