[meteorite-list] Park Forest impact locations by mass

From: Matson, Robert <ROBERT.D.MATSON_at_meteoritecentral.com>
Date: Thu Apr 22 10:21:09 2004
Message-ID: <AF564D2B9D91D411B9FE00508BF1C86901B4EBE9_at_US-Torrance.mail.saic.com>

Hi All,

I spent the better part of today and this evening modifying a
software model I wrote for propagating Columbia debris to the
ground into one that would simulate the impact locations of
Park Forest meteorites by mass. I had to make a number of
assumptions to simplify the simulation, but I don't think
these will affect the trend of the result -- only its
precision.

I started with the DoD vacuum impact point and velocity
vector, and then backed it up to the 100-km altitude point.
The DoD report does not provide an extinction point or any
altitude references, so the actual break-up altitude(s) is/are
unknown. However, based on witness reports I think it's safe
to assume that there were multiple fragmentation events, so
there's a wide range of realistic altitudes from which to
begin a ballistic propagation.

For a first hack, I chose that 100 km as an altitude at which
some fragmentation could have begun, and propagated masses
of 5, 10, 20, 50, 100, 200, 500, 1000, 2000, and 5000 grams
to the ground to see where they would end up. To simulate
random aerodynamic forces acting on individual masses, I
modeled a 0.5-degree dispersion cone around the initial
velocity vectors of all masses.

I used a standard density profile for the atmosphere, and
chose a nominal 3.5 g/cm^3 density for the meteoroid. (If anyone
has a more accurate value for the density of Park Forest, I'll
be happy to update my model accordingly, but I don't think it
will change the results much.) Finally, after much web surfing
I located the 300-mb North American jet-stream map for the date
and time in question, and estimated its velocity and direction
for the area south of Chicago.

The initial results are impressive! I end up with a distribution
that looks like this \ not this / -- in agreement with the trend
of mapped-out recoveries. All that remains is for me to
superimpose my impact coordinates on a map of the Park Forest
area to see how well it matches the locations of meteorites
recovered so far -- I'll do this tomorrow on my work computer.
I also want to play around with different starting altitudes
for the 20-km/sec initial velocity to see how much effect it
has on the strewnfield shape.

One of the ancillary products of my code is the time-to-impact
from my initial position and velocity. From the 100-km altitude
point, impact occurs 1.8-4.8 minutes later depending on the
fragment mass (5 kilos to 5 grams). Masses lighter than 5 grams
would take more than 5 minutes to impact.

Another code output is the amount of impact position shift due
to upper atmospheric winds -- the amount is ~not~ insignificant:
nearly 7 km for the 5-gram fragments! This confirms our
theory as to why the strewnfield runs SSE to NNW rather
than SSW to NNE, though my predictions also show a slight curve
to the strewnfield major axis. More tomorrow!

Cheers,
Rob
Received on Mon 28 Jul 2003 01:58:31 AM PDT