The year is 1908, and it's just after seven in the morning. A man is sitting on the front porch of a trading post at Vanavara in Siberia. Little does he know, in a few moments, he will be hurled from his chair and the heat will be so intense he will feel as though his shirt is on fire. That's how the Tunguska event felt 40 miles from ground zero. Today, June 30, 2008, is the 100th anniversary of that ferocious impact near the Podkamennaya Tunguska River in remote Siberia--and after 100 years, scientists are still talking about it. "If you want to start a conversation with anyone in the asteroid business all you have to say is Tunguska," says Don Yeomans, manager of the Near-Earth Object Office at NASA's Jet Propulsion Laboratory. "It is the only entry of a large meteoroid we have in the modern era with first-hand accounts." While the impact occurred in '08, the first scientific expedition to the area would have to wait for 19 years. In 1921, Leonid Kulik, the chief curator for the meteorite collection of the St. Petersburg museum led an expedition to Tunguska. But the harsh conditions of the Siberian outback thwarted his team's attempt to reach the area of the blast. In 1927, a new expedition, again lead by Kulik, reached its goal. "At first, the locals were reluctant to tell Kulik about the event," said Yeomans. "They believed the blast was a visitation by the god Ogdy, who had cursed the area by smashing trees and killing animals." While testimonials may have at first been difficult to obtain, there was plenty of evidence lying around. Eight hundred square miles of remote forest had been ripped asunder. Eighty million trees were on their sides, lying in a radial pattern. "Those trees acted as markers, pointing directly away from the blast's epicenter," said Yeomans. "Later, when the team arrived at ground zero, they found the trees there standing upright – but their limbs and bark had been stripped away. They looked like a forest of telephone poles." Such debranching requires fast moving shock waves that break off a tree's branches before the branches can transfer the impact momentum to the tree's stem. Thirty seven years after the Tunguska blast, branchless trees would be found at the site of another massive explosion – Hiroshima, Japan. Kulik's expeditions (he traveled to Tunguska on three separate occasions) did finally get some of the locals to talk. One was the man based at the Vanara trading post who witnessed the heat blast as he was launched from his chair. His account: Suddenly in the north sky… the sky was split in two, and high above the forest the whole northern part of the sky appeared covered with fire… At that moment there was a bang in the sky and a mighty crash… The crash was followed by a noise like stones falling from the sky, or of guns firing. The earth trembled. The massive explosion packed a wallop. The resulting seismic shockwave registered with sensitive barometers as far away as England. Dense clouds formed over the region at high altitudes which reflected sunlight from beyond the horizon. Night skies glowed, and reports came in that people who lived as far away as Asia could read newspapers outdoors as late as midnight. Locally, hundreds of reindeer, the livelihood of local herders, were killed, but there was no direct evidence that any person perished in the blast.


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"A century later some still debate the cause and come up with different scenarios that could have caused the explosion," said Yeomans. "But the generally agreed upon theory is that on the morning of June 30, 1908, a large space rock, about 120 feet across, entered the atmosphere of Siberia and then detonated in the sky." It is estimated the asteroid entered Earth's atmosphere traveling at a speed of about 33,500 miles per hour. During its quick plunge, the 220-million-pound space rock heated the air surrounding it to 44,500 degrees Fahrenheit. At 7:17 a.m. (local Siberia time), at a height of about 28,000 feet, the combination of pressure and heat caused the asteroid to fragment and annihilate itself, producing a fireball and releasing energy equivalent to about 185 Hiroshima bombs. "That is why there is no impact crater," said Yeomans. "The great majority of the asteroid is consumed in the explosion." Yeomans and his colleagues at JPL's Near-Earth Object Office are tasked with plotting the orbits of present-day comets and asteroids that cross Earth's path, and could be potentially hazardous to our planet. Yeomans estimates that, on average, a Tunguska-sized asteroid will enter Earth's atmosphere once every 300 years. "From a scientific point of view, I think about Tunguska all the time," he admits. Putting it all in perspective, however, "the thought of another Tunguska does not keep me up at night."

You can get a sense of the magnitude of this event by comparing observations made at different distances. Seismic vibrations were recorded by sensitive instruments as much as 1000 km (600 mi) away. At 500 km (300 mi), observers reported "deafening bangs" and a fiery cloud on the horizon. About 170 km (110 mi) from the explosion, the object was seen in the cloudless, daytime sky as a brilliant, sunlike fireball; thunderous noises were heard. At distances around 60 km, people were thrown to the ground or even knocked unconscious; windows were broken and crockery knocked off shelves. Probably the closest observers were some reindeer herders asleep in their tents in several camps about 30 km (20 mi) from the site. They were blown into the air and knocked unconscious; one man was blown into a tree and later died. "Everything around was shrouded in smoke and fog from the burning fallen trees."

