  The presence of debris in Earth's orbit poses a significant risk to human
activity in outer space. This debris population continues to grow due to ground
launches, loss of external parts from space ships, and uncontrollable
collisions between objects. A computationally feasible continuum model for the
growth of the debris population and its spatial distribution is therefore
critical. Here we propose a diffusion-collision model for the evolution of
debris density in Low-Earth Orbit (LEO) and its dependence on ground-launch
policy. We parametrize this model and test it against data from publicly
available object catalogs to examine timescales for uncontrolled growth.
Finally, we consider sensible launch policies and cleanup strategies and how
they reduce the future risk of collisions with active satellites or space
ships.
