The Truth About Meteors: A Horizon Special Horizon


The Truth About Meteors: A Horizon Special

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For the residents of the Russian city of Chelyabinsk,

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the morning of Friday, February 15th, 2013

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began like any other.

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As they set off to work, in what has become a craze throughout Russia,

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many recorded their journeys.

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But these cameras, usually used for capturing

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minor traffic incidents, were about to record history.

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A fireball brighter than the sun appeared from nowhere...

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..before exploding with the power of 30 Hiroshimas.

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A minute later,

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a shockwave blew in the windows of 4,000 buildings across the region.

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The broken glass accounting for most of the 1,200 injured.

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The people of Chelyabinsk had just experienced the most powerful

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meteor strike for more than a century.

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The meteor that exploded over Chelyabinsk is a spectacular

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reminder of just how exposed our world is.

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Earth is this tiny planet in a vast, violent cosmos.

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It is also a reminder of the powerful impact that these

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alien rocks can have on the fate of our planet

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and on us.

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This is not the first time it has happened.

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Over the last few years, scientists have examined many other

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devastating impacts in the earth's past.

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Using this knowledge, I want to answer the key questions

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that the Chelyabinsk meteor strike raises.

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Where did this alien rock come from? When will the next one strike?

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And can we do anything to protect ourselves?

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A fortnight after the impact,

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the meteor strike is still big news in Russia.

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In Chelyabinsk there is a popular new winter pastime -

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hunting for any fragments of the meteorite that remain.

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Scientists have also been out in force,

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particularly around Lake Cherbarkul

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where there is evidence of an impact in the ice.

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So many fragments have been found here,

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it has been called the Cherbarkul meteorite.

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They are trying to piece together exactly what happened,

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because the fact is no-one in the scientific world saw this coming.

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I was shocked. I was truly shocked.

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I never thought I would see an event like this over a major

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city during my lifetime.

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We could not predict this was going to happen.

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The piece of rock that entered the atmosphere was relatively small,

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maybe only a few metres across, and so we could not see

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this before it entered.

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When something like this happens, there is no doubt about it,

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it is frightening.

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But I have to admit, as a geologist

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witnessing a once-in-a-lifetime event,

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it is utterly thrilling.

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You only had to look at social media to see that scientists

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all over the UK and around the world were getting very,

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very excited about this as the news broke.

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It was exciting. It was exciting for me as a meteoriticist

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because you immediately want to know, what is it?

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What has landed? Is it a bit of Mars or a bit from an asteroid?

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I am almost ashamed that I had such great excitement about seeing

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this event and knowing that meteorites had fallen,

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because people had been injured.

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Chebarkul was the biggest meteorite to strike the Earth

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since we've had the technology to measure them.

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From its journey through the atmosphere

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to its spectacular end, every moment was captured.

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One of the best documented 16 seconds in science ever.

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'Professor Alan Fitzsimmons is one of the scientists'

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who has been examining the meteorite footage frame by frame.

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These are amazing images,

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but what can you get out of these as an expert?

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What it shows us, first of all, is a great record

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of the entry of the object into the Earth's atmosphere,

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so you see it right from the moment it really penetrated and there it is.

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-That is the edge of the atmosphere?

-That is it and it is coming down at a fairly shallow angle

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probably and as we play the movie on, what we see is, bang, there,

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it suddenly got brighter. So something has happened to the object.

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It is starting to break apart and as it breaks apart, it releases

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some of its orbital energy and that is causing that big flare up there.

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Is that because the atmosphere is denser, it is harder?

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That's right, it is finding it harder and harder to punch through the atmosphere.

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As we roll on, we suddenly get this bang, this huge flare up where

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suddenly the whole object is starting to fragment and break apart.

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That is where the majority of the energy is being released.

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If we look here.

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There are just little bits falling off.

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There is another flare up here and there is another flare up there,

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and now we can still see it's glowing, incandescent,

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some major fragment of the object is still falling down through the Earth's atmosphere.

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Underneath that trajectory you are going to have showers

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of bits of asteroid, essentially, falling down,

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and then finally 16.5 seconds later,

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-and what we are left with is this contour trail.

-The shockwave.

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The shockwave coming towards us, that is right.

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It is about a minute later that it has gone and reached the ground.

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-This guy driving doesn't know yet that the shockwave is on its way here.

-That is right.

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He is still happily listening to the radio on his drive to

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work in the morning.

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The explosion generated a shockwave

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so massive it was detected over 15,000 kilometres away.

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The low-frequency waves were picked up by monitoring stations.

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This is kind of like a listening network around the world.

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That is right. They're not set up for fireball or asteroid impacts,

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but set up to listen for nuclear explosions.

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What the monitoring stations picked up were some of the largest

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infrasonic waves ever recorded.

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Here they have been modified to make them audible.

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It has been detected down in Antarctica, we've got records of it

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up there in Alaska, so the pressure wave from the entry of the object

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and the explosive fragmentation was found, seen all over the world.

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So from the data that is coming in, it is early days, obviously,

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but from the data that is coming in, what is your best guess at the size of that rocky lump?

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Well, from the infrasound we know the energy released

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was something like 500,000 kilotons of energy, which is huge.

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-I was thinking it sounded a lot.

-That is right.

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And because we know it came in,

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from the video footage, at about 17.5 kilometres per second,

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we can combine that energy with that velocity to get a mass of the object.

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From that mass, we can get a size

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and it is probably about 15 metres across or so.

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That is a rarity, isn't it?

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We think these things come in maybe, in once every 50 or 100 years,

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that is all randomly, so this is a really special and really rare event of course.

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Meteor strikes as big as this may be rare

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but scientists have a surprisingly detailed knowledge of what

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meteorites are and where they come from.

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Long before the meteorite reached its explosive finale in full

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view of Chelyabinsk's dash cams, it had a very different existence

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and going by a very different name.

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Meteorites begin life in deep space,

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as part of much larger bodies called asteroids.

