What happened in the Skies above Tunguska in 1908?

This post is part of a series of guest posts on GPS by the undergraduate and graduate students in my Science vs. Pseudoscience course. As part of their work for the course, each student had to demonstrate mastery of the skill of “Educating the Public about Pseudoscience.” To that end, each student has to prepare a 1,000ish word post on a particular pseudoscience topic, as well as run a booth on-campus to help reach people physically about the topic.

______________________________________________

What happened in the Skies above Tunguska in 1908? by Lisa Jordan

Early in the morning of June 30th, 1908, in eastern Siberia, Evenki tribesmen are checking their traps and gathering their Reindeer herds for the day’s drive to new pastures. Russian settlers are opening their trading posts and tending to their farms and livestock. Suddenly, at about 7:15am, a series of thundering booms ring out,and something that looks like a bright blue tube moving from east to northwest seems to split the sky. From the small frontier town of Vanavara, the sky to the north seems to glow as if the whole forest was on fire. Along with the successive claps of deafening thunder come waves of rushing, superheated air that makes any witnesses who are outdoors and within 30 miles of the event feel as if they’d just stepped into a hot oven. Thatched huts are blown flat, people and livestock are tossed about, and windows are shattered in houses more than 100 kilometers away. Closer in, whole herds of Reindeer are flash-cooked alive while the storehouses of native herdsmen are reduced to ashes. Barometers all around the world record the pressure wave in the coming hours, the records of which will indicate that it traveled around the globe twice. Over the next several days Central Asia and Northern Europe experience “white nights”, where the sky glows bright enough to read a newspaper outdoors at midnight. The phenomenon will take nearly 30 days to completely subside. Something extraordinary has happened on the eastern Siberian plateau, and yet except for the sparse population near the great explosion, no one really knows about it, or even cares.

The first expedition to investigate the area of the event would not occur for almost two decades, mainly because Russia had more pressing problems to deal with in the intervening years, namely World War I, the Bolshevik Revolution, and the following Russian Civil War. When Leonid Kulik led the first expedition in 1927, he immediately ran into problems when his Evenki guides refused to enter the area. They believed that their vengeful thunder god Ogdi had planted his lodgepole in the Stony Tunguska and cursed the surrounding taiga. Once he procured new guides and finally reached the area of the event several days later, Kulik was struck with a sight he could not have imagined. Over 2000 square kilometers of trees were scorched and knocked flat in a radial “throw-down” pattern from a common epicenter where a large, roughly circular stand of trees had been scorched and, instead of being toppled, were completely stripped of their branches. The one thing he expected to find, though, was elusive. There was no crater, no evidence of the meteor impact that was assumed to be the cause. That is just the beginning of the mystery that has come to be known as the Tunguska Event.

It has been 106 years since the Tunguska Event. Hundreds of researchers and scores of expeditions have scoured the area for evidence, and dozens of hypotheses have been bandied about. However, even after the expenditure of all that time, sweat, and resources, the only thing that we can say for sure is that something seems to have exploded over the hilly marshes of east-central Siberia with a force of around 15 megatons. That is roughly equivalent to the Castle Bravo nuclear bomb test on Bikini Atoll in 1954,which was the most powerful thermonuclear device ever detonated by the United States. A definitive explanation for the Tunguska Event has become something of a Holy Grail of science and, as we’ll see, that explanation has proved just as elusive as that fabled artifact.

Anyone who tells you that they “know” what happened over the Tunguska taiga is either unjustifiably confident in a particular set of data, ignorant of the preponderance of conflicting evidence, or just plain lying to sound like they’re smart. There are two predominating hypotheses about what the “impactor” was (even though there’s no conclusive evidence that anything hit the ground, the object is referred to as such): a comet or a meteor. Let’s take a look at those two and see why we can’t say for sure that it was either one.

British astronomer F.J.W. Whipple first proposed the idea that the impactor was a comet in 1930, which gained great popularity with Soviet researchers by the 1960s. The body of a comet, which is primarily a collection of ice and dust, surely would have exploded completely before impacting the ground due to ram pressure forces of the shock-front that would build up as the object plunged through the atmosphere. That would explain the lack of a crater or fragments. Based on the prima facie evidence, it sounds good so far.

It was later suggested by Slovak astronomer Ľubor Kresák that the impactor could have been a dislodged piece of Encke’s Comet, a short-period comet with an orbital cycle of 3.3 years and the cause of the periodic Beta Taurid meteor shower, which was at its peak when Tunguska Event happened. Wow, this comet idea keeps getting better and better, right? But hold on, there’s a big problem that has to be addressed. Models of the trajectory of the impactor indicate that it entered the atmosphere at a relatively shallow 300 relative to the horizon, and that the explosion occurred 5-10 kilometers above the surface. Astronomer Zdeněk Sekanina published a refutation of the comet hypothesis in 1984, wherein he posits that the icy body of a comet, entering the atmosphere at such a shallow angle, simply could not have survived long enough to plunge into the lower atmosphere. He’s not alone in this assertion, and the observational evidence from the fragmented comet Shoemaker–Levy 9’s impact into Jupiter in 1994 indicated that they did indeed explode very high in the Jovian atmosphere. There are some other issues with the comet hypothesis, but this is the big one that renders all the others moot, so it looks like the comet hypothesis is a bust.

