Monday, June 30, 2008

A Century of Tunguska

One hundred years ago today, something exploded over Siberia.

The weather in central Siberia on June 30, 1908, a Tuesday, was warm and mostly clear. Ten days after the summer solstice, the days are not appreciably shorter than they are on that longest day. In Siberia, that means the days are very long. North of the Arctic Circle the sun does not go down at all. Instead, it approaches the south and rolls along the horizon for a few minutes at midnight before beginning to rise again. Five degrees south of the circle, it dips just barely below the southern horizon a little before eleven and rises back above it just after one in the AM. At midnight it is still light enough to read a newspaper outdoors on a clear night. People and animals adjust to the long days by rising early and staying up late. There will be time to sleep during the long nights of winter.

At a little after seven in the morning, settlers near the north end of Lake Baikal saw something bright appear in the sky, crossing to the northwest leaving a trail behind it. As it touched the horizon, it was transformed into a column of black smoke in which flames could be seen. Soon after, they felt a thump in the ground and heard a series of bangs that they compared to artillery in the distance. Other villagers to the west of them gave similar descriptions. Later scientific expeditions would locate the ground zero of the explosion in a remote area among the tributaries of the Podkamennaya (Stony or Upper) Tunguska River. Seismic stations around the world recorded the ground movement and set the time at precisely 07:17:11 AM.

The closest people to ground zero were Evenki reindeer herders camped along the Chambe River forty kilometers away. They reported being thrown in the air by the shock, along with their tents and belongings. They saw trees broken off by the shock and the forest set on fire. Several herders were injured, but the only reported death was an old man who probably had a heart attack. One herder, Ilya Potapovich, later reported that his brother was so shocked by the explosion that he didn't speak for years after. Their herds were scattered and many reindeer perished in the fire.

The devastated forest at Tunguska photographed twenty years later.

At the Vanavara trading post, seventy kilometers from ground zero, people were knocked to the ground, with enough force to lose consciousness. Windows were broken and buildings damaged. The heat was painful, but not hot enough to start fires. Two hundred kilometers south, the ground shock and wind were strong enough to knock people and animals off their feet. Six hundred kilometers southwest, an eastbound train on the Trans-Siberian Railroad shook so hard the engineer feared the train might be derailed and brought it to a screeching halt.

When all of the reports were collected in the 1920s, it revealed that the object in the sky was visible 700 kilometers away and the explosion was heard over 1200 kilometers away.

Sibir a regional newspaper published in Irkutsk was the first to make an official notice publishing on July 2:
In the N Karelinski village (200 verst N of Kirensk) [one verst equals 1.0668 kilometers] the peasants saw to the North-West, rather high above the horizon, some strangely bright (impossible to look at) bluish-white heavenly body, which for 10 minutes moved downwards. The body appeared as a "pipe", i.e. a cylinder. The sky was cloudless, only a small dark cloud was observed in the general direction of the bright body. It was hot and dry. As the body neared the ground (forest), the bright body seemed to smudge, and then turned into a giant billow of black smoke, and a loud knocking (not thunder) was heard, as if large stones were falling, or artillery was fired. All buildings shook. At the same time the cloud began emitting flames of uncertain shapes. All villagers were stricken with panic and took to the streets, women cried, thinking it was the end of the world.

Another Siberian paper, Golos Tomska, wrote more cynically on the fourth:
The noise was considerable, but no stone fell. All the details of the fall of a meteorite here should be ascribed to the overactive imagination of impressionable people. There is no doubt that a meteorite fell, probably some distance away, but it's huge mass and so on are doubtful.

A few other local newspapers added their stories through the rest of the short Siberian summer, but by late August, when the nights were growing noticeably longer and cooler, the mysterious blast of June was forgotten amid other concerns.

The most likely path of a meteorite based on witness recollections in the 1920s.

