In 1993 NASA scientists Christopher Chyba and Kevin Zahnle and their colleague Paul Thomas from the University of Wisconsin gave the asteroid theory for the Tunguska explosion new weight and rigour.
Their computer simulations to explain the pattern of trees blown down showed that the explosion released about 15 megatons of energy in the atmosphere at an altitude of about 8 kilometres, but did not crater the Earth's surface. They then examined the entry of three classes of asteroids (stony, iron and carbonaceous) and two classes of comets (short-period and long-period) starting with 15 megatons of kinetic energy. Their simulation showed that cometary nuclei and carbonaceous asteroids explode far too high in the atmosphere to account for the blast, and iron asteroids hardly fragment and so hit the ground at high speed. Only the stony asteroids would create a Tunguska-like explosion.
In their innovative analysis, the researchers included the effects of aerodynamic forces on an asteroid: as the asteroid moves deeper into the atmosphere, atmospheric drag on it increases. When the drag exceeds the asteroid's strength, the asteroid crumbles and begins to flatten like a pancake. The increasing surface of the fragmented asteroid experiences a sharper rise in drag. Increasing drag slows down the asteroid, which in turn spreads it even more. At the same time, atmospheric density rises with decreasing altitude, creating more drag. These increasing forces stop the asteroid abruptly in the atmosphere. The asteroid explodes like a bomb, and within a fraction of a second megatons of kinetic energy is converted into high temperatures and high pressure. The asteroid vaporises. A shock wave races outwards.
Taking account of these effects, the researchers calculated that a stony asteroid about 30 metres in diameter and moving at 54,000 kilometres per hour would explode at a height of about 8 kilometres, the same height at which the Tunguska body apparently exploded. A smaller one would have exploded much higher, and a larger one would have created an impact crater.
The weak, fast-moving, easily crumbled comets do not penetrate the atmosphere deeply and are unlikely to approach the altitude of the Tunguska explosion. 'Even if the comets had strengths comparable to stony asteroids, they still could not fit the Tunguska observation', the researchers maintained. For example, if a 15-megaton incident comet is assigned an anomalously low asteroidlike speed of 54,000 kilometres per hour, it would completely exhaust itself before it reached an altitude of about 16 kilometres. It would have also caused far less surface destruction.
The measurements of Halley's comet by the Giotto spacecraft in 1986 showed that it had a density of between 0.6 and 1 gram per cubic centimetre. In their simulation the Chyba trio used a density of 1, but said that values as low as 0.3 might be possible. Such lower-density objects would airburst even higher. 'Tunguska was probably a fairly strong, dense, asteroid-like object, but probably not as strong or dense as iron', the researchers concluded in their report in Nature. 'Carbonaceous asteroids and especially comets are unlikely candidates for the Tunguska object.' However, their simulation did not completely rule out an unusually fast iron asteroid or a very strong carbonaceous asteroid.
Chyba's team also supported Turco's 1983 observation that bright nights were caused by noctilucent clouds. They said that the air heated by the explosion injected enough water into the stratosphere for noctilu-cent clouds to be produced.
Further support for the asteroid theory came from Jack Hills and Patrick Goda of the Los Alamos National Laboratory. In their general study of fragmentation of asteroids, they found that a stony asteroid must be greater than 200 metres across, much larger than the Tunguska object, to hit the ground. They agreed that the Tunguska evidence ruled out a comet, and that furthermore there is no question but that the object was a stony asteroid and not an iron asteroid. Comets start fires more easily than asteroids, but the Tunguska asteroid generated enough heat to ignite pine forests. 'However, the blast wave from an impacting asteroid goes beyond the radius in which the fire starts', they said. 'The blast wave tends to blow out the fire, so it is likely that the impact will char the forest (as in Tunguska), but the impact will not produce a sustained fire.'
Henry J. Melosh of the University of Arizona in Tucson, commenting on the Chyba team's research, said that they had 'wrapped up' the most believable explanation for the Tunguska event. 'Substantial progress has thus been made in reducing the Tunguska explosion from the realm of the near-miraculous to a natural, although rare, occurrence', he said.
Chyba declared in Astronomy in December 1993: 'According to this picture, Tunguska goes from an exotic event demanding the invocation of UFOs or black holes to a completely normal, predictable outcome for a stony asteroid entering the atmosphere with a typical velocity. This understanding is important, for it allows us to assess the contemporary hazard posed by small asteroids and comets colliding with Earth.'
Fifteen years after proposing his original asteroid theory, in 1998 Zdenek Sekanina revisited his earlier analysis and concluded that the interpretation of the Tunguska event as a fall of a small asteroid is 'not only plausible, but virtually certain'. However, he said that one issue where he was not prepared to take sides was whether the object was a stony or a carbonaceous asteroid.
