Observation and interpretation of meteoroid impact flashes on the moon

The first unambiguous detection of meteoroids impacting the night side of t he Moon was obtained during the 1999 Leonid storm. Up to eight optical flas hes were recorded with CCD video cameras attached to small telescopes on No vember 18, 1999. Six impacts were videotaped by at least two independent ob servers at the same times and lunar locations, which is perhaps the stronge st evidence for their collisional nature. The flashes were clearly above th e noise and lasted for less than 0.02 s. Although previous observational ef forts did not succeed in detecting impact flashes, additional candidates ha ve been reported in the literature. The evidence accumulated so far implies that small telescopes equipped with high speed cameras can be used as a ne w tool for studying meteoroid streams, sporadic meteoroids, and hyperveloci ty collisions. In this review we discuss the various intervening parameters for detectability of flashes on the night side of the Moon (geometrical ef fects, contamination by scattered light from the day side, and properties o f the meteoroids such as speed and flux of particles). Particular emphasis is placed on the analysis of the observations in order to derive relevant p hysical parameters such as luminous efficiencies, impactor masses, and crat er sizes. Some of these parameters are of interest for constraining theoret ical impact models. From a simple analysis, it is possible to derive the ma ss distribution of the impactors in the kg range. A more elaborate analysis of the data permits an estimate of the fraction of kinetic energy converte d to radiation (luminous efficiency) if the meteoroid flux on the Moon is k nown. Applied to the 1999 lunar Leonids, these methods yield a mass index o f 1.6 +/- 0.1 and luminous efficiencies of 2 x 10(-3) with an uncertainty o f about one order of magnitude. Predictions of visibility of the major annu al meteor showers are given for the next few years. These include the forth coming 2001 Leonid return, for which we estimate detection rates in the vis ible.