[meteorite-list] Asteroid orbital evolution

[Alexander Seidel]

Excellent explanation by Rob M (..as we are used to always get from  
him..)!

Cheers,
Alex

Von meinem iPhone gesendet

Am 15.10.2010 um 18:43 schrieb "Matson, Robert D." <ROBERT.D.MATSON at 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
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