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These can range in size

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from just a few metres to more than 900 kilometres.

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The leftovers from the nebula that created our solar system

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some 4.6 billion years ago.

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And millions of them

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circle the sun in a trail known as the asteroid belt.

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Here, collisions create smaller fragments

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and when these fall towards Earth, they take on one of two forms.

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The smallest pieces will burn up in the atmosphere to become meteors,

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what we call shooting stars.

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Only the larger fragments that make it all the way to the earth's

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surface are called meteorites.

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The meteorite is a piece of rock from space,

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or a piece of metal from space, that falls through our atmosphere

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and actually hits the ground to be recovered.

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Technically, scientists love their words, it is

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not a meteorite before it is actually found and discovered.

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By collecting and comparing meteorites,

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scientists have been able to piece together a picture of how they form

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and these studies have revealed some of the most remarkable rocks

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in the solar system.

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Few places in the world have got as many

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meteorites as the Natural History Museum, meteorites like this.

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It is a cracker, isn't it?

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The ones that are out here on display are just a fraction.

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The bulk of the collection is behind the scenes

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and that is where the science goes on.

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So all these are meteorites in some shape or form?

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They are all either meteorites.

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'Professor Sara Russell is expert at decoding the messages hidden

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'within these fragments of space rock.'

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This looks quite rocky, what about that one?

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It looks like a humble rock but if you hold it -

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be very careful of this one, this is older than the Earth, it is

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the oldest thing you will ever hold.

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-This is older than what, 4.6 billion years?

-Yes.

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We have this number of 4.6 billion years of the age of the solar system,

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we know that from meteorites like this one,

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and from looking at the age of the components within it.

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If you can see, it has these rounded objects in it, which are 1mm to 1cm

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in size, these are called chondrules, and these were once free-floating.

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Before there were planets, these were free-floating in the solar system

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around the very young sun and then they slowly coalesced

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to make asteroids and larger and larger objects,

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until eventually planets were formed.

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These were the building blocks of planets.

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So the Russian meteorite, any news on what kind it is?

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Well, the early reports are that it is an ordinary chondrite

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and that means it will be similar to this one, so this is really exciting

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for us as scientists because we want to know how the planet is formed,

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what was around before the planets, what the environment was like

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and how the material that made up the planets first came together,

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and the chondrites are the best way of finding that out.

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Is this the most common in the solar system?

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It is the most common type to fall down to Earth.

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There is almost certainly a bias that the only material that we get

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to Earth is stuff that happens to cross the Earth's orbit.

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It has to be going in a slightly odd direction to cross the Earth

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anyway, so there is some kind of selection bias.

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This is a really special thing for you to kind of have in your career.

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Yes, if only something like this would happen in Britain

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so we could go and get it.

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I don't think there's too many people watching this programme

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that will be saying, "I wish it happened in the UK."

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-Obviously somewhere uninhabited.

-OK.

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How much of this stuff comes to us every year?

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Actually, huge amounts. The Earth is growing by

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at least 40,000 tonnes a year, so a huge amount of material is falling

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to Earth but we don't really notice most of it

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because the vast majority of it comes in the form of dust.

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Although several thousand meteorites actually land on Earth every year,

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most of those actually go unnoticed.

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They fall just too far away from people.

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If a meteorite falls maybe 15 feet away from you,

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you probably won't notice it.

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It will make a dull thud and that will be it, unless it is very large.

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This event is special because it was so large.

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There was no way you could not notice this meteorite falling.

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It really wanted to get noticed. It said, "Ta-da! I am here."

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Those events are spectacular and they give us scientists these

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important pieces of rock from which we can learn about the solar system.

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It is remarkable how we are able to build up this picture of what

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is going on millions of miles away in the solar system.

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It is one of the joys of science really, almost like a detective

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picking up on those tiny clues to tell a bigger story.

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So that the big question, the one that really needs answering,

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is why do some of these asteroids suddenly head straight towards us?

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Over 95% of asteroids are found in an orbit between Jupiter

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and Mars, called the main belt.

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It's almost 200,000,000 kilometres across

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and home to millions of these orbiting rocks.

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These asteroids have been following the same path for millions of years.

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So long as they remain here, they pose no threat to Earth

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but occasionally, one goes astray.

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Collisions are one of the reasons why this might happen.

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But in the last decade,

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we have learned that just a few rays of light are enough, because one

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scientist has tracked the orbit of just one of these millions of rocks.

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Steve Chesley of NASA's Jet Propulsion Lab in California

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has made a study of 200,000,000 tonne asteroid called Golevka.

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This is a model of Golevka.

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It is actually about 500 metres across,

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say the size of a football stadium.

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It rotates in this direction.

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As you can see, it has a very angular shape to it.

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He set out to investigate a 100-year-old theory that said

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asteroids were powered by the sun itself.

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It was called the Yarkovsky effect.

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The Yarkovsky effect is a very small acceleration and acts on asteroids

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and what it is is,

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if you take a model, the sun is hitting the asteroid,

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warming the surface, and as the asteroid rotates,

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that hot surface radiates the heat out in a different

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direction into space and that causes an acceleration, a very slight

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acceleration coming from the photons that are emitted from the asteroid.

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The idea is that this acceleration,

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slight as it is, can have significant effects upon

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the orbits of asteroids over millions of years.

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It was an intriguing idea.

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What sent asteroids out of their orbit and on a path towards

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Earth was photon propulsion, but what was lacking was proof.

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The Arecibo telescope is over 300 metres in diameter.

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It is one of the most powerful telescopes in the world and it

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uses radar to mark the precise position of objects in deep space.

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It was this telescope that would allow Steve Chesley to detect any

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tiny alterations in the orbit of asteroid Golevka,

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more than 15,000,000 kilometres out in space.

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We knew that it would be in one place

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if the Yarkovsky effect wasn't acting on it, and it would be over here

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if it was acting and our models were correct.

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When Steve and his team studied the data, the results were unequivocal.