June 30, 1908 In a remote part of Russia, a fireball was seen streaking across the daytime sky. Within moments, something exploded in the atmosphere above Siberia’s Podkamennaya Tunguska River in what is now Krasnoyarsk Krai, Russia. This event – now widely known as the Tunguska event – is believed to have been caused by an incoming meteor or comet, which never actually struck Earth but instead exploded in the atmosphere, causing what is known as an air burst, three to six miles (5–10 kilometers) above Earth’s surface. The explosion released enough energy to kill reindeer and flatten trees for many kilometers around the blast site. But no crater was ever found. At the time, it was difficult to reach this remote part of Siberia. It wasn’t until 1927 that Leonid Kulik led the first Soviet research expedition to investigate the Tunguska event. He made a initial trip to the region, interviewed local witnesses and explored the region where the trees had been felled. He became convinced that they were all turned with their roots to the center. He did not find any meteorite fragments, and he did not find a meteorite crater. Over the years, scientists and others concocted fabulous explanations for the Tunguska explosion. Some were pretty wild – such as the encounter of Earth with an alien spacecraft, or a mini-black-hole, or a particle of antimatter. The truth is much more ordinary. In all likelihood, a small icy comet or stony asteroid collided with Earth’s atmosphere on June 30, 1908. If it were an asteroid, it might have been about a third as big as a football field – moving at about 15 kilometers (10 miles) per second. Because the explosion took place so long ago, we might never know for certain whether it was an asteroid or comet. But in recent decades astronomers have come to take the possibility of comet and asteroid impacts more seriously. They now have regular observing programs to watch for Near-Earth Objects, as they’re called. They also meet regularly to discuss what might happen if we did find an object on a collision course with Earth. A full century after the mysterious Tunguska explosion in Siberia leveled an area nearly the size of Tokyo, debate continues over what caused it. Many questions remain as to what crashed into the Earth from above -- how big it was and what it was made of. Some question whether it even came from space at all, or whether it erupted from the ground instead. And there is always speculation that it was caused by a UFO or famed inventor Nikola Tesla's "death ray." The explosion near the Podkamennaya Tunguska River on June 30, 1908, flattened some 500,000 acres (2,000 square kilometers) of Siberian forest. Scientists calculated the Tunguska explosion could have been roughly as strong as 10 megatons to 20 megatons of TNT -- 1,000 times more powerful than the atom bomb dropped on Hiroshima. The longstanding theory regarding the cause of the event is a cosmic impact from an asteroid or comet. In the last decade, researchers have conjectured the event was triggered by an asteroid exploding in Earth's atmosphere and measuring roughly 100 feet wide (30 meters) and 617,300 tons (560,000 metric tons) in mass -- more than 10 times that of the Titanic. But recent supercomputer simulations suggest the asteroid that caused the extensive damage was much smaller. Specifically, physicist Mark Boslough at Sandia National Laboratory in Albuquerque, N.M., and his colleagues say it would have been a factor of three or four times smaller in mass and perhaps 65 feet (20 meters) in diameter. As the asteroid exploded as it ran into Earth's atmosphere, Boslough and colleagues calculate it would have generated a supersonic jet of expanding superheated gas. This fireball would have caused blast waves that were stronger at the surface than previously thought. At the same time, prior estimates may have overstated the devastation the event caused. The forest back then was unhealthy, according to foresters, so it would not have taken as much energy to blow down such trees. In addition, the winds from the explosion would naturally get amplified above ridgelines, making the explosion seem more powerful than it actually was. What researchers had thought to be an explosion between 10 and 20 megatons was more likely only 3 to 5 megatons, Boslough said. As to whether the impact was similar to a stony, carbonaceous asteroid or a comet, "while the community has pretty much accepted the view that it was a carbonaceous asteroid, I'm not sure it's a slam dunk," Boslough said. "The main argument against it being a comet is statistical. There are a lot more small Earth-crossing asteroids than comets at least by a couple orders of magnitude. While it's unlikely to be a comet, I'm not convinced it's physically impossible." Discovering the size and makeup of whatever hit at Tunguska could shed light on how often such a devastating impact might take place, explained NASA Ames Research Center planetary scientist and astrobiologist David Morrison. "As interesting though Tunguska is, I'm more interested in the next Tunguska," "We know small objects are far more numerous than large ones out there, so we want to see how much damage they might be able to do." MORE Huge Tunguska Explosion Remains Mysterious 100 Years Later A full century after the mysterious Tunguska explosion in Siberia leveled an area nearly the size of Tokyo, debate continues over what caused it. Many questions remain as to what crashed into the Earth from above -- how big it was and what it was made of. Some question whether it even came from space at all, or whether it erupted from the ground instead. And there is always speculation that it was caused by a UFO or famed inventor Nikola Tesla's "death ray." Death from above? The explosion near the Podkamennaya Tunguska River on June 30, 1908, flattened some 500,000 acres (2,000 square kilometers) of Siberian forest. Scientists calculated the Tunguska explosion could have been roughly as strong as 10 megatons to 20 megatons of TNT -- 1,000 times more powerful than the atom bomb dropped on Hiroshima. The longstanding theory regarding the cause of the event is a cosmic impact from an asteroid or comet. In the last decade, researchers have conjectured the event was triggered by an asteroid exploding in Earth's atmosphere and measuring roughly 100 feet wide (30 meters) and 617,300 tons (560,000 metric tons) in mass -- more than 10 times that of the Titanic. But recent supercomputer simulations suggest the asteroid that caused the extensive damage was much smaller. Specifically, physicist Mark Boslough at Sandia National Laboratory in Albuquerque, N.M., and his colleagues say it would have been a factor of three or four times smaller in mass and perhaps 65 feet (20 meters) in diameter. As the asteroid exploded as it ran into Earth's atmosphere, Boslough and colleagues calculate it would have generated a supersonic jet of expanding superheated gas. This fireball would have caused blast waves that were stronger at the surface than previously thought.

