Thursday, November 24, 2011


Internet Poster's Question:
I understand (drag etc) why the asteroid would disintegrate, but that is something different from actually exploding which, to me at least, implies that there is some kind of internal energy reserve which is released causing the asteroid to fragment violently from the inside. This is different from breaking up due to external forces (equivalent to the difference between how a bomb exlodes on hitting the ground and how a china teapot disintegrates on hitting the ground).

If I hit the water at 100 miles an hour I would, indeed, splatter quite unhappily, but I would not actually explode.

The Siberian example given by the speaker features an asteroid actively exploding, not just burning up. How does that happen?

My Answer:

The explosion is not a chemical explosion but a pressure-wave induced explosion. Take a look at this high-speed camera footage of a bullet going through an apple and a banana:

Inside the apple and banana, if the bullet were traveling slowly, it would just enter and exit leaving a hole-shaped burrow, much like slowly jabbing a pencil through chocolate cake. Because of the bullet's speed relative to the resistance of the medium, the bullet creates a pressure wave as it passes through.

This article talks some more about asteroids exploding in the atmosphere.

An explosion can be triggered by expanding gas, as with nuclear bombs that heat the air and cause it to expand, or with TNT which chemically combusts to create CO2 and O2 gases that expand from denser, solid matter (and heats the air since it's exothermic). But an explosion can be any shock wave moving outward from the center.

When an asteroid hits the hard rocky surface of the earth, an explosion occurs as the kinetic energy is transferred to the surrounding rock as shock wave energy. That's why a room-sized asteroid can leave a mile-wide crater.

Similarly, if an asteroid is moving fast enough, the atmosphere will seem "hard" enough for much of its kinetic energy to be converted into shock wave energy (and heat from friction). When a smooth asteroid fractures and breaks off into numerous jagged non-aerodynamically shaped objects, further shock waves are created as these non-aerodynamic objects suddenly transfer even more kinetic energy into shock wave energy.

P.S. Regarding your teapot v bomb analogy: If your teapot were moving at 3,000m/s, it'd have the explosive power of a kilogram of TNT (4.1 megajoules). Remember, kinetic energy = 0.5mv^2, so something moving 100 times faster has 10,000 times the energy.

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