The Meteoroid Flux Near 10_12 Joules: A Comparison Between Seismically Detected Impacts on the Moon and Earth-Based Observations

We determined the impact locations and energies of the 18 largest impact events detected by the lunar seismic station network. We estimate that the impact flux on the lunar surface near 10_12 Joules is given by log10n(E)&#610.99log10E+11,5, where is the number of meteroids having impact energies larger than E in Joules. This is compared to estimates of the meteroid flux from lunar crater statistics, terrestrial studies of large meteors, and asteroid detection rates of the Space Watch Camera. Our result is consistent with lunar crater statistics within the error limits.These error limits are large, though, as the crater scaling laws that relate crater size to impact energy are poorly known. However, our flux estimate is a factor of four to five times lower than terrestrial estimates. A possible reason for this discrepancy is that gravitational acceleration of meteoroids and focusing of meteoroid trajectories near Earth are not taken into account properly. The encounter velocities of the meteoroids must be known in order to consider this effect. If the velocities before gravitational acceleration were near 9 km/s, then, the difference between terrestrial and lunar flux would be large and this could explain the observed discrepancy. In contrast, though, mean encounter velocities are thought to be as high as 17 km/s, and this is not sufficient to produce a significant difference in the impact flux on the surfaces of the Moon and the Earth. The recent disovery by the Space Watch Camera of a "belt" of small asteorids with perihelia that cluster near 1 AU may resolve this mystery. The average encounter velocities with the Earth-Moon System of objects in such orbits would be lower than was believed earlier. If such objects are indeed the major contributors to the lunar impact flux near 10_12 Joules, lunar seismic and Earth-based meteoroid flux estimates would be reconciled