**31st Annual Meeting of the DPS, October 1999**

*Session 28. Near Earth Asteroids*

Contributed Oral Parallel Session, Tuesday, October 12, 1999, 2:00-3:30pm, Sala Plenaria
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## [28.01] Understanding the Distribution of Near-Earth Asteroids

*W.F. Bottke (Cornell U.), R Jedicke (U. Arizona), A. Morbidelli, B. Gladman, J.-M. Petit (Obs. de la Cote d'Azur)*

No accurate estimate of the orbital and absolute magnitude
distribution of the Near-Earth Objects (NEOs) currently
exists, largely because: (i) the known NEOs are biased by
complicated observational selection effects which favor the
discovery of bright or large objects that come close to
Earth; (ii) relatively few NEOs have been discovered, making
debiasing efforts difficult; (iii) NEO orbits are chaotic on
short timescales (< 1000 years); and (iv) the source
regions and replenishment mechanisms for the NEOs are not
well understood. For these reasons, observers are still
struggling to increase NEO detection rates, while the
interpretation of existing data continues to be problematic.

We propose a new method to attack this problem, one which
takes advantage of theoretical advances and new numerical
tools. To treat observational biases, we have applied a
model-independent, semi-analytical method for calculating
the probability that an asteroid observation program will
find a given asteroid in a (a, e, i, and H; semimajor
axis, eccentricity, inclination, and absolute magnitude,
respectively) bin per square degree at opposition at the
Vernal Equinox (Jedicke and Metcalfe 1998). To discover how
NEOs are replenished, we have used symplectic numerical
integration techniques which can track the orbital paths of
test bodies started in several potential NEO source regions
(e.g., 3:1 resonance, v_{6} resonance, multiple weak
mean-motion resonances). By merging the observational biases
with these NEO dynamical ``roadmaps" (and an NEO absolute
magnitude distribution), we get a probability distribution
which, if the sources have been weighted correctly, can be
directly compared to the known NEOs. By testing a range of
possible source combinations, we have produced a
``best-fit'' distribution which not only yields the
normalized and debiased NEO orbital and absolute magnitude
distribution (over various NEO sizes) but also the relative
importance of each NEO replenishment source.

These results have several important applications for NEO
observers and for studies of the impact rates of asteroids
onto the terrestrial planets. These issues are discussed in
an abstract by Morbidelli et al. (this issue).

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