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Eroica
2006-Nov-30, 08:50 AM
I am currently reading Meteors and Meteorites: Origins and Observations by Martin Beech (2006), a very interesting guide written with the serious amateur in mind. His description of what happens when a meteor-producing meteoroid passes through the Earth's atmosphere is at odds with what Phil writes in the book:


When a meteoroid begins to encounter molecules in the Earth's upper armosphere, at about 300km up, it becomes heated through direct surface collisions - essentially the kinetic energy of any colliding molecules is converted into heat energy at the surface of the meteoroid. The meteoroid will continue to heat up until a surface temperature of about 2,000C is reached. Once this temperature has been achieved, typically at an altitude of about 115km, material will begin to ablate from the meteoroid's surface, producing a trailing wake of electrons and atoms in an excited or ionized state. At this stage the meteoroid is beginning to lose mass and is also beginning to gradually slow down. Collisions between meteoric ions and atmospheric atoms result in the production of a plasma trail behind the meteoroid. The excited atoms in the plasma trail will eventually lose energy through the emission of photons, and the electrons will also eventually recombine with the ions to produce more photons. It is the emission of these photons that produces a meteor's light. Spectroscopic observations of meteor trails indicate that it is mainly atoms and ions from the meteoroid that produce the observed light - Martin Beech
When the meteoroid enters the upper reaches of the Earth's atmosphere, it compresses the air in front of it. When a gas is compressed it heats up, and the high speed - perhaps as high as 100 kilometers per second - of the meteoroid violently shocks the air in its path. The air is compressed so much that it gets really hot, hot enough to melt the meteoroid. The front side of the meteoroid - the side facing this blast of heated air - begins to melt. It releases different chemicals, and it's been found that some of these emit very bright light when heated. The meteoroid glows as its surface melts, and we see it on the ground as a luminous object flashing across the sky. The meteoroid is now glowing as a meteor.

... In reality, there is actually very little friction between the meteoroid and the air. - Philip Plait
It would be nice to know a few more details about those meteoroid chemicals that glow when heated. Phil's description is a little vague at this point.