[meteorite-list] Park Forest timing

From: Sterling K. Webb <sterling_k_webb_at_meteoritecentral.com>
Date: Wed, 26 Mar 2008 23:22:51 -0500
Message-ID: <018301c88fc2$38a78830$b35ee146_at_ATARIENGINE>

Hi, All,

    The speed of sound depends on a huge variety of factors.
At normal density and at 70 ?F, the speed of sound is
344 m/s, or 1230 km/h, or 770 mph, or 1130 ft/s. The
biggest variable affecting the speed of sound is temperature.
Since it's colder at high altitudes, sound moves slower there.
At the cruising altitudes of commercial jets, it's only 660 mph.

    The range of the variation is covered well by estimating a
range of speeds from 1000 to 1100 feet/sec. But the time of
falling, now, that is messy!

    Natural objects falling great distances all have a natural
stagnation velocity, different for each shape, mass, and density.
They are accelerated by gravity to that velocity and then remain
at that constant ("terminal") velocity until they reach the ground.

    A heavy streamlined object (like a very skinny bomb) dropped
from 42,000 feet breaks the "sound barrier" on the way down,
and achieves about a steady speed of 900 mph or better before
impact.

But, a mouse tossed out of that same heavy bomber at 42,000
feet gets an incredible view for a very long, long time, lands on
its feet, rolls over, shakes its head, and utters the small-rodent
expression for "What the heck was that all about?" and scampers
off unaffected. (A cat breaks a leg or rib; a man hits at 125 mph
or more and his survival depends on the target characteristics.)

    The timing of a meteorite's "dark fall" can be best
estimated by trying to figure the actual impact velocity from
the evidence at the scene of the impact and then compare that
to the best estimate of the altitude of its stagnation as if the
velocity were constant.

    Actually, in a very long fall the velocity will slow as the
air becomes more dense near the ground. A stone that lands
on your roof with a loud thump but doesn't penetrate its
surface would be moving at the speed of a well-thrown
baseball (fastball), 130-140 feet/sec. If it started from a
stagnation point at 100,000 feet, it could take 10-12 minutes
to fall. The 400 feet/sec stone that penetrates roof and the
drywall ceiling takes 3-4 minutes from 100,000 feet.

    The big uncertainty is determining the stagnation altitude.
Since almost all the freefall is at a constant velocity, Mr.
Newton not withstanding, even knowing the speed well
doesn't help very much. 100,000 feet is probably too high
for stagnation, which depends on the characteristics of the
meteorite itself, but is usually between 35,000 feet and
90,000 feet. The maximum retardation, based on air density,
is likely to occur about 45,000 feet, so that's a good guess,
in the absence of solid data.

    Both 2 minutes and 4 minutes are perfectly reasonable
fall times for Park Forest. "Analysis of audio and seismic
recordings clearly matched three distinctly visible fragmentation
events. The main fragmentation event, visible on a police-car
video, occurred at a height of 44 miles (70 km) and was
quickly followed by two smaller detonations at 22 and 16
miles (36 and 26 km)."
http://www.astronomy.com/asy/default.aspx?c=a&id=2075

    So, stagnation was below 85,000 feet (16 miles). If it was
as low as 45,000 feet, a roof-smasher would have taken 1.5
to 2 minutes to the ground and a porch or roof thumper would
have taken up to 6 minutes. The sound would take 45 seconds
(for 45,000 feet) to 60 seconds (for 60,000 feet) to reach the
ground if directly overhead, but of course the landing spot
of the meteorite is somewhat downrange and further away.

    All in all, the "minutes" time scale seems right and the
"seconds" time scale seems unlikely. Or so says the back
of my envelope.


Sterling K. Webb
---------------------------------------------------------------------------
----- Original Message -----
From: "Chauncey Walden" <clwaldeniii at comcast.net>
To: <meteorite-list at meteoritecentral.com>
Sent: Wednesday, March 26, 2008 7:10 PM
Subject: [meteorite-list] Park Forest timing


Bernd wrote:

> "He was not yet asleep when he experienced
> the bright light illuminating his
> bedroom and the tremendous explosions
> that followed. A *couple minutes* later
> he heard a disturbance outside, he said it
> sounded like something hit his house."
>
> In view of the tremendous fireball and
> the accompanying sound phenomena
> witnessed by various people here in
> Europe (especially Switzerland, Southern
> Germany and Eastern France) ... was
> it really "a couple minutes later" or was
> it "a couple seconds later"?

I would think it would be longer than that. If we have retardation at
100,000 feet, the sound would take about 90 seconds to reach the ground
and the meteorites at a terminal velocity of, say, 200 mph, would take 5
minutes, so the difference might be on the order of 3.5 minutes. I'm
sure someone will chime in with more exact figures.

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Received on Thu 27 Mar 2008 12:22:51 AM PDT