A recipe for estimating the frequency of asteroid close approaches to Earth

We present a simple recipe for estimating the frequency of asteroid close approaches to Earth as a function of theasteroid's absolute magnitude H and the close approach distance r from the geocenter. This formulation can be usefulin assessing how rare a close approach is compared to the background rate from the asteroid population, similarly tohow the Palermo scale (Chesley et al. 2002) is used to rank the risk posed by virtual impactors (Milani et al. 2005).