On June 30, 1908, an horrendous explosion occurred in the sky above the central Siberian wilderness near the Tunguska River. The concussion from the blast, estimated at 20 megatons of TNT, levelled trees in an area nearly 40 miles wide. Oddly, the detonation produced no crater nor other evidence of impact.
Over the past 80 years, dozens of explanations for the Tunguska Event have been proposed. These varied explanations have suggested comets, black holes, nuclear explosions, antimatter, asteroids, and even alien spacecraft as the cause.
Recently, Christopher Chyba (now at NASA-Goddard Space Flight Center), Paul Thomas (University of Wisconsin at Eau Claire), and Kevin Zahnle (NASA-Ames Research Center) conducted a computer simulation that strongly suggests that the culprit was a stony asteroid, the most common class of meteorite. The simulation indicated that a stony asteroid about 100 feet in diameter and moving at a speed of about 10 miles per second would disintegrate at a height of about 5 miles above the ground -- approximately the same height at which the Tunguska object is believed to have exploded.
In their analysis, the researchers determined that an iron-rich object, which is twice as dense as a stony asteroid, would probably not have exploded at all, meaning that a crater would have been produced. Lighter objects, such as a comet or a carbonaceous asteroid, which are about one-fourth the density of a stony asteroid, would explode too high, thereby not inflicting as much damage on the ground.
Chyba and associates explained that a stony asteroid would suffer "catastrophic fragmentation" on its trip through the atmosphere. This would increase the surface area of the meteorite and, hence, the heat generated by friction against the air. The heat would, in turn, cause large amounts of meteoritic surface material to be vaporized, and thus produce the explosion.
Assuming that the Tunguska event was, indeed, produced by the explosion of one of the most common classes of meteorites, what are the chances of another Tunguska-like event happening on Earth? Duncan Steel, of the Anglo-Australian Observatory in Coonabarabran, New South Wales (and a member of the committee that produced the NASA Report of the Near-Earth-Object Detection Workshop) comments: "The thing which may be wrong with most of the discussion is that it is generally assumed that such objects hit the Earth randomly in time. This is daft in that we know that a good fraction -- perhaps even the majority -- of the mass influx of smaller meteoroids hit the Earth in showers (meteor showers).
"These occur as the Earth passes through the meteoroid stream produced by an asteroid or comet. They recur each year since the smaller particles produced by the cometary decay are spread around its orbit from the comet nucleus. However, there is a concentration, especially of the larger particles, close to the cometary nucleus.
"The above would lead to the following occurring: a few random incoming Tunguska-type objects every few centuries, but every millennium or so there will be a phase in which every few years/decades there is a large number of Tunguska-type events spread over a week or so. This I call 'Coherent Catastrophism,' and catastrophic it would be. Indeed, I believe, it 'has been,' judging from the historical record, since this is what is going on at the moment, with us now (late 20th century) being in a hiatus between mass influxes."
Copyright © 1993 by Fallbrook Gem and Mineral Society, Inc.
The preceding article was originally published in the February 1993 issue of Lithosphere, the official bulletin of the Fallbrook [California] Gem and Mineral Society, Inc; Richard Busch (Editor).
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Last updated: 18 September 2002