An Article In Meteorite Times Magazine
by Jim Tobin

The Fall

The last few months have been very exciting for those of us interested in meteorites. Falls have been happening every few weeks. The United States getting its fair share has been very exciting indeed. Each time one of these huge bright fireballs is seen there is the immediate gathering of hunters to try and find fragments of the cosmic body. But, there are many factors which can influence how many pieces or if any are recovered.

More and more often the video cameras which surround us everywhere now are catching the passage of the meteor through the atmosphere. This is where investigation for me begins. I don’t usually have the opportunity to go and hunt. But, I enjoy doing some analysis from my computer of the events. The characteristics of the meteor’s passage hold a lot of clues about what may or may not be found.

For example, the Peekskill fireball showed fragmentation but no explosive air burst. The one large fragment which struck the car was the only piece found. Yet, the visual evidence would support the possibility of at least a few more pieces which have never been recovered. The fragmentation during flight shows several quite large fragments in the meteor’s trail.

In contrast to Peekskill other meteorite forming fireballs show large brilliant explosions often with the accompanying detonation sounds being heard on the ground. Though there is no hard and fast rule established these that explode often produce dozens even hundred or thousands of fragments that can be recovered. Buzzard Coulee of last year produced large numbers of individuals. The Holbrook body produced something in the area of 30,000 individual pieces. Something like 14,000 were picked up immediately but they continue to be found still nearly a hundred years later. The same is true for Pultusk with even larger numbers and in the long past L’Aigle with several thousand. But, such is not always the case. Ensisheim from the best we know was a single large individual. Zagami was a single 18kg individual. The list is long for falls of only one or a hand full of meteorites. Clearly there are factors which must influence whether many are formed or a small few.

What are the characteristics of speeding chunks of rock passing through our atmosphere? I am sure this is not a complete list but lets start with the obvious. Speed, size of meteoroid, angle of entry, strength of the rock to stress, presence of natural faults and flaws in the mass, and shape of the incoming body, these seem like factors to begin the discussion with.

The listed factors are so intertwined that the easiest way to look at them all is by formulating a couple different scenarios First, an iron body approaching the Earth from behind at a low atmospheric entry speed and traveling tangent to the surface so that it travels hundreds of miles at high altitude slowing gradually like a returning Space Shuttle. I can visualize this body surviving the passage nearly intact. The iron is inherently strong and it has some ductile properties allowing it to deform without shattering. One of our factors is shape and this body might have knobs or points on it that would be high drag areas. These projections might be torn off. These pieces could show in the trail of the meteor as fragmentation. But, this type of low stress entry by a strong object might yield only a few specimens on the ground.

Going to the other extreme, a stone meteoroid approaches the Earth from space traveling much more head on thus entering the atmosphere with tremendous speed. This object also is coming in at a steep angle. It will pierce the atmosphere in a matter of seconds. It is a rough sphere in shape presenting a large front surface to the atmosphere and great drag. This object encounters almost immediately pressure on it leading surface. This pressure increases as the Earth’s atmosphere thickens with decent. Soon the rock which is almost totally lacking in malleability is compressed and stressed until it fails structurally. In this explosion the body relieves itself of all the imposed compression and stress. The heat from the explosion is sufficient to melt the outside of most fragments which now have very low mass and quickly slow. They fall in simple decent to Earth. They are no longer glowing and this part of their passage to the ground is therefore called “dark flight.” Their exterior cools and in fact if they are large enough they may be cold if found immediately. The very low temperatures of space may contribute to the brittleness of the material and add a minor component to their ease of failure under stress. In this type of entry a meteoroid may fragment into hundreds or thousands of pieces. Also, a large portion of its volume will become dust in the explosion. Another portion of all meteoroids is lost due to ablation and is often seen as the “smoke trail” a meteor leaves behind. After all the damage done in the atmosphere a much reduced amount of material reaches the ground as meteorites. Even when thousands of pieces are found the percentage recovered is probably small compared to the original mass before atmospheric entry.

Between these two extremes of atmospheric passage are countless variations. Each body entering the atmosphere will however be subjected to the factors discussed. None can pass through without some stress or without melting. Some will break up, some will explode, some will survive more or less intact, And then there is Carancas.

To me the fall of the Carancas meteorite has not been satisfactorily explained. This was not a large body upon reaching the ground. Guesses are  tons at most. It was stone yet appears to have survived passage through the atmosphere to reach the ground with energy enough to form an explosion crater. Since it did not have tremendous mass the energy must come from it velocity at impact. How did a small stone body survive with very high velocity all the way to the ground. I am sure there are numerous researchers still working this, but what comes to my mind first is the shape of the meteoroid. A long slender needle shaped body would have very low drag and low stress overall. The point would have great stress and suffer great heating. Maybe such a shape can make it through the atmosphere to reach the ground and form an explosion crater. I don't know maybe we will find out someday. The Carancas body was not able to survive the explosion however. It was reduced to a small number of good size fragments and the remainder peas size and smaller tiny bits.

Meteorite hunters are currently working in Arizona and Pennsylvania to find meteorites from recent fireballs. Arizona has yield an undisclosed number of fragments. It is yet to be seen if pieces can be found in the more vegetation covered state of Pennsylvania. Good luck to all the hunters and I am sure much will be learned from these falls. Each fall is unique but each begins with a fiery passage through the atmosphere. The visual event more and more often recorded on video gives many good clues about what may or may not be found on the ground.

As always if you have any comments about this or other articles you can always email me at jim@meteorite.com