Tuguska Fireball meteor fragments found

Mark- More than 100 years ago, back in 1908, a huge blast ripped open the sky in a region of Siberia called Tunguska. It flattened more than 2,000 square kilometres of forest all around.

Eyewitnesses said that they saw this huge object tearing through the atmosphere and then exploding, as it turned out probably a few kilometres above the ground. It's estimated that this explosion was the equivalent of about 3 to 5 megatons of TNT. That's 5 times more powerful than the biggest US nuclear weapon.

Obviously, the leading theory for this is that something came from space - a chunk of asteroid or perhaps a comet and broke up on entry. But the trouble is, scientists have never actually found any residues. There's no fragments been recovered despite the fact that there's many scientific expeditions, and that's left the way open for lots of speculation about other possible causes - anti-matter, black holes, aliens, all sorts of things. I think it's even featured in an episode of the X Files. What's happened now is that a group led by Ukrainian scientist called Victor Kvasnytsya has identified very small fragments of rock, less than a millimetre wide each that look very much like they came from a meteor. They're in the right泥炭层将日期完全right time that the explosion happened. These looked like our strongest candidates ever of rock from that explosion.

Kate - I actually gave this a bit of a Google before we had a chat and there are pictures on there of thousands and thousands of trees just sort of lying down on their sides. If we think it's such a massive collision, why haven't we found a large number of rocks? Why are we only finding these tiny fragments?

Mark - Well, that's the thing. What happens if you have something coming space that was basically an iron-based asteroid then it will be dense enough to survive this fall through the Earth's atmosphere and you would expect to find large chunks actually of iron strewn around the region. You'd also expect to see a massive crater like the huge crater in Arizona for example.

What scientists think with the Tunguska event was that, it was a more fragile body basically. It wasn't as dense. It wasn't as strong and that's why it broke up. And when it broke up in this huge explosion, it just would've almost vaporised all the material into tiny, tiny fragments.

Now, previous expeditions have found hundreds of what they call 'black magnetic spherules', tiny balls of material basically in soils, but they've never really been unequivocally proved to be part of the Tunguska meteor. These fragments in contrast because they're bigger, you can get an awful lot more information out of them. They've got a form of carbon in them called lonsdaleite and that's got a crystal structure - so, halfway between graphite and diamonds. Only forms under extreme heat and pressure, and you do often find it in meteorites. They've also found iron based minerals, one of them, an iron-nickel alloy, that's veined through the carbon and it's in proportions in a composition that's very, very similar to that of other iron rich meteorites.

Kate - Is this comprehensive though? Are all asteroids the same? Is there a certain signature that we can say that definitely comes from a meteor?

Mark - Well, there's one thing that doesn't fit which is why there's still going to be debate about this. Almost all meteorites tend to have very high levels of quite rare metals called iridium and osmium which on the Earth, all tends to be deep below the Earth's crust. You don't find much of it on the surface. So, where you find high concentrations of it, very often, they have come from objects from space. These samples have got levels of iridium and osmium that's halfway between what you'd expect to find in terrestrial rocks and what you'd expect to find in space rocks. So, there's some uncertainty about that. The team are now going back to do further isotope tests to try and determine that extra-terrestrial origin.

Kate - But until that happens, I suppose the conspiracies will continue.

Mark - They're certainly fun. So, I'm sure they will.