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W.F. Bottke (SwRI), A. Morbidelli (Obs. Nice), J.M. Petit (Obs. Nice), R. Jedicke (U. Arizona)
Recently, Bottke et al. (2000, Science 288, 2190) modeled the orbital and size distribution of the near-Earth asteroids (NEAs) using a combination of numerical integration results, observational biases, and the orbits and sizes of NEAs discovered by Spacewatch. They estimated there are ~900 H < 18 (i.e., km-sized) NEAs, 29%, 65%, and 6% which reside on Amor, Apollo, and Aten orbits, respectively. Most NEAs were found to come from three so-called intermediate sources (IS), regions which are constantly fed material escaping the main belt. In this abstract, we concentrate on characterizing the Intermediate-source Mars-Crossers (IMCs), the IS population located adjacent to the main belt. IMCs have orbital parameters q > 1.3 AU, 2.06 AU \leq a \leq 2.48 AU or 2.52 AU \leq a < 2.8 AU, i below the \nu6 resonance, and a combination of (a, e, i) values such that they cross the orbit of Mars during a secular oscillation cycle of their eccentricity. They provide many of the impactors which strike Mars. To estimate the size of the IMC population, we assume the IS and NEA regions are in steady state. This forces the combined inflow/outflow flux from the IS regions to equal the inflow/outflow flux from the NEA region. We estimate that the IMC outflow flux is the number of NEAs derived from the IMC region (250 H < 18 bodies) divided by the mean lifetime of the IMCs in the NEA region (3.85 Myr), or 64 H < 18 asteroids Myr-1. This flux must equal the population of IMCs capable of evolving into NEAs multiplied by \tau, the fractional decay rate of the IMC population per Myr. Since numerical integration results suggest \tau = 0.018 Myr-1, we predict that the ``active" part of the IMC population contains 3500 H < 18 asteroids. Roughly one-third of the IMC bodies we integrated for 100 Myr, however, encountered Mars yet failed to enter the NEA region. Including this ``non-active" population in our estimates, we predict that the total IMC population contains ~5300 H < 18 bodies, nearly 6 times the estimated size of the NEA population. The accuracy and implications of this result will be discussed.
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