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Re: (meteorobs) Meteorite/Fireball Occurences?

> in regards to "meteorite-dropping-fireballs" (and I
> don't have the references handy) but there are peak
> periods for these, as well, and I believe they are
> May-June, close to the peaks Ed mentioned,


Hi Bob a.o.,

Never mind the possibility of influence of "meteorite streams" and
non-random orbital distributions for meteoroids with an asteroidal source,
the meteorite fall frequency is clearly related to the altitude of the
antapex for the hemisphere you live in, i.e. to hemisphere season and not
solar longitude (=non-random orbital distributions). This can be clearly
seen from comparing northern hemisphere fall distributions above +20 degrees
latitude, and southern hemisphere fall distributions above -20 degrees
latitude(data taken from the Catalogue of Meteorites):

http://home.wanadoodot nl/marco.langbroek/falfreq1.gif

As can be see, the two fall distributtions mirror each other, pointing out
that it is not time of the year in the sense of earth orbital position
(i.e., non-random orbital distributions), but instead hemisphere season
which determines the number of meteorite falls.

If we take the northern hemisphere dataset and compare the fall frequency
variation to the variation of the maximum antapex declination (which
corresponds to a maximum altitude the antapex can obtain in the sky,
resulting in highest "rates" from this source, similar to a meteor radiant),
it can be seen that the two mimic each other, but that the peak in meteorite
falls lags the maximum declination by about two months:

http://home.wanadoodot nl/marco.langbroek/falfreq2.gif

If anybody can explain this two-month lag, I am very interested (I have
never quite believed the "more people out in the summer season" argument to
be of prime influence. But perhaps this effect has some slight visible
effect in the august-october period nevertheless).
My explanation for it would be that most meteorites actually do not come
exactly from the antapex, as there are only two strict types of orbits in
accordance with the latter situation:

1. meteoroids in an orbit similar to that of earth.
2. meteoroids in orbits with their perihelion at the earth-orbit
intersection, so they come in exactly parralel to the earth movement.

Option 1 is not likely for an asteroidal object. Option 2 is a valid
selection effect. However, this does not exclude meteoroids that do not
exactly abide to this rule. In fact, most meteoroids will come in under a
slight angle to the earth movement, but many of these will still have low
enough velocities to make a dropping possible. So I think while
theoretically meteorids from the antapex have the highest chances, in
reality teh majority of meteorites does not come exactly from the antapex.
This might reuslt in the two-month off-set of the peak, although I do not
yet grasp why that off-set should be in a particular direction instead of
randomizing around the maximum antapex declination.

- Marco

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Marco Langbroek

marco.langbroek@wanadoodot nl
meteorites@dmsweb.org
http://home.wanadoodot nl/marco.langbroek

"What seest thou else
 In the dark backward and abysm of time?"

                            William Shakespeare
                            The Tempest act I scene 2
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