Relative encounter velocities between planetesimals. In panel 1 we compare the relative velocities in the case of small planetesimals at different times. There is a significant time variability in the impact velocity which, however, remains high all the time. In panel 2 we illustrate the relative velocities for R = 25 km planetesimals at t = 10⁴ yr. The black lines in both plots show the average impact velocity computed in small radial bins The 2 horizontal lines display the limiting Δv values for the accretion/erosion frontier, as deduced from Leinhardt & Stewart (2012), for 5 km impactors and 25 km ones.