Planetesimal collision rate
= Planetesimal collision rate
For an impactor number density per size $n(D_{\rm im})$, relative speed $v_{\rm rel}$, and collision cross-section $\sigma(D,D_{\rm im})$, the differential collision rate of one target is $n(D_{\rm im})v_{\rm rel}\sigma(D,D_{\rm im})dD_{\rm im}$.