[Prev][Next][Index][Thread]
(meteorobs) Meteorite/Fireball Occurences?
Below is a couple questions that I presented to Peter Brown and following the
questions is his response. Just thought some folks would be interested.
George Zay
-------------
>>Peter,
I've ran into some more stuff about fireballs that has puzzled me. Perhaps
you can shed some light? I have this book entitled "Rocks From Space" by
O.R.Norton. In discussing what time of year do Meteorite dropping fireballs
occur, it states that of 581 falls documented between 1800 and 1960, the
maximum number of falls occurred in May and June. The falls drop to a minimum
in December and January, with a further dip in March. It reasoned that since
most of these meteorite falls were observed in the Northern Hemisphere, these
data must have been influenced by the Northern Hemisphere's winter season,
when there are fewer people outside on cold winter nights. However, if the
fall of meteorites is uniform throughout the year, then the numer of reports
during the spring and fall months should be essentially the same...but they
are not. This survey seems to be not in agreement with what is stated in
IMO's Meteor observers Handbook. I am puzzled as to why there should be a
discrepancy?"<<
>>Also in the same book where it discusses what time of day meteorite
dropping fireballs is most likely to occur, using 469 recovered meteorites
whose falls were observed, the American meteoriticist Frederick C. Leonard
plotted a graph showing an obvious peak around 3 pm and an equally obvious
minimun twelve hours later at 3 am. In IMO's Meteor Observers Handbook, it
states that the most meteorite dropping fireballs occurs near 18h local and
the minimun around 6h local. Again I am puzzled as to why there should be a
discrepancy? <<
>>Do you have any knowledge that would explain why these two surveys are not
in sync? I would appreciate any kind of enlightenment.
George Zay<<
----------------
And Peters response:
>>George--
(1) I believe that the fall variation throughout the year of meteorites
reflects real differences in the distribution of the nodes of the bodies
involved - i.e there may be a significant "meteorite-stream" component
to the flux and thus the difference. The interpretation Norton gives seems
at the surface most reasonable, but as you point out it cannot account for
the statistically significant difference in fall numbers around the
equinoxes - hence I believe it is a true orbital distirbution difference as
opposed to an effect caused by pure observational biases.
(2) The difference between the IMO Handbook and Leonards famous
statistic is simply the difference between empirical and theoretical
results - the IMO statement is based purely on theory (if meteorite
producing bodies moved along essentially random orbits, than the altitude
of the antapex locally would determine the number of meteorites which
make it to the ground - hence 18h and 6h local). Leonards results (and
more recent ones by Wetherill (1968) and Wetherill (1985)) show
that the distributions are not entirely random - rather than meteorite
proiducing bodies we see at the Earth have low inclinations and tend
to have perihelia distributed close to 1 AU as a result of their
dynamical evolution and selection effects as to when they have the
largest collision cross-section with the Earth). A good discussion of
all of this can be found in Wetherill and Chapman (1988) in the boks
Meteorites and their Parent Bodies (In the University of Arizona Series).
Cheers.....Peter<<
Follow-Ups: