Apex/antapex ratio of the crater density generated by the current impactors in related works compared with ours. Our relation between N_c(apex) /N_c(antapex) and a_M with v_enc = 22.1 km s^-1 and α_p = 1.75 (solid curve) is close to the empirical one in Le Feuvre & Wieczorek (2011), which works for a_M = 20−60 R_⊕ (Eq. (2); dashed curve); the former with v_enc = 19.7 km s^-1 and α_p = 2.59 (dotted curve) is even nearly equal to the latter in its working range. Simulation results from Gallant et al. (2009) and Ito & Malhotra (2010) assuming the current lunar orbit (triangle and square) are also indicated.