Tunguska: 100 Years and Counting
The Tunguska event didn't leave a crater — in fact, to this day investigators have never recovered a trace of the impactor itself — but its calling card was unmistakable. The interplanetary intruder came from the southeast, cutting a white-hot swath across the sky before exploding a few miles above the ground with a thousand times more destructive force than the nuclear blast that leveled Hiroshima, Japan.
The resulting shock wave toppled trees across more than 800 square miles, felling them in a radial pattern that pointed the way back to ground zero. A fireball incinerated anything closer in, including more than a thousand reindeer. Yet, owing to the area's remoteness, only one nomad lost his life.
Tunguska also left its mark throughout the Eastern Hemisphere. A seismograph in St. Petersburg, 2,500 miles to the west, recorded tremors in the ground. The night sky, lit up with tons of stratospheric dust, remained bright enough to cast shadows. Halos were common around the Sun.
Over the years, all kinds of extraordinary causes have been invoked to explain the Tunguska event: a titanic explosion of subterranean methane, nuclear fusion from the deuterium in a comet's ices, collisions of Earth with a small black hole or antimatter — and, of course, the crash of an alien spacecraft.
Whatever the particulars, impact specialists now realize that Tunguska was a wake-up call to the scale of devastation that even a small asteroid or comet could wreak. And cosmic oddsmakers now predict that collisions of this magnitude occur every 1,000 years or so. So, in some sense, we're lucky that Tunguska happened so recently. Scientists have been able to assess the devastation firsthand and ponder how to avoid a "next time."
To mark this remarkable anniversary, Sky & Telescope asked a select group of solar-system specialists to reflect on Tunguska's legacy. Here's what they had to say: