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. Last Updated: 07/27/2016

The day the world shook

The effect was much like that of a great volcanic eruption, yet there had been no eruption. The only objective indication of the extraordinary event was a quiver on seismographs in the Siberian city of Irkutsk, indicating a moderate quake some 1,000 kilometers north, in a remote region called Tunguska.

Scientists did not come to Tunguska for another 19 years, apparently reluctant to travel to a site so swampy and remote. When they did finally come, they were rewarded with a stunning vision of utter devastation, with scorched trees lying in rows that stretched to the horizon. They searched for a crater but found none. They searched for fragments of a meteorite -- an asteroid or a chunk of one -- but found none. All they found were eyewitnesses in neighboring villages who told of a fireball streaking through the sky, horrifying noise and a blast that knocked people off their feet. Clearly something unprecedented had occurred at Tunguska, but the trees were the only tangible proof that remained.

For 88 years the mystery of Tunguska has attracted a swarm of theories, some eminently reasonable, such as meteorite impact, and some considerably less so, like conjectures about an exploding spaceship. While the debate continues as to precisely what happened there back in 1908, compelling evidence has recently turned up that could finally put an end to most questions.

Researchers have found embedded in Tunguska trees tiny particles with an extraterrestrial signature. Combined with computer simulations, the evidence points to a meteorite born of an asteroid that fragmented in the atmosphere. For many researchers, the debate is no longer whether the cause was a meteorite but, rather, exactly what kind of meteorite.

Tunguska is the kind of mystery scientists will try to solve just for the thrill of the challenge, working after-hours at laboratories and paying for the travel and fieldwork out of their own pockets. Yet there's a very practical aspect to the work. The impacts of comets or asteroids -- collectively known as bolides -- are an important part of the history of the solar system. Bolides have peppered the Earth in the past -- a particularly big one probably finished off the dinosaurs 65 million years ago -- and today they pose an unknown risk to human civilization. Reconstructing that fiery day over Tunguska in 1908 may give us the best picture we have of what future the sky holds for us.

The first scientist to make the journey to Tunguska was Leonid Kulik, a Russian geologist who for years had collected meteorite fragments in other parts of Siberia. When, in 1927, Kulik first saw the vast panorama of charred, downed trees, his immediate impression was that an immense fire had begun simultaneously over a wide area.

Over the next 14 years, Kulik led four more Tunguska expeditions. His teams photographed the flattened trees and dragged swamps and bogs for meteorite fragments, but they found nothing. From dozens of eyewitnesses they interviewed, they collected contradictory accounts: almost half the witnesses claimed seeing a fireball traveling on a northerly path, while the others claimed it was northwesterly or westerly. Still, despite this paltry, confusing evidence, Kulik was certain that a meteorite had caused the inferno.

Kulik died in 1942 as a prisoner of war. Not until the late 1950s would scientists visit Tunguska again. The initiative would come from Alexander Kazantsev, a Soviet engineer and an army colonel who in 1946 wrote a short story in which he suggested that only a nuclear explosion could have caused the bizarre wreckage at Tunguska. And, he suggested, since humans obviously couldn't have managed such an explosion in 1908, it must have been caused by an exploding spaceship. Over the years, the story was reprinted several times in the Soviet Union, most successfully in 1958 in a popular book called "Guest from Space." Young Siberian scientists were intrigued by the notion and by Kazantsev's claim that there should still be measurable levels of radioactivity at Tunguska. "We wanted to find out whether the book was true," added Victor Zhuravlyov, a specialist in ionizing radiation in Tomsk, Siberia.

Led by Gennady Plekhanov, a physician and physicist, a group of Tomsk scientists and some hangers-on set out in the summer of 1959 for a wilderness adventure. "We had an appetite for scientific tourism," says Dmitri Dyomin, a retired Novosibirsk physicist. The group paid their own way to Tunguska, where they spent 40 days scouring the site for bolide fragments from comets or asteroids and measuring radioactivity levels in the soil. "We thought that in a year or two the Tunguska problem would be solved," Plekhanov recalls. After two field seasons, however, Plekhanov had found no evidence of elevated radiation levels and no credible bolide fragments. "We realized the situation was far more complicated than we had thought," he says. In spring 1961, he delivered a report on the expeditions to a packed audience at the Kurchatov Institute of Atomic Energy in Moscow, one of Russia's top scientific institutes. Despite drawing a blank on the mystery that Plekhanov had naively thought would be solved within two years, his group's field work got a warm reception from the Kurchatov scientists. Sergei Korolyov, Russia's top missile designer at the time, was so impressed that he ordered a state helicopter sent from Moscow to Vanavara, a trading post 90 kilometers south of Kulik's base camp, to aid Plekhanov's team.

Since then Russian scientists have gone to Tunguska every summer. One of the most important fruits of the ongoing work is a detailed map of the tree fall pattern. The scientist in charge of this effort is Wilhelm Fast, a 60-year-old Tomsk State University lecturer of probability theory. Fast cuts a striking figure, with long white hair and a white beard that reaches halfway down his chest. As a teenager, Fast wanted to be an astronomer, but his father -- imprisoned for years because his Old Believer views prevented him from volunteering for the Soviet Army -- opposed such a high-brow occupation. "He said to me, 'In prison camp, nobody needs an astronomer'," says Fast, who as a 13-year-old spent three days imprisoned with his mother in a tiny isolation chamber in Tomsk waiting to be shot.

Fast escaped that fate and, as a graduate student in mathematics at Tomsk State in 1959, he crossed paths with Plekhanov. "The first time I saw the fallen trees, it impressed me very much. I was in awe," he says. During the course of his eight trips to Tunguska, Fast and others have methodically mapped by compass the entire 1,360 square kilometer region of tree fall. It is this map, so painstakingly put together over 35 years, that has allowed other scientists to calculate that the trees must have been knocked down by a blast about six kilometers above the ground with an energy of 10 to 20 megatons of TNT as the object traveled an east-to-west course.

For three decades, Tunguska remained an exclusively Russian scientific investigation, chiefly because the two nearest cities, Tomsk and Krasnoyarsk, were centers for research into military technology and were closed to foreigners. But in 1989, with the end of the Cold War, outside researchers were at last able to begin studying the site. Among them was Italian physicist Menotti Galli of the University of Bologna. His idea was that the growth rings of trees that survived the Tunguska explosion could provide similar information, on environmental conditions after the blast.

Galli missed the expedition that summer, but expedition co-leader Nicolas Vasilyev made sure he wasn't left too far away on the sidelines. Vasilyev put Galli in touch with Korado Korlevic, a Bosnian biologist who sent Galli a chunk of spruce. By no means was the sample museum quality; marring its pattern of concentric rings was a dead branch enveloped by the trunk many years ago, like a splinter beneath the skin. Dried resin -- exuded to ward off infections to the living tree stem -- surrounded the branch, which had died prior to the 1908 explosion.

To Galli, the sample was pure gold. He guessed that particles might be trapped in resin like prehistoric bugs in amber. The resin between each yearly growth ring, meanwhile, would not have been exposed to the atmosphere. "The best place to find particles would be in the resin around the dead branch," Galli says.

Galli's ideas intrigued a friend of his in the physics department, Guiseppe Longo. In July 1991 they flew to Vanavara, and from there an Aeroflot helicopter ferried them to Kulik's isolated camp. During the hot days at Tunguska the researchers slaked their thirst with brown swamp water laced with mosquito larva, a far cry from sipping espressos under Bologna's endless porticoes. Galli looks back and sighs, "Those were 10 difficult days."

The Italians found six spruce that had survived the blast within an eight-kilometer radius of the epicenter. From these trees they took 13 samples, the size of healthy carrots, from trunks that had grown around withered branches. For comparison, they sampled two other trees near the epicenter -- a pine and the roots of a larch uprooted during the explosion -- as well as a spruce in Tomsk, 1,100 kilometers southwest of the epicenter.

Back at their lab, the Italians analyzed particles, extracted from the resin's surface, under a scanning electron microscope to determine their size and shape. Over the next two years they gathered data on 5,854 particles from resin exuded in three periods in the six spruce and the pine near the epicenter, as well as 1,138 particles in the larch roots and the Tomsk spruce. The results were striking: In the period from 1902 to 1914, the density of particles of a certain metallic composition was more than 10-fold greater than during the other time periods.

"The great and important question was how these particles are related to the Tunguska event," Longo says. Were the particles from the cosmic body itself or the dirt kicked up by its impact?

To answer this question, the scientists compared the particles found in the living Tunguska trees to those in the larch roots. Because the blast wave uprooted the larch, resin around its roots would have captured mainly particles thrown up from the ground, Longo says. The resin from the standing trees, meanwhile, would differ in composition, because it would have captured particles from the bolide as well as the ground. Their hunch proved correct.

But what Longo calls the "main hint" that his team had found bolide particles was the smooth, spheroidal shape of the particles found in the resin from 1902 to 1914, "indicating that they were heated and melted by a great temperature," he says. "The blast wave would not have melted particles in the ground, where the conductivity is too low," says Longo. "That means the melted particles came directly from the cosmic body." Other Tunguska sleuths agree. "I find Longo's claim that he has found remnants of the Tunguska object compelling," says Christopher Chyba, an astrophysicist and visiting lecturer at Princeton University.

