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----- Original Message ----- From: "Meteorites USA"
<[email protected]>
To: <[email protected]>
Sent: Wednesday, August 11, 2010 11:34 PM
Subject: Re: [meteorite-list] Meteor shower meteorite dropping events
Thanks for posting this Chris... This sounds like a good topic
for an
article for my magazine. If you're interested in it, and/or would
like to
write for the mag on this topic let me know. Anyone have a
working theory
based on evidence of this associative phenomena? I've heard many
people
suggest that meteor showers don't drop meteorites. Then I've
heard people
associate meteorite falls that happen during meteor showers with
said
shower. And I've also heard that people believe that there is ZERO
connection and it's purely coincidence.
So which is it? yay or nay, or maybe? or no one really knows...?
Eric
I have been following the discussion on whether a meteor from a
cometary meteor shower can drop a meteorite. This is not likely to
occur. Cometary Shower meteors are rather friable dust balls,
having been ejected from a Comet as it nears the Sun and over many
years gets spread more or less in the same orbit as the parent
Comet. The entry velocities of shower meteors is quite high ranging
from 71 km/sec for the Leonids down to 23 km/sec for the October
Draconids. Fast meteors travelling at 70 km/sec become visible at
around 110-115 kms. Slow meteors become visible at around 90 kms or
so and burn out around 60 kms. They just don't survive and burn up
in the atmosphere. A very bright meteor sometimes referred to as a
fireball or bolide originating from a cometary shower can become
visible at around 140 kms and burn out at around 40 kms. They enter
at too high a velocity to survive as a meteorite. A meteorite
dropping fireball, not associated with comets but the asteroid belts
have an average velocity of around 17 km/sec or so. These are
asteroidal solid fragments ranging from iron types to chondrites or
stones. Their survivability depend on their velocity, angle of
entry, and whether the object is solid enough not to fragment and
burn up. Some of course do fragment and survive to the ground as a
meteorite resulting in a multiple sample elliptical drop zone. Lets
look at some of the known surviving meteorite falls. Pribram,
initial velocity 20.9 km/sec, end point velocity ~7 km/sec. First
observed at 98 km with an end point of 13.3 km. Lost City, initial
velocity 14.2 km/sec, end point velocity 3.5 km/sec. First observed
at 86 km with an end point of 19.5 kms. Innisfree, initial velocity
14.5 km/sec, end point velocity well below 4 km/sec. First observed
at 62.4 kms with an end point of 19.5 kms. These and other surviving
meteorites originate from the Asteroid Belt, NOT Comets.
Well you ask, what about Shoemaker/levy 9 and it's entry with
Jupiter. This impact was an actual Comet head, which fragmented into
several pieces by Jupiter's gravitational attraction, not the usual
cometary ejecta along a Comets orbit. Some speculate that Tunguska
was the result of a Cometary collision with the earth. In this case
as far as I am aware no fragments were recovered from this event.
Comet impacts along with asteroid impacts are a worry to us
earthlings as this could result in severe damage perhaps ending in an
extinction if the impacting object is large enough.
As I understand things a normal meteor shower meteor will Never
survive to the ground as the collision with our atmosphere will end
its life high in the atmosphere. Impacts of such meteoroids on the
Moon during meteor showers is a completely different matter as there
is no atmosphere to cause it to burn up. I'd love to get a spectrum
of such an impact but most of these are too faint to result in a
spectrum as the light is spread out. I'm still hoping! ;-) Oh,
meteorites can drop during shower dates as Chris Spratt points out
but these are just a random fall not associated with cometary meteor
showers. I base my conclusion on the many professional papers kindly
sent to me by meteor scientists. Greatly appreciated!
Ed Majden - Amateur Meteor Spectroscopy
Courtenay, B.C.
Canada.
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