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We knew from the radar measurements where Golevka was within a few

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tens of metres and yet it was actually 12 or 15 kilometres

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away from where it was predicted to be without Yarkovsky effect,

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so these very precise radar observations allowed us to see

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the 12-kilometre displacement caused by the Yarkovsky effect.

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So photons, those elementary mass-less particles of light,

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really can create a tiny force.

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The force is about one ounce on earth,

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say that the weight of a shot glass, that is

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the force on this huge asteroid, the size of a football stadium.

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Even for me it is truly remarkable, it is dramatic that a force

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so slight can have such dramatic changes on individual asteroids'

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orbit over millions of years.

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The Yarkovsky effect is subtle.

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It takes many millions of years to gently nudge an asteroid

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out of its regular orbit.

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But once that orbit has been disturbed,

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the consequences can be profound.

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Now it can come increasingly under the influence of the solar system's

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largest planet - Jupiter.

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Jupiter has a mass 300 times bigger than Earth's

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so there is a huge gravitational field.

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Often that works to our benefit.

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Stray objects can be swept up in Jupiter's gravity,

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drawing them into the planet.

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We've actually observed Jupiter acting as a shield in this way.

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This photograph, from the Hubble Space Telescope,

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shows the fragments of a comet torn apart by Jupiter's gravity...

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as the pieces were drawn to the planet's atmosphere

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the impacts left blast scars - some as big as the Earth...

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..but there is a downside to Jupiter...

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it can also deflect asteroids into orbits that cross the Earth's path.

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The Chelyabinsk meteor appears to be one of these typical

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Earth-crossing events.

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The likelihood is that it was thrown out of its regular orbit

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by either one or a combination of the known causes -

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collision, the Yarkovsky effect, Jupiter's gravity.

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It continued its new orbit for hundreds, thousands,

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even millions of years before meeting its fateful end.

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We can even begin to trace the exact path that the Chelyabinsk meteor

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took on its collision course with Earth.

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Within the 16 seconds of action are all the clues we need.

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Now, from just one vantage point it's not clear...

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exactly how far up it is or how far away it is

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but that's what we get from looking at other vantage points.

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So, here we are, again, at a different angle.

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The object is coming in, almost out of the sun, there,

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and by combining this video clip with the other video clips,

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what we can do is trigonometry.

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Basically, you can figure out how high up the object was

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and how far away it was.

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And if you catch the object early enough

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then you actually know where it was in the atmosphere

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the first time you saw it.

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So, you're kind of, triangulating to get that fixed position

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-and it changes over time so you get the trajectory?

-That's right.

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In the first part of the trajectory, what you've got there

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is a path that is relatively unaffected by the Earth's atmosphere.

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So, we can use that part of the video footage to track back

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and figure out where this object came from in the solar system.

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I love watching this because I now know where it is going to come

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and you see it just hitting the edge of the atmosphere.

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It's going to be...just...

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Come on, come on...

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-There it is!

-Yup.

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It's about 90 kilometres up, at that stage,

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travelling at 17.5 kilometres per second.

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Using the different camera positions,

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scientists have pinpointed the exact position

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at which the meteor entered the atmosphere...

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and, by tracking the speed and angle of the shadows

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that the meteor casts,

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they've calculated its velocity.

0:22:050:22:09

Together this is enough to track back the asteroid's path

0:22:090:22:13

from deep space.

0:22:130:22:14

Although the asteroid and Earth orbits are different durations

0:22:160:22:20

and at angles to one another

0:22:200:22:23

their clockwork regularity means that we were bound to collide.

0:22:230:22:27

So, this shows, speeded up, obviously, three and a half hours,

0:22:290:22:33

the last three and a half hours of the life of this little rascal.

0:22:330:22:36

Yeah, it's nice to see it from the asteroid's point of view.

0:22:360:22:39

The thing to remember is that this asteroid has been in its orbit,

0:22:390:22:42

going around the sun, roughly once every two years, we believe...

0:22:420:22:46

-Minding its own business.

-Absolutely.

0:22:460:22:49

..and, unfortunately, on February 15 it found a planet in the way.

0:22:490:22:53

Sure enough,

0:22:580:23:00

at 09:20 hours the neat yet entered our atmosphere above Siberia.

0:23:000:23:06

On this path and at time

0:23:060:23:09

it was Chelyabinsk that took the full impact...

0:23:090:23:12

..but could there have been another scenario?

0:23:160:23:20

The meteorite landed at a latitude of 55 degrees north,

0:23:220:23:27

had it arrived just a few hours later

0:23:270:23:30

we would have been directly in its flight path.

0:23:300:23:33

So, was this a near miss for us?

0:23:330:23:36

If the asteroid had been in a different part of its orbit,

0:23:360:23:40

so it didn't hit this year but it hit next year,

0:23:400:23:43

it would have still hit us on February 15th

0:23:430:23:45

but instead of coming in over Russia

0:23:450:23:47

it would have come in over the UK and Ireland

0:23:470:23:49

and would have entered the Earth's atmosphere,

0:23:490:23:52

in fact, entered the North Atlantic Ocean.

0:23:520:23:56

In order for the meteorite to strike anywhere near Britain

0:23:560:23:59

our paths through space would have had to be fundamentally different.

0:23:590:24:03

So we know where asteroids come from

0:24:080:24:10

and the forces that shape their date with destiny...

0:24:100:24:14

but what exactly happens next?

0:24:140:24:16

The moment that a meteor strikes?

0:24:160:24:19

And what determines just how devastating that strike will be?

0:24:190:24:23

When the Chelyabinsk meteor reached our atmosphere

0:24:250:24:28

it was travelling at more than 65,000 kilometres per hour...

0:24:280:24:33

and measured more than 15 metres across.

0:24:330:24:37

Apart from some unconfirmed reports

0:24:370:24:39

of craters at the bottom of Lake Chebarkul

0:24:390:24:41

there's surprisingly few signs of an impact.

