In the early morning hours of June 30, 1908, in Central Siberia, a giant fireball was seen moving rapidly across the sky. Where it touched the horizon, an enormous explosion took place. It leveled some 2,000 square kilometers of forest and burned thousands of trees in a flash fire near the impact site. It produced an atmospheric shock wave that twice circled the Earth. For two days afterwards, there was so much fine dust in the atmosphere that one could read a newspaper at night by scattered light in the streets of London, 10,000 kilometers away.
The government of Russia under the Czars could not be bothered to investigate so trivial an event, which, after all, had occurred far away, among the backward Tungus people of Siberia. It was ten years after the Revolution before an expedition arrived to examine the ground and interview the witnesses. These are some of the accounts they brought back:
Early in the morning when everyone was asleep in the tent, it was blown up into the air, together with the occupants. When they fell back to Earth, the whole family suffered slight bruises, but Akulina and Ivan actually lost consciousness. When they regained consciousness they heard a great deal of noise and saw the forest blazing round them and much of it devastated.
I was sitting in the porch of the house at the trading station of Vanovara at breakfast time and looking towards the north. I had just raised my axe to hoop a cask, when suddenly . . . the sky was split in two, and high above the forest the whole northern part of the sky appeared to be covered with fire. At that moment I felt a great heat as if my shirt had caught fire . . . I wanted to pull off my shirt and throw it away, but at that moment there was a bang in the sky, and a mighty crash was heard. I was thrown on the ground about three sajenes away from the porch and for a moment I lost consciousness. My wife ran out and carried me into the hut. The crash was followed by a noise like stones falling from the sky, or guns firing. The Earth trembled, and when I lay on the ground I covered my head because I was afraid that stones might hit it. At that moment when the sky opened, a hot wind, as from a cannon, blew past the huts from the north. It left its mark on the ground . . .
When I sat down to have my breakfast beside my plough, I heard sudden bangs, as if from gun-fire. My horse fell to its knees. From the north side above the forest a flame shot up . . . Then I saw that the fir forest had been bent over by the wind and I thought of a hurricane. I seized hold of my plough with both hands, so that it would not be carried away. The wind was so strong that it carried off some of the soil from the surface of the ground, and then the hurricane drove a wall of water up the Angara. I saw it all quite clearly, because my land was on a hillside.
The roar frightened the horses to such an extent that some galloped off in panic, dragging the ploughs in different directions, and others collapsed.
The carpenters, after the first and second crashes, had crossed themselves in stupefaction, and when the third crash resounded they fell backwards from the building onto the chips of wood. Some of them were so stunned and utterly terrified that I had to calm them down and reassure them. We all abandoned work and went into the village. There, whole crowds of local inhabitants were gathered in the streets in terror, talking about this phenomenon.
I was in the fields . . . and had only just got one horse harnessed to the harrow and begun to attach another when suddenly I heard what sounded like a single loud shot to the right. I immediately turned round and saw an elongated flaming object flying through the sky. The front part was much broader than the tail end and its color was like fire in the day-time. It was many times bigger than the sun but much dimmer, so that it was possible to look at it with the naked eye. Behind the flames trailed what looked like dust. It was wreathed in little puffs, and blue streamers were left behind from the flames . . . As soon as the flame had disappeared, bangs louder than shots from a gun were heard, the ground could be felt to tremble, and the window panes in the cabin were shattered.
. . . I was washing wool on the bank of the River Kan. Suddenly a noise like the fluttering of the wings of a frightened bird was heard . . . and a kind of swell came up the river. After this came a single sharp bang so loud that one of the workmen . . . fell into the water.
This remarkable occurrence is called the Tunguska Event. Some scientists have suggested that it was caused by a piece of hurtling antimatter, annihilated on contact with the ordinary matter of the Earth, disappearing in a flash of gamma rays. But the absence of radioactivity at the impact site gives no support to this explanation. Others postulate that a mini black hole passed through the Earth in Siberia and out the other side. But the records of atmospheric shock waves show no hint of an object booming out of the North Atlantic later that day. Perhaps it was a spaceship of some unimaginably advanced extraterrestrial civilization in desperate mechanical trouble, crashing in a remote region of an obscure planet. But at the site of the impact there is no trace of such a ship. Each of these ideas has been proposed, some of them more or less seriously. Not one of them is strongly supported by the evidence. The key point of the Tunguska Event is that there was a tremendous explosion, a great shock wave, an enormous forest fire, and yet there is no impact crater at the site. There seems to be only one explanation consistent with all the facts: In 1908 a piece of comet hit the Earth.
In the vast spaces between the planets there are many objects, some rocky, some metallic, some icy, some composed partly of organic molecules. They range from grains of dust to irregular blocks the size of Nicaragua or Bhutan. And sometimes, by accident, there is a planet in the way. The Tunguska Event was probably caused by an icy cometary fragment about a hundred meters across – the size of a football field – weighing a million tons, moving at about 30 kilometers per second, 70,000 miles per hour.
If such an impact occurred today it might be mistaken, especially in the panic of the moment, for a nuclear explosion. The cometary impact and fireball would simulate all effects of a one-megaton nuclear burst, including the mushroom cloud, with two exceptions: there would be no gamma radiation or radioactive fallout. Could a rare but natural event, the impact of a sizable cometary fragment, trigger a nuclear war? A strange scenario: a small comet hits the Earth, as millions of them have, and the response of our civilization is promptly to self-destruct. It might be a good idea for us to understand comets and collisions and catastrophes a little better than we do. For example, an American Vela satellite detected an intense double flash of light from the vicinity of the South Atlantic and Western Indian Ocean on September 22, 1979. Early speculation held that it was a clandestine test of a low yield (two kilotons, about a sixth the energy of the Hiroshima bomb) nuclear weapon by South Africa or Israel. The political consequences were considered serious around the world. But what if the flashes were instead caused by the impact of a small asteroid or a piece of a comet? Since airborne over-flights in the vicinity of the flashes showed not a trace of unusual radioactivity in the air, this is a real possibility and underscores the dangers in an age of nuclear weapons of not monitoring impacts from space better than we do.
A comet is made mostly of ice – water (H2O) ice, with a little methane (CH4) ice, and some ammonia (NH3) ice. Striking the Earth’s atmosphere, a modest cometary fragment would produce a great radiant fireball and a mighty blast wave, which would burn trees, level forests and be heard around the world. But it might not make much of a crater in the ground. The ices would all be melted during entry. There would be few recognizable pieces of the comet left – perhaps only a smattering of small grains from the non-icy parts of the cometary nucleus. Recently, the Soviet scientist E. Sobotovich has identified a large number of tiny diamonds strewn over the Tunguska site. Such diamonds are already known to exist in meteorites that have survived impact, and that may originate ultimately from comets.