IMCA Insights – January 2008
Meteorite Myths
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:

Compression/Motion
Motion/Friction
Friction/Heat

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 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 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 in frost!

***

Myth 3: Meteorites are radioactive

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 make noises

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 speeds.

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 information.

Adzhi-Bogdo (LL3-6)
"Black objects that produced crackling sounds like gunshots, flew eastward through the heavens trailing green smoke."

Allegan (H5)
"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."

Crumlin (L5)
"...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."

Felix (CO3)
"...his attention was attracted by a loud rumbling noise sounding very much like thunder."

Honolulu (L5)
"...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 minutes..."

Holbrook (L6)
"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."

Juancheng (H5)
"They heard a roaring sound for 2 to 3 minutes, ..."

Karakol (LL6)
"Suddenly we heard a strong report and then a sharp noise and whistle."

Mooresfort (H5)
"Appearance of moving cloud and sounds like thunder."

Noblesville (H4)
"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, ..."

Pontlyfni (WIN)
"...he heard a rushing, whistling sound."

Rowton (IIIA)
"a strange rumbling noise was heard in the atmosphere, followed almost instantaneously by a startling explosion resembling a discharge of heavy artillery."

Stratford (L6)
"A whistling sound was heard."

St. Robert (H5)
"At least one observer noted electrophonic sounds heard simultaneously with the passage of the fireball."

Trebbin (LL6)
"The fall was accompanied by a loud hissing noise, ..."

Warrenton (CO3)
"...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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