2007-03-18 The Fire Next Time

In the early morning of June 30, 1908, a fireball flew across the Siberian sky and exploded in a 15 megaton blast that flattened 2,150 acres of Siberian forest. In the years that followed, scientists correlated atmospheric pressure readings, reports of unusually bright sunsets and "night glows" in the skies over northern Europe, recordings of seismic waves, and eyewitness accounts, concluding that the cause was probably a stony asteroid that entered the earth's atmosphere and broke up explosively 8 kilometers above the Earth at 7:14 am local time.

Verma's story doesn't end there, of course, or "The Tunguska Fireball" would be a fairly short book. As it is, Verma uses the Tunguska event to embark on an entertaining discussion of how scientists came to understand what had probably happened in the skies over Siberia. The investigations into this remote area were difficult and the findings yielded many interesting theories, ranging from fairly plausible ideas about the arrival of a stony asteroid or comet, to more exotic hypotheses involving black holes, antimatter, mirror matter, volcanoes, ball lightning, and "geometeors," to really bizarre notions about crippled alien spaceships, laser beams from other planets and death rays secretly invented by Nikola Tesla (really). The Tunguska event offers a great excuse to digress among a number of interesting ideas, although I confess that I find Verma's explanations of the underlying science to be a tad murky at times.

When the dust settles (so to speak), I'll place my bets on the stony asteroid theory, with a sentimental vote for the killer comet--the other hypotheses seem to require too much special pleading to be a compelling way to think about the event, at least based on the information we have in hand today. That said, the most sobering revelation in Verma's book is his report of the "mini-Tunguska" event of September 24, 2002. A US satellite spotted an object that entered the earth's atmosphere, but lost it as it fell below 30 kilometers; a few moments later, another satellite reported a fireball exploding in the cloudy skies above Siberia. The explosion flattened 100 square kilometers of forest with the energy of a small atomic bomb, but no one witnessed the fireball and, as far as we know, no one was killed or injured. The story would have been very different if the object, whatever it was, had exploded above a populated area.

Verma's books makes entertaining and sobering reading. "The Tunguska Fireball" will make you wonder how many more objects are floating around in the void with Earth's name on them.