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(meteorobs) Re: Brightest lunar meteors probably large




David Dunham <dunham@erols.com> wrote:

>     There was a suggestion that sunglints from
>artificial satellites might be involved, but this is
>unlikely since the observations were made late at night
>local time when most of these would be deep in the Earth's
>shadow.  Also, with six events simultaneously recorded...


David, we know of course that satellites CAN be observed throughout the night,
at least from subtropical latitudes that I'm familiar with... A night of meteor
observing in southern Florida would immediately confirm this. Thus, don't rely
on this first argument. However, the argument that some of the simultaneously
observed events displayed too little parallax to be as close as geosynchronous
orbit would clearly be convincing. Therefore congratulations on what looks like
it might be an excellent capture by your observers!

If the meteoric nature of these events continues to be indicated by successive
tests, I guarantee that meteor (and lunar) observers will take a closer look at
less extraordinary - but still high-rate - annual showers like the Geminids,
Perseids or Quadrantids in future... These showers generally display nothing
approaching the peak ZHR of 1999's Leonids, but can show ZHRs within an order of
magnitude of it, especially during outbursts. (Impacts from 1993's Perseids,
e.g., should have been worth watching for under these assumptions!)


=======================================

However, one thing I did want to ask about in an email you forwarded:

Jay Melosh <jmelosh@LPL.Arizonadot edu> wrote:

>>I just heard from Paul Weissman that he estimates that your m = 3 flashes
>>must have been made by an object "about half a meter" in diameter.  I have
>>to agree with this estimate--which implies masses *much* larger than you
>>have mentioned!  (a half meter diameter projectile would mass about 500
>>kg).  The problem is that the luminous efficiency of an impact onto a solid
>>surface is *much* lower than the ca. 10% Mike Mazur estimates for a bolide.


As an amateur meteor observer, I'm used to seeing in discussions and papers that
meteoroids from cometary streams (such as the Leonids) are on average *FAR* less
dense than Dr. Melosh appears to imply! The most common values I've heard are of
order 0.5-0.8 g/cm^3... This would then imply a mass for a 25-centimeter radius
sphere of cometary debris of only 30-50 kg!

I have seen older papers (little discussed, at least in amateur circles, in the
past 5 years or so) which seemed to imply greater densities for cometary debris,
but they are generally predicated on data from invasive collection methods
(e.g., earth-collected meteorite material!) which all seem to prejudice density
estimates to the high end. Even these numbers are of order 3 gm/cm^3.

If the putative large impactors were in fact from the Leonid stream, and current
estimates of Leonid meteoroid density are anywhere within an order of magnitude
of the 0.5 g/cm^3 estimate (the highest published value I've noticed for the
LEOs was an upper-bound assumption of 4 gm/cm^3), this would imply a larger
source object, based on Dr. Melosh's other assumptions...

Back-of-envelope calculations suggest a diameter of perhaps 100 cm or more,
using Dr. Melosh's assumption of a 300 kg body for one observed flash.

Based on both estimated and reported mass distribution indices ('s' values) for
the Leonids - both of this year's display as observed from earth, and of what
ever substreams the moon was predicted to pass through - it might be interesting
to estimate the number of potential objects of such a size which the LEOs might
throw in the path of a moon-sized surface during the observed period! Would it
agree reasonably well with the number of distinct events observed?


BTW, don't hesitate to qualify or correct any of the assumptions/calculations I
make above! And thanks again for keeping the readers of the 'meteorobs' mailing
list informed of this ongoing analysis, David...

Clear skies!
Lew Gramer


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