Posted on July 23, 2004

Soapbox Seminar #2

Star Wars

Scientists have always understood that
[the Tunguska object] was a comet or asteroid.

— Tom Gehrels, principal investigator for Operation Spacewatch

All assumptions must be critically examined;
arguments from authority are worthless.

— Carl Sagan, Cosmos, Chapter XIII: Who Speaks for Earth?

When we left off last time, Tunguska studies had just taken a giant step forward with help from a science-fiction story. Once it was corroborated by Zotkin’s scale-model simulations, Kazantsev’s notion that the Tunguska Object might have exploded in mid-air had pretty much solved the no-crater problem. Meteors and comets were both back on the table.

It was the first real breakthrough in Tunguska research — the one everybody’d been waiting for. Unfortunately, it was also the last.

You’d think all there was left to do was pick up the pieces: Find enough in the way of evidence to decide between comet and meteor, declare victory, and go home. Didn’t happen that way.

Then as now, the most painstaking fieldwork on site, the most meticulous analysis back in the lab, never succeeded in turning up more than a few trace elements in the soils, peats, and tree resins of Tunguska. And there was, and is, no way to know for certain those traces weren’t just by-products of the flash fire that incinerated thousands of square miles of taiga the morning of the Event, or a specially heavy concentration of the normal background infall of meteoric dust and debris that rains down all over the earth, day in and day out.

It’s for sure nobody’s ever found a chunk of stuff big enough to see with the naked eye, much less heft in your hand — and that’s from an object that, going by the blast wave energy, supposably weighed anywhere from fifty to a hundred thousand tons!

No, regardless of how long you paw over it, there just isn’t enough evidence on the ground to tell for sure if the object that exploded high over the taiga back in 1908 was a meteorite, a comet, or something else entirely.

So, what do you do in a situation like that, where there’s just no telling one way or the other? How can folks like Tom Gehrels be so sure it “was a comet or asteroid” when, for all the hard evidence there is, it could just as well have just as well been Aleksandr Kazantsev’s little green men?

Well, wouldn’t you know it, science has got a rule for what to do in case of a toss-up. It’s called Ockham’s Razor[1], and what it says is: “do not multiply causes beyond necessity.” Or, to put that in plain, ordinary Texan: if all else fails, pick the simplest explanation that fits the facts — where “simplest” can mean, ’mongst other things, “most commonplace.”

And let’s face it, folks, comets and asteroids are a lot more commonplace than teensy black holes (which have never been observed), not to mention way stranger stuff like antimatter or “mirror matter” or even UFOs. In other words, in the absence of any evidence whatsoever, you’re best off going with a comet or an asteroid.

So, are we done yet? Can we all go home now? Not quite, because it’s like Albert Einstein once said: “Everything should be made as simple as possible, but not simpler.” No matter how simple an explanation is, it’s not going to be worth the paper it’s written on if it doesn’t fit the facts. And it so happens that, while there isn’t any hard evidence for either of the two leading contenders, there is a certain amount against them.

And that right there changes the groundrules. If you’ve got any evidence pro or con, Ockham’s Razor goes right out the window along with the strap (for more on Ockham’s Razor, click here). At that point, like Carl Sagan used to say:

The critical issue is the quality of the purported evidence, rigorously and skeptically scrutinized — not what sounds plausible ...

Now, Carl was talking about really “extraordinary claims” (read: UFOs) when he said that. But just because meteorites and comets happen to sound less extraordinary — more plausible — than flying saucers, doesn’t mean we let those theories off the hook altogether. Somewhere along the way, even the most commonplace explanations have got to stand up to that same rigorous and skeptical scrutiny. Sure, maybe you give them more of the benefit of the doubt, but only as long as they aren’t flying in the face of the facts.

So, just what are the facts? The facts that’ve kept the comet-vs.-meteorite wars raging down through the decades.

Well, lacking any knock-down physical evidence one way or the other, the two camps have been throwing everything else into the pot, from Zdenek Sekanina’s 1983 distillation of everything written on Tunguska up to that point, to Mark Boslough’s 1994 computer simulations of blast patterns, to Luigi Foschini’s 2001 declaration of a “solution” based on orbit probabilities ... the list goes on.

