Bad Astronomy: Misconceptions and Misuses Revealed, from ...

METEORS, METEOROIDS, AND METEORITES, OH MY! 137
as I said above, this is the usual explanation given in books and on
TV. However, it’s wrong. In reality, there is actually very little friction
between the meteoroid and the air. The highly heated, compressed
air stays somewhat in front of the meteoroid, in what
physicists call a standoff shock. This hot air stays far enough in
front of the actual surface of the rock that there is a small pocket
of relatively slow-moving air directly in contact with it. The heat
from the compressed air melts the meteoroid, and the slow-moving
air blows off the melted parts. This is called ablation. The ablated
particles from the meteoroid fall behind, leaving a long glowing
trail (sometimes called a train) that can be kilometers long and can
stay glowing in the sky for several minutes.
All of these processes—the huge compression of air, the heating
of the surface, and the ablation of the melted outer parts—happen
very high in the atmosphere, at altitudes of tens of kilometers. The
energy of the meteoroid’s motion is quickly dissipated, slowing it
down rapidly. The meteoroid slows to below the speed of sound,
at which point the air in front is no longer greatly compressed and
the meteor stops glowing. Regular friction takes over, slowing the
meteoroid down to a few hundred kilometers per hour, which is
really not much faster than a car might travel.
This means that it takes a few minutes for an average meteoroid
to pass the rest of the way through the atmosphere to the
ground. If it impacts the ground, it is called a meteorite.
This leads to yet another misconception about meteors. In practically
every movie or television program I have ever seen, small
meteorites hit the ground and start fires. But this isn’t the way it
really happens. Meteoroids spend most of their lives in deep space
and are, therefore, very cold. They’re only heated briefly when
they pass through the atmosphere, and they’re not heated long
enough for that warmth to reach deep inside them, especially if
they are made of rock, which is a pretty decent insulator.
In fact, the hottest parts ablate away, and the several minutes
it takes for the meteoroid to get to the ground let the outer parts
cool even more. Plus, it’s traveling through the cold air a few kilometers
off the ground. By the time it impacts, or shortly thereafter,
the extremely frigid inner temperature of the meteoroid cools the