Tunguska, one century later
Published on Jun 30, 2008 at 4:10 pm.
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Filed under meteors.
On the morning of June 30, 1908, people throughout the world were minding their own business. Then, a great fireball streaked across the sky over a remote part of Asia. Soon after a titanic explosion rocked Siberia. The explosion was heard for great distances, and it was even detected by its overpressure at sites around the world as the pressure wave circled the globe more than once. Debris in the atmosphere turned days and nights into twilight across the northern hemisphere for weeks afterwards.
The remote location delayed word reaching scientists in major cities. The remote location also meant that travel to the site was an expedition rather than just a trip. Considerable planning was needed, as well as gathering of supplies. Before scientific expeditions could make it into the area, the world fell into war. The Great War, now known as World War I, pretty much kept everyone occupied for a number of years. After the war, Russia was deep into the throes of revolution. So, it was over two decades before scientists made it into the area. What they saw shook the world. An entire forest was devastated by the explosion.
Right away, speculation began to run rampant. Nobody had seen something like this. But, just a few years later, that changed. During World War II, weapons scientists began to realize that for very large bombs, the overpressure can do more damage than the immediate explosive fireball. And, to maximize the coverage of that overpressure, the bomb should be detonated above the ground. The blast damage from the atomic bombs dropped on Japan at the end of the war displayed similarities to the Tungaska blast pattern, only the Tunguska blast was much, much larger. Soon, a favored hypothesis was that the blast was caused by some sort of air burst. But, an air burst of what?
The Solar System is a shooting gallery. There are a lot of things flying around out there. Most of these things are small, so when they run into Earth, they simply appear as meteors (shooting stars). A few, though, survive passage through Earth’s atmosphere to strike the ground. These are meteorites. The larger the meteorite, the bigger the explosion when it hits the ground, and the bigger the crater. Earth has plenty of craters.
But, the Tunguska event shows signs of an atmospheric explosion, not a crater. There have been numerous attempts to find a crater, but so far all have been fruitless. At present, there are still a few claims that have yet to be evaluated by the scientific community, but most today feel that there is no crater. So, how could something so big hit Earth and not leave a crater? Well, it obviously had to be something that did not make it to the ground. So, what could that be?
One of the early contenders was that it may have been something that would not survive the high temperature and pressure of passing through the Earth’s atmosphere. A comet was suggested as fitting the bill. After all, comets are icy bodies, so they would tend to vaporize during entry into Earth’s atmosphere. Of course, it would have to be a smaller body than most comets, so perhaps it was a piece of a comet that had broken off. An likely parent body was even postulated: Encke’s Comet. Comet Encke was known to shed pieces now and then. And, Encke’s Comet comes close to Earth. In fact, in June, Earth is quite near the comet’s orbit, passing through a swarm of debris shed by the comet. This debris gives us the Beta Taurid Meteors, which peak in late June and early July. Furthermore, the bodies approach Earth from the daylight side of the planet, not unlike the object that created the blast. However, this hypothesis has gradually fallen into disfavor.
The top hypothesis today is that a stony asteroid was the progenitor of the Tunguska blast. But, how can a huge chunk of rock not make it through the atmosphere? Well, remember the asteroid Itokawa. That is an example of a rocky asteroid that is not a solid chunk of rock. It is at best a pile of rubble. Such a body would hit the atmosphere moving at dozens of kilometers per second and shatter into billions and billions of pieces from the shock of the sudden deceleration. Those pieces would separate, the debris would pancake, and the air in front of the body would be compressed and heated into a great fireball. The energy released by the ensuing explosion would be huge. For a body of only tens of meters across, the resulting explosion could easily be equivalent to that of a hydrogen bomb. Even for a fairly solid rock, the stress of hitting the atmosphere would be an awful lot to stand. For rocky bodies, the tiny ones burn up. The small ones make it to the ground as meteorites. The medium sized ones blow up in the atmosphere. The large ones, miles across, would probably make it to the ground. Favoring the asteroid hypothesis is dust found at the impact site consistent with the composition of asteroids.
So, both the asteroid and comet hypotheses are still alive, but the scientific community is leaning heavily towards an asteroid, based on the evidence currently available. Also, Earth crossing asteroids in that size range are very common. Comets or comet fragments of the right size are quite rare by comparison.
But, just how big was the explosion? For many years, I had heard estimates of about the equivalent of 25 megatons of TNT. But, in recent years, the estimate had dropped considerably, with about 12 MT being about average. There have been suggestions that the trees of the area were easier to knock over than had been thought, and so the blast damage was overestimated. I have heard estimates of blast strength as low as about 3 MT. That is about what you’d expect from a hydrogen bomb. This happened a century ago. But, there is a reason to try to nail down the size of the blast beyond simple curiosity. The smaller the blast, the smaller the body that caused it. And, there are a lot more small bodies flying around than large ones. So, knowing the size of the body responsible for the blast gives us an idea of how likely it is happen again anytime soon.
Estimating the size of the body causing the explosion is difficult. For one thing, we don’t know how big the blast really was. For another, we don’t know how fast the impacting body was moving. Recently, we’ve been able to compute the atmospheric effects far better, and that suggests a much smaller body may have been responsible than had generally been assumed. If so, then the risk of another Tunguska event goes up. The smallest bodies that I’ve seen proposed are believed to strike Earth perhaps once every couple hundred years. Now, that doesn’t mean that we are safe for another hundred years. Ask the people in Iowa that have had their second hundred year flood in under two decades. A hundred year flood simply means a 1% chance of flooding each year. A once every couple hundred year chance of impact really means is that there is a 0.5% chance of impact each year. Of course, I’ve heard other estimates that were far more comforting, such as a chance of impact once every thousand years or so (about a 0.1% per year).
Over the years, of course, there have been far wilder suggestions of what caused the impact, ranging from a miniature black hole to a chunk of antimatter. And, there have been suggestions of non-natural causes, too, such as a crashing flying saucer or a scientific experiment gone awry. But, the simplest and far most likely scenario is of an impact by an asteroid or comet.
-Astroprof
Images courtesy Wikimedia Commons
