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November 19, 2004

UFOs and Tunguska

by Michael Shermer

A story has been unfolding over the past couple of months about a Russian expedition that claims to have uncovered evidence of an extraterrestrial spacecraft that crash landed at the site of Tunguska where, in 1908, scientists believe a meteor exploded. What follows is a story about the Russian expedition and the entry for Tunguska from the Skeptic Encyclopedia of Pseudoscience, by my friend and colleague, Alan Harris, a Jet Propulsion Laboratory space scientist.

The “artifact” in the photograph being held by the expedition leader looks like a rock to me, but check it out yourself here.

alien cartoon

The photograph of the spacecraft half buried in the dirt is, to my eyes, risibly ridiculous, and has to be seen to be believed that in this day and age of photoshop anyone would try to pass this off as real.

To me it looks much like the half-buried UFO model at the Roswell museum.

Given what we know about what happens to aircraft when they hit the ground at high speed, and the fact that we’ve all seen pictures of the debris fields of such crashes where the craft is disintegrated into countless tiny pieces, UFOlogists seem to prefer to present these UFOs as crashing completely intact, along with alien bodies, which are typically shown scattered about the crash site with a few cuts and bruises (human remains from airline and spacecraft crashes are so destroyed that it takes considerable scientific testing to determine identity).

Finally, I find it interesting that the folks behind this story have chosen to retain the best scientific explanation of a meteor impact, and graft onto it the UFO element, with the aliens “saving” us by zapping the meteor high enough above the ground to prevent total extinction of humanity. It’s a nice touch that harkens back to science fiction films of the 1950s where the aliens intervene just in time to save us from ourselves (as in The Day the Earth Stood Still).

The following is an article that appeared on on August 12, 2004. View the entire article online.

Did Aliens Save the Planet in 1908?

Members of a Russian scientific team researching the site of the Tunguska meteorite crash of 1908 say they have found remnants of an extraterrestrial spacecraft, report a variety of Russian news agencies.

The object appeared to be a large metallic block, according to the reports. The researchers chipped a piece of the object and will now test its composition.

One scientist said based on his calculations, the mass of the space object headed for Earth in 1908 was nearly 1 billion tons. He believes the meteorite was blasted by the spaceship at an altitude of 10 kilometers to prevent the destruction of all humanity on the planet.

“I am fully confident and I can make an official statement that we were saved by some forces of a superior civilization,” Yuri Lavbin said. “They exploded this enormous meteorite that headed towards us with enormous speed,” he said. “Now this great object that caused the meteorite to explode is found at last. We will continue our research,” he said.

Lavbin says that the results of this year’s expedition give him hope that the Tunguska mystery will be solved before the phenomenon’s 100th anniversary. To do this, Russian researchers plan another large-scale expedition to the Eastern Siberia.

The scientific team says the Tunguska event was an aerial explosion that occurred near the Tunguska River in Siberia June 30th, 1908. The blast felled an estimated 60 million trees over 2,150-square kilometers. Local residents observed a huge fireball, almost as bright as the Sun, moving across the sky. A few minutes later, there was a flash that lit up half of the sky, followed by a shock wave that knocked people off their feet and broke windows up to 400 miles away.

The explosion registered on seismic stations across Eurasia, and produced fluctuations in atmospheric pressure strong enough to be detected by the recently invented barographs in Britain. Over the next few weeks, night skies over Europe and western Russia glowed brightly enough for people to read by. In the United States, the Smithsonian Astrophysical Observatory and the Mount Wilson Observatory observed a decrease in atmospheric transparency that lasted for several months.

The size of the blast was later estimated to be between 10 and 15 megatons. Until this year members of numerous expeditions have failed to find any remains of the object that caused the event.

The following is an article by Alan Harris that appeared in the Skeptic Encyclopedia of Pseudoscience


by Alan Harris

Early in the morning of June 30, 1908, in a remote region of Siberia about 1,000 km (600 miles) north of Irkutsk, an immense explosion occurred, centered about 8 km (5 miles) above the forest below. Trees were flattened over an area about 50 km in diameter, several times larger than the area encircled by the Beltway around Washington DC. Trees near the “epicenter” of the event were ignited and a forest fire covered the inner third or so of the area of devastation. Sonic booms were heard as far away as 1,000 km; magnetic disturbances were recorded in the area, and the barometric disturbance from the explosion was recorded world-wide, and a seismic disturbance equal to a magnitude 4.5-5.0 earthquake was recorded. So sparsely populated was this region that apparently no one was killed. There were enough witnesses within a few hundred km of the event to establish that the cause of the explosion came from the sky, apparently a giant meteoroid slamming into the atmosphere from the southeast. (Figure 1)

At the time, there was little controversy over the nature of the event. The fact that “stones fall from the skies” was established a century earlier and by 1908 was well accepted. It was logical to assume that this was just an enormously larger than average meteoritic event. Unfortunately these were troubled times in Europe generally, and Russia in particular, with World War I on the horizon and the Russian Revolution soon to follow. It was not until 1927 that the first scientific expedition, led by L. A. Kulik, visited the region. Kulik expected to map out the ground damage and collect meteorite specimens. In this and successive expeditions Kulik, and later his successor E. L. Krinov, did the former in great detail. They discovered an area several times larger than the urban area of Washington DC inside the Beltway where most trees had been blown down radially outward from an epicenter, with evidence for a forest fire covering the central area about equal to that inside the Beltway. But they failed to find any traces of meteoritic material, and thus began the mystery.

