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As those at the June meeting realize, there was no way to cover so broad a field as meteorites in a short session. So here's some more whether you want to hear it or not.

First, a clarification of some points and errata.

As Antoine pointed out, the orbital speed of the Earth is 18 miles per second, or about 30 kilometers per second. I gave this number as 30 mps. Thus, the statement that the highest measured meteoroid velocity at the Earth should read 72 kilometers per second. Any solid body moving faster than the remaining 42 kps will have exceeded the escape velocity of the solar system.

I have not been able to locate a hardness measurement for the nickel iron meteorites, If I find the info I will pass it along. Incidentally, all known iron meteorites also contain some percentage of nickel.

Some authorities state the Barringer crater in Arizona resulted from an iron meteorite of about 64,000 tons. Older books quoted about 1 million tons.

More recent books on the subject seem to indicate that meteoroids greater than a few hundred feet in diameter will be completely destroyed at impact by the immense pressure and heat. Bodies of this size are scarcely slowed by the atmosphere. The pressures are enormous, well above the strength of the body. At impact the parent body is almost completely vaporized. Apparently this explains why some huge craters (astroblemes-literally 'star wounds') here on the Earth have yielded so few large meteorites. Indeed, they yield zillions of small ones. The Hoba West at about 64 tons is the largest yet found.

The fusion crust of most meteorites is very thin, less than a millimeter. The fast motion through the atmosphere and the low conductivity of stone prevents more than just a skin of molten material. The few meteorites found immediately after impact were cool to the touch, unless the body was of considerable size.

As intuitively odd as this sounds, a meteoroid impacting the Earth at any angle above about 15 degrees will produce a round crater.

The Earth's atmosphere will slow meteoroids of less than a ton or so to terminal velocity at a height of several miles. It will thence drop to the Earth in about a straight line at speeds of up to 300 mph. This explains why the impact of a meteoroid is not so high energy as to destroy it or dig a deep crater..

Iron meteorites are susceptible to a 'disease' called Lawrencite. Apparently, chlorine gets in the tiny cracks and combines with water to form hydrochloric acid. The acid slowly dissolves the layered metal and it can fall apart in sheets. I'll look into this further. Some museums place the specimens in nitrogen filled containers to alleviate the problem.

The auto struck by a meteoroid in New York a few years ago sold to a meteorite collector for $10, 000. It may have since been sold to a collector for more.

The current price for some SNC meteorites is a few thousand dollars a gram.

Far fewer stony meteorites are found than iron ones, as they look more like terrestrial rocks. They also weather faster.

For most of history, in fact till about the 18th century, there was no scientific acceptance of meteors as extra-terrestrial. They were thought to originate in volcanic blasts or were carried aloft by high winds (?)

For the question of the day who allegedly said" I'd sooner think two Yankee professors lied than believe stones fell from the sky"?

When the French Academy of Science was meeting around 1800 to ridicule the idea of stones falling from the sky, a rain of stones fell on a French city. Not long after this a similar meteoritic fall occurred in Italy. This left the scientists in a a bit of a sticky wicket. The stones fell from a clear sky and were witnessed to fall by hundreds of people right after " a bright object lit up the sky leaving a huge smoke trail, and thunderous noises were heard by many."

The craters in the Henbury region of Australia were created by a large meteoroid, which split into several large pieces. The local aborigines have a fear of the place, and refer to it as sun-fire-stone-from-air or something to that effect. Apparently, the fall was witnessed by their forefathers (for those with a political correctness obsession, read 'forebears')

The noctilucent clouds, visible at heights of 60 miles or so after sunset, are thought to be the dusty remnants of meteoroids. After the Tunguska fall, the sky was described as having an eerie glow for days, and the early nights were lit up by many high level 'clouds', which were dust particles many miles high lit up by the setting sun.

There are several thousands of meteorites in collections. However, this number counts a fall as one. Therefore, there are millions of them.

It is estimated that about 20 tons of iron/nickel fragments from the Canyon Diablo crater area were removed by iron prospectors and melted down to make various 'useful' tools and appliances before it was recognized what the stones were. The crater itself was first believed to be meteoritic in origin by Barringer, who dropped several shafts in an attempt to find the main mass.

Though no main mass exists, many tons of small specimens still did not convince scientists of the reality of the crater being associated with the meteorites. Until Eugene Shoemaker ( the same as in the Shoemaker/Levy comet impact on Jupiter, tragically killed in a car accident) investigated the area and found shocked rock and 'impactite', a quartz mineral subjected to huge stress, the crater was thought to be either a limestone sinkhole or a volcanic remnant. There are several of both within a hundred miles of the crater.

Many theories of the Tunguska event have hit the market. A small piece of antimatter, a quantum black hole, even a spaceship. The currently accepted comet impact theory may be proven wrong as recent analysis also supports an iron meteoroid.

Why doesn't the Earth show impact scars like the moon and many other solar system bodies? Aside from normal weathering, there are plate tectonics at work which alter the visible surface in just a few hundred million years. There are also glaciers associated with ice ages, volcanoes and earthquakes altering the appearance. Impact scars on the Earth would be considerably less than on the moon due to the larger gravity and the damping effect of the atmosphere. A large astrobleme in Canada is estimated to be 2 billion years old, and new eveidence is being found of older craters.

Enough for now.

If anyone has any ideas for future material in this column please let me know. I'm running out of ideas related to our hobby.

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