On Fri, 21 Jun 2002, The Silent Observer wrote:
> If we presume that impacts are chondrite asteroidal bodies, and that
> most such are composed as "gravel piles" akin to the one examined by the
> recent NEAR mission, we might expect this to be the norm for impacts in
> this size range...
Eros, which is what NEAR ended up at, is *not* a gravel pile -- it appears
to be more or less solid rock, as best we can tell from the admittedly
limited information. (We really need penetrating radar on these
spacecraft, to get some information about the interior.)
Mathilde, which NEAR flew by along the way, *may* be a gravel pile. Its
density is awfully low for it to be solid rock. There are other possible
explanations for the low density, though -- in particular, it may have an
ice core. See above complaint about the lack of penetrating radar. :-)
That said, though, note that an object *doesn't* have to be a gravel pile
to break up in the atmosphere. In fact, in the Tunguska size range, most
anything except a nickel-iron meteorite will break up. The stresses
produced by the air drag overwhelm the structural strength of a stone or
chondrite meteorite, and when it starts to break up, positive feedback
sets in, so it disintegrates almost instantly into small bits even if it
started out as solid stone. Only the nickel-irons are strong enough to
survive to impact.
In fact, the Tunguska breakup altitude matches a stone meteorite much
better than a rubble pile, a comet head, or a chondrite.
> It would be interesting to find out if there are records, perhaps
> legends, of great disasters in remote parts of the world (such as, in
> those days, South America, central Asia, or much of Africa) from prior
> to, say, 1850, that might be linked to impacts similar to Tunguska.
There are a few that people have speculated about, but the physical
evidence is basically nonexistent and nobody can be sure.
Henry Spencer
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