[meteorite-list] Zig zags and spinning

[MexicoDoug at aol.com]

>>zig zagging<<
>>Some years ago, I read that the forces necessary to make a fast 
>>moving meteoroid have a visual directional change of any kind will
>>immediately destroy it.  George Zay

Wouldn't that be an overly generalized statement, with it's application here? 
 There are enough examples of what looks like, sounds like and feels like 
"zig zag" (such as the Pasamonte example posted) appearing trails to show it is 
not completely true.

But the overstatement certainly has some truth in it as well under other 
conditions.  As the bolide spirals or gradually zig zags, odds are that it it 
sloughing off non-ablated material as it goes along.  I would guess that that is 
the tossed material is specifically at the pressure points which cause the zig 
zaging to begin with, and from the meteor's point of view is like being hit 
along the edges with a hammer.

Meteorites recovered on earth are almost always made much smaller than the 
incoming body principally by this destructive action - right?, and not by smooth 
ablation which I have no references for, but would suppose accounts for not 
much more than several centimeters depending on composition and momentum as 
well.  The exceptions are the well oriented pieces.

In studying the changing of direction -"zig zagging"- it is probably 
important to consider that these changes in direction are cycles are slipped into 
gradually, not instantly, allowing a much greater disippation of energy.  In other 
words, like the puttering jalopy sounds I mentioned, the entire cycle takes 
place over seconds, and in the Pasamonte example as well as the one I have 
researched, the clever choice of words of the Australian witness as "gradual zig 
zags" which are very noticable, is caused by a rock that spins or tumbles at a 
relatively gentle rate given the situation, and with each tiny advance in time 
the overall sheer pressure the rock experiences is basically continuous - 
tangentially increasing or decreasing in a sense.  Should the rock quickly flip 
or not loose enough momentum, then it might explode in a major way - like 
Pasamonte eventually did, captured right at that moment after zig zagging, which so 
enchanted Nininger.

It is worth adding that in space visual changes of direction at meteoritical 
speeds thankfully are possible, with no problem (Cassini orbital insertion, 
etc.).  Undoubtably your old reference was for meteorite entry into the lower 
atmosphere.  Taking into consideration the exponential nature of the mass 
distribution (scale height) of the atmosphere, it seems clear that ablation 
processes dominate meteorites traveling straight as an arrow would (ie, 
parabolically), removing "cosmic velocity" until the atmospheric soup gets thick and 
sloughing begins to takes over from ablation for most convient sized meteors.   So 
the zig zagging would be most likely beginning after the initial loss of much 
momentum, at the lower most visual altitudes for meteors that are interestingly 
enough most probable to survive as meteorites.

As an after thought, a meteorite which is still smoking and glowing somewhat, 
creating a smoke trail, at say 1000 miles per hour, that weighs 100 pounds 
still only needs only a negligable force applied perpendicular to it's entry 
angle (and moreover, it could be downward) to zig zag.  A very noticable zig in 
the velocity vector, since cosine of up to 8 degrees is over 99% conserved and 
only affects the momentum (or energy) in this corresponding little bit.  Just 
like a man on a swing really going at it can be pushed laterally by a small 
child and do (quickly damped by other forces) zig.  Or a batter in baseball 
doesn't even feels the tipping of a fastball yet it goes over the Catcher's head.

And finally, the nice photos - and wood carvings - which appear to have 
famous bolide families traveling in straight lines - like Peekskill or Newark on 
Trent.  But a good question in those cases, is why they all aren't 
colinear...clearly something has happened at quite high speed that allowed a change of 
direction up or down which appreciable-sized fragments survived.  There may be a 
counter argument that they started out as a group.  I don't believe that for 
the majority of those cases.  I can't believe small objects in space have much 
chance of stayying so close together after millions of years.  Then there are 
always the bolides that hit earth at cosmic velocities and manage to "skip" off 
the atmosphere.  It is hard to buy that they hit at perfecty tangents, and 
the one from California 8 years ago only broke in two - and you know it was 
going fast as I believe it practically completed one orbit...

Saludos
Doug
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