As shown byChesley and Spahr (2004)and inFig. 1there are
important differences between the impactor population and the
NEOs. The impactors have orbits with lower semi-major axis, inclination and eccentricity. This has the effect of decreasing the Earth
encounter and impact velocity (v1andvimp respectively) for the impactors relative to the NEOs. The decreased impact velocity has
implications for modelling the impact crater size-frequency distribution on the Earth and Moon since lower impact energies per unit
mass require larger impactors to create equivalent-size craters.