Tunguska Event Centennial

In a few days, we in the U.S. will be celebrating the nation's anniversary with plenty of fireworks. But today is itself the 100th anniversary of a particularly spectacular natural fireworks display that occurred in the middle of Siberia, Russia: the famed Tunguska Event.

If you'd been in Siberia on June 30, 1908, then sometime between 7 and 7:30 am you'd have seen a very bright shaft of bluish light coming down in your sky. Perhaps ten minutes later, you'd have heard an immense series of booms, as loud as an artillery barrage. The ground under your feet would have shaken violently, and a billow of smoke erupted over the distant forest.

Follow up:

If you were close to the Tunguska River, less than a hundred miles from the event, it might have been still more spectacular. Here you might have seen the sky above the forest erupt into bright flames, and felt a scorching wind pass you. The trees around you would have been falling over, some with their tops on fire. Of course, you yourself would probably be knocked off your feet!

Hundreds of miles away from the event, trees fell, houses shook, windows were broken. Thousands of miles away, seismic instruments recorded an earthquake. So much dust was thrown into the air that the reflected light from it made it possible to read by the sky-glow at night, even as far away as London, England.

What happened?

Based on the evidence we have, the most likely scenario is that a rocky body (asteroid) some 60 meters across came tearing through our atmosphere that fine summer morning, heated by the friction of its passage through the air at over 10 miles per second. The heat and pressure builds up in front of a body slamming through the atmosphere that way. (Think of how it hurts to belly-flop into the water.) In many cases, it's so intense that it can destroy the incoming object before it hits the ground. But it doesn't have to actually hit the ground to do damage...

In this case, perhaps five miles above the ground, the incoming asteroid probably exploded with an energy of 10-20 megatons. The asteroid seems to have disintegrated completely in mid-air, since no crater was found afterward. However, the explosion would also have created a tremendous shock wave traveling outward. When this "blast wave" hit the ground, it had sufficient force to flatten trees over 800 square miles - about 80 million trees in all. Ouch! And to top it off, the heat of the explosion would also have set some of those trees on fire - double ouch!

It's really fortunate that this event occurred in a remote region of Siberia, where few were around to suffer the results. (Only one death was recorded.) As it was, some local villagers thought the world was coming to an end. Had that blast occurred over a city, the results could have been catastrophic. Fortunately, there is, even today, much more uninhabited land (and even more ocean) than there is heavily populated land, so the odds of hitting a city dead on are moderately low.

If you're interested in what you might expect from different sizes of asteroids and comets plummeting in toward Earth, I recommend the Earth Impact Effects calculator. See what happens when you explode a 60-meter rock over your hometown...