In the witness box

A tiny comet with a long tail. The available 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. The collision of comet Shoemaker-Levy 9 with Jupiter has shown us that the mass of a comet that enters a planet's atmosphere needs to be more than 100 million tonnes in order to trigger a powerful explosion at the end of its journey. The Tunguska fireball's pre-explosion mass is believed to be 1 million tonnes. It is inconceivable that such a small comet could survive the intense atmospheric pressure on its journey to the Siberian sky. On the other hand, an asteroid could survive such an arduous journey. It has to be an asteroid, a stony asteroid.

A stony asteroid. The facts that prove that the colliding object was a comet include its unusually loose structure, which led to its break-up in the atmosphere; its dust tail, pointing away from the Sun, which caused unusual sunsets; and the nature of its orbit. The Sun's glare prevented any sighting of the comet before it hit Earth, because its direction and the angle of strike towards Earth were from behind the Sun. It was not an asteroid for another reason: the absence of reasonably large asteroid fragments. No fragments, no asteroids. It was certainly a comet, a ball of ice and dust.

A mini black hole. It's not theoretically impossible that a mini black hole could pass through Earth, but the black hole theory was a good try by a couple of theoretical physicists. It flopped when no one could find any record of a black hole's exit on the other side of Earth through the North Atlantic. Surely it was a big rock, not an invisible black hole.

An anti-matter rock. The trouble with these rocks is that they can't survive the journey through the atmosphere. Anti-matter-matter annihilation also annihilates this theory.

An invisible mirror matter rock. A good effort to test drive the new mirror matter theories. The effect on Tunguska, however, has been invisible. It wasn't a rough rock, it was a beautiful ball.

A lightning ball that turned into a killer. No one has yet explained how a tiny lightning ball could turn into a giant fireball. But it's easy for a cloud of methane.

A volcanic blow-out. The radial pattern of fallen trees shows that the shock waves came from above, not from the rattling of Earth. What about the fireball that eyewitnesses saw streaking across the sunny Siberian sky? Try a geometeor (but the same arguments can be applied against that theory).

Ghostly geometeors. Meteors that do not come from the sky, but from the ground under your feet? A plasmoid sounds more plausible.

A plasmoid. A sort of bottle filled with plasma and surrounded by a strong magnetic field? A spaceship disguised as a plasma container? Why not try the real thing?

An alien spaceship. Was it on its way to the Star Wars studios and got caught in a space-time vortex which spewed it onto the set of The X-Files: Tunguska? A laser beam is a better bet.

A sharp laser beam. You are not alone, earthlings. Read your next postcard from space carefully, otherwise you'll be zapped by a death ray.

A deadly death ray. A yarn that seems to have been spun on April Fools' Day. Our benevolent Dr Tesla fired his death ray to deflect the asteroid that was on its way to destroy his home city of New York. The death ray changed the course of the rogue rock and it exploded in almost empty Siberia. Thank you, Dr Tesla.

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