[meteorite-list] Fwd: Meteorite-list Digest, Vol 84, Issue 18
From: Ed Majden <epmajden_at_meteoritecentral.com>
Date: Sat, 14 Aug 2010 16:24:24 -0700 Message-ID: <9D24291E-3D8A-4BBC-9A6B-3907BF727A0D_at_shaw.ca> >> --------------------------------------------------------------------- >> --- >> ----- 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. Received on Sat 14 Aug 2010 07:24:24 PM PDT |
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