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Week of July 20, 1998

I have something very different for this week's Snack. I am learning how to program using JavaScript, and as a test I started fooling around with entering and manipulating numbers. What better way than to use some simple equations to figure out the energy of an asteroid impact?

When an asteroid or comet impacts the Earth, the size of the explosion depends on a lot of factors, but mostly on the mass of the impactor and the velocity of the impact. The routine (which is really only two short lines of math) calculates that energy, then converts it to megatons, which is equivalent to one million tons of TNT exploding.

It's easy to use. You need a browser that is JavaScript 1.1 compliant; check your owner's manual. Netscape 3 and 4 should handle it. Anyway, first choose what kind of impactor you'd like: a rocky asteroid, an iron asteroid, or an icy comet. Then enter the radius in meters (1 meter is about a yard; there are about 1600 meters in a mile). Note that the density and mass of the asteroid will immediately come up. Then enter a velocity in kilometers per second (1 kilometer is about 0.6 miles). The energy released on impact will come up, in megatons. For reference, I believe the largest nuclear bomb ever tested (at least, the largest we the public know about) had a yield of 57 megatons. See what combination of mass and velocity gives you that figure. I bet you'll be surprised!

Some incidentals: a density of 1 is that of water (and roughly that of ice), rock has a density of about 2, and iron about 7. The units are grams per cubic centimeter. For velocity, bear in mind that 11 kilometers per second is the minimum velocity of an incoming rock: that's the Earth's escape velocity. Anything coming in would actually be traveling faster. Values of 11 to 70 are physically most likely. Enjoy!

1) Type of the impactor: Rock Iron Comet
2) Radius of the impactor in meters:
3) Velocity of asteroid in km/sec:
Mass and Impact energy:
4) Density of asteroid (grams/cm^3):
5) Mass of asteroid (tons):
6) Impact energy in megatons:

©2008 Phil Plait. All Rights Reserved.

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