Oral, Chairs: D. Durda, P. Michel, Saturday, December 1, 2001, 11:05am-12:35pm, Regency E

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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 nu_{6}
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 H_{t}, beyond which we assume that
the families magnitude distribution is background-like. We
find that H_{t}=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/cm^{3}, 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.