Oral, Chairs: B. Buratti, C. Hergenrother, Tuesday, 2000/10/24, 10:30-11:10m, C106

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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 \nu_{6} 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.

The author(s) of this abstract have provided an email address for comments about the abstract: bottke@boulder.swri.edu

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