[meteorite-list] Asteroid orbital evolution

From: Matson, Robert D. <ROBERT.D.MATSON_at_meteoritecentral.com>
Date: Fri, 15 Oct 2010 09:43:47 -0700
Message-ID: <9180F6B27399C541B10663E21C8BDE9269FE11_at_0461-its-exmb09.us.saic.com>

Hi Bob and List,

> When I give presentations to groups about meteorites, I often get
asked this
> question, "After all this time, what would cause an asteroid to depart
from
> its orbital confines in the "asteroid belt" and to end up crossing the
> Earth's orbit?"

> Now I can give a graphical answer by pointing to the (new) images in
this
> most recent article:

>
<http://www.dailymail.co.uk/sciencetech/article-1320385/Asteroid-collisi
on-90million-miles-Earth-caught-NASA-camera.html>

(Massive collision between two asteroids 90 million miles from Earth
caught
on camera for first time)

While collisions in the Main Belt provide a potential mechanism for
producing
asteroids in earth-crossing orbits, this is not the main source of
near-earth
asteroids. Usually, Main Belt inter-asteroid collision velocities are
quite
slow (a few hundred meters per second), which is far too low a delta-V
to
transform a Main Belt orbit into a planet-crossing one.

The real delivery mechanism is orbital resonance, the most efficient of
which is the nu-6 secular resonance. (When the Greek letter nu is
transcribed
to English, you'll usually see this resonance written as v6.)

The v6 resonance zone is at a distance of roughly 2 a.u., and the most
likely asteroid family to inject asteroids into the v6 resonance is the
well-populated Flora family. Once in the v6 resonance zone, the
eccentricity
of an asteroid's orbit starts to get "pumped up": as the centuries go
by,
the orbit shape becomes less circular and instead more elongated. The
asteroid's perihelion progressively decreases from 2 a.u., to 1.9, 1.8,
1.7 and so on, while its most distant point from the sun progressively
increases (2.1, 2.2, etc.) Eventually, the perihelion has decreased so
much that the asteroid crosses Mars' orbit (mean distance 1.52 a.u.),
possibly even impacting Mars itself, or getting flung by Mars' gravity
into an even more eccentric orbit that crosses that of earth. This
series of events is the main way that meteorites are delivered
to earth. To summarize:

1. Inter-asteroid collisions in the Main Belt produce asteroids with
orbits that evolve into Flora-like orbits (inner Main Belt)

2. Further collisions or perturbations of inner Main Belt objects
nudges them into the nearby v6 secular resonance.

3. v6 secular resonance operates quickly (roughly a million
years), transforming the orbit into a Mars-crossing one.

4. One or more close encounters with Mars further perturb the
orbit into an earth-crossing one.

5. Eventually the earth-crossing asteroid and the earth itself
happen to be at the same place at the same time and voila:
meteorites!

The most common meteorites on earth (L chondrites, representing
38% of all meteorites) are believed to have come directly from
the Flora family.

--Rob
Received on Fri 15 Oct 2010 12:43:47 PM PDT