[meteorite-list] fireball speed

From: Sterling K. Webb <kelly_at_meteoritecentral.com>
Date: Thu Apr 22 10:31:25 2004
Message-ID: <40395BCB.64F21E32_at_bhil.com>

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 mental math and posting = a good way to pass
> the time :)
>
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Received on Sun 22 Feb 2004 08:47:58 PM PST