While the Italians have been sorting through particles, American researchers have been creating computer simulations of Tunguska, trying to use the laws of physics to recreate a bolide that could have produced the known evidence. Chyba's group at NASA Ames Research Center in California developed a computer model in which a bolide, moving at supersonic speeds, begins to flatten like a pancake as it hurtles through the atmosphere. Air, acting as a brake, exerts a pressure on the bolide's front that is much greater than the pressure on its sides and rear. Under such uneven pressures, bolides on the order of 10 to 100 meters in diameter will crumble.

The fragmenting and braking feed off each other and accelerate so rapidly that the bolide, by this time a cloud of debris, appears to explode in mid-air. "If a lot of the debris fell into the southern swamp, it would be extremely difficult to find fragments 20 years after the explosion," Longo says, referring to the fact that Kulik's first scientific expedition to Tunguska wasn't until 1927.

Chyba's group homed in on the two most common meteor types: stony meteors and their more infrequent cousins, carbonaceous chondrites, which are peppered with carbon and other organic matter that makes them less dense. Both kinds of meteors are spun off after collisions between asteroids in the main asteroid belt lying between Mars and Jupiter.

For Tunguska modelers, the crucial difference is how strongly a meteor resists breaking apart as it travels through the atmosphere. A meteor's strength depends on its composition and size. Because a carbonaceous chondrite is less dense than other stony meteors, "it's slightly weaker," says physicist Evans Lyne of the University of Tennessee at Knoxville. Using the fragmentation model, Lyne's group found that the best explanation for Tunguska is a carbonaceous chondrite.

Many Russians who have been wrestling with Tunguska for years view the American efforts with skepticism. "Most Russian scientists think Tunguska must have been a comet," says Gennady Andreev, a senior scientist at Tomsk State University. The main reason behind this belief, says astronomer Vitaly Bronshten of the Committee for Meteorites of the Russian Academy of Sciences, is that Russian scientists who search the site every year for meteorite fragments return empty-handed. Bronshten disagrees with Longo's assertion that it would have been difficult to find meteorite fragments, if they existed in the southern swamp. Bronshten contends that "reasonably large fragments" should have been discovered at the site.

But many Western scientists scoff at the comet theory. If the Tunguska body were a comet with a mass of a million tons, it would have shed billions of tons of dust and gas during its atmospheric flight, says Zdenek Sekanina of NASA's Jet Propulsion Laboratory. "The effects on life on Earth would have been horrendous. ... It would have been a global catastrophe, comparable with a nuclear winter."

While a carbonaceous chondrite is the leading suspect, most Russian scientists refuse to rule out more exotic theories. One new idea is that an earthquake was responsible for at least part of the Tunguska phenomena. Its author, Andrei Ol'khovatov, a radiophysicist at the Radio Instrument Industry Research Institute in Moscow, points out that earthquakes sometimes do more than simply shake the ground; they can also release lightning-like flashes and sounds such as whistles, hisses and hums.

In his analysis, published earlier this year in the journal Science in Russia, Ol'khovatov faults the inconsistent eyewitness accounts that give three possible trajectories for the bolide. "It couldn't have come from so many directions," Ol'khovatov says. "This is one of the weakest and most puzzling points of the meteorite theory." Instead of accepting the east-to-west trajectory suggested by Fast's tree fall maps, Ol'khovatov found that all three trajectories closely follow fault lines running through the region, and that the epicenter is smack in the middle of an ancient volcanic crater. "Very likely, the locals observed glowing effects set off by awakening tectonic processes," he says.

One idea that is being challenged in the West is that Tunguska-class objects should worry us at all. Asteroids with the potential to decimate countries or even snuff out most life on Earth would require a diameter of greater than 1 kilometer, more than 10 times that of the Tunguska bolide. According to Alan Harris of the Jet Propulsion Laboratory, the explosion in 1908 "is a more honest picture" of what to expect from future Tunguska-sized impacts: "A spectacular blast in an isolated place, killing almost nothing."

Reason dictates, however, that the real lesson of Tunguska can't be learned until there's firmer evidence in hand. As each year passes, that evidence becomes more difficult to find, so scientists are working harder on Tunguska than ever before.

For many, the scientific challenge is as rewarding as the anticipation of answering the riddle. "If we found a meteorite, we would immediately bury it," jokes physicist Nina Fast, who met her future husband, Wilhelm, at Tunguska in 1960. "We enjoy the paradoxes and the contradictions."