Galactic exploration by directed Self-Replicating Probes, and its   implications for the Fermi paradox
  This paper proposes a long term scheme for robotic exploration of the galaxy,and then considers the implications in terms of the `Fermi paradox' and our search for ETI. We discuss the parameter space of the `galactic ecology' of civilizations in terms of the parameters T (time between ET civilizations arising) and L, the lifetime of these civilizations. Six different regions are described. 
Martin T. Barlow
physics.pop-ph