[meteorite-list] Fw: Last on Adamana for a while (I hope)

From: Sterling K. Webb <sterling_k_webb_at_meteoritecentral.com>
Date: Fri, 2 Mar 2007 13:07:34 -0600
Message-ID: <093801c75cfe$09e17e90$32ea8c46_at_ATARIENGINE>

T'm re-posting this, as the first try didn't go
through, think it was too long with all the previous
messages in the thread still attached. Sorry if it's
a second copy.
----------------------------------------------------------
Hi,

    At the risk of stepping into a private argument
and collecting a wild punch, I just wanted to point
out something about meteoric entry.

    The "stone" is most likely to fragment at the
point of maximum dynamic pressure from the
atmosphere (or Max Q). The dynamic pressure
equals (density) x (velocity)^2 / 2. Now, the square
of the object's velocity decreases exponentially, that is
to say very rapidly, from the drag created by that
rising pressure. A good chunk of rock is going to
be slowing down at anywhere from 50 gees to perhaps
200 gees. We can measure the actual deceleration of
meteors and we can test existing meteorites to determine
their crushing strength, and that is the range we find.
The average is about 80 gees. The density of the
atmosphere increases linearly in proportion to altitude,
so the pressure builds up mostly in the later stages
of the entry.

    These three factors (rapid slowdown, weak stones,
and atmospheric density) combine to USUALLY result
in a low altitude fragmentation. If the stone is extremely
weak ("friable") it will fragment at a higher altitude than
a "normal" stone. Stones that fragment into a very large
number of pieces (like Holbrook) seem to do so because
they are very weak. Thus, Holbrook could be considered
atypically weak and that could produce some odd behavior.

    While Jason is correct that the maximum pressure
is exerted on the "nose" of a "nose cone," that point
is also the most stable and the least subject to vibration.
The external shock waves in hypersonic flight could have
folded smoothly over the ablating cone-shaped portion
of the mass and then become turbulent further back along
the more irregular and less ablated main body of the object,
producing buffeting and vibration that caused the main
portion of the mass to shatter and break, while the "nose"
managed to transition the hypersonic-subsonic boundary
more or less intact, leaving the "second" stone to
re-fragment and re-fragment, ablating until they too could
also drop to subsonic velocities. It's an unusual scenario,
not the "normal" breakup (if there is such a thing as a
normal meteoric breakup). If you ever seen an ablating
entry (or re-entry), there can be an amazing amount of
tumbling and gyrations of fragments after they break
loose from a larger mass.

    This could all be a wild fantasy but, interestingly,
there is this paper that claims that a mathematical analysis
of the distribution of sizes of fragments found in a meteorite
fall can reveal such details as the number of breakups the
object went through or if the shape of the original body
deviated from the spherical:
http://www.iop.org/EJ/article/0295-5075/43/5/598/node4.html
by L. Oddershede (Technical University of Denmark ), A. Meibom
(University of Odense, Denmark ) and J. Bohr (Hawai'i Institute of
Geophysics and Planetology, University of Hawai'i at Manoa)

    The authors say: "A known example is the Holbrook
shower, where the presence of different thicknesses of
the fusion crust shows that the meteoroid was subject
to at least two fragmentation processes. The mass
distribution of fragments from the Holbrook shower...
seems S-shaped which might be consistent with a
superposition of two power laws with different cut-off
masses... The mass distributions could equally well or
better be a result of three (or more) fragmentations."

    They are talking about the fragments called "Holbrook"
only, but it is clear that the statistics suggest a "stepped"
process in which a big rock breaks into two rocks, one of
which breaks into multiple fragments, the largest of which
could in turn break into smaller multiple fragments...

    They studied a number of "showers" and found some
to be the result of a single fragmentation event and some
to be the result of multiple fragmentations. Quite incidentally,
the equations also imply the volumetric coefficient of the
original shape. The Mbale Object was almost spherical
(with Vc=3) while the original Sikhote-Aline meteoroid
was a long cylinder (Vc=1.8). Hey, no wonder it had such
a bumpy ride! A big iron splinter.

    Jason would be "right" in that it is counter-intuitive
and does not follow the "usual" course of events for the
many Holbrooks and the Venus Stone to be part of the
same mass, but there are many indications that this may
be an unusual fragmentation event, in which case all the
usual bets could be off.

    Theory is one thing, but the proof is always on the
ground (or in it, sometimes). Keep hunting!


Sterling K. Webb
------------------------------------------------------------------
----- Original Message -----
From: "DNAndrews" <dna1 at cableone.net>
To: "Meteorite-list" <meteorite-list at meteoritecentral.com>
Sent: Thursday, March 01, 2007 11:09 PM
Subject: [meteorite-list] Last on Adamana for a while (I hope)


Hi again, Jason...
Received on Fri 02 Mar 2007 02:07:34 PM PST