Tunguska - Whodunit?

The Tunguska Explosion took place over 101 years ago, but the controversy on what caused it is still very much alive and kicking - which is a good thing, because if everyone were of the same opinion, it would be boring and progress would come grinding to a halt.

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Leaving aside the flamboyant, exotic theories, we are left with the most likely scenario that an object with a diameter of between 40 and 80 meters came hurtling out of space, entered the atmosphere at a fairly steep angle and exploded at an altitude of about 8 km above a remote region in Siberia on the Upper Stony Tunguska river. This released an energy equivalent to about 10 to 15 megatons of TNT (the yield of a fairly large thermonuclear device, though both Soviets and Americans have dropped larger clangers than that) with disastrous consequences for the immediate surroundings of the epicentre - luckily largely unpopulated.

It is well-known that massive objects smaller than about 100 meters of diameter will disintegrate as they encounter denser atmospheric layers while still travelling at hypersonic velocities. Metallic bodies constitute an exception; they are denser and more solid and stand a fair chance of reaching the surface still largely in one piece.

No controversy so far. However, the altitude at which the explosive disintegration will take place depends on numerous parameters and is difficult to quantify. These parameters are:

1. Entry velocity: There can be significant variation here. Earth-crossing asteroids typically enter at around 15 km/s, whereas comets, especially long-periodic ones, can reach 50 km/s. In the worst case, a parabolic, retrograde comet might enter at staggering 72 km/s. The kinetic energy of the entering body (this is then shed in form of heat during the explosion) grows with the square of the velocity, so does the dynamic pressure, which defines the aerodynamic forces. The heat flux will even follow a cubic function of the speed. Conversely: The faster the object flies, the shorter the time during which heat and pressure wreak their destructive effect.
2. Entry angle: A steeper entry means higher peak loads. A shallow entry implies lower peak loads but longer exposure duration. Therefore, the total, "integrated" loads typically are higher for a shallow entry than for a steep one. Therefore, even though the peak loads remain fairly moderate, a body performing a shallow entry may sill be destroyed by the accumulated thermal effects. This is precisely the problem facing designers of spacecraft that shall enter a planetary atmosphere, such as manned capsules. 
3. Object size: The larger the object, the closer to the surface it gets. Large bodies make it to the surface and leave a crater. Small bodies explode or are vaporized while still high up. Between these two there are those sizes that lead to a disintegration altitude of around 8 km, which was computed to be consistent with the damage patterns and other features observed at the Tunguska site.
4. Composition and Homogeneity: The higher a body's yield strength, density and thermal capacity, the more deeply in the atmosphere will it penetrate.  Metallic asteroids (iron/nickel) mostly reach the ground. Silicate (stony) asteroids are less strong; with the diameters assumed here they will disintegrate in several kilometers altitude. Carbonaceous bodies disintegrate somewhat higher up and comets, with their high ice content, even higher. This looks deceptively straightforward. Things get complicated when you consider that minor bodies need not belong clearly to one class or  the other. They can be composed of a mix of different materials and also contain empty spaces like a flying rubble pile. There is no reason whatsoever to assume homogeneous interior compositions.

A recent scientific paper claims to offer proof that it must have been a small cometary nucleus. The water released in the disintegration of the comet's ice is interpreted as the source of noctilucent clouds observed over the northern hemisphere. A somewhat speculative transport mechanism is postulated to explain how this water got back to high altitudes and the observed wide spatial distribution.

Then, there is this widely cited, older paper: Chyba et al, Nature 361, 1993. Its authors apply extensive calculations applying models in which some of the above parameters are varied and claim that the Tunguska event must have been caused by a stony asteroid. A metallic body would have produced a crater rather than an airburst, while a carbonaceous or icy body would not have reached 8 km of altitude but would have disintegrated much higher up.

However, it appears that Chyba et al. assumed a high degree of homogeneity in the interior composition. Also, they seem to have assumed that if it was an asteroid, it would have to have originated in the population of near-Earth asteroids. This constrains the entry velocity to around 15 km/s. Obviously, both assumptions impinge on the obtained results.

If you accept the assumptions, the paper and its conclusions are consistent and clear. So - case closed, end of the matter? Well, therein lies the rub; it's the assumptions that should be questioned.

Strange Coincidences Galore

Nothing of what follows is my own idea; the suggestions all come from experts on minor planets like Dr. Duncan Steel, Vice President of the Spaceguard Foundation. All I did is to verify the consistency of the conjecture with the data.

The possibility of a connection between the annual beta taurid meteorite shower and the Tunguska event has been pointed out already decades ago, e.g., by Arthur C Clarke. The beta taurids peak at the end of June, and the Tunguska explosion occurred on June 30, 1908. The beta taurids probably split off from the taurids; both are thought to be related to comet 2P/Encke.

The above figure shows comet Encke's orbit in the solar system. This comet is remarkable in many respects. Its orbital period is very short, only 3.3 years, and the perihelion is very low. Encke gets closer to the Sun than Mercury does, so enormous thermal loads are incurred during each perihelion pass. It is not clear how Encke got into this orbit, though, frankly, I find it less surprising that the authors of this paper apparently do. Certainly, this orbit is subject to significant perturbations, which would be consistent with massive past fragmentation events. The original nucleus would have been much larger than the 3-5 km diameter remaining today. The balance would be due to outgassing but also to fragmentation, leaving a stream of objects large and small trailing the comet's orbit and creating the taurids as the debris encounters the Earth.   