0:24:410:24:44

Little of the 7,000 tonnes of space rock that entered the atmosphere

0:24:460:24:50

have been recovered...

0:24:500:24:51

..perhaps 300 fragments...

0:24:530:24:55

..and yet, the effects were felt over 3,000 square kilometres.

0:24:590:25:05

The question is how can apparently so little do so much harm?

0:25:050:25:10

There's a clue from the last time

0:25:140:25:16

Earth experienced a meteor strike on this scale.

0:25:160:25:19

On June 30th, 1908,

0:25:310:25:34

a huge explosion tore through the forest of Tunguska, Siberia.

0:25:340:25:41

It was 20 years before the Russians mounted an expedition to the site.

0:25:410:25:46

What they found astonished them...

0:25:460:25:48

..60 million trees across an area the size of London

0:25:530:25:58

had been levelled.

0:25:580:25:59

Scientists thought it has been caused by a meteorite strike...

0:26:020:26:06

..but then why was there no sign of any kind of impact crater?

0:26:090:26:12

The answer is that the devastation had to be caused by a meteor attack

0:26:150:26:20

of a very particular kind.

0:26:200:26:22

Physicist Mark Boslough has been fascinated

0:26:240:26:27

by how so much destruction can be caused

0:26:270:26:29

without any apparent direct contact.

0:26:290:26:34

The explosion at Tunguska was caused by an asteroid

0:26:340:26:36

that entered the atmosphere, got close to the surface

0:26:360:26:39

and exploded before it hit the ground.

0:26:390:26:41

And that explosion created a blast wave with hurricane force winds

0:26:410:26:46

that knocked trees over for thousands of square miles.

0:26:460:26:50

Scientists call it an airburst -

0:26:530:26:57

a massive explosion in the atmosphere,

0:26:570:26:59

rather than on the ground.

0:26:590:27:01

As it enters the atmosphere at speeds of up to 24 metres per second

0:27:030:27:08

the air resistance decelerates the asteroid so fast

0:27:080:27:12

it breaks apart in a huge explosion.

0:27:120:27:14

Most of the damage from an explosion like this is actually the blast wave,

0:27:220:27:26

it's the very high winds.

0:27:260:27:27

Mark created a simulation to see what size

0:27:290:27:32

an asteroid would need to be to generate such destructive power.

0:27:320:27:38

In this simulation I include more of the physics to be more realistic.

0:27:380:27:42

We can see that the main shockwave

0:27:420:27:44

doesn't come out of the point of the explosion

0:27:440:27:46

but it comes out of the point where the fireball descends to.

0:27:460:27:49

So, by the time the shockwave gets to the ground

0:27:490:27:52

it's much stronger than it would otherwise be

0:27:520:27:54

and there's more damage on the ground

0:27:540:27:56

because the destructive power was carried downward.

0:27:560:27:59

Based on Mark's calculations, the devastation at Tunguska

0:28:030:28:07

could have been caused by an asteroid,

0:28:070:28:09

perhaps as small as 30 to 50 metres in diameter...

0:28:090:28:12

..and this carries a worrying implication.

0:28:140:28:17

Smaller asteroids are more dangerous than we used to think

0:28:190:28:22

and because there are so many more smaller asteroids

0:28:220:28:25

than bigger asteroids

0:28:250:28:26

we need to take that risk more seriously than we used to.

0:28:260:28:30

The lesson of Tunguska helps explain why in Chelyabinsk

0:28:320:28:37

there's so much damage but very little meteorite to be found.

0:28:370:28:41

If we go back to the video footage and we see the object coming in,

0:28:410:28:46

when it's in the high atmosphere it suffers very little effect

0:28:460:28:51

but just here you get this huge flare-up

0:28:510:28:53

and that's because the atmosphere has become so dense

0:28:530:28:57

that it's almost impossible for it to push through any more.

0:28:570:29:01

And, basically, something's got to give, and the asteroid gives,

0:29:010:29:04

and it, basically, just breaks apart

0:29:040:29:07

in a huge catastrophic fragmentation effect,

0:29:070:29:12

and that is what creates a shockwave,

0:29:120:29:15

which we hear as this sonic boom.

0:29:170:29:18

EXPLOSION

0:29:180:29:21

Really it's a balance between the size of the object,

0:29:210:29:26

its speed into the atmosphere and, critically,

0:29:260:29:29

the altitude at which it explodes.

0:29:290:29:31

Too high, if it's too small and it explodes too high

0:29:310:29:35

the shockwave has little effect on the ground.

0:29:350:29:38

If it's...quite low in the atmosphere, it's a large object,

0:29:380:29:43

then that shockwave is completely devastating.

0:29:430:29:45

Actually seeing it in real life really brings home to you

0:29:450:29:48

the energy that these things carry

0:29:480:29:50

and, even though it exploded tens of kilometres, perhaps, up in the air,

0:29:500:29:55

so, quite a long way from the ground, the force of the explosion,

0:29:550:29:58

the shockwave, was able to damage buildings over a huge area and injure people,

0:29:580:30:03

and that was quite a shocking thing to see.

0:30:030:30:06

The destructive power of an air blast is immense

0:30:060:30:09

but, in a way, the people of Chelyabinsk are lucky

0:30:090:30:13

because out there in the cosmos is a different kind of asteroid,

0:30:130:30:17

one that poses an even greater threat.

0:30:170:30:20

I've seen the evidence of what one of those can do,

0:30:200:30:23

the damage that it leaves behind,

0:30:230:30:25

and what you realise is the Earth's own destructive forces -

0:30:250:30:28

you know, the great earthquakes, the volcanic eruptions -

0:30:280:30:31

seem trivial in comparison.

0:30:310:30:34

This is Barringer Crater, Arizona...

0:30:410:30:44

..the 50,000-year-old remnant of a massive meteorite impact.

0:30:460:30:50

'This place really gives you a sense of the destructive power'

0:30:520:30:55

of incoming meteorites.

