The main belt as source of Near-Earth Asteroids
Marzari F., Farinella P., Vanazi V.
Abstract
We investigate the flux of
main-belt asteroid fragments into resonant
orbits converting them into near-Earth
asteroids (NEAs), and the variability of this
flux due to chance interasteroidal
collisions. A numerical model is used, based
on collisional physics consistent with the
results of laboratory impact experiments. The
assumed main-belt asteroid size distribution is
derived from that of known asteroids extrapolated down
to sizes of ~40 cm, modified in such a way to yield a
quasi-stationary fragment production rate over
times ~ 100 Myr. The results show
that the asteroid belt can supply a few hundred km-sized NEAs per year, well enough to
sustain the current population of
such bodies. On the other hand, if our
collisional physics is correct, the
number of existing 10-km objects
implies that these objects either have very
long-lived orbits, or must come from a
different source (i.e., comets). Our model
predicts that the fragments supplied from the asteroid belt have initially
a power-law size distribution somewhat steeper than the observed one,
suggesting preferential removal of small objects. The component of the NEA
population with dynamical
lifetimes shorter than or of the order of 1 Myr
can vary by a factor reaching up to
a few tens, due to single large-scale collisions
in the main belt; these fluctuations
are enhanced for smaller bodies and faster
evolutionary time scales. As a consequence, the
Earth's cratering rate can also change by
about an order of magnitude over the 0.1 to
1 Myr time scales. Despite these sporadic
spikes, when averaged over times of 10
Myr or longer the fluctuations are
unlikely to exceed a factor two.