So it looks like we’re left with an asteroid. I mean, we’ve pretty much eliminated the other most-likely culprit, right? But the meteor hypothesis has some pretty big gaps in the evidence as well, starting with the lack of an impact crater. That’s not to say that there haven’t been some researchers who have claimed to have actually found that “smoking gun” though.Professor Giuseppe Longo, et al. from the University of Bologna in Italy identified nearby Lake Cheko as the crater in 2007. They cite the soundings of the lake floor which revealed a “conical shape” that would be expected in an impact crater, as well as evidence from sediment core samples which indicate (only to them, apparently) that the sediment below one meter is chaotic and indicative of a catastrophic impact event. The fact that this research was published first on the university’s website and later in the journal Terra Nova (which is not known for rigorous peer review) should tell you something about the scientific validity of their claims.

A little over a year later, G.S. Collins, et al. published a refutation in that same journal which points out the lack of any damage to the trees surrounding the lake, which should have been knocked flat if there had been an impact, the lack of any impactor material being found at the site, and the lack of a characteristic raised rim, among others. The biggest problem with the Italian hypothesis though is that records of eye-witness accounts of the event indicate that Lake Cheko existed before 1908, and was in fact a well-known watering hole and landmark used by herdsmen in the area.

Most recently,researchers from the Russian Troitsk Institute identified the so-called “Suslov crater” as the impact site in 2010, but this too has been deemed spurious at best by other researchers of the Tunguska event, due largely to reasons similar to those posited by G.S. Collins. Additionally, this site was originally investigated by our friend Leonid Kulik, who “after a laborious exercise in draining one of these bogs (the so-called “Suslov’s crater”, 32 metres [105 ft] in diameter), he found there was an old stump on the bottom, ruling out the possibility that it was a meteoric crater.” I don’t know if you, dear reader, have ever been to Meteor Crater in Arizona, but I have, and it is one big damn hole in the ground, much larger than Lake Cheko or the Suslov “crater.” The impactor that made it was a ferrous meteor significantly smaller than the proposed size of the Tunguska impactor, and there are quite a few sizeable fragments of it that have been found.

There have been other proposed causes of the Tunguska Event, including a botched “death ray” experiment supposedly conducted by Nicola Tesla and an exploding alien spacecraft, an idea taken by a few journalists as a real hypothesis from Russian UFOlogist and author Alexander Kazantsev’s fiction short stories “A Visitor From Outer Space” and “The Explosion”, and his fiction novel “Burning Island.”

One rather curious hypothesis was proposed, mainly on a lark, by University of Texas professor Albert Jackson and then-PhD candidate Mike Ryan in 1973. They asked the question “What if the Tunguska impactor was actually a primordial micro-black hole (PBH),and what would the properties of an impact by such an object be?” Their idea was that a substellar black hole, created during the Inflationary Epoch following the Big Bang and having a mass of only a quintillion tons or so, might be able to cause the same effects of the Tunguska Event, while explaining the lack of an impact crater. So they did the math, wrote it up, and submitted their paper to the journal Nature, never expecting it to actually be published. Well it did get published in September of 1973, and Jackson and Ryan found themselves in the middle of a furious storm of criticism from the rest of the astronomy world. The main argument against their idea was the fact that such a tiny object, about the size of an atom, would have passed right through the Earth and shot out the other side in the Atlantic Ocean somewhere near the Azores Islands, causing similar effects in its departure as witnessed upon its arrival. No such event was recorded, and Jackson and Ryan quickly abandoned the idea altogether.

I could write a great deal more about the myriad proposals and rebuttals regarding the Tunguska Event, but this is already running long, so I’ll leave you with these parting thoughts. The Tunguska Event was unique in recorded human history, an extraordinary occurrence that requires an extraordinary explanation, which in turn requires extraordinary evidence. Therein lies the strength, and even beauty, of skeptical thought. It does not settle for appeals to authority, or to tradition, or to what makes us feel safe and comforted. As Dr. Carl Sagan once said about this very incident in the “Heaven and Hell” episode of the original Cosmos series, “The critical issue is the quality of the purported evidence, rigorously and skeptically scrutinized — not what sounds plausible…” As for the Tunguska Event, there are a great many claims of explanation coming from a great many scientists with literally centuries worth of academic and research experience between them, and yet thus far there is no broad consensus on exactly what happened because none of the crucial evidence has been able to withstand rigorous, skeptical scrutiny. And this is a good thing, because it is a far better world that takes the long path to truth rather than the short path to speculation.