0:30:550:30:57

The blast here would have vaporised a city larger than London

0:30:570:31:02

but the lump of rock that did it measured barely 15 metres across.

0:31:020:31:07

Down on the ground the scale of the impact is even more breathtaking...

0:31:190:31:24

..the crater is more than a kilometre across

0:31:290:31:32

and nearly 200 metres deep.

0:31:320:31:35

The forces here were enormous,

0:31:480:31:51

the impact turned this solid rock

0:31:510:31:53

into this pulverised mush.

0:31:530:31:56

It just...bursts out in your hand.

0:31:560:31:58

I mean, look at that.

0:31:580:32:00

They started out as the same kind of rock.

0:32:000:32:04

The meteor that struck here was about the same size

0:32:070:32:09

as the one that flattened Tunguska

0:32:090:32:12

but there is a critical difference...

0:32:120:32:16

at Barringer the meteor didn't explode in the atmosphere,

0:32:160:32:20

it struck ground.

0:32:200:32:22

So, this is just a fragment of the true devastation unleashed here.

0:32:220:32:26

Fortunately, to understand exactly why ground strikes

0:32:350:32:39

are so very destructive

0:32:390:32:41

we don't have to wait for another Barringer to happen...

0:32:410:32:45

because today we can simulate this kind of impact.

0:32:450:32:50

And that's thanks to the research of Pete Schultz...

0:32:500:32:53

and one very special piece of equipment.

0:32:530:32:56

So, so, this was serial number one, it was built during the Apollo time.

0:33:000:33:05

I guess because they thought there would be several of them made

0:33:050:33:08

but this is the first one and the last one.

0:33:080:33:10

And is the only one like it, in the world.

0:33:100:33:13

This is NASA's Vertical Gun Range.

0:33:160:33:19

It was built to study how impacts affected the moon

0:33:190:33:22

as the astronauts prepared to make the first lunar landing.

0:33:220:33:26

We are armed, gated and reset.

0:33:260:33:29

Today, Professor Pete Schultz uses it to model precisely

0:33:320:33:36

the dynamics of an asteroid impact.

0:33:360:33:38

We know that these...asteroid impacts are bad

0:33:380:33:43

but you want to understand really how bad.

0:33:430:33:46

Peter uses the NASA gun to fire projectiles at very high speed

0:33:480:33:53

to simulate an asteroid hitting the Earth.

0:33:530:33:56

So, for this experiment we're going to fire

0:33:560:33:58

this tiny quarter-inch aluminium sphere at very high speeds,

0:33:580:34:02

up to around five kilometres per second,

0:34:020:34:04

and then we will see what type of crater it produces.

0:34:040:34:07

The target it will hit is made of sand.

0:34:090:34:13

So, we use sand because it records the shock affects very clearly.

0:34:130:34:18

Outside of the impact chamber are super high-speed cameras

0:34:210:34:25

that can film at up to 1,000,000 frames per second,

0:34:250:34:27

capturing every detail of the impact and the aftermath.

0:34:270:34:31

-OK, lights out. Everything good?

-Yeah.

-OK, we're out of here.

0:34:330:34:38

We have high voltage, the paddle is in, the warning lights...

0:34:450:34:51

and...rolling.

0:34:510:34:53

ALARM BUZZES

0:34:530:34:55

Oh, perfect. Perfect, perfect.

0:35:130:35:16

Now we're seeing the fireball come in - it's brighter than the sun

0:35:160:35:20

and then, "Kapow!", it hits the surface. Jeez!

0:35:200:35:23

This whole region, downrange, would have been incinerated.

0:35:230:35:29

It would have been incinerated just by this plasma,

0:35:290:35:32

this exploding vapour plume engulfing everything.

0:35:320:35:37

There would have been winds that would have been going so fast

0:35:370:35:40

it could pick up houses and spread them hundreds of kilometres away.

0:35:400:35:45

This would have been Armageddon.

0:35:470:35:49

Experiments like this reveal several important things.

0:35:550:35:59

One is that it's not just the impact,

0:35:590:36:01

it's all that vapour that runs downrange.

0:36:010:36:04

In fact, you can see areas, here,

0:36:040:36:06

where there was so much wind it actually carved out

0:36:060:36:11

pieces of this landscape.

0:36:110:36:12

So, what these experiments help us do,

0:36:120:36:15

they actually allow us to witness the event -

0:36:150:36:18

see it in real time -

0:36:180:36:20

and try to understand the processes that are going on.

0:36:200:36:24

It's really complex but we have to see it to understand it.

0:36:240:36:28

So, asteroid impacts unleash a trail of destruction far greater

0:36:300:36:34

than suggested by the footprint of the crater alone.

0:36:340:36:38

Comparing the effects of an airburst with a ground strike,

0:36:440:36:49

it seems the Chelyabins got away lightly.

0:36:490:36:51

It's estimated that the largest piece to hit the ground

0:36:560:36:59

weighed 500 kilos,

0:36:590:37:00

a fraction of the asteroid's original mass of 7,000 tonnes.

0:37:000:37:05

Now if a piece of rock that big had hit that area of Russia,

0:37:060:37:10

it would have produced a huge impact crater.

0:37:100:37:12

Then that kinetic energy is then delivered into the ground

0:37:120:37:16

and we see things like seismic shock.

0:37:160:37:18

So, you get... People would feel earthquakes on the ground.

0:37:180:37:22

So, the fact that it was an airburst actually limited the consequences

0:37:220:37:26

for the people on the ground.

0:37:260:37:27

So, yes, still quite dramatic,

0:37:270:37:29

still, you know, obviously, causing injuries

0:37:290:37:32

but it could have been a lot worse, had it survived down to ground.

0:37:320:37:36

Ground strikes are amongst the most destructive

0:37:470:37:50

natural hazards we know of.

0:37:500:37:52

When viewed from space,

0:37:530:37:55

Earth's encounters with giant asteroids in its deep history

0:37:550:37:58

are revealed.

