Dynamical Evolution of Near-Earth Asteroids
M. C. Nolan (Arecibo Observatory)
Over the last few years, numerical integrations by many workers have
shown that asteroids are perturbed into Earth-crossing orbits very
quickly ( 
y). It is, however, apparently more difficult for
asteroids to be captured into orbits similar to the observed population. In
addition, some ``special'' populations have been suggested, such as
asteroid streams and an overabundance of small low-eccentricity
objects.
There are a number of Solar-system resonances that affect objects in
the terrestrial-planet region of the Solar system. Integrations have
shown that the 
, 
, 
and Kozai resonances
affect most Earth-crossing asteroids over the course of a few million
years. There are certainly other resonances as well. Such resonances
could (and do) make some orbits more stable than others, which could
be responsible for these effects.
I integrated the orbits of 2400 asteroids in Earth-crossing orbits for 2 to 10 million years, in an attempt to identify (or rule out) the sources of these special populations. Earlier work showed that the low-eccentricity objects mix with the general population in a few million years, but did not examine how such objects could be generated. In addition, those integrations showed some regions of orbital-element space where the population should be enhanced, but the number of particles examined was too small to be definitive. The current integrations examine many more particles for a much longer time.
These integrations show that, under some initial conditions,
population enhancements do occur. In some cases, these regions are
dynamic: there are always extra particles there, but individual
particles move in and out, due to resonances and to close approaches
to the Earth. However, in some of these cases, the initial conditions
were somewhat artificial. More integrations and analysis are in progress.