Briefly, the controversy can be summed up by the legal term “habeas corpus” (“[you] have the body”). All of the observed phenomena corresponded to expectations for a giant meteor, except that no “body” could be found. The immediate presumption of Kulik and Krinov, and indeed most Russian researchers to this day, was that the cosmic body that caused the Tunguska event was a small comet rather than an asteroid. They reasoned that a comet might be composed mostly of ice and therefore would not leave physical traces in the form of meteorites. It was generally expected that if the offending body were made of stone, it would leave at least some fragments behind, and none were found. Following World War II, the “mystique” of Tunguska heightened. The atomic bomb demonstrated that explosions of the energy of Tunguska (now reckoned to be about 20 Megatons equivalent of TNT) could be of artificial origin, and the flurry of flying saucer reports beginning in 1947 led to speculation that Tunguska could be the explosion of a nuclear powered alien spacecraft.

On February 12, 1947, another giant meteor struck, again in Siberia. This one, known as Sikhote-Alin after the nearby mountain range, was much smaller than the Tunguska event. This event was first investigated only a few days after the event, over 300 eyewitnesses were interviewed, and a picture of the event was drawn by a landscape artist who happened to be doing a painting of the scene at the time. The documented characteristics of this event were very similar, although on a smaller scale, to Tunguska, with one exception: habeas corpus—this time we “have the body.” Sikhote-Alin was an iron meteorite fall, and left numerous iron meteorites scattered over an area of about a km long and a few hundred km wide. There can be no doubt of the nature of this event. One might expect this similar event to have resolved the controversy over Tunguska, but instead it only served to heighten the mystery, because of the lack of a “body” at Tunguska.

In more recent times, a couple oddities of modern physics have been added to the brew: the suggestion that Tunguska was the impact of a “mini-black hole,” or the impact of an antimatter “meteor.” To be sure, none of these more far-out suggestions have been seriously embraced by the scientific community. The serious scientific controversy, such as it is, involves only the question of whether the cosmic body that entered the atmosphere in 1908 was a stony body (an asteroid) or an icy one (a comet).

The debate over the nature of the Tunguska Cosmic Body (TCB), as it is often referred to, has been carried out at great length, both in time and words. Most of the early literature is in Russian. Krinov published a lengthy article in English in 1963, and a full book in 1966, describing the Tunguska and Sikhote-Alin events. Both are out of print but may be found in university libraries. Gallant (1995) has written a descriptive book for young readers, which unfortunately is also out of print. A more recent summary of the current status of Tunguska research is published by Valilyev (1998, and web site). This article is the introductory article to an entire proceeding of a scientific meeting devoted to the study of the Tunguska event, held in Bologna, Italy in 1996. These references provide a thorough description of the physical evidence surrounding the Tunguska event, including the estimates of the path of the incoming body, the visual and sonic phenomena, tree fall and burn patterns, and the results of careful examination and sample collecting from the sites.

A few additional facts have emerged from recent expeditions, perhaps most significantly the identification of traces of meteoritic material, including small dust grains embedded in tree sap dating from the time of the event and anomalous concentrations of the element iridium in the peat bog that lies directly under the explosion site, in a layer corresponding to the time of the event. Excess iridium is associated with cosmic impacts, most notably the one 65 million years ago that led to the extinction of the dinosaurs. This evidence quite conclusively argues that Tunguska was not an alien spaceship crash or impact of a black hole or antimatter, but was instead a more ordinary arrival of a small asteroid or comet. Unfortunately, a comet, although containing some icy material, would also have plenty of cosmic dust so the distinction between comet or asteroid is not resolved by this discovery.

In 1993 two scientific papers were published that would seem to lay to rest the controversy over comet or asteroid. Chyba et al. (1993) and Hills and Goda (1993) each reported similar results based on computer calculations of the detailed physics of atmospheric entry of high-speed projectiles. One can easily imagine the original purpose of such sophisticated computer codes, and how they came to be openly available about this time. Both groups concluded that a cosmic body, entering the Earth’s atmosphere with about the expected speed, and bringing with it the amount of energy (20 MT) known to have been associated with the Tunguska event, would have to have the strength of a fairly hard stone. An object of the strength of ice would blow up far too high in the atmosphere, and an object of iron, like the Sikhote-Alin meteoroid, would punch on through to the ground, leaving a crater. Only “hard stone” fits the observed explosion height.

