0:00:09 > 0:00:13For the residents of the Russian city of Chelyabinsk,
0:00:13 > 0:00:18the morning of Friday, February 15th, 2013
0:00:18 > 0:00:20began like any other.
0:00:20 > 0:00:23As they set off to work, in what has become a craze throughout Russia,
0:00:23 > 0:00:27many recorded their journeys.
0:00:27 > 0:00:30But these cameras, usually used for capturing
0:00:30 > 0:00:33minor traffic incidents, were about to record history.
0:00:39 > 0:00:43A fireball brighter than the sun appeared from nowhere...
0:00:50 > 0:00:53..before exploding with the power of 30 Hiroshimas.
0:01:12 > 0:01:13A minute later,
0:01:13 > 0:01:18a shockwave blew in the windows of 4,000 buildings across the region.
0:01:19 > 0:01:23The broken glass accounting for most of the 1,200 injured.
0:01:31 > 0:01:35The people of Chelyabinsk had just experienced the most powerful
0:01:35 > 0:01:38meteor strike for more than a century.
0:01:39 > 0:01:43The meteor that exploded over Chelyabinsk is a spectacular
0:01:43 > 0:01:47reminder of just how exposed our world is.
0:01:47 > 0:01:52Earth is this tiny planet in a vast, violent cosmos.
0:01:52 > 0:01:55It is also a reminder of the powerful impact that these
0:01:55 > 0:01:58alien rocks can have on the fate of our planet
0:01:58 > 0:01:59and on us.
0:02:03 > 0:02:05This is not the first time it has happened.
0:02:05 > 0:02:08Over the last few years, scientists have examined many other
0:02:08 > 0:02:11devastating impacts in the earth's past.
0:02:14 > 0:02:17Using this knowledge, I want to answer the key questions
0:02:17 > 0:02:20that the Chelyabinsk meteor strike raises.
0:02:24 > 0:02:30Where did this alien rock come from? When will the next one strike?
0:02:32 > 0:02:35And can we do anything to protect ourselves?
0:02:52 > 0:02:54A fortnight after the impact,
0:02:54 > 0:02:56the meteor strike is still big news in Russia.
0:03:01 > 0:03:05In Chelyabinsk there is a popular new winter pastime -
0:03:05 > 0:03:07hunting for any fragments of the meteorite that remain.
0:03:30 > 0:03:32Scientists have also been out in force,
0:03:32 > 0:03:35particularly around Lake Cherbarkul
0:03:35 > 0:03:39where there is evidence of an impact in the ice.
0:03:39 > 0:03:41So many fragments have been found here,
0:03:41 > 0:03:43it has been called the Cherbarkul meteorite.
0:03:45 > 0:03:49They are trying to piece together exactly what happened,
0:03:49 > 0:03:53because the fact is no-one in the scientific world saw this coming.
0:03:55 > 0:03:57I was shocked. I was truly shocked.
0:03:57 > 0:04:01I never thought I would see an event like this over a major
0:04:01 > 0:04:03city during my lifetime.
0:04:03 > 0:04:05We could not predict this was going to happen.
0:04:05 > 0:04:09The piece of rock that entered the atmosphere was relatively small,
0:04:09 > 0:04:13maybe only a few metres across, and so we could not see
0:04:13 > 0:04:15this before it entered.
0:04:23 > 0:04:25When something like this happens, there is no doubt about it,
0:04:25 > 0:04:28it is frightening.
0:04:28 > 0:04:30But I have to admit, as a geologist
0:04:30 > 0:04:34witnessing a once-in-a-lifetime event,
0:04:34 > 0:04:37it is utterly thrilling.
0:04:37 > 0:04:41You only had to look at social media to see that scientists
0:04:41 > 0:04:43all over the UK and around the world were getting very,
0:04:43 > 0:04:45very excited about this as the news broke.
0:04:45 > 0:04:49It was exciting. It was exciting for me as a meteoriticist
0:04:49 > 0:04:51because you immediately want to know, what is it?
0:04:51 > 0:04:56What has landed? Is it a bit of Mars or a bit from an asteroid?
0:04:56 > 0:05:00I am almost ashamed that I had such great excitement about seeing
0:05:00 > 0:05:02this event and knowing that meteorites had fallen,
0:05:02 > 0:05:05because people had been injured.
0:05:05 > 0:05:07Chebarkul was the biggest meteorite to strike the Earth
0:05:07 > 0:05:10since we've had the technology to measure them.
0:05:10 > 0:05:12From its journey through the atmosphere
0:05:12 > 0:05:15to its spectacular end, every moment was captured.
0:05:22 > 0:05:26One of the best documented 16 seconds in science ever.
0:05:26 > 0:05:29'Professor Alan Fitzsimmons is one of the scientists'
0:05:29 > 0:05:34who has been examining the meteorite footage frame by frame.
0:05:34 > 0:05:36These are amazing images,
0:05:36 > 0:05:38but what can you get out of these as an expert?
0:05:38 > 0:05:41What it shows us, first of all, is a great record
0:05:41 > 0:05:46of the entry of the object into the Earth's atmosphere,
0:05:46 > 0:05:51so you see it right from the moment it really penetrated and there it is.
0:05:51 > 0:05:55- That is the edge of the atmosphere? - That is it and it is coming down at a fairly shallow angle
0:05:55 > 0:06:00probably and as we play the movie on, what we see is, bang, there,
0:06:00 > 0:06:05it suddenly got brighter. So something has happened to the object.
