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(meteorobs) Fireball Questions

I asked Peter Brown some questions regarding fireballs..dot below are my
questions and his answers to them...enjoy!
George Zay
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GeoZay>>It is said in the Meteor Observers Handbook about Fireball rates that
they
have preferred periods where most are seen....Namely around the Vernal
Equinox and in the early evening for the Northern Hemisphere.  I was
wondering if this is in reference to sporadics only...or is this a broad
statement that includes the various major meteor showers such as the
Perseids, Quadrantids, Leonids and the Geminids that don't occur near the
Vernal Equinox? Or does this refer to only the meteorite dropping fireballs
that are sporadics and asteroidal? It mentions that Rendtel and Knofel proved
that the rates for visual and photographic fireballs are about three times
the rates seen around the autumnal equionox....again, is this in reference to
the slow moving asteroidal sporadic fireballs, or does this include the
faster shower related cometary fireballs?<<

Peter Brown>>(1) The annual variation in fireball rates has also been looked
at by
Halliday in addition to the work done by Rendtel and Knoefel. Although
I have not read these papers in sometime it was my understanding that
they included all fireballs in this analysis which were not obviously
related to showers. This means there still may be some fireballs in
the statistics from the bigger showers, but that (for example) the
huge blips in activity on Aug 12 and Dec 13 were removed. Halliday also
found the same trend as R&K, namely a peak at the time of the vernal
equinox. In a broad sense both cometry and asteroidal fireballs are
included in the stats (again with the exception of the largest, obvious
showers). From Ceplechas work we know that there are roughly
equal proportions of cometary, chondritic and carbonaceous - related
fireballs (types III, I and II in his classification scheme). The
annual variation I believe to be intrinsic to the distribution of
such bodies and NOT a seasonal effect of radiant visibility. The
sporadic rates (for comparison) show the OPPOSITE trend with peaks
nearer the authumnal equinox and lows iin the Spring. Suggests that the
two populations have different source bodies or source body
distributions.

GeoZay>>When a Fireball meteor of asteroidal source enters the atmosphere in
the early evening, it appears to be very slow in it's velocity because it's
entering from the earth's antapex side....Does the earth also meet up with
asteroidal type meteoroids head on in the morning? Do these kind of meteroids
orbit the sun in an opposite direction?<<

Peter Brown>>(2) The asteroidal type fireballs (related to Apollo or Aten
asteroids)
orbit the sun in the same sense as the Earth - prograde. Hence they
have a strong tendancy to collide on the evening side and produce
low velocity fireballs (which have a higher chance of producing meteorites).
Note that it is certainly still possible to have asteroidal fireballs
in the morning hours - the odds of collision in this geometry are just
slightly lower, the chances of producing meteorites also are smaller
as the geocentric velocity of the fireballs is lower. Note also that the
recorded fall patterns of meteorites show a strong maximum in the
late afternoon/early evening. Wetherill (1968) used this fall pattern
in conjunction with his modelling of the delivery mechanisms
of (originally) Main Belt asteroids to Earth to show that these
meteorites must originally have evolved from the Main Belt. (Sorry - I
noticed a typo earlier in the paragraph - the odds of producing a
meteorite in the morning hours are lower as the geocentric velocity is
*higher*, of course, not lower).<<

GeoZay>>Another question...These slow moving asteroidal fireballs taht have
long durations...such as those lasting 15 to 60 seconds, can they travel thru
our skies upon entry in an assorted number of directions? That is, can they
travel North to South, South to North, East to West, West to East and the
sort? Is there anything that dictates they have to travel in certain
directions only?<<

Peter Brown>>(3) The slow moving fireballs of asteroidal origin have an
excess of
apparent radiants in the South West in the early evening (see 
Simonenko (1969)). By pure geometry, depending on the time of day
the fireballs occur they can only appear "slow" if they have radiants
in certain directions. Slowest of all are the fireballs having apparent
radiants in the W, NW and SW in the early to mid-evening - these all
have corrected radiants with small elongations from the antapex. If a
fireball comes from the East in the early evening it MUST have a higher
elongation from the antapex and hence a hugher speed. Note that a fireball
occuring at 18h local time and going from S -> N will have a low
speed, while a fireball occuring around local noon will have a low
speed if coming from the East. The directions of travel for the slow
fireballs simply reflect the changing location of the antapex in the sky
(which culminates around 18h local and sets near midnight, depending
on latitude and time of the year). For high latitude observers in the
early morning near the summer solstice, for example, it is possible
to have fireballs coming from radiant in the N which still have low
velocity. At lower latitudes, the antapex is below the horizon from
midnight-noon and hence fireballs seen at this time must have higher
velocities. The extremely long-duration events you speak of are
indicative of both asteroidal-type fireball collisions as well as
grazing geometries associated with such fireballs.<<

Hope this answers some of your questions.
Best Regards......Peter
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