[meteorite-list] ANOTHER QUESTION Fireballs

[Mr EMan]

There are a few too many myths about meteor shower's
to try to reverse them all in one post, but that
rarely stops me from trying. There are lots of reasons
your average cometary debris stream rarely could
produce meteorites large enough to be seen with the
naked eye.

As already raised, 99.99% of fireballs don't arrive on
the ground as a fireball. Any fireball that reached
the ground is going to be crater making.  To remain
incandescent all the way to the ground means it is
still moving at cosmic velocity and will be yielding
kilotons of shock energy.  

Conventional science is that a meteor goes into dark
phase(i.e., Retardation point) most always at no less
than 5 miles above sea level. (3.8 to 4.2 miles has
been reported for high speed stones arriving in one
retrograde fall)

Meteor light energy output is a function of velocity
and mass which lead to items like efficiency of
conversion.  A fast moving small mass can produce as
much light as a medium mass stony object moving at
slowed speeds. The major difference is all of the mass
of a cometary particle is converted to light in a shor
time.  Cometary fireballs frequently "look" to be able
to produce meteorites as we tend to perceive them as
larger and closer. Yet, we know from radar ranging
that the typical cometary meteor extinguishes between
120 and 80 Miles above sea level. 

Of the dramatic fireballs in our life time where
meteorites were recovered they tend to be carbonaceous
which are associated theoretically with comet-like
origins.(Allende, Murchison, Moss, Tagish Lake  etc.)
Point being all "fireballs" are not equal. 

I do use the term bolide as originally used as a
fireball that explodes producing an audible report aka
BOOM.  The term fireball is also overused as large
longer lasting meteors that are "spectacular" also get
classified as fireballs by the typical observer.

As to football and basketball sized chunks, a
carbonaceous meteor has to be on the order of a cubic
meter or larger to stand the chance of producing even
a small meteorite.

A meteoriod stream's meteor producing ability each
shower is governed by many factors including recency
of comet passage, and the number of times the earth
has gone thru the debris field in the past. It is also
distorted by Jupiter's orbit. Unless a comet has
broken up, the bulk of the debris is dust and not
sports equipment-sized masses. Just because a comet
has recently passed doesn't mean it is off-chucking
larger sized objects.
 
So this is why the a cometary debris stream(aka
shower) hasn't produced a single identifiable
meteorite. One or more showers seem to be stony
asteroidal in origin but I don't have the details at
hand.  I believe we do encounter asteroidal debris
streams that do consistently drop fireballs
cyclically.

Elton


--- "jwb7772 at netzero.net" <jwb7772 at netzero.net> wrote:

> Hi All!
>      I have another question concerning meteor
> showers.  That is-----If showers do not produce
> meteorites, then how come fire balls are often seen
> during the Perseid's?  Fireballs often hit the
> ground providing that they do not blow up. 

 I
> believe that Temple replenished the field not too
> long ago so there has to be a lot of bigger rocks up
> there.  Yes most are just dust, rice sized grains
> and pea sized rocks but I like to think that base
> ball, foot ball, and basket ball sized rocks are
> just lurking around and just waiting for us to look
> up and watch them come screaming down in a blaze of
> glory!  Jim Balister
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