[meteorite-list] Meteoroids from the asteroid belt to earth

From: Matson, Robert <ROBERT.D.MATSON_at_meteoritecentral.com>
Date: Thu Apr 22 09:54:05 2004
Message-ID: <AF564D2B9D91D411B9FE00508BF1C8698E59FC_at_US-Torrance.mail.saic.com>

Hi Rick,

> The asteroid belt to my knowledge is 168,000,000
> million miles away from the Earth.

I assume you're talking about the closest part of the "belt",
or maybe the average distance. The belt is actually VERY wide,
so some parts of it are quite a bit closer than others. And of
course, there are many hundreds of asteroids in earth-crossing
orbits: orbits that are elliptical, with their perihelions
closer to the sun than earth, but their aphelions further than
Mars. But continuing with your thought:

> If the average speed of the meteoroid coming out of the belt
> where 10 mph that would mean in 24 hours it traveled 240 miles
> one week 1680 miles one year 87600 miles. In 100 years 8760000
> miles To do 168,000,000 miles would take about 1917 years. So
> If I found a meteorite in 2002 the meteoroid had to start it's
> journey in 85 A.D. (if using AD - Anno Domini is to heavy for
> this list let me know) I wanted to present this write up as
> a sales pitch to the general public. Is this on or off track??

Quite a bit off track, I'm afraid. What you're neglecting is that
meteorites nudged out of the belt by either impacts or close
encounters with one another cannot just make a bee-line for
earth. They still have to follow gravitational laws. This means
that any meteoroid on an intercept trajectory with the earth
must follow an elliptical path (or hyperbolic, but I won't
unnecessarily complicate this) with the sun at one focus of
the ellipse. The velocity of the meteoroid along its trajectory
varies considerably depending on its distance from the sun.

If a meteroid has an aphelion in the Main Belt, and a perihelion
inside or at the same distance as the earth's roughly circular
orbit, the closing velocity between the two during an "intercept"
will be quite considerable -- over 10 kilometers per second.

The point I'm getting at is that "average velocity" doesn't
really tell the story very well. If a meteoroid coming from
the asteroid belt is going to hit the earth on a particular
orbit, it will do so in less than 3 years transit time. (For
manmade satellites we call this a "Hommann Transfer"). If
instead, the meteoroid happens to miss the earth on one orbit,
then it has to go all the way back out to the asteroid belt
before making another attempt. Obviously everything has to
be right in this cosmic pinball game for an impact to occur,
so these earth-crossing rocks can orbit for hundreds, thousands,
even millions of years before their orbits line up just so.

Cheers,
Rob
Received on Sat 16 Feb 2002 10:41:16 PM PST