From: James Rogers (jamesr@best.com)
Date: Tue Jan 14 1997 - 18:41:57 MST
At 05:09 PM 1/14/97 -0500, Michael Lorrey wrote:
>Thanks James. Given this, what is your guesstimate of what an 11 km/s
>one ton ferrous rock could do, with a 15 ton impact force?
You mean 15 ton (nuclear) impact ENERGY?
>Size of crater?
>If preshaped for penetration, effective depth?
>effective radii of shock wave?
>anything else?
>
I did some research on known meteor impacts and the various relevant values,
and I was able to generate a baseline for meteor-nuclear equivalence. Due
to rounding, density estimations, and other error-inducing techniques, I
estimate the error at +/- 25%.
ENERGY WISE:
10Mt (nuclear) = 900kton (mass @ 11000 m/s) = ~60m diameter ferrous rock
(BTW, I verified the impact velocity for the sake of completeness.)
Impact parameters, assuming 10Mt (energy, nuclear equivalence) impacts on earth:
CRATER DEPTH: Nuclear: 60 meters Rock: 250 meters
CRATER DIAMETER: Nuclear: 2000 meters Rock: 1500 meters
CONCLUSION:
While the nuclear impact will impart greater damage over a larger area, a
rock has *much* better penetration (good-bye, all but the hardest, deepest
targets). The crater diameter is mostly a function of hydrostatic shock.
The spherical nature of a nuclear explosion will naturally produce better
horizontal damage than a falling rock, which only generates maximal
atmospheric compression in front of it. However, the crater diameter *is*
an indication of how the shockwave radii scale. The cratering effects are
known to scale roughly as a function of the cube root of impact/explosion
energy. You can make your estimations from there.
It is obvious from the data that the size of the rocks you have to throw for
large-scale nuclear impact equivalence is pretty large.
IMPORTANT NOTE: According to sources, a rock must have a mass >500 tons in
order to impact at the above assumed velocity. Rocks with less than 500
tons mass are slowed by atmospheric drag to velocities less than 1000 m/s.
A one ton rock will not work.
FYI, My primary data source for falling rocks was the Barringer crater in
Arizona. The following is the data I collected from that impact independant
of data I calculated:
IMPACT VELOCITY : 11000 m/s
DIAMETER : 50 meters
COMPOSITION : Nickel-Iron (I assumed 7g/cc for density)
CRATER DEPTH : 200 meters
CRATER DIAMETER : 1200 meters
Calculated Impact Energy ~ 5-6 Mt (energy, nuclear equivalence)
For additional energy information, you might want to check out Tunguska,
Siberia in 1908. That impact (although it never actually touched the
ground) had an estimated damage equivalence to a 20-40Mt nuclear airburst
(estimates vary) and the estimated size of the incoming object ( density
estimated at 1-4g/cc ) is 50-100 meters diameter. Impact velocity estimates
range from 7-17 km/s.
Factual damage figures: 2100 km^2 flattened by shockwave, 200 km^2 scorched.
This is for speculation only as the information available on the Siberian
impact is insufficient for useable calculations.
Enjoy.
-James Rogers
jamesr@best.com
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