Furthermore, both groups found that the breakup of the incoming body, which occurs when the pressure of the atmosphere on the front face of the object exceeds its strength, is catastrophic and instantaneous; the meteoroid “explodes” into small particles only a few centimeters or smaller in size. Such tiny pieces, mostly dust, sand-sized, and none larger than gravel, would be easily lost in the swampy bog beneath the explosion site in the nearly twenty years from the time of the event until the first expedition arrived to search for traces of the impacting object. As is often the case in scientific debates, these results have not fully settled the argument. With some exceptions, most Russian investigators still cling to a preference for a comet-like TCB, while those in the west mostly favor an asteroid-like TCB. Perhaps the most realistic assessment was provided by E. M. Shoemaker at the Bologna meeting when he declared, “Anyone who says he knows what a comet is, is kidding himself.”

Further progress on this issue may have to wait until we have better knowledge of the physical properties of comets. But the scientific debate over whether the TCB was a comet or an asteroid is a mere quibble for specialists rather than a serious call for alternative, more bizarre explanations. In the last decade or two, we have become more acutely aware that we are living in a cosmic shooting gallery. There are about a million asteroids the size of the TCB in orbits, which cross the Earth’s orbit around the Sun, that are potentially capable of striking the Earth.

A collision of that size is expected every several hundred years, thus the Tunguska event, about one hundred years ago, is not at all unusual. That it occurred over land for at least a few people to see is a bit more unusual. For such an event to hit a heavily populated area would be even more rare, so it is not inconsistent with the historical record that no such massive catastrophe has ever been recorded. It is certain though that such an explosion of 20 MT energy over a populated area could cause millions of deaths. Still larger impacts occur, but less frequently.

As mentioned above, a giant impact of an asteroid or comet perhaps 15 km (10 miles) in diameter is now thought to have ended the rein of the dinosaurs 65 million years ago. The crater scar has been found, about 200 km in diameter, lying beneath later limestone sediments, on the tip of the Yucatan peninsula. Based on the number of such large asteroids known (we can see and track things this large with telescopes from the Earth), such a huge collision should occur only about once in 100 million years, again consistent with the last big one 65 million years ago. There is not much worry about such an event lying in our immediate future—we believe we have discovered and cataloged all the really big Near-Earth Asteroids (NEAs).

There is concern though over NEAs just smaller than that, in the size range of one to a few km in diameter. Some of these are not yet cataloged and could have global consequences if one were to hit the Earth. It is estimated that impacts of this size occur about once in a million years, and could cause climatic effects similar to “nuclear winter,” causing a global failure of agriculture for a year or two leading to massive famine worldwide. Such an event is a most extreme class of natural disaster, in terms of numbers of fatalities it could cause, but also in terms of the low frequency of occurrence. For this reason, cosmic impacts are all but ignored by the public, compared to earthquakes, volcanic eruptions, floods, and other natural disasters, yet the “death rate” (number of deaths per event divided by the average time between occurrences) is comparable.

Finally, at the bottom end of the scale, meteoroids delivering kilotons of explosive energy arrive all the time; events with the energy of the Hiroshima atomic bomb occur about once a year. Why no fatalities from these events? We can thank our atmosphere for that. Even hard stony meteoroids of this size explode high up in the atmosphere so usually not even a muted shock wave reaches the ground. The smallest size stony body that can cause any ground damage at all is only slightly smaller than the TCB. Below about 5 MT of energy, the explosion occurs so high in the atmosphere that no damage occurs at the surface of the Earth.

References & Notes
  • Chyba, Christopher F., Peter J. Thomas, and Kevin J. Zahnle. 1993. “The 1908 Tunguska Explosion: Atmospheric Disruption of a Stony Asteroid.” Nature 361: 40-44.
  • Gallant, Roy A. 1995. The Day the Sky Split Apart. NY: Athenaeum Books.
    See also
  • Hills, Jack G., and M. Patrick Goda. 1993. “The Fragmentation of Small Asteroids in the Atmosphere.” Astronomical Journal 105, 1114-1144.
  • Krinov, E. L. 1963. “The Tunguska and Sikhote-Alin Meteorites.” In The Moon, Meteorites and Comets. Edited by B. M. Middlehurst and G. P. Kuiper. Chicago: University of Chicago Press, 208-234.
  • Krinov, E. L. 1966. Giant Meteorites. Oxford: Pergamon Press.
  • Vasilyev, N. V. 1998. “The Tunguska Meteorite Problem Today.” Planetary & Space Science 46, 129-150. Condensed version at
  • The Unnatural Museum: The Great Siberian Explosion.
  • The University of Bologna Physics Department Tunguska Home page.

Figure 1. A photo taken by the Kulik expedition in 1928 of the pattern of fallen trees within the Tunguska area of devastation.

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