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February 5, 2002

All In A Writer's Day

From the Mail Box

Somebody in Japan sent me an e-mail asking what the effect would be of a 0.5 million ton body coming in at 40,000 mph and impacting on Mount Fuji. Would there be much of it left? It's straightforward enough to work out that the kinetic energy of such an object would be 82.3 x 10E15 joules. But what does this represent in explosive terms? I spent the next half hour or so thumbing through the various references and sources on my bookshelf only to find to my surprise that nowhere was there a conversion factor from regular energy units to Megatons. Physicists and engineers, it seemed, lived in their world, the military in theirs, and the two didn't come together. So I called Brent Warner, an old friend at NASA Goddard who has been of inestimable help with details like this in many of the books I've written, and left a message with the answering system giving the problem. Twenty minutes later he called back with the vital information that a one-megaton explosion equals 4.184 x 10E15 joules, from which it follows that the event in question would approximate to 20 Minuteman II warheads, or 1,000 Hiroshima bombs.

Brent's source for this indispensable fact was a book published by the IEEE (their Order No. PP4044) entitled Metric Units and Conversion Charts by Theodore Wildi. I obtained a copy from AMAZON.COM. It turned out to be a goldmine of facts for anyone who has wondered how to convert acre-feet, barrels of oil (US or UK), or drams of volume into teaspoons, hogsheads, and registered tons, needed to know the speed of light in feet per second or millimeters per hour, agonized over the gram-centimeter squared equivalent of one pound-force foot second squared of Moment of Inertia . . . and pages more. It now sits confidently above my desk, awaiting the next weird reader challenge.

[Incidentally, for those who were wondering, at typical meteorite densities, a half-million-ton body would be around 50 meters in diameter. The rule of thumb for lunar cratering is that the crater will typically be 50 times the impacting body's diameter and a tenth as deep, which on a flat surface would be 2500 meters (1.5 miles) across and 250 meters (820 feet) deep. But this is based on an incoming velocity of 15 kilometers per second against the 18 that we have above. Since the effect goes up as the square of velocity the results would be somewhat greater. On the other hand I'd guess Fuji to contain considerably more material than the amount excavated above too, with the result that a respectable chunk of it would be left?which is not as devastating as I had guessed intuitively.]

Note added April 13, 2003
In this connection, Dan McGregor also called my attention to a reference gold mine entitled 12) joules.

 
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