And, while nobody’s managed to come up with a proof of their own position, be it pro-meteorite or pro-comet, they have managed to put some dents in the other side’s theory along the way. Here’s a sampling:

• The absence of physical evidence on the ground has itself been taken as evidence against the stony-meteorite hypothesis by V. Bronshten. According to him,

  [T]he complete lack of stony fragments over the area affected by the shock waves generated either by the meteorite itself or by its explosion is in itself sufficient to reject the hypothesis that the Tunguska body is an asteroid...

  [N]either the radiation from the fireball nor the interaction with the air flow after the explosion can evaporate stony fragments of the hypothetical body of asteroidal nature. Therefore, this body could not be stony. Only the icy nucleus of a comet could explode without leaving large fragments.”

• But Sekanina turns that argument on its head: The “icy nucleus” of a comet, he says, is way too fragile to have survived a plunge into the lower atmosphere — the heat of atmospheric friction should have vaporized it at an altitude of 200 kilometers, much too high to cause the observed blast effects on the ground.

• Those blast effects raise another problem for Academician Nikolai V. Vasil’ev of Tomsk University. According to him, careful analysis of the treefall pattern hints that some part of the object continued on course after the first, multi-megaton air-burst. That’s really hard to square with the fragile nature of a cometary nucleus, they’re not much denser than water, after all. And a meteorite doesn’t come off looking much better — because if a piece of rock or iron made it through the main explosion intact, then where is it?

• Then, there’s the geomagnetic storm that raged for four hours after the Event. There are problems seeing how a comet could have stirred that up. As for the magnetic anomalies L. Weber recorded at Kiel University on the three nights leading up to the impact, it’s hard to see how a comet or an asteroid could have generated those — not from a couple million miles out in space.

I could go on, but you get the idea. Maybe any one or two of these disproofs could be shrugged off. But taken all together, they were enough to make that grand old man of Tunguska studies, Academician Nikolai Vasil’ev, throw up his hands in despair. His 1992 article on “Paradoxes of the Tunguska Meteorite Problem” winds up like this:

Since a final resolution of the question of the Tunguska phenomenon’s nature has yet to be found, and since it must be acknowledged that the perennial attempts to interpret it within the framework of the classical paradigm have so far brought no decisive success, it seems expedient to examine and test alternative ways of explaining it.

In other words, if your pet theories aren’t either of them panning out, why not try something different? Sounds like pretty good advice, doesn’t it?

But it fell on deaf ears. Seems like alternative explanations are just what neither side — pro-asteroid or pro-comet — wanted to hear. And the reason for that isn’t quite what you’d expect.

Before I get into that, though, let me say right up front that the folks at Operation Spacewatch are doing good work, trying to spot the asteroids and comets that might someday hit us — and spot them earlier enough for us to do something about it. Trying to keep us, in other words, from going the way of the dinosaurs. And that with only a handful of dedicated staffers and a funding level that wouldn’t pay NASA’s electric bill for a week!

Matter of fact, long’s I’m perched up here on my soapbox, I’m going to take the opportunity to ask you all to send them a few bucks. It’s easy enough to do — just click here.

Your great-grandchildren will thank you.

On the other hand, doing good work’s no excuse for, well, doing not-so-good science — for letting your commitment overrule your objectivity. And, much as it pains me to say so, I get the feeling that explains a lot of the present-day Tunguska debate — not the comet vs. asteroid part, the comet and asteroid vs. everything else part.

Because, in some strange way, it’s become what you might call “politically incorrect” to claim that Tunguska could’ve been caused by anything other than a meteorite or a comet.

Somewhere along the way, the Tunguska Event’s become a poster child for the notion that there are giant space rocks out there just waiting to clobber us. Seems it kind of brings that whole idea into focus, makes it more believable. To hear NASA’s Dave Morrison tell it [2], “if it hadn’t been for Tunguska, we might not be aware today that there’s an impact hazard at all.”

Well, all’s I can say to that is, Dave ought to get out more. Out to where he can catch a glimpse of the crater-pocked face of the full moon on a clear night, for instance. That’d raise his impact-hazard consciousness for sure.

Or, if that’s not good enough, how about Shoemaker-Levy? Here, a solid stream of comets goes and slams into Jupiter. Kicks up an impact plume the size of the whole world — with the whole world watching. And we still don’t get it?

But, no, we’ve just gotta have Tunguska too. As a comet or a meteorite, it’s our planetary wake-up call. If, Lord help us, it turned out to be anything else, we’d all just hit the snooze button and go back to sleep. Or so the theory goes.

