[meteorite-list] Why Meteorites Are Unlikely To Cause Fires

From: Marcia Swanson <MJSOfArc_at_meteoritecentral.com>
Date: Thu Apr 22 10:28:37 2004
Message-ID: <17047-3FA1A98F-3866_at_storefull-2155.public.lawson.webtv.net>

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Dear Ron, Charley V., Tracy, George Z., Kevin
fly Hill, Elton and List,

I want to thank you all for the help and reference material you gave me
in answer to my question.
As a learner, it was very appreciated. Thank-you.
There was one post that never made it to the List, though I was lucky
enough to get it off-line. Elton sent it, but says he's had trouble
getting his posts through the last few times he tried to post the List.
Anybody have any idea what the problem is, and could you please contact
him?
He so thoroughly explained and answered my every question, and then
some, considering the naitivity in laymans terms, that I used, when I
posted my question. For the people like me with limitted knowledge it's
content is invaluable, and I applaud his effort. He has given me
permission to pass his post to the List, as his was lost in transit
somehow.Thank-you Elton, Thank you List, Best Regards, Marcie


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To: Marcia Swanson <MJSOfArc_at_webtv.net>
Cc: meteorite-list_at_meteoritecentral.com
Subject: Re: [meteorite-list] Why Meteorites Are Unlikely To Cause Fires
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Hello List, Marcia, Ron

I didn't take the time to convert units of measure so the values here
are commingled: Metric and English-you do the math.
Here is the input you asked for. Also check the archives 4-5 years back
where we discussed the possibility that tektites were hot enough on the
right surface to have started a fire. We extensively discussed the
plausibility or implausibility of a meteorite causing a fire.

Marcia Swanson wrote:
> Would the same principles be present in a large asteroid
> disintigtating (exploding/vaporising) just before it impacted on
> earth within our atmosphere,

Yes and No, The principle is the same only there isn't enough time or
distance for the momentum to be dissipated entirely for larger bodies.
There is a term called cosmic velocity(initial entry speed). This,
coupled with mass, relates to the amount of mechanical work that has to
be applied to a meteoroid to dissipate the energy/momentum (mass X
velocity= kinetic energy). The disintegration of an asteroid liberates
heat from a different source(shear) as discussed below. Some mass is
lost through ablation and fragmentation, Yes. Both of these allow more
air pathways to come to bear on the meteoroid. This spreads the load
and speeds the slowing of the mass.

Most meteors (i.e. lower mass) expend all that cosmic energy
compressing the atmosphere. The atmosphere is as dense as a brick wall
when taken in total. At some point gravity becomes the big player and
accelerates the meteoroid at the rate of gravity-- 32 ft per sec^2. This
rate is further moderated by the air resistance(drag) of the lower
atmosphere and the maximum velocity a meteoroid can reach is around
400mph but more typically around 200mph.

Lets look into the actual mechanism of slowing a meteoroid. A
meteoroid's cosmic velocity (and therefore energy) is spent/
counteracted by displacing a column of air of equal mass. For the
moment, disregarding mass loss via ablation, each "foot second" of
travel amounts to a certain reduction in velocity of the meteoroid as
the kinetic energy is converted to mechanical energy(foot pounds). The
mechanical energy is from moving air molecules out of it's path or
squeezing them if they don't move fast enough. This isn't applied all at
once but, it is cumulative all along the flightpath. The mechanical
energy compresses gas molecules which in turn give up existing heat. Be
it Remembered that ...Gas, Solid, or Liquid, all molecules above
absolute zero vibrate. When molecules bump into each other they liberate
heat. Because there is more space between gas molecules they don't
bump into each other as much. In fact they store a fair amount of heat
energy hopping around all that free space. However, when something comes
along really fast-- before the molecules can flow aside, it supper
squeezes them, they have no choice but to shed heat. How this ablates
the meteoroid is discussed further below.