Although the Tunguska blast did not raise any attention outside Siberia, the effects of the blast were noticed across the Northern Hemisphere. The dust from the blast was injected into the stratosphere and circled the globe creating spectacular sunsets and bright night skies for the next few days. In London, cricket games continued after midnight. In Scotland, farmers used the extrra daylight to harvest the hay crop. In the cities and countryside of Northern Europe, intelligent observers wondered about the cause of the displays. Newspapers in London, Berlin, Prague, and New York sent reporters to interview astronomers. The most common answer given was that the bright skies were probably unusual auroral displays brought on by energetic eruptions on the sun. A few experts admitted that the displays didn't show the normal characteristic sheets and scintillations of an aurora, but they didn't have a better explanation on hand. Some of the older observers, expert and amateur, came closer to the truth when they compared the skies to the displays that followed the eruption of Krakatau in 1883. The mystery was soon forgotten in the face of other news. There were crises in Central Europe to worry about and it was the regatta season.

European Russia paid even less attention to the mysteries of nature than their neighbors to the West. Ever since the disastrous war with Japan in 1904-05 the empire had been in a state of crisis. The stress of the war brought about a revolutionary situation that was only calmed by the Tsar agreeing to the formation of a parliament. Three separate elections were held during 1906-07 before a government could be formed that could work with the Tsar. Meanwhile, unrest continued in the countryside with peasants burning manor houses and the army burning peasant villages right up till the end of 1907. At the same time as the government tried to establish some kind of domestic peace, it also had to establish a safe diplomatic space in which to rebuild its military strength. Russia's diplomats tried unsuccessfully to balance between Germany and Austria on one side and Britain and France on the other without committing to either alliance. As if all of that wasn't enough, the constitutional ideas of the Revolution of 1905 appeared to have infected Russia's neighbors to the South, Turkey and Persia, adding more crises and instability for the rulers to deal with. The result was that in the cities of European Russia the ruling elites had little attention to spare for exploring Siberian mysteries. No official attention was directed into the explosion at Tunguska until after the World War, the Revolutions of 1917, and the following Civil War were over and the country had had a few years to catch its breath.

The man who brought the mystery of the Tuguska explosion to the attention of the world was Leonid Kulik. Kulik had ideal credentials for the job. Although of a bourgeois background, the son of a doctor, he had been expelled from school for pro-Bolshevik revolutionary activities. He continued his education in forestry, physics, and mathematics in the years before WWI, with occasional breaks to be arrested for continuing revolutionary activities. Working in the Urals as a forester, he had come to the attention of Vladimir Vernadsky and acquired an interest and quick education in geology. The Revolution found him in Tomsk, Siberia teaching mineralogy. He joined the Red Army and served till the end off the Civil War, when he was discharged with honor. He took a museum job in Petrograd (the less German sounding name given to St. Petersburg in 1914) studying under Evgenii Krinov, the country's leading authority on meteors. Thus he came to the problem with a broad education, influential intellectual patrons, impeccable revolutionary credentials, and a familiarity with Siberia.

In 1921 the Soviet Academy of Sciences approved an expedition to Siberia to collect information on meteors. The Purpose of the expedition was not particularly scientific. Some had the idea that utilizing the concentrated ore of iron meteorites they might jump-start the recovery of Soviet industry. No doubt the people who approved the expedition were influences by a number of discoveries of giant iron meteors in recent years. In 1894, the Arctic explorer Robert Peary located the Cape York meteorite in Greenland consisting of nearly 35 tons of almost pure iron-nickel alloy. In 1902, an American entrepreneur located the Willamette meteorite in Oregon, which weighed in at 15.5 tons. Most impressive of all, in 1920 a farmer near Grootfontein, Southwest Africa (now Namibia) found the Hoba meteorite with 66 tons of alloy. Kulik was made the head of the Siberian meteorite expedition.

As Kulik was boarding his train to the Far East one of his colleagues handed him a page torn from an old calendar that had a news article about a meteor fall in Siberia in 1908. Almost every detail of the article was wrong. It described a red glowing stone landing near the railroad and curious passengers standing around to watch it cool. However, from the date and location in the story, Kulik was able to figure out what had happened. He prepared a questionnaire about the event that he gave to people across Siberia. Comparing witness accounts he was able to calculate approximately where the streak in the sky should have touched down. Kulik had no doubt that the 1908 explosion was a meteorite.