0:37:580:38:01

And there is evidence from our planet's past

0:38:010:38:03

of a truly devastating meteorite strike

0:38:030:38:06

that decisively altered the course of life on Earth.

0:38:060:38:09

Today, millions of years after the impact,

0:38:110:38:15

the evidence for that crater is well hidden.

0:38:150:38:18

SHE SHOUTS

0:38:210:38:23

This is a gateway to the cenotes,

0:38:260:38:30

the unique cave system of Mexico's Yucatan Peninsula.

0:38:300:38:33

Wow!

0:38:370:38:38

Look at the size of this!

0:38:400:38:42

This is magnificent!

0:38:430:38:44

That is beautiful.

0:38:450:38:47

'This cave may be stunning,

0:38:490:38:52

'but it provides the evidence for one of the greatest catastrophes

0:38:520:38:55

'in the Earth's history.'

0:38:550:38:58

And that water, it's so clear!

0:38:580:39:00

Lower the gear, please!

0:39:020:39:04

There's actually much more to this amazing cavern

0:39:060:39:09

than first meets the eye.

0:39:090:39:11

But to understand the scale of what happened here,

0:39:120:39:15

you have to go deeper still.

0:39:150:39:17

Underwater.

0:39:170:39:18

OK?

0:39:210:39:22

I'm not sure if I'm ready for this.

0:39:230:39:25

I've got all the equipment, but...

0:39:250:39:28

there's something about going down into water

0:39:280:39:30

when you're not quite sure where your exit is...

0:39:300:39:33

But I trust Bernadette completely here.

0:39:330:39:36

HE CHUCKLES

0:39:360:39:37

She knows what she's doing.

0:39:370:39:38

So I'm as ready as I'll ever be.

0:39:380:39:40

-Ready?

-All right.

0:39:400:39:41

Descending into the depths of the cenote is like entering a new world.

0:39:570:40:02

Fewer people have visited some of these drowned caverns

0:40:100:40:14

than the surface of the moon.

0:40:140:40:16

As divers have explored further,

0:40:260:40:28

they've discovered the cenotes are actually part of a huge complex

0:40:280:40:33

of tunnels and caves.

0:40:330:40:35

In fact, when you look from above,

0:40:430:40:45

you can see there are cenotes

0:40:450:40:47

scattered across hundreds of kilometres.

0:40:470:40:50

And when they're mapped,

0:41:010:41:03

it becomes clear that they follow

0:41:030:41:05

a distinctive circular course through the jungle.

0:41:050:41:07

They mark out the rim of a giant crater.

0:41:100:41:14

Scientific instruments show

0:41:160:41:18

the structure of the underlying rock has been deformed,

0:41:180:41:22

revealing the boundaries of a colossal meteorite impact crater.

0:41:220:41:27

This amazing cavern is part of a bigger story, a much bigger story.

0:41:380:41:44

65 million years ago, THIS was the site

0:41:440:41:47

of one of the most catastrophic impacts in Earth's history.

0:41:470:41:51

What became known as the Chicxulub meteorite landed here.

0:41:510:41:56

And THAT triggered the extinction of the dinosaurs.

0:41:560:42:00

The meteorite was 15 kilometres across,

0:42:030:42:07

enough to cause utter devastation

0:42:070:42:10

across the whole planet.

0:42:100:42:12

It exploded with a force of 100 million million tonnes of TNT.

0:42:140:42:19

The blast sent a giant plume of vaporised rock out into space.

0:42:220:42:27

A crater was punched 30 kilometres into the Earth's crust.

0:42:290:42:33

It was above this rim of weakened rock that these cenotes formed,

0:42:350:42:40

millions of years later.

0:42:400:42:41

The blast would have been ferocious.

0:42:440:42:47

But it was what happened next

0:42:550:42:56

that made the impact a global catastrophe.

0:42:560:43:00

The blast plume that shot into space fell back to Earth.

0:43:020:43:06

Billions of molten particles superheated the air

0:43:090:43:12

to a temperature of hundreds of degrees.

0:43:120:43:15

Fires swept the planet,

0:43:180:43:20

choking the atmosphere with soot and dust.

0:43:200:43:23

The dinosaurs, and most other creatures, were doomed.

0:43:230:43:28

That discovery, back in the 1980s,

0:43:300:43:32

about what happened at Chicxulub,

0:43:320:43:34

changed everything.

0:43:340:43:36

Up until then, we thought

0:43:360:43:38

that the Earth had changed only through grindingly slow processes,

0:43:380:43:41

but now we knew that there was also sudden, violent catastrophes

0:43:410:43:45

that made the Earth the way it was.

0:43:450:43:48

Of course, what that meant

0:43:480:43:49

was that something like this could happen again.

0:43:490:43:52

At any moment.

0:43:520:43:53

Luckily, the very biggest asteroids are few and far between.

0:43:560:44:01

But there are still plenty of rocks out there

0:44:020:44:05

that represent a significant danger to us.

0:44:050:44:07

So, at the summit of an extinct Hawaiian volcano,

0:44:130:44:16

Professor Nick Kaiser and his colleagues

0:44:160:44:18

are searching the skies for killer asteroids.

0:44:180:44:21

Each night, using a revolutionary billion-pixel sensor,

0:44:250:44:30

the team scans a vast swathe of the sky.

0:44:300:44:34

Follow me up to the next floor,

0:44:340:44:36

you'll see a better view

0:44:360:44:37

of the telescope itself.

0:44:370:44:39

They are looking for any unidentified objects

0:44:430:44:45

that could be heading our way.

0:44:450:44:48

By capturing several images of the same patch of sky,

0:44:480:44:51

separated by several minutes,

0:44:510:44:53

the team can see if anything's changed

0:44:530:44:55

against the background of stars.

0:44:550:44:57

You can see that there's a dark thing and a white thing.

