Monday, October 12, 2009

Near earth objects

Tunguska-sized explosions occur on Earth about once per century, and larger explosions the size of the largest H-bombs, occur about once per millennium. Many of these explode in the atmosphere and cause devastation over tens of kilometers, but don't leave long-lasting craters. If we want to imagine the effect of impacts we need to calculate the energy release of an A-bomb and compare it to the energy of a NEO impact.
First, we have to know the energy liberated by an A-bomb. The Hiroshima bomb expended the energy of roughly ten thousand tons of TNT, or 18 "kilotons" in military parlance. One kiloton (1 KT) is about 4.2 x 1012 joules (the joule is the unit of energy in the Standard International, or "SI," set of scientific units). The Hiroshima bomb thus represented roughly 8 x 1013 joules of energy.
Now all we have to do is calculate the energy of the meteoroid. In freshman physics courses, you learn that the kinetic energy of a moving object is 1/2mV2.
The trick in using any equation like this is to be sure to use the correct units. In SI, the units are meters, kilograms, and seconds, so that mass m must be in kilograms and velocity V must be in meters/second.
Thus, right away we can say that V in the equation will be V = 15 km/s or 1.5 x 104 m/s.
To get the mass, we have to figure out the mass of a 30-meter wide rock. Rock has a density of about 3000 kg per cubic meter, so we need to calculate the volume of the rock and multiply times this density. Thus we have,
m = (4/3) PI R3 (3000 kg/m3) = (4/3) PI (15 m)3 (3000) = 4.2 x 107 kg.
For a two kilometer asteroid going 28 km/sec:
m = (4/3) PI R3 (3000 kg/m3) = (4/3) PI (1000 m)3 (3000) = 1.3 x 1013 kg.
Thus the total energy is,
E=1/2 ( 1.3 x 1013 kg) (2.8 x 104 m/s)2 = 5.1 x 1021 joules.
To be safe, let's imagine that half the kinetic energy is lost to noise, heat, slowing, and fragmentation of the meteoroid before it explodes. That still leaves about 2 x 1021 joules for the Asteroid 2002 NT7 explosion, compared to about 3 x 1013 joules for the Hiroshima A-bomb.
Thus, my estimate is that the NT7 had an explosive energy roughly on order of 6.6 x 107 TIMES the A-bomb. That’s 66,000,000 Hiroshima bombs. All at once, all in the same place.
By the way, all those Sci-fi shows where folks look up and see the incoming? The speed of light electro magnetic output of the object in the atmosphere will instantly crisp all line of sight organic matter (you). You won’t see it because you won’t be, anymore, instantly, forever or for billions of years which ever is longer.

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