Despite his confidence, it would take Kulik six years to convince the academy to fund another expedition. During that time he corresponded with other researchers in that part of Siberia and published his theory. S.V. Obruchev, a geologist, interviewed some of the locals in 1924. A.V. Voznesensky, former director of the Irkutsk observatory, did his own interviews and calculated the ground zero for what he also believed was a giant meteorite. He published his findings in 1925. In 1926, I.M. Suslov, an ethnologist working among the Evenki people conducted over sixty interviews and published his work. The force of this collected testimony enabled Kulik to overcome the Academy and get funds for an expedition in the spring of 1927 to ground zero. At this point, no outside had seen the damage caused by the explosion, though Suslov's witnesses described a great area where the trees no longer stood.

In February of 1927, Kulik and an assistant traveled from Leningrad (as Petrograd had been renamed) to Taishet, a station on the Trans-Siberian Railroad 900 kilometers from the believed epicenter of the Tunguska explosion. From there they traveled by horse-drawn sledge and cart to the Vanavara trading post. After recovering for a few days, Kulik began negotiating for a guide to take him to the center of the blast. The local Evenki had developed a strong aversion to the region which they believed had been cursed by Ogdy, their god of thunder. Eventually, Kulik convinced Ilya Potapovich, the brother of the herder who was probably closest to the explosion, to take him there.

After a false start, the group traveled by horse and reindeer north to the River Makirta. As soon as they crossed the river, they began to find whole stands of trees broken off and knocked over. On April 15, Kulik and his group climbed the highest hill they could find to survey the forest. From that vantage, Kulik reports that as far as he could see to the North, the forest had been leveled and burned, with all of the fallen trees pointing south. In all, 80 million trees were flattened over an area of over 2000 square kilometers. Potapovich and a second guide refused to go any further.

Kulik returned to Vanavara and hired some laborers to take him to the center of the blast area. He expected to find a large crater there, like the one that was becoming a famous attraction in Arizona. He found the exact opposite. Kulik could tell, from the direction the trees had fallen, where the approximate center was. Instead of a crater, he found a circular stand of trees still upright, but with all their branches blown off. Kulik deduced that his meteor had broken apart above the ground and the force of that shattering had blown downward, stripping the trees directly below and knocking over the trees further away. Kulik also noticed circular potholes, sometimes tens of meters across and filled with water. He decided these must be the small craters that the fragments of the meteor made. Unfortunately, the summer thaw was now well underway and the countryside was turning into a sea of mud. After taking some photographs, Kulik's group returned to Vanavara.

Back in Leningrad, Kulik's report got considerably more attention than his 1921 report had. Western scientific journals wrote him up. Observatories and scientists began combing through records from 1908 to find out how they could have missed the explosion. The mysterious bright nights of the summer of 1908 were recalled and declared solved. In London, C.J.P. Cave unveils a set of records from a device called a microbarograph. This device invented in 1903, was able to ignore normal changes in air pressure, such as storm fronts, and record tiny fluctuations in pressure. Cave shows that six of these devices recorded four waves of pressure in rapid succession exactly five hours after the explosion in Siberia.

With the whole world watching, the honor of Soviet science was at stake and the Academy quickly approved another expedition for the next year. At first the 1928 expedition didn't seem to have much hope of adding to the information gained the previous year. Kulich, three assistants, and a film maker surveyed the blast area, but the equipment they brought was not up to the task of drilling in the soggy ground or detecting buried metallic objects. When food began to run low, Kulik sent the others home.

They couldn't have arrived at a better time. Umberto Nobile's attempt to reach the North Pole by zeppelin had ended in a crash on the return leg and the crew was left stranded on the Arctic icepack. The Soviet Union dispatched a state-of-the-art icebreaker to rescue them. Day after day the world watched the dramatic rescue take place. No sooner was the drama over, than three scientists arrived from the wastes of Siberia to announce that their leader had refused to leave his work and was stranded there. They had a nice documentary film to go with their story. The press corps turned their attention ninety degrees and looked to Siberia for the next adventure.