0:45:000:45:05

What that means is, in these two exposures,

0:45:050:45:08

there was an asteroid, which was here in the first exposure

0:45:080:45:11

and there in the second one.

0:45:110:45:13

It's kind of cute, here's another one in the same image.

0:45:130:45:16

And, in fact, we'll detect hundreds of asteroids in a single exposure.

0:45:160:45:20

Their observations are collated

0:45:230:45:25

at the nerve centre of asteroid detection -

0:45:250:45:28

the Minor Planet Centre,

0:45:280:45:30

just outside Boston.

0:45:300:45:32

Its director is Tim Spahr.

0:45:420:45:45

And his job is to keep track of every asteroid in the solar system.

0:45:450:45:50

Tim has developed a map to visualise their location.

0:45:580:46:01

And, on that map, the most important are the Near-Earth Asteroids,

0:46:010:46:06

the ones closest to the planet.

0:46:060:46:08

On the screen here is a map of the solar system.

0:46:090:46:13

And I've got the sun in the centre

0:46:130:46:15

and the third planet out here would be that of the Earth.

0:46:150:46:18

The red dots in here are actually Near-Earth Asteroids,

0:46:180:46:21

the green ones are the regular Main-Belt Asteroids.

0:46:210:46:24

There are over 9,000 Near-Earth Asteroids.

0:46:270:46:31

But there's one type they're particularly concerned to locate...

0:46:310:46:34

..those asteroids that are over one kilometre in diameter.

0:46:360:46:39

An Earth impact with one of these would spell disaster.

0:46:420:46:46

Tim's data reveals

0:46:530:46:55

that there are 900 asteroids bigger than a kilometre

0:46:550:46:58

in those dangerous near-Earth orbits.

0:46:580:47:01

But he has some good news.

0:47:020:47:04

Right now, there's no information

0:47:060:47:08

that any of those large objects will hit the Earth in the next 100 years,

0:47:080:47:12

so we're safe from impact of those objects for at least 100 years.

0:47:120:47:16

So there are no catastrophic asteroid impacts on the horizon.

0:47:170:47:21

But there are still dangers out there.

0:47:230:47:25

On 6th October 2008,

0:47:360:47:38

asteroid hunter Richard Kowalski saw something that would change

0:47:380:47:42

the assessment of threats presented by asteroid impacts.

0:47:420:47:45

The night was proceeding normally

0:47:450:47:48

and up on the screen came another asteroid.

0:47:480:47:52

As I continued to make observations throughout the night,

0:47:520:47:55

it appeared to be moving slightly faster.

0:47:550:47:57

And this indicates that the object is close to the Earth.

0:47:570:48:00

As with any other asteroid,

0:48:020:48:04

Richard reported what he'd found to the Minor Planet Centre.

0:48:040:48:07

I got up in the morning about seven o'clock

0:48:120:48:14

and I had a message on the computer saying,

0:48:140:48:16

"Could not compute an orbit for a particular object."

0:48:160:48:20

I grabbed the observations of this object and I computed an orbit

0:48:200:48:24

and it was immediately apparent, right then,

0:48:240:48:28

that that object was going to hit the Earth.

0:48:280:48:30

And, sort of ominous fashion,

0:48:300:48:33

it said it was in 19 hours.

0:48:330:48:35

Following a strict written protocol,

0:48:380:48:40

Tim quickly reported the findings to NASA's asteroid investigation team,

0:48:400:48:44

in California.

0:48:440:48:46

We got a call from Tim Spahr, at the Minor Planet Centre,

0:48:470:48:49

saying we had an impacter coming in, in less than 24 hours.

0:48:490:48:54

That woke me up.

0:48:540:48:55

NASA's expert on asteroid orbits, Dr Steve Chesley,

0:48:570:49:01

raced to verify the data.

0:49:010:49:03

The first thing I saw was a 1.000,

0:49:030:49:06

a 100% probability of impact

0:49:060:49:08

in less than a day's time.

0:49:080:49:09

I'd never seen anything like this

0:49:090:49:11

outside of simulations and software testing.

0:49:110:49:13

An asteroid strike would create a huge explosion.

0:49:150:49:19

NASA feared this might even be mistaken for a nuclear bomb.

0:49:190:49:22

We wanted folks to know this was a natural event,

0:49:220:49:25

by Mother Nature rather than some sort of man-made

0:49:250:49:28

event like a missile or something dreadful.

0:49:280:49:30

Information passed rapidly up the chain of command.

0:49:320:49:35

NASA headquarters notified the White House that this was coming.

0:49:350:49:39

Everyone wanted to know where it would strike.

0:49:400:49:43

NASA predicted a remote area of the Nubian Desert.

0:49:450:49:48

At 2:45 in the morning, NASA were proved right.

0:49:590:50:02

The explosion created a vast fireball burning as hot as the sun.

0:50:040:50:08

It was so big and so hot this image was captured by a weather satellite.

0:50:100:50:14

And yet the object that caused it was only four metres across.

0:50:170:50:20

Smaller than the asteroid which exploded over Chelyabinsk.

0:50:200:50:25

I definitely think the impact was a wake-up call.

0:50:250:50:29

I have to admit I never thought I'd see that in my career,

0:50:290:50:32

where we would discover something and it would hit the Earth later that day.

0:50:320:50:35

What was worrying about that impact was that the asteroid was too

0:50:400:50:43

small to detect until it was very, very close to the Earth.

0:50:430:50:47

Of course, for Chebarkul, it wasn't even spotted until it was already here.

0:50:470:50:51

But we are getting better at spotting smaller asteroids.

0:50:530:50:56

On the same day that Chebarkul was hit, another asteroid,

0:50:570:51:01

similar in size to the object that created the Barringer Crater,

0:51:010:51:04

came within just 28,000 kilometres of the Earth.

0:51:040:51:08

Approaching from beneath the planet, asteroid 2012 DA14,

0:51:100:51:15

passed inside the orbit of our geostationary satellites

0:51:150:51:18

before heading off to the north.

