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A. Morbidelli (Observatory of Nice, France), W.F. Bottke (South West Research Institute, Boulder, Co.), R. Jedicke (Lunar Planetary Laboratory, Tucson, Az.), P. Michel (Observatory of Nice, France), E.F. Tedesco (TerraSystems Inc., Lee, NH)
Our NEO orbital-magnitude distribution model (Bottke et al., 2001, Icarus, in press) relies on 5 main intermediate sources for the Near Earth Object population: the nu6 resonance, the 3:1 resonance, the outer portion of the main belt (I.E., 2.8-3.5 AU), the Mars-crossing population adjacent to the main belt, and the Jupiter family comet population. The model establishes the relative contribution of these sources to the NEO population, in each region of the NEO orbital space. Therefore, by computing the albedo distribution of the bodies in/close to each source, we can deduce the albedo distribution of the NEO population, as a function of their orbital location. An important caveat is that the albedo distribution of main belt asteroids may change with the absolute magnitude, because asteroid families and background populations have different albedo and magnitude distributions. In our model we extrapolate the observed absolute magnitude distributions of the families up to some threshold value Ht, beyond which we assume that the families magnitude distribution is background-like. We find that Ht=15 provides the best match to (I) the color vs. heliocentric distance distribution observed by the SLOAN survey and with (II) the observed albedo distribution of NEOs.
Our model predicts that the debiased ratio between dark and bright (albedo smaller or larger than 0.089) NEOs with diameter larger than 1km is 0.8 . We estimate that the total number of NEOs larger than a kilometer is 834 which, compared to the total number of NEOs with H<18 (963), shows that the usually assumed conversion H=18~<=>~D=1km is slightly pessimistic, on average. The right statistical correspondence should be H=17.82~<=>~D=1km.
Combining our orbital distribution model with the new albedo distribution model, and assuming that the density of bright and dark bodies is 2.7 and 1.3 g/cm3, respectively, we estimate that the Earth should undergo a 1000 megatons collision every 64,000 years. The NEOs discovered so far carry only 18% of this collision probability.
We thank NASA and ESA for support.