The ethnologist I.M. Suslov, who had worked among the Evenki and knew the area, led the rescue expedition. He took along an army of reporters. When they arrived, Kulik put everyone to work digging for meteorite fragments. No one found anything. At the end of the summer, the whole group returned to Leningrad to examine their findings. Kulik and Tunguska were world famous.

There was no question of the Academy refusing a 1929 expedition. This time Kulik had an army of engineers and fifty carts of supplies. Kulik's boss at the museum, Krinov, also came along. They spent two summers and a winter making major excavations at a number of sites around ground zero. The lack of meteor fragments was causing some controversy. Arctic geologists pointed out that the pothole formations were a permafrost structure found all over the North. Krinov and Kulik got into a fight over the exact epicenter of the blast and stopped talking to one another. By the end of the summer, it looked like Tunguska research was over for a while. Kulik's health had been severely degraded by the rough trips. Russia was pulling into isolation under Stalin. The World Depression and new crises in Europe had taken over the interest of the news reading public. Finally, there was no money left for research.

Kulik managed one more expedition in 1939. By now aerial surveys had precisely mapped the blast area and a road and airstrip had been built to make travel to the site a holiday compared to his journey's of the previous decade. Kulik was planning another series of annual research trips when the war interrupted. His short 1939 trip was his last. When the Nazis invaded the Soviet Union in 1941, Kulik, who was almost sixty, joined the militia and fought in the defense of Moscow. He was wounded and captured by the Germans and died of typhus in a prison camp in April 1942. When the Soviets mapped the back of the moon, they named a crater for him.

Leonid Kulik memorial stamp issued in 1958 on the fiftieth anniversary of the Tunguska explosion. The cause of the blast was still very much a mystery when the stamp was issued.

To Kulik and many others, it was obvious from the very beginning that the Tunguska explosion was the result of some type of meteorite. However, a large portion of the world scientific community resisted the idea. The lack of a crater or fragments of the impactor were major sticking points, to be sure, but even if they had been found, many would have tried to explain them away. They just didn't like the idea of big things hitting the Earth. Their hostility came from two scientific battles fought in the early part of the nineteenth century.

The founding of the science of geology was made especially difficult because it seemed to go in the face of the Biblical narrative in Genesis. Prior to the Enlightenment, it was commonly believed in the Western world that the Earth was only a few thousand years old. The whole idea of a separate history of the Earth was meaningless, because the Earth was only a few days older than human history. The formation of the Earth was a simple matter of divine will and the structure of the Earth's surface was all conveniently explained as the scars of the Mosaic Flood. The Copernican revolution created the first cracks in this cozy view of the universe. By making the universe much bigger than the Biblical-Aristotelian worldview allowed, astronomers created a need for more time to let cosmic processes work their way out. An older universe had more time for gentle processes, like erosion, to explain the shape of an older Earth. The idea that gradualistic processes were sufficient to explain the mountains and the seas was particularly hard resisted by English speaking Protestants. In fighting to establish geology on a naturalistic basis, scientists in Britain and the United States took an equally hard line against anything that appeared miraculous or catastrophically sudden.

At the same time geologists were fighting to establish the Earth sciences on a rationalistic basis, astronomers were settling a long-standing controversy over the nature of meteors. It may come as a surprise to some to discover that the very idea of meteorites is a new one that was strongly resisted by many in the scientific community. According to the Aristotelian science that was endorsed by the Church, meteors were an atmospheric phenomenon similar to the Northern Lights. When Western thinkers began to reject the miraculous, they lumped reports of rocks falling out of a clear sky together with rains of blood or frogs as something not to be believed. The streaks of light called meteors had nothing to do with things falling from the sky.

