Dynamical consequences of meteorite impacts on the moon

The magnitudes of the excitation of free precession of the lunar spin axis about the position defined by Cassini's laws, free libration in longitude, and free wobble are determined as a function of meteorite angular momentum relative to the lunar center of mass and the position of impact on the lunar surface. Angular-momentum conservation suffices for the estimates of precession and libration excitation, but a cratering model for the ejecta distribution is necessary for the estimate of the wobble excitation. The simultaneous excitation of free wobble is always associated with the excitation of precession, and the angular amplitude is at least comparable to, but may exceed, that of the induced precession by a factor of 3 or 4. It is possible to excite a free libration in longitude with no first-order excitation of free wobble, but generally, all three free motions are excited simultaneously. The induced libration will nearly always have the largest amplitude. For crater sizes scaled as powers of impact energy, impacts leaving craters as small as a few kilometers in diameter can excite free motions which will ultimately be observable by the lunar laser-ranging experiment.