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(meteorobs) ROCKS from Space cont'd

Keith>>I have heard it said that a meteor's journey through the atmosphere 
is
akin to a metallurgical refining process.  The resultant meteorite is
supposedly a crudely-refined material with an iron base. <<

George>>Keith, I think the only parts of a meteorite that may go thru any
significant heating as it passed thru the atmosphere, would be found only
on the outside edges...or the crust. While a glowing meteor, the object
would be quite cold inside I would think?  Therefore I don't think the
internal structure would be changed much if any?  The raw materials would
still be intact.  George Zay<<


George, without having any real meteor experience I can still comment based
on some fundamentals of physical science.  We all know that NASA reentry of
various vehicles including the space shuttle is only possible because of
highly insulating refractories (ceramics) that are adhered to the vehicle
surface.  The heat transfer coefficients of metals are far too great and
without the refractory tiles, the contents of the vehicle (including
people) would become extremely hot.  If meteors are largely metallic or at
least similiar to iron ore, then the heat transfer coefficeints would
insure that heat IS transferred to the meteor interior rather rapidly.  I
imagine that the friction between the meteor and the atmosphere raises the
temperature of the surface meteor material to a level that it is melting
and fluxing off.  Is this fluxing responsible for the "tails" that we see
on many meteors?  Does anyone know where I can find any actual physical
data regarding the compositions and/or heat transfer coefficients of
meteorites?  How about information regarding the temperatures that meteors
experience during their flight?

Thanks alot, Keith.

Keith, I don't have any source materials at hand, I'm sure other will 
supply some, but Three factors must be considered. The meteor/ meteorite 
process is more closely related to the old capsule heat shields than the
shuttles ceramic tiles. In fact the outer layer melts, and is ablated away, 
removing the heat with it...this is part of what we see as a meteor. 
Another is, these objects have been in deep space for a long time and are 
at space temperature...-200 C or so (?). The other factor is the amount of 
time required. A capsule (or in fact the shuttle) takes minutes to pass 
through the atmosphere; whereas a meteors minimum velocity (due to the 
earth's gravity) is 11 km per second. At the most, therfore the entire 
process takes only a few seconds, not giving much time for heat to be 
conducted (and remeber the heat is going away with the ablated material. 
For meteor shower meteors, where the geocentric velocity is more like 35 to 
72 kps, well it happens mighty fast!! Now two factors work in the other 
direction; most meteorites (meteoroids that reach the ground) consist of 
more substantial material, of semi mettalic- asteroidal origin. And second, 
they are travelling at that minimum speed; otherwise they are usually 
shattered by the dynamic pressures of the rapid deceleration and do not 
fall to the earth in large pieces.
A very interesting topic of discussion!

Wayne