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Projects: Apophis Mission Design CompetitionThe Tunguska Riddle: How Powerful was the Greatest Asteroid Impact in Recorded History?by Amir Alexander
A column of bluish light as bright as the Sun streaked through cloudless skies above the Taiga forest. Minutes later, local herdsmen and recent settlers saw a brilliant flash followed by the sound of explosions, like an artillery barrage from a great battle raging over the horizon. An ashen cloud rose in the distance, and could be seen from hundreds of miles away. And then – silence. The place: the Podkamennaya (Lower Stony) Tunguska River in central Siberia, northeast of Lake Baikal; the time: 7:14 a.m. on the morning of June 30, 1908. Within minutes the "Tunguska Event," the largest asteroid impact in modern recorded history, was over. Times were hard in Russia at the turn of the last century, with wars, revolutions, and civil war following each other in quick succession. And so it was not until 1927 that a Soviet scientific expedition led by Leonid Kulik arrived at the Tunguska site. What it found was a landscape of devastation: millions of trees from the taiga forest were uprooted for miles around, all pointing away from the apparent point of impact. Overall, Kulik estimated, 2150 square kilometers (830 square miles) of forest were flattened, or a circle of 25 kilometers around the epicenter. In the years since Kulik's expedition, scientists have struggled repeatedly to reconstruct the events of that fateful day. This is only natural for researchers who specialize in asteroid impacts, and for whom the Tunguska Event is truly a goldmine. As the only sizable asteroid impact that had been witnessed in recent history, whose location is known with certainty, and where living eyewitnesses could – until quite recently – be interviewed, it offers unparalleled opportunities for study. But there is more at stake here than scientific curiosity: if the Tunguska asteroid had struck a populated area rather than a wild Siberian forest and caused a similar-sized explosion, it could have wiped out a large city with all its inhabitants. How likely is such a catastrophe, and can it be predicted or defended against? By reconstructing the Tunguska event, scientists hope to gain insight into the dynamics of asteroid impacts, and begin to answer these questions. From Tunguska to HiroshimaOne of the first things researchers noted about the Tunguska site was the curious fact that no actual meteorite was found there. In the 1920s and 30s Kulik had believed that he was at the site of a physical impact by an asteroid, and he worked hard to unearth its remains. All in vain: despite the unmistakable evidence of explosion and the devastation all around, no space rock was ever found at Tunguska. In the absence of such physical traces, researchers concluded that the Tunguska asteroid never, in fact, struck the ground. What the locals had seen and heard was in all likelihood the mid-air explosion of the incoming impactor, which took place several thousand meters above the ground. This explosion in turn caused a massive "airburst," which was strong enough to flatten trees for dozens of kilometers in every direction.
Once it was widely accepted that the Tunguska Event was a massive airburst explosion in the atmosphere rather than a physical impact on the ground, the challenge for scientists became determining the magnitude of the explosion. The question is crucial, because the magnitude of the explosion is a good indicator of the frequency of Tunguska-like events all over the Earth. A very powerful explosion naturally requires a very large impactor, whereas less powerful ones are caused by smaller space rocks entering the atmosphere. But the larger the space rock entering the atmosphere, the rarer it is. This means that if the devastation at Tunguska was caused by a massive explosion comparable to a giant nuclear blast measuring hundreds of megatons, then we have relatively little to worry about: asteroids large enough to cause such an explosion are extremely rare, and the probability of our planet encountering another one in the coming millennia is near negligible. But if the devastation at Tunguska was caused by a smaller explosion of only a few megatons, then there is greater cause for concern. Impactors of sufficient size to cause airbursts on that scale may strike our planet as often once every 400 years. But how does one determine how powerful the explosion unleashed by the Tunguska impactor actually was? In order to come up with a reliable estimate, scientists have taken their cue from what seems to be the closest manmade equivalent of an asteroid impact – the detonation of nuclear bombs. Extensive studies during the Cold War