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In Brief:
At 7:17 AM on the morning of June 30, 1908, a mysterious explosion occurred in the skies over Siberia. It was caused by the impact and breakup of a large meteorite, at an altitude roughly six kilometers in the atmosphere.
What was the explosion?
Because the meteorite did not strike the ground or make a crater, early researchers thought the object might be a weak, icy fragment of a comet, which vaporized explosively in the air, and left no residue on the ground. However, modern planetary scientists have much better tools for understanding meteorite explosion in the atmosphere. As a meteorite slams into the atmosphere at speeds around 12 to 20 km/sec or more, it experiences a strong mechanical shock, like a diver bellyflopping into water. This can break apart stones of a certain size range, which explode instead of hitting the ground. Some of them drop brick-sized fragments on the ground, but others, such as the one that hit Siberia, may produce primarily a fireball and cloud of fine dust and tiny fragments. In 1993 researchers Chris Chyba, Paul Thomas, and Kevin Zahnle studied the Siberian explosion and concluded it was of this type -- a stone meteorite that exploded in the atmosphere. This conclusion was supported when Russian researchers found tiny stoney particles embedded in the trees at the collision site, matching the composition of common stone meteorites. The original asteroid fragment may have been roughly 50-60 meters (50-60 yards) in diameter.
If asteroids hit Earth, why don't we see more such explosions? Many asteroidal fragments circle the Sun; the Siberian object was merely the largest to hit the Earth in the last century or so. Had it hit a populated area, devastation would have been enormous. If there are many asteroid fragments, why don't we see more hits? We do! The problem is that they have not been understood until recently. Current studies reveal that such explosions may happen every couple of centuries; however, six out of seven happen over the ocean, and few happen over populated land. A key to the phenomenon is: the larger the impact the rarer it is. An Air Force satellite in the 1990s detected a smaller explosion over the Pacific. In 1972, a 1000-ton object skimmed tangentially through Earth's atmosphere over the Grand Tetons in Wyoming, and then skipped back out into space, like a stone skipping off water. It was photographed by tourists and detected by Air Force satellites. Had it continued on into the atmosphere, it could have caused a Hiroshima-scale explosion over Canada, somewhat smaller than the Siberian blast. Even larger objects have hit Earth, but they are more rare. For example, an iron asteroid fragment perhaps 100 m across hit Arizona about 20,000 years ago, leaving the kilometer-wide "Arizona Meteor Crater," which is open to visitors; and a 10-km asteroid hit Earth 65 million years ago, ending the reign of dinosaurs. Brick-sized interplanetary stones fall from the sky in various locations every year. Several houses and a car have been hit in recent decades. Tiny dust grains are even more common; they can be seen every night if you watch long enough; they are the bright streaks of light sometimes called "shooting stars." Interplanetary space contains many small bodies of different sizes. All of them move in elliptical orbits around the sun as prescribed by Kepler. Occasionally their orbits intersect those of planets, leading to a collision. Large enough bodies leave sizable craters on planets or satellites. This explains why impact craters are present on surfaces of planets and moons throughout the solar system. If we continue to study asteroids and build more telescopes for detecting and tracking them, we will have better information about the frequency of such asteroid impact-explosions, and more chance to have warning about impending impacts.
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The Tunguska event was an explosion that occurred at 60°55′N 101°57′E, near the Podkamennaya (Under Rock) Tunguska River in what is now Evenk Autonomous Okrug, at 7:17 AM on June 30, 1908. The event is sometimes referred to as the Great Siberian Explosion.
The explosion was probably caused by the airburst of an asteroid or comet 5 to 10 kilometers (3–6 mi) above the Earth's surface. The energy of the blast was later estimated to be between
10 and 20 megatons of TNT, which would be equivalent to Castle Bravo, the most powerful nuclear bomb ever detonated by the US. It felled an estimated 60 million trees over 2,150 square kilometers (830 sq mi). An overhead satellite view centered at 60.917N 101.95E (near ground zero for this event) shows an area of reduced forest density, with a fully visible irregular clearing of somewhat less than one square kilometer in area.
In recent history, the Tunguska event stands out as one of the rare large-scale demonstrations that a full doomsday event is a real possibility for the human race.
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