[meteorite-list] Impact Duration Time

From: mexicodoug at aim.com <mexicodoug_at_meteoritecentral.com>
Date: Wed, 19 Mar 2008 11:54:18 -0400
Message-ID: <8CA57F09F65E988-14D0-303_at_webmail-nd19.sysops.aol.com>

Robert W. wrote:
"VERY rough ESTIMATE of the spread of time by simply dividing the
length of the strewnfield by a max free fall speed of ~ 200mph??? ...
IF all the above is even halfway accurate, could one safely say that
during a "typical" fall, meteorites are impacting down the length of
the strewnfield for approximately 1 to 5 minutes? Or am I way off base?
"


Hi Robert, Listees,

I think the times you mention may be in the ball-park, but not
necessarily for the logic of dividing by 200mph the length of the
strewn field. Remember the free fall velocity is downward (vertical)
and the strewn field is a horizontal section of it. So the strewn
filed major axis lengthnwould have at least two main drivers:

1) How quickly and at what height fragmentation happens (and as you say
incident angle but during the period the fragments still have cosmic
velocity).

2) The difference in Free Fall speeds caused mainly by the different
sizes. Remembering the cross sectional area inverse and mass have
proportionalities to free fall velocity, I think the question is best
understood by considering that with cosmic velocity in rarer atmosphere
the dispersion is much faster - indeed still incandescent - and the
differences in timing for "impacts" (or maybe we should call them
landings in light of Peru) will be determined by the differences in
vertical speed - not horizontal. The smaller particles fall more
slowly of course as they have a higher surface area to mass ratio in
most cases. So your times may be accurate but only in a minor sense
have to do with the length of the strewn field major axis. On the
contrary, I would expect that the biggest piece falls first furthest
along the axis, and then the little pieces fill in, having traveled
less distance but at a slower velocity in the tail end, which might not
even fall in line with the azimuth of entry in many cases, especially
falls that do not contain multiple fragmentations during incandescence
(incl. small falls).

If we say the difference in speed between pieces we can hold (and not
powder and tiny grains which will remain suspended for very long times
- just look at the photos, esp. the recent one from Peru of the
persistent "smoke".) So maybe take 200 mph as a difference between the
biggest pieces and the smallest size pieces of interest to recoverers,
and then your numbers work out in a general sense to be ok - but
looking at the mean vertical direction mostly during free fall...

So I say major axis length is a secondaryt factor.

Best wishes and Great Health,
Doug

-----Original Message-----
From: Robert Woolard <meteoritefinder at yahoo.com>
To: meteorite-list at meteoritecentral.com
Sent: Wed, 19 Mar 2008 8:40 am
Subject: [meteorite-list] Impact Duration Time



Hello List,

  Would any members be able to help with a question I
have regarding the "impact-duration-time" (???) of
some typical falls? I don't know if that's the best
terminology to use, but what I mean is: What would be
a "typical" measure of time between the impact of the
FIRST (known) individual at the start of the
strewnfield, and the fall of the LAST (known)
individual? I can't remember ever having read of any
instances where these times were actually determined.
I know the odds for accurate recordings of both of
these times would be very slim, but does anyone know
IF they have ever been recorded for a fall(s)?

  Also, is it a relatively simple matter of getting a
VERY rough ESTIMATE of the spread of time by simply
dividing the length of the strewnfield by a max free
fall speed of ~ 200mph??? I know that the angle of
entry, the wind speed, the density of the meteorite,
etc., would all affect the results, and the answer
from such a simple equation would truly give ONLY an
estimate if I'm right. But WOULD this give an
approximate duration time?? For instance, if a
strewnfield is ~ 7 miles long, would it be fairly
accurate to say that meteorites were impacting over
about a 2 minute spread of time during the fall? And
if the strewnfield was ~20 miles long, impacts may
have occurred over ~ 6 minutes?

(7 miles/200mph = 0.035 hours,or = ~2.1 minutes)

(20 miles/200mph =0.1 hours, or = ~ 6 minutes)


   Finally, IF all the above is even halfway accurate,
could one safely say that during a "typical" fall,
meteorites are impacting down the length of the
strewnfield for approximately 1 to 5 minutes? Or am I
way off base? If this is right, I never thought about
the concept of meteorites hitting the ground for 5
solid minutes (or more!) during a fall. That puts
thing in a different perspective, to me at least.

  Thanks,
  Robert Woolard









       
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Received on Wed 19 Mar 2008 11:54:18 AM PDT