attempted to predict the extent of damage that would be caused by the detonation of nuclear bombs of different explosive power. Scientists studying the Tunguska event relied on these studies, but reversed the question: given the amount of damage seen in Tunguska, they asked, how big must the explosion have been? While the method seems promising in principle, the goal of determining the size of the Tunguska explosion has remained elusive, and estimates of its power have varied widely. At the high end of the scale, researchers in 1982 suggested that the power of the explosion was equivalent to 700 million tons of TNT, or 700 megatons for short. That would be nearly twenty times the power of "Tsar Bomba," the most powerful nuclear bomb of the Cold War, which was estimated at 30 to 40 megatons. At the opposite end some researchers have suggested an explosive energy of the Tunguska airburst was only 3 to 5 megatons, about one tenth of the power of Tsar Bomba, but still about 300 times more powerful than the bomb dropped on Hiroshima in 1945. Most scientists today accept a middle ground estimate of the Tunguska impact of between 10 and 15 megatons. Pancakes and FireballsBut in an article forthcoming in the International Journal of Impact Engineering Mark Boslough and David Crawford of the Sandia National Laboratories in New Mexico challenge this growing consensus. The airburst from an exploding asteroid works very differently from an explosion of a nuclear bomb, they argue, and is far more destructive. It follows that the massive devastation seen in Tunguska was likely caused by a relatively modest explosion, considerably smaller than most scientists had suspected. This in turn would suggest that Tunguska-like events are not as rare as one might hope, and the risk of a similar event occurring in the coming centuries is relatively high.
According to Boslough, "Previous models used by researchers to estimate the Tunguska Event were oversimplified, stripped of some very important physical effects." In most cases, he said, researchers estimating the power of an asteroid explosion in the atmosphere simply assumed that the event was similar to a detonating bomb. The destruction on the ground caused by asteroid airburst would therefore be similar to the destruction that can be expected from a bomb of the same power detonated at the same height above the ground. It is a reasonable-sounding analogy, which has been at the heart of estimates the power of the Tunguska impact in recent decades. Unfortunately, according to Boslough, it is incomplete and therefore highly misleading. Asteroids and bombs, Boslough and Crawford argue in their paper, explode very differently and destroy their surroundings in very different ways. When a sizable rock rock enters the atmosphere form space it travels at hypersonic speeds, and the air resistance it encounters causes it to increase its diameter and "flatten out." This in turn increases the air resistance even more, which causes even more flattening, and so on. The overall effect on the asteroid as it streaks through the atmosphere is known as "pancaking" because of the flat and thin shape that is inexorably forced upon it. Throughout this process the pancaking space rock heats up exponentially until it fragments and vaporizes in a big explosion at a distance from the ground known as the "airburst altitude." For the Tunguska Event Boslough estimates this altitude at around 12 kilometers. Now if an asteroid explosion were analogous to a detonating bomb, this is where the story would end: the force of the explosion at the airburst altitude would wreak havoc on the ground in exactly the same way as an equally powerful bomb detonated at the same height. But in fact, according to simulations run by Boslough and Crawford, this initial explosion is only the beginning of the devastation brought on by an impacting asteroid. As the space rock blazes through the atmosphere, it leaves behind it a hot low-density wake of air mixed with rock vapor, which can reach far behind the rock and shoot out into space. Then, once it reaches the airburst altitude, the mass of the asteroid fragments and vaporizes in a massive explosion, and takes the form of a giant fireball of vaporized rock and air. Even then – and this is the chief difference between Boslough's and Crawford's simulation and previous ones – the fireball continues speeding towards the ground, driving a massive shockwave before it. At this point the fireball is moving much slower than the asteroid had been prior to the explosion, but it is still traveling at supersonic speeds. And it is the fireball and its accompanying shockwave, say the article's authors, not the initial bomb-like explosion, which cause most of the damage on the ground.