Larger bodies (e.g over 1 meter ) retain some cosmic velocity for
reasons like a cross sectional density that exceeds the amount of
counter-pressure the air can generate. In another way, a larger body
contains more kinetic energy than the atmosphere is deep enough to
totally dissipate. Aerodynamic pressure acts on a falling body such
that the front of a large body is trying to slow down while the rear of
the asteroid is trying to push through. These internal stresses called
"shear" can cause the body to disintegrate. I have surmised that the
colder the meteoroid is inside, the more rigid it is and the more
brittle it becomes when exposed to these internal stresses. This could
account for why a malleable metal like Iron which one can't break with
a hammer at room temperature, can fracture into shrapnel/shards on
reentry. Look to the high altitude breakup of Silhote-Alin for an
example. This shearing breaks chemical and crystal bonds causing a
release of heat energy in a very short time. . If the body is still
larger, it reaches the ground in some degree of integration and
retaining some degree of cosmic energy. The heat release from the
instantaneous conversion of kinetic energy(momentum) to mechanical to
thermal on this scale CAN start fires as it behaves like a nuclear
explosion.

> the pieces, metal bearing ( heat conducive), would be subject to our
> gravitational and friction factor, depending on what velocity and
> distance, they are spewn apart to ? Friction is a factor in our
> atmosphere, once it explodes here, frozen or not, isn' it ?

See above, however, this question border's on "thermal conductivity
coefficients discussion" which I'll save for another time, but to say --
yes there is a heat storage potential but not within the brief time of
passage. If you are asking if additional friction applys to a breakup,
yes, (per above) it is a factor for slowing the total mass down by
bringing to bear more surface area to work against. Being frozen or
boiling hot makes no difference in velocity.

> How much heat, due to our friction, if any, does a shockwave
> accumulate? I know there is no way of measuring an exact answer for
> this question, as every occurance and strewnfield is different, but

A shockwave per se doesn't accumulate heat-- it emits energy in the form
of sound which is a result of mechanical compression. I make a
distinction between shockwave and compression wave. Remember when you
compress a gas the molecules give up vibrational momentum in the form of
heat. The heat radiates into both the atmosphere-- forming plasma and
into the meteoroid's surface flash melting it. This "ablation" actually
acts to cool the meteoroid via evaporation. A meteoroid doesn't
"store"much heat during passage but sacrifices small bits of itself to
carry away molten-ergo heat laden, droplets. The existence of flow
lines on an oriented meteorite are proof of melting and that there is a
cushion of air molecules between the surface of the meteorite and the
slip stream.

> I really don't understand why, under these circumstances, a margin of
> credability doesn't exist? The proverbial, exception to the rule?

I do, the math just isn't there. Any heat liberated by slowing down
remains largely within a cloud of mist and a column of air 120 miles
long. If you look at the depth of scorching behind the fusion crust of
a typical there is no thermal signature more than a few nanometers wide.
This should be the strongest indicator that the typical meteorite is not
capable of retaining enough heat to cause a fire--little alone in
suddenly re- radiating that .

> Could it be that the shockwave, not the meteorite fragments, itself
> could create under the right climate ( hot dry) the ability to
> generate enough heat to ignite combustible earth matter?

No. The shockwave proper is a dynamic waveform which goes to near zero
overpressure when the meteoroid drops from supersonic levels. The plasma
you are possibly referring to stops being produced when the meteoroid
slows to between 2-4 kilometers per second. Ok, I am still ahead of you
hot meteorite folks-- you say what about the fragments hitting another
rock causing a spark and setting dry grass on fire...maybe yes-- but
that is from a spark and not from a hot meteorite.

Finally before anyone asks about heat storage within a meteorite-- you
are facing an uphill battle. The internal temp. is calculated to be just
a few degrees about absolute zero. That requires even more calories
/BTUs to raise the skin temp to kindle anything...Burrrrrrr. If anything
you might get a "freezer"burn from picking up a freshly fallen meteorite!

Good Night Meteorite Lovers Everywhere!
Long Live the Cold Meteorite Clan,
Elton



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Received on Thu 30 Oct 2003 07:15:11 PM PST