[meteorite-list] strewnfield size vs entry angle

From: Rob McCafferty <rob_mccafferty_at_meteoritecentral.com>
Date: Sun Mar 5 15:17:28 2006
Message-ID: <20060305201724.58138.qmail_at_web50913.mail.yahoo.com>

Chris

I dare say you are right about it being a biased
formula and the data set we had to work with was very
limited as you point out. Thoroughly documented falls
leading to strewn fields of known dimensions may even
have been less than 5 at the time but memory isn't
that great after these years. The equation did work
remarkably well, though and a lot of theoretical work
went into the original equation.

Sadly, a purely theoretical method didn't even produce
fragmentation events in many cases, with frictional
forces increasing all the way to impact rather than
causing fragmentation and we had to modify the
equation based on fragmentation at a certain pressure
rather than at maximum resistance. This allowed large
pieces to break up and smaller ones to survive. It
wasn't tidy and I never could really satisfactorily
justify the "Normalisation factor value" for each type
of meteorite, for which I was marked down heavily. The
fact that it worked was irrelevant to my rather
demanding task-master. It is most likely that we did
something wrong in the theory. The major problem we
found is the one you have, there's not enough data to
properly test a theoretical model. When we tried, it
failed miserably so we did an empirical one instead
since we had to at least complete the task given.

Winds were soething we took into account I seem to
recall but I think we calculated that unless the winds
are really high, they didn't make much difference.
Only the really small pieces tend to get blown about
by more than a couple of hundred yards and since many
strewn fields were huge we generally ignored them and
only worked with pieces larger than a certain size. (I
don't remember how small but I'm pretty sure I have a
meteorite sample that's bigger than it). Of course,
it's not beyond the realms of possibility that we got
that terribly wrong too.

Rob McCafferty

--- Chris Peterson <clp_at_alumni.caltech.edu> wrote:

> Hi Rob-
>
> I think your methodology probably resulted in a
> biased formula. Falls
> connected with witnessed fireballs are strongly
> associated with shallow
> entry paths. Shallow paths produce multiple
> fragmentation events, or single
> fragmentation events that extend over a long ground
> path. This results in a
> strewn field that is more closely aligned with the
> entry path. As the path
> becomes steeper, high altitude winds become more
> significant. For entry
> angles greater than about 60? (from the vertical),
> winds are the dominant
> factor in predicting the shape of the strewn field.
>
> I need to use radiosonde data to estimate the
> potential strewn field for
> most of the fireballs I track. Actually, the wind
> data is essential to
> predict the _location_ of the field with respect to
> the fragmentation, and
> usually important to predict the orientation and
> size of the strewn field.
>
> Of course, my analysis is theoretical, not
> empirical. There simply isn't
> enough data available to test the theory with any
> degree of statistical
> significance. The number of falls that produced
> strewn fields with multiple
> meteorites, and for which both atmospheric wind data
> and trajectory data are
> available, can be counted on one hand.
>
> Chris
>
> *****************************************
> Chris L Peterson
> Cloudbait Observatory
> http://www.cloudbait.com
>
>
> ----- Original Message -----
> From: "Rob McCafferty" <rob_mccafferty_at_yahoo.com>
> To: <Meteorite-list_at_meteoritecentral.com>
> Sent: Sunday, March 05, 2006 9:31 AM
> Subject: Re: [meteorite-list] strewnfield size vs
> entry angle
>
>
> I know a formula does exist for this because I did
> my
> 3rd year undergraduate project on exactly this and I
> and another student wrote it.
>
> It involved a lot of empirical evidence and
> formulating a formula which fitted the very few
> properly observed falls and seeing if it could be
> extrapolated to other strewn fields.
>
> The formula predicted very well the size
> distribution
> for these and even allowed us to predict the
> incoming
> angle for unobserved meteorite falls. The formula
> also
> has a modification factor depending on whether it
> was
> a stony or iron meteorite.
>
> Obviously, I don't have a copy of this to hand it
> having been 13 years ago and I don't have the
> resources (nor the ability, which has waned sadly in
> the intervening period) to re-derive it.
>
> However, if you were to find an address or e-mail
> for
> Professor David Hughes from Sheffield University,
> UK,
> I dare say he sitll has it lying around in his
> office
> as he was the Planetary Astronomer I studdied under.
>
>
> Rob McCafferty
>
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>
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>


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Received on Sun 05 Mar 2006 03:17:24 PM PST