However ... At the same time, prior estimates may have overstated the devastation the event caused. The forest back then was unhealthy, according to foresters, so it would not have taken as much energy to blow down such trees. In addition, the winds from the explosion would naturally get amplified above ridgelines, making the explosion seem more powerful than it actually was. What researchers had thought to be an explosion between 10 and 20 megatons was more likely only 3 to 5 megatons, Boslough said. As to whether the impact was similar to a stony, carbonaceous asteroid or a comet, "while the community has pretty much accepted the view that it was a carbonaceous asteroid, I'm not sure it's a slam dunk," Boslough said. "The main argument against it being a comet is statistical. There are a lot more small Earth-crossing asteroids than comets at least by a couple orders of magnitude. While it's unlikely to be a comet, I'm not convinced it's physically impossible." Discovering the size and makeup of whatever hit at Tunguska could shed light on how often such a devastating impact might take place, explained NASA Ames Research Center planetary scientist and astrobiologist David Morrison. "As interesting though Tunguska is, I'm more interested in the next Tunguska," Morrison told SPACE.com. "We know small objects are far more numerous than large ones out there, so we want to see how much damage they might be able to do." Death from below?

Instead of a cause from above, in the last decade some researchers have suggested the Tunguska explosion actually came from below. Astrophysicist Wolfgang Kundt at the University of Bonn in Germany and others have suggested that an eruption of natural gas from kimberlite, a kind of volcanic rock best known for sometimes holding diamonds, could be to blame. "It would have come from the molten earth, some 3,000 kilometers deep (1,864 miles)," Kundt said. "The natural gas would be stored as a fluid that deep, and when it reaches the surface it would become a gas and expand by a factor of thousand in volume, for a huge explosion." For support, he cited the pattern the trees fell in, as well as chemical anomalies. Wilder theories have been bandied about over the years regarding what caused the Tunguska explosion, including:

A UFO crash. Struck by the similarity of Tunguska and Hiroshima decades later, a science fiction writer named Alexander Kazantsev wrote a story in which the Tunguska blast was the exploding nuclear power plant of a spaceship from Mars. A few Russian scientists took up the cause and claimed to find various bits of evidence -- never substantiated -- for a civilized alien explanation. The annihilation of a chunk of antimatter from space. This does not account for mineral debris the explosion left behind. A black hole zipping through Earth. This also does not account for mineral debris the explosion left behind, and there was no subsequent explosion as such a black hole, having tunneled through the Earth, would have shot back out through the surface of the Atlantic. A Nikola Tesla "death ray." The man who pioneered radio and modern alternating current electric power (AC) systems was often seen as a mad scientist. One story alleges he test-fired a death ray on the evening of June 30, 1908, and once he found out about the Tunguska event, he dismantled the weapon, deeming it too dangerous to remain in existence. All the speculation concerning Tunguska is to be expected, Boslough said.

"Lots of theories are going to pop up -- it's like a crime scene, and everyone wants to have a hand in solving the mystery," he commented. "It's fun to speculate."