0:51:180:51:20

This asteroid had been successfully tracked for a year.

0:51:220:51:26

Despite its proximity, scientists knew that it posed a threat.

0:51:260:51:30

So we know we are safe for at least 100 years from most near-Earth asteroids over a kilometre in size.

0:51:350:51:41

We are better at detecting objects down to 50 metres across,

0:51:430:51:46

like DA14.

0:51:460:51:48

But for asteroids smaller than that, like the one which exploded over

0:51:530:51:57

Chelyabinsk, we still have little or no warning.

0:51:570:52:00

There are still some we haven't found.

0:52:020:52:04

So there's this unknown bit of the equation where we are still looking

0:52:040:52:08

for some, we know they are there but we don't know where they are.

0:52:080:52:11

So this is a threat, but hopefully as technology moves on,

0:52:110:52:15

we'll always have a much better idea whether one's going to pose a risk to the Earth.

0:52:150:52:19

We could see an event tomorrow or in 10 or 20 years time,

0:52:190:52:24

that we hadn't previously detected.

0:52:240:52:26

That is always the risk we face.

0:52:260:52:28

Until we can catalogue and identify all the hazardous

0:52:280:52:32

objects in the solar system, that risk will always remain.

0:52:320:52:36

And there's one other factor that can make it particularly hard to spot an incoming object.

0:52:390:52:44

It's the reason why no-one saw the asteroid that was hurtling towards Chelyabinsk.

0:52:440:52:48

It came in in the daytime sky out of the sun.

0:52:510:52:54

Right.

0:52:540:52:55

We've got telescopes looking out there for these objects,

0:52:550:52:58

but they only work at night.

0:52:580:53:01

Radar doesn't help either, because to really use radar,

0:53:010:53:04

to find these objects, you have to know exactly where to look.

0:53:040:53:08

If you don't know what's coming in, you don't know where to look.

0:53:080:53:12

Because of that then, this thing and objects like this,

0:53:120:53:15

if they come in at that particular direction they're always going

0:53:150:53:18

to take us by surprise at the moment with our current survey system.

0:53:180:53:21

But even if we can spot an asteroid heading towards us

0:53:240:53:27

and in good time to prepare,

0:53:270:53:30

what if anything can we do?

0:53:300:53:33

There's different options for deflecting asteroids and it is a bit sci-fi at the moment.

0:53:340:53:40

The idea of shooting it out of

0:53:400:53:42

the sky with a nuclear weapon

0:53:420:53:43

would really be a dreadful idea.

0:53:430:53:45

It would just shower us with radioactive debris,

0:53:450:53:49

and it would just be... do more harm than good.

0:53:490:53:53

What would be much better would be to push it, nudge it

0:53:530:53:55

slightly off its course so that it wasn't then going to collide.

0:53:550:53:59

So how do you gently nudge an asteroid?

0:53:590:54:02

There's lots of different techniques to push it.

0:54:020:54:04

So... The one I love is called a mass driver.

0:54:040:54:08

There's a machine, which sits on the asteroid and throws off rocks,

0:54:080:54:12

so it is accelerating rocks that way

0:54:120:54:15

and that makes the asteroid gradually move in the opposite direction.

0:54:150:54:20

You can paint one side of the asteroid white.

0:54:200:54:23

That reflects the sun and there's this weird effect that makes the asteroid gradually drift across.

0:54:230:54:30

We can launch a mission now, which is essentially can impact an asteroid

0:54:300:54:37

and then deflect it - a bit like a billiard shot or a snooker shot.

0:54:370:54:41

We just hit the asteroid extremely fast with a spacecraft

0:54:410:54:45

and that small impact is sufficient to just alter its course

0:54:450:54:49

so that it misses the Earth.

0:54:490:54:51

When you consider Earth's history, stretching over billions of years,

0:54:550:55:00

it's clear that meteorite impacts, far from being unexpected,

0:55:000:55:04

are just a normal part of the life cycle of our planet.

0:55:040:55:07

But that is not how they seem to us.

0:55:160:55:18

The Chebarkul meteorite is a reminder of something

0:55:230:55:26

we would probably rather not think about too often -

0:55:260:55:29

how a sudden, apparently random event could have devastating consequences.

0:55:290:55:34

EXPLOSION AND SCREAMING

0:55:380:55:41

But this time we have been lucky.

0:55:430:55:45

Although it was terrifying for those who witnessed it,

0:55:450:55:49

this meteor struck without causing any fatalities.

0:55:490:55:53

And close enough to be captured on multiple cameras.

0:55:530:55:56

So it's given us a huge amount of information to help us

0:55:560:55:59

prepare for the next one.

0:55:590:56:01

I think perhaps the real lasting legacy of the Russian meteor

0:56:030:56:07

will be the effect it has had on the popular consciousness and perhaps on politicians.

0:56:070:56:12

Scientists have been saying for decades now that these things do happen from time to time,

0:56:120:56:16

that they could be dangerous if they happened over populated area.

0:56:160:56:20

But now we have actual proof, we have an event we can point to.

0:56:200:56:25

We know it could've been worse than this.

0:56:250:56:28

So, I think if this leads to more vigilance and perhaps,

0:56:280:56:32

the detection of future impacting events, that'll be a good outcome.

0:56:320:56:36

When a bit of an asteroid, comes through the atmosphere and lands on

0:56:360:56:41

the Earth as a meteorite, it reminds us that the solar system is a dynamic place.

0:56:410:56:45

It's... It's not finished.

0:56:450:56:49

It's still working. It's still evolving and still changing.

0:56:490:56:54

So next time you look up at the night sky, spare a thought

0:56:540:56:58

for those thousands of rocky lumps whizzing across our path.

0:56:580:57:02

A few of them have got our name on them,

0:57:020:57:04

but the thing is by analysing in detail the data from the meteor,

0:57:040:57:08

it means that next time, and there will be a next time,

0:57:080:57:11

we will be much better prepared.

0:57:110:57:14

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