The next image shows a close-up of the inner solar system. The comet, like the planets, orbits the Sun counter-clockwise. There are two intersections with the Earth orbit, one on the way down to the perihelion, on the upper right hand. The Earth passes this point in late October/early November, the time of the main taurid meteor shower. The other intersection is passed on the way back up from the perihelion. This happens in late June/early July. The comet itself does not get dangerously close to the Earth at either of these locations - at least not in the recent past and near future. However, the orbits of its retinue of debris have been subject to various changes due to perturbations. The debris envelopes the comet's orbit like a tunnel. Individual debris objects do not necessarily have to be close to the nucleus, but over the course of time can have drifted to very distant points of their similar orbits. The debris does intersect the Earth, which is why we see the taurids in October/November and the beta-taurids in June/July. One can see that the beta taurid meteor showers have to occur during daytime because they are caused by objects that are on trajectories that lead out, away from the sun.

I have computed the expected entry conditions based on the assumption that the debris moves on an orbit similar to that of comet 2P/Encke. The results are shown in this diagram (Click on it to obtain a larger view). Admittedly, the diagram may appear somewhat difficult to comprehend at a first glance, but it's quite straightforward, really, and it contains a wealth of salient information. On the horizontal axis is the local solar time, on the vertical axis the geographical latitude. I plotted the possible entry points for entry angles of -15, -30, -45 and -60 degrees. You can assume that the point of disintegration is close to the point of entry, especially for steep entry, so let's not bother with any of those fine distinctions here.

The diagram also shows the terminator, the radiant (the apparent point in the sky from where the streaks in a meteor shower appear to emanate) and the position of the Sun. For the Tunguska case, we're only interested in the location in that diagram at a latitude of 61 degrees North and a local time of around 7:15 - this denotes location and time of the Tunguska event. The diagram then shows that if the object that exploded over Tunguska is related to the beta taurids, it must have entered the atmosphere at an angle of around -45 degrees. A shallow entry would have led to entry at an earlier time of the day, a steeper entry to a time that is too late - and also, the latitude of 61 degrees would not have been reached. Furthermore, the entry velocity is in excess of 34 km/s, not 15, as assumed rather arbitrarily in the "Chyba et al." paper.

The direction of flight would have been more or less due West, meaning that to observers, the object would appear to arrive from the East. The radiant is at a declination of 21 degrees exactly in the constellation Taurus, as would be expected for "taurids". All of the above follows directly from an application of the known orbital layout of comet Encke and the beta taurids. If I'm wrong here, please disprove me. What's more, my results are entirely consistent with the observations.

Just what am I getting at here?

Obviously nothing I said up to now constitutes solid proof that a beta taurid object caused the Tunguska explosion. It is possible that an Earth-crossing asteroid chose exactly the combination of date, time and direction of flight that would make it appear like a large object swimming along in the stream of debris causing the beta taurids. Is that a likely scenario? No, but unlikely does not mean impossible. As the saying goes: "If it looks like a duck, quacks like a duck and walks like a duck, it might still be a dragon disguised as a duck". (And the usual corollary goes "... but it ain't very likely.")

Still - I do think that in view of the strong indications we have here it would make sense to at least entertain the notion that the Tunguska explosion was due to a fragment of comet 2P/Encke. Can we really exclude the possibility that large fragments of a comet nucleus may harbour large chunks (with diameters of a few dozens of meters) of silicate rock or carbonaceous chondrite? Does it really constitute an "exotic hypothesis" to assume that  a comet - especially an unorthodox one like 2P/Encke -  may also be composed of a relatively small fraction of rock? Is it really implausible that such a rock would be protected by the ablation of surrounding ice to reach an altitude of 8 kilometers, where it disintegrates? By the way, this theory would explain the formation of noctilucent clouds in the aftermath even without requiring speculative transport mechanisms: The entering body would have shed ice during entry, not during the explosion.

When the "Chyba et al." paper was published in 1993, we knew next to nothing about the interior composition of cometary nuclei. I mean "to know" in the sense of verifiable knowledge based on observations, not just vague theories. The only close-up observations done up to then were those made by the Giotto spacecraft during a 600 km flyby of comet 1P/Halley in1986. This surprised scientists by proving that volatile material was ejected in discrete jets, not over the entire surface. After that, we had further missions: Deep Space 1, Stardust and Deep Impact. Still, our knowledge in this respect is still very patchy and far from complete. Otherwise, why would the European Space Agency have launched the Rosetta Mission that will perform a long-term observation of an active nucleus for the first time ever?

So we shouldn't disregard plausible scenarii if we do not have a sufficiently solid scientific basis to allow us to make an informed decision.

Further Information

C.F.Chyba, P.J. Thomas, K.J. Zahnle: The Tunguska Explosion: Atmospheric Disruption of a Stony Asteroid, Nature 361, pp. 40-44, January 1993 (Abstract)

H.F. Levinson, D. Terrell, P.A. Wiegert, L. Dones, M.J. Duncan: On the Origin of the Unusual Orbit of Comet 2P/Encke, Icarus 2005.12.016

  culprit conspiracy

 

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