Whoops!
It's those tricky decimal points again. Or long division, perhaps. Either
that, or the back of this envelope is not big enough. Maybe I should have used
one of those two PDP-11's you have connected with a serial cable?
And the really silly thing is that three years ago (02-20-01), I posted the
correct answer to this List myself. The best thing about keeping 30,000 emails
in your computer is the fun of finding one. At the risk of repeating myself,
here it is:
Then there's the question of how large a stone could
possibly fall and survive?
Nothing can get through the atmosphere to the ground
without impacting at destructively high speeds unless
its mass per unit area is less than the atmosphere's mass
per unit area (from the top of the atmosphere down to the ground).
Assuming a stone three times denser than water, the
theoretical upper limit is a sphere of about three
meters diameter, or 40,000,000 grams (40 metric tons).
To reach this upper limit, everything would have to be
perfect. The stone would have to be strong, no cracks
or fissures, well consolidated (porosity of 1% or less),
so it is strong enough not to fracture under the dynamic
pressure of re-entry. It would probably be an achondrite.
It should be of a regular shape so turbulence wouldn't
make it oscillate and saw it apart. It should have the
lowest possible entry velocity and a low angle of
incidence for a long grazing re-entry, so it will reach
its stagnation point at a very low altitude, near the
ground, so it doesn't pick up much speed in the dead
drop phase of its fall. It shouldn't land on rocks, which
would fragment it, but soft soils. Is that all? What else
do you want?
That's all. That's the perfect meteorite.
So if anyone notices a ten-foot ball of rock half-buried
in the cow pasture and covered with fresh black fusion
crust, they should definitely phone it in.
Richard Norton estimated that, even in the best case, a
meteroid on it way to being a meteorite loses 90% of its
mass to ablation on the way down, so maybe the 4 ton Jilin
started out high in the atmosphere as the "perfect" 40 ton
chondrite.
Here's some comparisons:
Mechanical (crushing) strength: Carbonaceous chondrites
from 0.1 bar to 10 bar. Ordinary Chondrites from 62 bar
to 3700 bar. Achondrites from 2500 bar to 4000 bar. And
irons from 3200 bar to 4400 bar.
Dynamic pressure of the atmosphere = density of air times
velocity of meteorite squared. Fireballs in meteor showers
break up at 0.1 bars to 10 bars. Sporadic bolides at 30 to
50 bars. Tracked and recovered stones (like Lost City and
Innisfree) never reached 200 bars of dynamic pressure. The
Tunguska object (whatever it was) disrupted at 200 bars.
Cratering will occur when the object impacts at a speed
greater than the speed of sound in the material of the
impactor. You would think the speed of sound might have
been measured in many meteorites, but it hasn't. The
only values I could find are: for shear waves 600 to
1200 meters/sec and for transverse or pressure waves,
2000 to 4200 meters/sec. This is considerably less than
for terrestial rocks.
Meteorites are much more porous than terrestial rocks
also. Ordinary chondrites have porosities of 0.7% to
18.3%. Carbonaceous porosities up to 25% (like a sponge).
Even achondrites run 4.3% to 15.1%. Similar terrestial
rocks would probably not exceed 1% porosity. Meteorites
are poorly consolidated.
More than you ever wanted to know, I guess. At least epoxy dries faster than
paint. And, you got the answer right before it dried! I bet you used one of
those PDP-11's.
Sterling
--
"stan ." wrote:
> > By the way, the maximum weight for a stone to reach the
> > ground is less than for an iron, only 40 tons or so. That's a
> > stone roughly spheroidal and about 25 feet in diameter! So, if
> > you notice a fusion crusted rock, say, 9 meters across, be
> > sure and check it with a magnet.
>
> thats a bit overly optimistic with regards to diameter I'm afraid... a
> shperiod with a radius of 9 meters would have 382 million cubic centimeters
> of volume. at the low end of 3 g/cc (juvinas) that would be about 1145
> metric tons. 25 feet in diameter is still 402 metric tons.. to get 40 tons,
> assuming a light eucrite, you would need a rock of about 147cm in radius, or
> 11.6ft across so remember, if you are walking down a path and spot what
> looks like a eucrite boulder, dont bother checking it unless it's less than
> 12ft in diameter! ;) now if only lunars came in that size!
>
> yes I'm bored and on the 'net... unfortunatly uv cure epoxy doesnt cure any
> faster when you watch it, hence