In his new analysis, Sekanina noted that the collision of comet Shoemaker-Levy 9 with Jupiter in 1994 showed that the mass of a comet that enters the atmosphere of a planet, such as Earth or Jupiter, apparently needs to be more than 100 million tonnes in order to trigger a powerful explosion at the end of its journey. Smaller comets are likely to dissipate their mass in atmospheric flight and would end up with no appreciable mass at low altitudes. By contrast, the study of a number of fireballs pointed out that initial masses as small as 10 kilograms exhibited terminal flares. Various studies of the interrelation between a fireball's altitude, its pre-explosion speed, and the aerodynamic pressure at which it explodes corroborate the Tunguska fireball's pre-explosion mass of 1 million tonnes.
The question of a cometary hypothesis, Sekanina implied, 'now becomes mute'. He also noted that the Tunguska fireball was dwarfed by the comet Shoemaker-Levy 9 when measured by the amount of released energy. 'On the other hand, the Tunguska event directly involves the issue of a threat to our civilization', he said.
Sekanina concluded his analysis by summarising eight points that made his asteroid theory broad-based and not based merely on eyewitness reports, as claimed by his critics: (1) its mid-air explosion is similar to the terminal flares of fireballs observed photographically; (2) the pressure at the point of explosion, estimated at about 200 times normal atmospheric pressure, is consistent with a value that is expected from similar fireballs; if the object were a comet, the pressure would be about 2,000 times atmospheric pressure, entirely out of a plausible range for a fragile comet; (3) the pre-explosion velocity of 36,000 kilometres per hour is the same as determined by seismic observations and a laboratory simulation of the uprooted forest; (4) this velocity range rules out a comet-like orbit; (5) existing limited evidence on the event is inconsistent with a fragmentation pattern typical for comets; (6) a comet of such a magnitude would have been extremely rare, perhaps 10 to 100 times more so than an asteroid of the same explosion energy; (7) the limited evidence on the object's orbit is consistent with the orbits of the Earth-crossing asteroids, but not with the orbits of short-period comets; and (8) this orbital information is particularly unfavourable to the hypothesis that associates the object with Encke's comet.
In 2001 a team of Italian scientists looked at the Tunguska object from a different angle, based on an idea of the late Paolo Farinella (1953-2000). Using data from detailed analysis of all the available scientific literature, including unpublished eye-witness accounts that have never been translated from the Russian, and the survey of directions of more than 60,000 flattened trees, the Italian scientists plotted a series of possible orbits for the object. Of the 886 valid orbits that they calculated, 83 per cent of them were asteroid orbits, with only 17 per cent being orbits that are associated with comets.
This overwhelming data showed that the Tunguska object was indeed an asteroid. But if it was an asteroid why did it break up completely? According to one of the team, Luigi Foschini of the University of Trieste: 'Possibly because the object was like asteroid Mathilde, which was photographed by the passing Near-Shoemaker spaceprobe in 1997. Mathilde is a rubble pile with a density very close to that of water. This would mean it could explode and fragment in the atmosphere with only the shock wave reaching the ground.'
As the 25th anniversary of his 1983 paper - and the 100th anniversary of the Tunguska event - approaches, Sekanina reflects upon his ground-breaking conclusion that the Tunguska object was an asteroid:
I have not changed my views on the subject in the least, still considering the Tunguska's asteroidal nature as virtually certain . My involvement with the subject was in fact quite peculiar. I would not have begun my research on this subject in the early 1980s, if it were not for the fact that the papers published through the 1970s - all strongly pro-
comet - had attracted my attention. I am a comet physicist and I felt that if it was a piece of a comet, one should be able to learn something on the properties of cometary nuclei from the event. Thus, I began to study the topic believing that this was indeed a comet: otherwise I would have never got involved and I surely cannot be accused of initiating my research on this subject with a bias against the Tunguska's cometary origin.
My conclusion was simply the result of my findings: the more work I did, the more obvious it became to me that this was not a comet (just take the huge dynamic pressures to withstand!). By the time I was convinced completely, I invested so much time that I felt it would be a shame to drop the subject, even though it became essentially irrelevant to my own scientific interests (I never found asteroids to be much fun). And this was the sole reason behind my writing the 1983 paper. The rest is history.
He adds: 'Of course, the cometary-origin hypothesis still has its old proponents such as Vitalii Bronshten, but no new admirers.' And, of course, the major contribution of Russian scientists in the development of the comet theory is well recorded in the history of the enigma known as Tunguska.
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