My book, Comets: Visitors from Deep Space, will be published October 1 by Cambridge University Press. Here’s a taste of an introductory chapter that describes the impact dangers from near-Earth objects, and specifically, in this chunk, the famous Tunguska event of 1908 . . .
What’s come to be known as the Tunguska event occurred along the Podkamennaya Tunguska River in central Siberia, in the present-day subject of Krasnoyarsk Krai, a region occupying 13 percent of Russia’s territory. Suddenly, on June 30, 1908, an enormous explosion thundered over the mostly uninhabited, forested landscape, flattening perhaps 80 million trees over 830 square miles (2,150 square kilometers). An enormous flash and bang were seen and heard over a huge area of the country, stunning hundreds of thousands of people. The shock wave from the blast would have registered about 5 on the Richter scale.
Tunguska was first noticed about 7:17 a.m. that morning when indigenous Evenks of the Russian North saw a blindingly bright column of bluish light sweeping across the sky. Some 10 minutes later came an immense flash of light and dull, heavy thudding akin to distant cannon fire. The sound rolled over the landscape in the direction of east to north. The air heated by the trail of the incoming object expanded and broke windows hundreds of kilometers distant. Fluctuations in atmospheric pressure were detected as far away as England. High-altitude ice particles floated, locked in suspension, and caused an eerie glow high in the night sky for several evenings after the event. Atmospheric transparency decreased for several months due to substantial dust kicked high into the atmosphere.
Eyewitness testimony was varied and recorded with enthralling details. The newspaper Sibir reported: “We observed an unusual natural occurrence. In the north Karelinski village 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 spot 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 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 started 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.”
The Tunguska event, blast, or explosion, as it was variously known, largely remained a mystery — substantially because of the then incredibly remote place where it occurred. Not until 13 years later did the Russian mineralogist Leonard Kulik (1883–1942) first visit the region. (Of course, delays also took place due to events that took precedence — World War I, the Russian Revolution, and the Russian Civil War.) When he arrived on scene in 1921, undertaking a survey for the Soviet Academy of Sciences, Kulik deduced the blast must have been caused by the impact of a giant object from space. Astonished by the enormity of the forest destruction and lured by the possibility of a large amount of recoverable iron from the offending meteorite, Kulik persuaded the academy to fund a proper expedition to investigate.
When his expedition returned in 1927, Kulik talked the local Evenks into taking him to the site of the central impact. The journey was long and difficult, and the locals would not pass all the way to the impact zone, fearful from freshly created superstitions. Kulik pressed on and found, amazingly, that he could not find a crater at all. Instead, he discovered a huge zone of scorched, upright trees whose branches had burned off; this spanned some 5 miles (8 kilometers) in diameter. (The surrounding trees for many kilometers in every direction were flattened like matchsticks.) More than 30 years later, researchers determined the blast created a butterfly-shaped zone of devastation whose “wings” stretched across 43 miles (70 kilometers) and whose “body” covered 34 miles (55 kilometers).
Over the next decade, Kulik led three more expeditions to the remote Siberian forest. He found holes and bogs he attributed to meteorite strikes but later discovered stumps in some of them, ruling out meteoritic craters. The first aerial images of the blast zone were taken in 1938. Later research trips uncovered tiny silicate and magnetite spherules in the soil. The high percentage of nickel found in some of these spheres strongly suggested a meteorite as the impactor. With no impact crater, however, it seemed clear that whatever whizzed into Earth from space exploded in the air above the Siberian woods, raining debris down violently from above.
Later scientific analyses of bog areas in the region also uncovered evidence of an extraterrestrial impact. Layers of sediment in the bogs were analyzed and found to contain different amounts of various carbon, hydrogen, and nitrogen isotopes than the layers from before and after 1908. The layer corresponding to the year 1908 also contained much larger amounts of iridium than normal, in an analogy to what would later be found in the K-Pg sedimentary rock.
So if the Tugunska event was caused by an impact, what happened to the body that struck Earth? Researchers could never find any evidence of solid bodies, meteorites, recovered from the site. Clearly, a significant object came hurtling in from space, exploded violently, and left almost no trace of itself save for the widespread damage that it caused. From 1908 onward, scientists were left with quite an impact puzzle.
In 1930, British astronomer Francis J. W. Whipple (1876–1943) — unrelated to the American Fred Whipple — proposed that perhaps the Tunguska impactor was not an asteroid but a comet. In contrast to an iron-nickel or rocky meteorite, a predominantly icy comet may have largely vaporized as it slammed into the lower part of the atmosphere, most of it disappearing into gas. The fact that Europeans saw a brighter than normal sky for several nights after the impact might also support this idea, the icy particles, water vapor, and dust scattered by the decomposition of a comet lingering in the upper atmosphere before being dispersed.
Since then, the idea that Tunguska was a comet that exploded in an airburst has washed back and forth, generally finding support but also coming into question. In 1978, Slovak astronomer Ľubor Kresák (1927–1994) linked the Tunguska impactor to the well-known periodic comet designated 2P/Encke, the parent body of an annual meteor shower called the Beta Taurids, which peaks in intensity during the last days of June. Kresák pointed out that the Tunguska event occurred during that shower’s activity and that the orbital trajectory of the impactor would have matched a stray fragment from that meteor shower.
A 1983 paper by American astronomer Zdenĕk Sekanina argued that the Tunguska object could not have been a comet because a comet would have disintegrated long before it approached the ground. American astronomer Christopher Chyba suggested that a stony asteroid could enter the atmosphere at high velocity, encounter a force that flattens and pancakes the leading edge, and create an explosion that blows the object apart, releasing nearly all its energy and leaving no crater. And a team led by Italian physicist Giuseppe Longo found that resin trapped in the trees of the Tunguska impact region contained grains typical of stony asteroids and atypical of comets. They also identified Lake Cheko, a small body of water in the region, as a possible crater tied to the Tunguska event. This may have been caused by a small fragment sent reeling downward by the Tunguska airburst.
The comet hypothesis received a boost in 2010 when Russian-American physicist Vladimir Alexeev studied the Suslov Crater, one of the bog-like holes found by the Kulik expeditions, this one spanning 105 feet (32 meters). Alexeev’s team concluded this crater was indeed formed by the impact of a body from above. The surface was covered by permafrost, but lower layers revealed disturbed ice and, at the lowest levels, icy debris that may have come from the impactor itself, supporting the comet hypothesis.
Planetary scientists still debate whether the Tunguska event was caused by a cometary nucleus that shattered in an airburst or an asteroid that violently exploded and left little trace — and, of course, the lines between comets and asteroids are becoming increasingly blurred. What’s clear is that a small body of the solar system some 330 feet (100 meters) across fell rapidly toward the Siberian forest and, 3 to 6 miles (5 to 10 kilometers) above the ground, exploded violently with the force of 1,000 Hiroshima bombs.
Never before or since have humans recorded such an event of exactly this type.