[meteorite-list] rock on the noggin story

From: Sterling K. Webb <kelly_at_meteoritecentral.com>
Date: Sun Jul 3 14:05:39 2005
Message-ID: <42C828D0.7AE223FB_at_bhil.com>

Hi,

    About HOT ROCKS from space, the glowing meteorites of legend and bad movies,
consider the meteoroid, still in flight immediately after it has reached the
stagnation point where air resistance slows it to a subsonic crawl or even a
near-stop before it then speeds up again in free fall toward the Earth.

    At that one specific moment, its outer surface, for one millimeter or five
is still hot molten rock that is about to or just has cooled below the re-entry
temperature, which is at or above the boiling point of rock. A few millimeters
below that is merely warm rock, but only a few millimeters, because rock is such
a poor conductor of heat.

    The vast majority of the mass of the "hot" rock is still at the ambient
temperature of outer space, a stable 50 or 100 or 150 degrees absolute. The
object, at this point, is a very long way from thermal equilibrium, so it's
impossible to speak about the "temperature" of a meteorite, ranging as it does
over 1000 C degrees or more at differing spots in it and on it.

    Very shortly, that pervasive inner cold and the cooling air of subsonic
flight will have cooled the outer surface below its melting point, forming and
hardening that lovely fusion crust we like so much. Having just weakly
solidified is why crust chips so easily.

    Almost every meteoroid will become a meteorite landing with a cool surface.
A number of accounts record the appearance shortly thereafter of heavy frost or
ice on the cold meteorite body as the space-cold interior is warmed to Earth
temperatures, a passing phase of only a few minutes.

    But, it is possible, in one rare case, with a steep trajectory, which
produces rapid deceleration, a very low altitude stagnation point just above the
Earth's surface producing only a short drop to the ground, for the stone to
arrive with a still sub-molten very hot exterior layer, in other words, a "hot
rock"!

    A person located very near that landing point would, of course, receive a
good dose of the sonic boom that precedes stagnation, so it's possible only
persons with newly scrambled brains have ever witnessed the close fall of a "hot
rock" and so have only silly stories about the event.

    Just kidding. But the scenario is theoretically possible, however rare.

    The smaller the stone, the greater the ratio of surface to volume and the
deeper and deeper the stone will be warmed in the hottest phase of its re-entry,
until at a certain size it would reach melt temperature all the way through. At
that point it would be a molten, or at least soft, drop and become a spheroid
instead of the usual irregular shapes of larger meteorites.

    The tiniest pieces of a well-fragmented fall, like a PULTUSK "pea" or
HOLBROOK "anthill recovery," are just that: often perfect spheroids that are
essentially a re-solidified drop of "crust." However, the high surface to volume
ratio would also cool them much more rapidly and make it impossible for them to
arrive "hot."

    The fact that such tiny meteorites never show the flight deformations from a
sphere that tektites do, tektites being hot molten drops at some point, is proof
that very small meteorites cool very rapidly indeed. (Don't a small handful of
the immense number of tiny HOLBROOK's show a flattened facet, as if they were
still malleable on impact? I think saw some pictures of that. Any one know?) The
rare patch of "glassy" crust is another indicator of a more rapid cooling as
well, I would think.


Sterling K. Webb
-------------------------------------
Dave Harris wrote:

> Hi,
> I just wondered regarding this much publicized story of this chap just missing
> being hit by a "meteorite" and "It was silver and it kind of had red and black
> on the back of it and smoke."
>
> and so on...
>
> Has any of our eminent listees emailed the news channels that carried this
> story that they do not land "glowing" or "smoking" or whatever and therefore
> this is most certainly not a meteorite?
>
> But I also agree with Dr Branch's observation that as long as people say
> things like this we know 100% for certain that it ain't a meteorite and that
> they are subscribing to Hollywood's perception of what meteorites are like.
> This saves us a lot of time!
>
> Best
>
> dave IMCA #0092
> Sec.BIMS
> www.bimsociety.org
>
>
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Received on Sun 03 Jul 2005 02:05:04 PM PDT