Think I’m kidding, don’t you? After all, these are scientists we’re talking about. The only thing that’s supposed to matter to them is, like Carl says, “the quality of the purported evidence, rigorously and skeptically scrutinized.” Not whether some theory or other looks to be plausible, or popular, or profitable, or even politically correct enough to save the world. Right?

Well, it sure doesn’t sound that way sometimes. Give a listen to what rocket scientist James Oberg has to say about the Tunguska UFO “theorists”:

[T]o defend against such future cosmic bombs [i.e., near-earth objects which might hit us] it is first necessary to recognize them for what they are. Here the spaceship theory and the distortions, omissions, and fabrications of its proponents (both well-meaning and otherwise) remain a major obstacle, and a major danger.

The fictional science and pseudoscience of the “Tunguska spaceship” should be discredited and dismissed as quickly as possible so that we can get on with defending the earth against future “Tunguska comets.”

Now, I’ll be the first to agree that anyone who plays as fast and loose with the facts as the flying saucer crowd does deserves to be “discredited and dismissed.” But does that make them “a major obstacle, and a major danger”? C’mon, Jim — lighten up!

It’s not the words so much, it’s the tone. I don’t know about you, but that doesn’t sound like a man of science talking to me. Sounds more like a man with a cause!

A cause that tells us we’ve got to keep a lid on any alternative explanations — maybe with one of those good old arguments from authority, like Tom Gehrels’ “scientists have always understood it was a comet or asteroid.”

And that right there’s the problem: Because anytime you start using science in the service of a cause — even the best, most worthy cause — it stops being science. Because science has already got a cause. It’s called the truth.

And the truth is that, much as I may not like them having an axe to grind, that doesn’t make the comet/meteorite theorists wrong. The thing of it is, it doesn’t make them right either. And yet, over the years, all that arguing from authority, all that smug self-confidence has pretty much driven any competing hypotheses off the field.

Because, like the characters in the funny papers, the meteorightists and the cometarians could always take time off from beating on one another to join hands and beat on anybody else who came along with a new idea.

As two guys name of Jackson and Ryan were about to find out, back in 1973.

[Notes and Further Reading]

[1] After William of Ockham, the 14th century English philosopher who thought it up. [Return to text]

[2] As quoted in “The Day The Earth Got Hit,” Cambridge-Conference Digest, 14 November 1997, at: http://abob.libs.uga.edu/bobk/ccc/cc111497.html . [Return to text]

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V. Bronshten, “On the nature of the Tunguska meteorite,” Astronomy and Astrophysics, 2000, No. 359, pp. 777–779.

P. Farinella, L. Foschini, C. Froeschle, R. Gomczi, T. J. Jopek, G. Longo, and P. Michel, “Probable asteroidal origin of the Tunguska Cosmic Body,” Astronomy and Astrophysics, 2001.

Luigi Foschini, “A Solution for the Tunguska Event,” astro-ph/9808312v2, 16 December 2001 at: http://www.arxiv.org.

Tom Gehrels, “Collisions with Comets and Asteroids,” Scientific American, March 1996, pp. 54-59.

James Oberg, UFOs and Outer Space Mysteries, Donning Press, 1982. (Chapter Seven, “Tunguska Echoes” is available at http://www.jamesoberg.com/ufo/tungus.html.)

Carl Sagan, Cosmos, Random House, 1980.

Zdenek Sekanina, “The Tunguska event: no cometary signature in evidence,” Astronomical Journal, 1983, vol. 88, No. 1, pp. 1382-1414.

Zdenek Sekanina, “Evidence for Asteroidal Origin of the Tunguska Object,” Planetary and Space Science, 1998, vol. 46, No. 2/3, pp. 191-204.

Nikolai V. Vasil’ev, “Paradoxes of the Tunguska Meteorite Problem,” Proceedings of the Higher Educational Institutes, No. 3 “Physics,“ 1992, pp. 111-117, at: http://www.tunguska.ru/obzor/stat/.

L. Weber, “On the lightshow in the night sky at the beginning of July,” Astronomische Nachrichten, 1908, Vol. 178, No. 4262, pp. 239-40:

David Whitehouse, “Mystery space blast ‘solved’,” BBC Sci/Tech, Tuesday, 30 October, 2001, 18:13 GMT, at: http://news.bbc.co.uk/hi/english/sci/tech/newsid_1628000/1628806.stm

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