The conversion of the scientific world to the idea that rocks really did fall out of the sky came rather quickly. Between 1794 and 1803 a number of meteor falls were well documented in Italy, England, and India. The climax was a meteor shower over L'Aigle, Normandy on April 23, 1803 that dropped over 2500 stones. So many rocks witnessed by so many people was more than could be denied.

To fit the idea of meteorites in with the non-catastrophic, or uniformatarian, view of geology a compromise was struck: it was agreed that meteors really existed, but that they were always small and insignificant as a geological process. This was easy to believe at first. When no one knew where meteors came from, it was easy to say they were by definition small, for example, they might be rocks tossed out by lunar volcanoes. A better understanding of comets and the discovery of asteroids challenged this compromise by adding big rocks in strange orbits to the prevailing model of the universe.

The nebular hypothesis of the origin of the Earth also presented a challenge. In this theory, the Earth was formed by the agglomeration of comets and asteroids in the early solar system. The uniformitarian paradigm absorbed this idea by making the age of bombardment a one-time era in the distant past. Large rocks once hit the Earth, but they don't do that anymore. All visible craters, therefore, were judged to be volcanic in origin and all large meteorites discovered, like the Willamette, Hoba, and Cape York meteorites, were judged to be very ancient, relics of the formation of the Earth. This resistance to allowing the possibility of large meteorites in modern time was mast extreme among American scientists. The Barringer Crater in Arizona was dismissed as volcanic in origin, despite a total lack of evidence, and all of the craters on the Moon were explained as ancient volcanoes.

The resistance to the idea that a large meteorite might be behind the Tunguska explosion created an opening where wild ideas were allowed to proliferate. The ideas can easily be divided into three types: natural objects falling from the sky that are natural but stranger than normal meteorites, natural causes coming up from the earth, objects of intelligent design coming from Earth or space.

The most commonly mentioned natural but strange object from space is a tiny black hole. This idea was first stated by A.A. Jackson and Michael P. Ryan in a 1973 article in journal Nature. Jackson and Ryan described a black hole with a event horizon radius less than a millimeter across. It entered the atmosphere creating a tube of superheated air that created the visible passage across the sky and the damage to the forest with no crater. The black hole itself passed at an angle through the Earth and exited in the North Atlantic. Its exit point off the western end of the Azores should have created an equally spectacular display, but none was reported.

When Kulik was still alive, a fellow Russian, Vladimir Rojansky suggested the possibility the Earth could be bombarded by small anti-matter meteors. In 1965 two American physicists, again in the pages of Nature explored the possibility of an anti-matter meteor explaining the puzzling aspects of the Tunguska blast. The authors admitted that an anti-matter meteor would erode during the entire course of its passage through our atmosphere rather than save most of its energy for blast at the end.

The idea of mirror matter, a type of matter with the particle spins reversed, was proposed in 1956 by two Chinese-American physicists. Mirror matter provides a mathematical solution to some problems of symmetry, but it should be completely undetectable. In a self-published book in 2002, Robert Foot, an Australian physicist, suggested a mirror matter asteroid exploding the upper atmosphere could have provided the energy for the Tunguska blast while leaving no trace of itself. The biggest problem with this theory is that mirror matter has never been found and isn't even widely accepted as necessary by most physicists.

In 2001, a German writer suggested leaking methane from a natural gas field might have caused the explosion. Andreii Olkhovatov of Moscow, postulates something he calls geometeors, an eruption from the Earth caused by an electrical linkage between some kind of meteorological activity and an earthquake. The key problem with all from-below theories is that they ignore the testimony of the dozens of people who saw some thing streak across the sky toward Tunguska before the explosion.

Jack Stoneley, in his 1977 book Cauldron of Hell: Tunguska brought up the possibility that the explosion was the largest incident of ball lightening ever seen, but even he wasn't that enthusiastic about the idea, only mentioning it before going on to give the most space to the idea of a spaceship crashing in Siberia. Others have developed variations on this idea comparing the collapse of the ball to a natural atomic bomb.

