[meteorite-list] Top 5 Cosmic Myths - #5

From: Robert Verish <bolidechaser_at_meteoritecentral.com>
Date: Thu Apr 22 09:53:34 2004
Message-ID: <20021213212440.64048.qmail_at_web80310.mail.yahoo.com>

>From a Space.com article earlier this year:

<http://www.space.com/scienceastronomy/top5_myths_020903-5.html>

Top 5 Cosmic Myths (cont.)

Myth #5 - Meteors are still very hot when they hit the
ground

You'd expect that something heated up so much that it
glows would still be hot a couple of minutes later.
Actually, the situation is a bit more complicated.

The super-hot air in front of the meteoroid is not
actually in contact with the particle. (A particle can
still be referred to as a meteoroid as it races
through the atmosphere, while "meteor" is meant to
describe the whole glowing phenomenon.)

The meteoroid's quick motion sets up a shock wave in
the air, like from a supersonic airplane. The shocked
air sits in front of the meteoroid, a few centimeters
away (depending on the meteoroid's size) in what's
called a standoff shock. Between the shocked air and
the surface of the meteoroid is a relatively
slow-moving pocket of air.

The surface of the meteoroid melts from the heat of
the compressed gas in front of it, and the air flowing
over it blows off the melted portion in a process
called ablation. The meteoroid's high velocity
provides the energy for all this heat and light, which
rob it of speed. When it falls below the speed of
sound, the shock wave vanishes, the heating and
ablation stop, and the meteoroid then falls rather
slowly, perhaps at a couple of hundred mph (or a few
hundred kilometers per hour).

It's still pretty high up in the atmosphere at this
point, and takes several minutes to fall to the
ground. Remember, this tiny bit of rock spent a long
time in space, and the core is pretty cold. Also, the
hottest parts were melted and blown off. Even more,
the air up there is cold, which chills the rock as
well.

All of these things together mean that not only is the
rock not hot when it hits the ground, it can actually
be very cold. Some meteorites (what a meteoroid is
called after it impacts) have actually been found
covered in frost!

-------------------------------------------------

Philip Plait is the author of "Bad Astronomy" (Wiley &
Sons, 2002). For more about these and other astronomy
misconceptions, you can buy his book or visit his Bad
Astronomy website.

-------------------------------------------------

Visit SPACE.com to explore a new science feature each
Tuesday. Science Tuesday Archive



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Received on Fri 13 Dec 2002 04:24:40 PM PST