(meteorobs) Fwd: Meteorite-list Digest, Vol 84, Issue 18

Ed Majden epmajden at shaw.ca
Sat Aug 14 19:24:24 EDT 2010


>> --------------------------------------------------------------------- 
>> ---
>> ----- Original Message ----- From: "Meteorites USA"  
>> <eric at meteoritesusa.com>
>> To: <meteorite-list at meteoritecentral.com>
>> 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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