The most famous alternative theory is the crashing spaceship, which can incorporate anti-matter or an atomic explosion coming from the spaceship engines. Aleksandr Kazantsev, an engineer who also writes science fiction, wrote the earliest version of this scenario. In 1946, Kazantsev published a short story called "The Blast" about aliens coming to steal water from Lake Baikal whose ship malfunctions and explodes. Kazantsev was clearly influenced by descriptions of the atomic bombing of Hiroshima and Nagasaki the year before. In 1963 Kazantsev developed his idea into a book length work of non-fiction. This is a favorite theory of the UFO crowd and of science fiction fans and has received dozens, if not hundreds of treatments over the last sixty years.

Nikola Tesla: A Serbian with a death ray?

One of my favorites is what I call the oops theories. There are two of these. The first, which appeared in Russia in 1964, is that aliens on a planet orbiting the star 61 Cygni saw the explosion of Krakatau and thought we were trying to communicate with them. Their answer, sent by a super tight laser pulse, burned up a big chunk of Siberia. Messages like that are the extraterrestrial equivalents of sending e-mails written with the Cap-Lock on. The second is home grown. In this theory the brilliant physicist and electrical engineer Nikola Tesla accidentally blew up the Tunguska forest while testing a death ray at his lab at Wardenclyffe on Long Island. When he realized how dangerous it was he dismantled his lab. Of course it would have taken Tesla twenty years to realize how dangerous it was because that's when the first reports from Kulik's expeditions reached the West.

The UFO magazine Nexus published a novel explanation in their 2004 and 2005 issues. The forests at Tunguska, they tell us, are home to mysterious underground installations built by sophisticated ancients. Every few centuries these installations come to life to defend the Earth from rogue meteors and alien invasions. The 1908 blast was just the latest battle waged by these heroic machines.

In describing the Tunguska explosion, a literary tradition has been established of comparing it to a nuclear weapon and describing its power in megatons. The Tunguska blast was equal to about twenty megatons (million tons) of TNT. Hiroshima was about twenty kilotons (thousand tons), one one-thousandth the size of Tunguska. There are good reasons for this comparison. A very hot, concentrated blast, above the surface of the ground accurately describes both the Tunguska and Hiroshima explosions.

But the main reason for the comparison probably has more to do with literary sensationalism than accuracy. Atomic blasts are one of the most frightening things that the post WWII generation can imagine (after clowns). The nuclear weapon comparison allows writers to paint vivid scenarios of what would have happened if the object had detonated over a populated area. The area of blown down trees is approximately the same size as a number of major cities including Washington, DC. It is frequently pointed out that the blast was at the same latitude as the Russian capital of St. Petersburg. If the Earth had been turned four and a half hours further when it met the object, the explosion would have happened over the northern industrial suburbs of the city and burned the entire metropolitan area to the ground.

In one way, the comparison is very false. Whatever exploded at Tunguska, it was not radioactive. The heat of the blast was great enough that it emitted x-rays, elements were ionized, and strange chemical reactions occurred, but these were the result of extreme heat, not of radioactive decay or fission. The atomic comparison brings on thoughts of modern weirdness and opens the door to ideas of strange forms of matter, spaceships, or super weapons. A more natural comparison would be to rank it next to other natural disasters.

In earthquakes, a twenty-megaton explosion releases about the same energy as an earthquake measuring 7.0 on the Richter Scale. The world sees about a twenty each year that are this strong or worse. The World Series earthquake that hit San Francisco in 1989 was in this range. The San Francisco earthquake of 1906 unleashed about 1000 megatons of energy and the Alaska earthquake of 1964 unleashed about 30,000 megatons.

Even a moderate hurricane has many times more power than that. Hurricane Katrina had an energy equivalent of over 8000 megatons or 400 times the size of the Tunguska blast. There are two important differences here: size and time. A hurricane spreads that energy over several thousand square kilometers and builds gradually over hours. Tunguska spread out from a spot only a few meters across and traveled three or four times faster than the strongest hurricane.