Boslough is an old hand at simulating asteroid impacts, and his evaluations carry the weight of experience. Back in 1994, when it became clear that comet Shoemaker-Levy 9 was on a direct collision course with the planet Jupiter, Boslough and fellow scientists at Sandia used a computer simulation to predict the impact's effect on the giant planet. The simulation showed that after entering the Jovian atmosphere the comet would flatten out in what became known as "pancaking," and a low-density wake would trail behind it. Based on the simulation, the Sandia scientists predicted that the impact will be observable from Earth in the form of firey jets shooting out into space above Jupiter's dense cloud coverage. When the fragmenting comet struck the giant planet between July 16 and 22 of that year, the event was observed from Earth and Boslough and his colleagues were proved correct. Because Jupiter is composed almost entirely of a gassy atmosphere, Boslough explained, the impact dynamics of an asteroid on that giant planet are analogous to what happens in Earth's atmosphere when it is penetrated by a massive space rock. In the case of Tunguska, Boslough's and Crawford's simulations show, the fiery wake of the impacting asteroid produced a plume that shot out into space. When it reached the airburst altitude of around 12 kilometers, the asteroid vaporized and transformed into a giant fireball but continued moving towards the ground at supersonic speed. All the while, it was driving a massive shockwave before it. In the event, the Tunguska fireball did not reach the surface. We know this for a fact, because some trees can be seen standing at the epicenter of the storm in some of Kulik's photographs—trees that would have been instantly incinerated had the fireball touched the ground. But the shockwave traveling before the fireball did reach the ground, bringing with it high winds that flattened trees for dozens of miles in each direction in the taiga forest. That is what Kulik and his companions saw when they arrived at the site 19 years later. Impact at TunguskaBoslough and Crawford ran repeated simulations of the Tunguska event, trying to reproduce the scale of the devastation seen in the region. From this they concluded that even if the airburst explosion took place at an altitude of 12 kilometers or higher, it probably had a magnitude of only 3 to 5 megatons. This is considerable less than the popular figure of 10 to 15 megatons, and less than 1% the high end suggestion of 700 megatons. That such a comparatively small explosion could cause this much damage is due to the fact that it was not a point explosion at the airburst altitude that caused the damage, but the fireball and shockwave that continued moving towards the surface. At a height of around 4 kilometers, the simulations show, the fireball came to a stop, but the shockwave continued on. It was the shockwave, not the explosion at the airburst altitude and not even the streaking fireball, which was responsible for most of the devastation in the forest.
Tunguska, in its way, was fortunate that it was spared direct contact with the supersonic fireball, but in other times and places our planet was undoubtedly scorched by by fireballs that made it all the way to the surface. "There is an energy threshold," explained Boslough, "that separates impacts in which the fireball comes to a stop in the lower atmosphere from impacts in which the fireball actually hits the ground." The former are Tunguska-like events, which leave no trace in the geological record, and the damage they cause is erased within a few decades. Even the Tunguska event itself would probably never have been known if it had not been witnessed, and word about it disseminated through modern means of communication. To the second type belong only those rare giant impactors who possess sufficient energy to send a fireball hurtling all the way to the ground. In those cases simulations show that the fireball can make direct contact with the surface over an area of hundreds of square kilometers. At the point of impact sustained wind speeds will exceed the speed of sound, and the combined effect will melt the very rocks in the impact area. Boslough and Crawford believe that just such an event less than a million years ago may have been responsible for the strange deposits in Laos known as Muong-Nong tektites. A similar fireball 29 million years ago, produced by an asteroid around 120 meters in diameter, may have been responsible for the striking-looking stones known as "Libyan Desert Glass." But the most significant result of Boslough's and Crawford's simulations is the relatively modest size of the explosion that flattened the thousands of square kilometers of Siberian forest. According to some estimates, asteroids big enough to cause a 3 to 5 megaton explosion in the atmosphere may strike Earth as frequently as once every few centuries. This suggests that we or our descendants may well have another Tunguska-like event in store for us, and while the 1908 impactor merely flattened trees in a Taiga forest, the next sizable asteroid to take aim at our planet is just as likely to strike in the heart of New York, Paris, or Beijing. Boslough himself is not particularly concerned: "if a major natural disaster takes place, it is far far more likely to be a hurricane, earthquake, tsunami, or something like that" he said. Even if the estimate of a Tunguska-like event once every few centuries is correct, he points out, it still makes asteroid impacts immeasurably rarer than more traditional harbingers of disaster. Nevertheless, those of us who are concerned less with the statistical likelihood of an impact and more with the awesome power of such an event should take heed: one of these days we may look up and see a column of bluish light more brilliant than the Sun streaking through the sky.
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