IMCA Insights – January 2008
by Jeff Kuyken
The following Meteorite
Myths 1 & 2 come from an article published by
www.space.com called "The Top 5 Cosmic Myths". Some very interesting
reading is presented below.
Myth 1: Meteors are
heated by friction as they pass through the atmosphere
This one makes sense,
which is why it's so pernicious. But it's still wrong. Meteoroids are
tiny bits of dust, rock, ice or metal that have the unfortunate luck of
having their orbits intersect the Earth's. When they pass through our
atmosphere, they are heated so ferociously that they glow (and at this
point are called meteors), and are visible for hundreds of miles.
However, it is not friction that heats them. Think of it this way: a
Space Shuttle's tiles are extremely delicate; they crumble easily in
your hand. If they were heated by friction as the shuttle de-orbits and
enters the atmosphere, the tiles would disintegrate. That's not a very
good design characteristic.
In reality, it isn't friction, but ram pressure that heats the
meteoroid. When a gas is compressed it gets hot, like when a bicycle
pump is vigorously used to inflate a tire. A meteoroid, moving at 33,500
mph (15 kilometres a second) or more compresses the air in front of it
violently. The air itself gets very hot, which is what heats the
meteoroid. That's the fact, not friction.
Russ Kempton has made a post to the Meteorite Central Mailing List on
the subject. Here is his post:
"Compression is correct - but - compression is motion, motion produces
friction and thus heat. And, you are right -- it's not just semantics.
It identifies a process making it a wonderful teaching model.
As the air (gas) molecules are compressed, they move. Any "gas" that
suddenly has to move (compression) has more and more molecules banging
into each other - heat from friction. The causal relationship is:
What happens afterwards is a complex process called gas dynamic drag
which deals with the thermal effects of gas in motion. Ultimately the
resultant heat is partitioned away from the meteoroid through ablation."
Myth 2: 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 centimetres 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
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 kilometres 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
Myth 3: Meteorites
The plain and simple
truth of this one is simply: meteorites are usually not
radioactive! In fact, most meteorites are less radioactive than most
Earth rocks as they do not contain radioactive elements. They are also
much older than Earth rocks meaning that even if they were radioactive,
they would have lost any dangerous radioactivity long, long ago.
The most radioactive meteorite recovered thus far is the lunar
KREEP-rich breccia SaU 169 with about 32ppm Thorium and 8ppm Uranium.
However, even these values are far below those of the granitic rocks
that make up the Black Forrest, and similar igneous mountain ranges, and
thus even the most radioactive meteorite is not dangerously radioactive,
and you really don't need to be concerned by radiation while handling
your favourite rocks from space.
Myth 4: Meteorites
As silly as this may
sound, some believe in mystical properties of meteorites, to the point
where they might hear ‘sounds’ from them. Well, they do make sound but
that’s when they’re hurtling through Earth’s atmosphere at phenomenal
When a meteorite is witnessed to find its way to Earth, it will present
a very impressive visual show as it burns through the atmosphere. Even
less common though, are the occasions when witnesses are able to
actually hear audible sounds associated with the falling meteor. These
have been described in many various ways such as: whistling, popping,
booming, thunderous, whizzing, whirling, whirring, crackling, drumming,
rumbling, humming, roaring and more.
Sometimes these sounds are the result of the meteor's high velocity and
the resultant 'sonic boom' which can also be felt. Occasionally
witnesses have described the ground shaking or windows rattling from
these booms. Below is a list (selection) of various recovered meteorites
which were witnessed to fall and also presented the observers with an
impressive sound show. Thanks to Bernd Pauli for compiling much of this
"Black objects that produced crackling sounds like gunshots, flew
eastward through the heavens trailing green smoke."
"The explosion was reported as cannon-like, and was followed by a
hissing sound compared with that of an engine blowing off steam."
Cabin Creek (IIIA)
"People in the town of Dardanelle, about 25 km southeast of the landing
site, were startled by an unusually loud report, accompanied by a
peculiar whizzing sound as if hot metal had come in contact with water."
Campos Sales (L4)
"Most observers reported astonishment by the fireball, the sonic boom,
and the whizzing ('like the sound of bullets') of the falling stones."
"...a noise like thunder or the rolling of drums broke overhead.
...described the report as twofold and followed by a whizzing noise or
the sound of escaping steam."
"...his attention was attracted by a loud rumbling noise sounding very
much like thunder."
"...a terrific noise came from the cloud, as if a multitude of ships had
begun firing their cannons. Thunderclaps followed each by other, as if
opponents were exchanging broadsides. This noise stopped after several
"The noise it created was very loud and lasted for at least half a
minute and sounded somewhat like distant thunder, or the booming of a
cannon in the distance."
"They heard a roaring sound for 2 to 3 minutes, ..."
"Suddenly we heard a strong report and then a sharp noise and whistle."
"Appearance of moving cloud and sounds like thunder."
"No light or sound except for the whirring sound as it passed and the
thud in the ground was noticed."
Piplia Kalan (EUC)
"The meteorite was accompanied by three loud widely heard detonations, a
hissing sound along the trail, ..."
"...he heard a rushing, whistling sound."
"a strange rumbling noise was heard in the atmosphere, followed almost
instantaneously by a startling explosion resembling a discharge of heavy
"A whistling sound was heard."
St. Robert (H5)
"At least one observer noted electrophonic sounds heard simultaneously
with the passage of the fireball."
"The fall was accompanied by a loud hissing noise, ..."
"...a whistling noise"
Wold Cottage (L6)
"...heard various noises in the air, like pistols, or distant guns at
sea, felt two distinct concussions of the earth, and heard a hissing
noise passing through the air..."
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