Volcanoes make the best comparison to meteors because they come in all sizes and can leak their energy out or deliver it in an explosion. Mount St. Helens delivered about seven megatons in its May 18, 1980 eruption. Krakatau's 1883 explosion is estimated to have been about 100 megatons. The explosion that took away most of the Greek island of Thera in the seventeenth century BC was six to ten times stronger than that.

And Having Writ... by Donald R. Bensen. A humorous alternative history novel published in 1978. The story centers on the extraterrestrial crew of the spaceship that almost crashed at Tunguska. Now stranded on Earth, they do everything they can to jump start WWI in order to advance human technology to the point where they can build a new spaceship and go home. Unfortunately, their best efforts keep causing peace to break out. When this book was published, the idea of the Tunguska spaceship was so familiar to me that using it for an alternate history made perfect sense and needed no explanation.

Over the years, the wilder explanations of the cause of the Tunguska blast have ensured it a place in popular culture. It has appeared in novels by writers as diverse as Stanisław Lem (a spaceship), Spider Robinson (Tesla), Larry Niven (a black hole), and Thomas Pynchon (an Awakened Chthulu like force). It has been mentioned in television shows and movies like Stargate SG-1, Star Trek, Dr. Who, Hellboy, Ghostbusters, Buffy, the Vampire Slayer, and the latest Indiana Jones. Tunguska was a major plot element in two episodes of the fourth season of The X-Files. An adventure game "Secret Files: Tunguska" is published by Deep Silver. It is even now the name of an herbal energy drink.

Tunguska Blast energy drink made from the same herbs that grow in the mysterious Tunguska region.

While these ideas have kept the name Tunguska alive in the popular mind, conventional researchers have continued to visit the site and collect evidence of a more prosaic meteorite. They have gathered what most believe is convincing evidence that a large body entered the atmosphere from the southeast and exploded at an altitude of some kilometers over the ground zero stands of trees that remained vertical. The main argument among most scientists is over the nature of the meteorite, whether it was a comet, a stony asteroid, or an iron asteroid. The composition of the meteor determines the altitude and power of the blast. Computer models have shown that most of the features of the blast can be explained by an airburst, just as Kulik believed. One of the strongest supports for the comet theory is the timing of the blast. Every year, in late June and early July, the Earth passes through a meteor stream called the Beta Taurids, which are fragments of Comet Encke. The strongest evidence for a stony asteroid is fine particles of dust found embedded in tree sap from the region and dated to 1908.

The most recent revelation from the site came from a team in Italy who examined Lake Cheko eight kilometers north of ground zero. They believe a fragment of the exploding object made it to the ground and is in the bottom of the lake. They will be returning in 2009 to excavate the lake bottom and see if they can recover something that will end the debate.

Besides simply being an persistent mystery that has engaged minds for a century, Tunguska has real relevance to our lives. In 1908, the world had many places like Tunguska that were so remote a giant meteorite could strike and still go almost unnoticed by the outside world. Today, there are fewer places where that is possible. A strike like Tunguska today could have global consequences. Obviously, a strike in a populous region could kill millions, but even the most remote regions are now tied into the global economic infrastructure. Siberia is criss-crossed by oil and natural gas pipelines. Even if people and infrastructure are missed, a large strike could have temporary climactic effects resulting in major food or economic crops failing for one or more years. An increasing number of historians are beginning to believe that meteorites and volcanic eruptions explain certain plagues, famines, and even the fall of empires in the past.

The smooth survival of our civilization might depend on getting to know more about the objects with which we share our solar system. But even if we never have to face a civilization busting threat from space, the knowledge we gain from Tunguska is priceless and the story is a great yarn. Happy birthday, old rock, snowball, spaceship or whatever you were. It's been a fun century and the adventure shows no sign of stopping.

Postscript: Looks like I'm not the only one who thought to do an anniversary piece. Here is the BBC science page.

Update: I have made some minor editing corrections thanks to alert readers pointing out the need. I also added a picture of Tesla that, for some reason, didn't want to upload yesterday.

1 comment:

john mckay said...

Hi John
I am also John J. McKay a science teacher born un NYC now living in PA. Are we kin?