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What caused the massive 1908 explosion in Tunguska, Siberia?

February 28, 2003

Dear Cecil:

I've just been rereading Rupert Furneaux's 1977 book The Tungus Event about the massive explosion that occurred in the Tunguska region of Siberia in 1908. The book does not come to any firm conclusion about the cause, although a meteorite is probably the main suspect. What is the latest thinking on this? I have tried Google but what information there is seems to have been hijacked by the UFO brigade. What caused the explosion?

Cecil replies:

Probably a comet or an asteroid, though the question hasn't been definitely settled. This answer disappoints some people, who'd prefer something more in an X-Files vein (Tunguska figured in an X-Files episode, in fact). My feeling is, we're talking about an extraterrestrial object with a mass of at least 100,000 tons that exploded with a force of 10 to 40 megatons of TNT and devastated 2,000 square kilometers of forest. This isn't cool enough?

The Tunguska whatever-it-was detonated six to eight kilometers above a remote section of the central Siberian plateau on June 30, 1908, at 7:14 AM local time. Herdsmen and traders 60 kilometers away saw a fireball brighter than the sun, felt fierce heat, then heard a deafening explosion. Thirty kilometers away huts were flattened, and people were flung into the air and knocked unconscious. At ground zero herds of reindeer were incinerated and forest fires burned for weeks.

News of the blast was slow to reach the outside world, but there were widespread signs that something odd had happened. The flash was visible 700 kilometers away and tremors from the explosion were recorded at a seismic station over 5,000 kilometers away in Jena, Germany. Anomalies in atmospheric pressure were observed by meteorologists in western Europe and North America, and the night skies over Europe glowed with an abnormal light due to a rare phenomenon called noctilucent ("night-shining") clouds.

Initial reports filtering out of Siberia suggested that a meteorite was responsible. But details were so fragmentary, the site so distant, and conditions in Russia so unsettled that the event wasn't investigated until 1927, when Soviet scientist Leonid Kulik led an expedition to the blast zone, a vast region of scorched and flattened trees with their trunks all pointing toward ground zero. Kulik was sure he would find a large meteoric body buried there but never did, despite repeated drilling. He never found an impact crater either. In fact, aside from some dust-size bits of mineral called "spherules," whose presence may or may not be connected to the blast, no one has ever found a piece of the mass that exploded.

In attempting to account for these anomalies, scientists have proposed a number of theories:

  1. The blast occurred when the nuclear engine of a Martian spaceship blew up. This idea is crackpot, of course, but when Soviet engineer Aleksander Kazantsev advanced it in a science fiction story published in the aftermath of Hiroshima the nuclear angle provoked some interest. Kazantsev's story had the added merit of suggesting that the explosion occurred some distance above the ground, a notion now accepted by virtually everyone.
  2. The object was a comet and thus produced minimal debris, since a comet's head is mostly dust and ice. This theory remains popular, although critics argue that comets are too flimsy to plunge deep enough into earth's atmosphere to create such a blast.
  3. It was a stony asteroid that disintegrated before reaching the ground. Initially proposed by a Russian researcher in 1960, this idea was revived to great fanfare in 1993 by American scientists Christopher Chyba, Paul Thomas, and Kevin Zahnle, designers of a computer model that shows that an object 60 meters in diameter moving through the atmosphere at 15 kilometers per second would have produced a Tunguska-like catastrophe. Many in the media pronounced the mystery solved, but other scientists say the case is far from proven.
  4. It was a chunk of antimatter, an idea seriously proposed in 1941 and not ruled out until the mid-60s.
  5. It was a black hole. Proposed by two American scientists in 1973, this theory is dismissed by most other investigators as hopelessly naive.
  6. It was a laser shot from an extraterrestrial civilization trying to contact us, etc. At this point we get back into the realm of those UFOlogists you mention, about which we will say no more.

Informed lay opinion presently seems to favor an asteroid, while the majority of scientists appear to prefer the comet scenario. A quick conclusion to the debate seems unlikely. But don't despair. More than 150 cosmic impact craters have been identified on the earth's surface. Between 1975 and 1992 military satellites detected 136 objects between 30 and 50 meters in diameter exploding in the upper atmosphere. An asteroid 13 meters in diameter missed us by just 105,000 kilometers in 1994, and there are lots more where that one came from. If you want another blast to compare to Tunguska, just wait.

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