0:06:05 > 0:06:09It is starting to break apart and as it breaks apart, it releases
0:06:09 > 0:06:13some of its orbital energy and that is causing that big flare up there.
0:06:13 > 0:06:16Is that because the atmosphere is denser, it is harder?
0:06:16 > 0:06:20That's right, it is finding it harder and harder to punch through the atmosphere.
0:06:20 > 0:06:27As we roll on, we suddenly get this bang, this huge flare up where
0:06:27 > 0:06:31suddenly the whole object is starting to fragment and break apart.
0:06:31 > 0:06:36That is where the majority of the energy is being released.
0:06:36 > 0:06:37If we look here.
0:06:37 > 0:06:39There are just little bits falling off.
0:06:39 > 0:06:42There is another flare up here and there is another flare up there,
0:06:42 > 0:06:47and now we can still see it's glowing, incandescent,
0:06:47 > 0:06:51some major fragment of the object is still falling down through the Earth's atmosphere.
0:06:51 > 0:06:54Underneath that trajectory you are going to have showers
0:06:54 > 0:06:57of bits of asteroid, essentially, falling down,
0:06:57 > 0:07:00and then finally 16.5 seconds later,
0:07:00 > 0:07:05- and what we are left with is this contour trail.- The shockwave.
0:07:05 > 0:07:07The shockwave coming towards us, that is right.
0:07:07 > 0:07:11It is about a minute later that it has gone and reached the ground.
0:07:11 > 0:07:15- This guy driving doesn't know yet that the shockwave is on its way here.- That is right.
0:07:15 > 0:07:17He is still happily listening to the radio on his drive to
0:07:17 > 0:07:18work in the morning.
0:07:22 > 0:07:24The explosion generated a shockwave
0:07:24 > 0:07:28so massive it was detected over 15,000 kilometres away.
0:07:30 > 0:07:35The low-frequency waves were picked up by monitoring stations.
0:07:35 > 0:07:38This is kind of like a listening network around the world.
0:07:38 > 0:07:41That is right. They're not set up for fireball or asteroid impacts,
0:07:41 > 0:07:44but set up to listen for nuclear explosions.
0:07:50 > 0:07:54What the monitoring stations picked up were some of the largest
0:07:54 > 0:07:56infrasonic waves ever recorded.
0:07:57 > 0:08:00Here they have been modified to make them audible.
0:08:03 > 0:08:07It has been detected down in Antarctica, we've got records of it
0:08:07 > 0:08:12up there in Alaska, so the pressure wave from the entry of the object
0:08:12 > 0:08:17and the explosive fragmentation was found, seen all over the world.
0:08:17 > 0:08:20So from the data that is coming in, it is early days, obviously,
0:08:20 > 0:08:25but from the data that is coming in, what is your best guess at the size of that rocky lump?
0:08:25 > 0:08:29Well, from the infrasound we know the energy released
0:08:29 > 0:08:34was something like 500,000 kilotons of energy, which is huge.
0:08:34 > 0:08:37- I was thinking it sounded a lot. - That is right.
0:08:37 > 0:08:38And because we know it came in,
0:08:38 > 0:08:42from the video footage, at about 17.5 kilometres per second,
0:08:42 > 0:08:46we can combine that energy with that velocity to get a mass of the object.
0:08:46 > 0:08:49From that mass, we can get a size
0:08:49 > 0:08:52and it is probably about 15 metres across or so.
0:08:52 > 0:08:54That is a rarity, isn't it?
0:08:54 > 0:08:58We think these things come in maybe, in once every 50 or 100 years,
0:08:58 > 0:09:02that is all randomly, so this is a really special and really rare event of course.
0:09:05 > 0:09:08Meteor strikes as big as this may be rare
0:09:08 > 0:09:11but scientists have a surprisingly detailed knowledge of what
0:09:11 > 0:09:14meteorites are and where they come from.
0:09:16 > 0:09:20Long before the meteorite reached its explosive finale in full
0:09:20 > 0:09:25view of Chelyabinsk's dash cams, it had a very different existence
0:09:25 > 0:09:27and going by a very different name.
0:09:32 > 0:09:35Meteorites begin life in deep space,
0:09:35 > 0:09:39as part of much larger bodies called asteroids.
0:09:41 > 0:09:42These can range in size
0:09:42 > 0:09:46from just a few metres to more than 900 kilometres.
0:09:46 > 0:09:49The leftovers from the nebula that created our solar system
0:09:49 > 0:09:52some 4.6 billion years ago.
0:09:54 > 0:09:56And millions of them
0:09:56 > 0:09:59circle the sun in a trail known as the asteroid belt.
0:10:03 > 0:10:05Here, collisions create smaller fragments
0:10:05 > 0:10:10and when these fall towards Earth, they take on one of two forms.
0:10:11 > 0:10:16The smallest pieces will burn up in the atmosphere to become meteors,
0:10:16 > 0:10:18what we call shooting stars.
0:10:20 > 0:10:23Only the larger fragments that make it all the way to the earth's
0:10:23 > 0:10:25surface are called meteorites.
0:10:28 > 0:10:30The meteorite is a piece of rock from space,
0:10:30 > 0:10:34or a piece of metal from space, that falls through our atmosphere
0:10:34 > 0:10:38and actually hits the ground to be recovered.
0:10:38 > 0:10:40Technically, scientists love their words, it is
0:10:40 > 0:10:44not a meteorite before it is actually found and discovered.
0:10:50 > 0:10:52By collecting and comparing meteorites,
0:10:52 > 0:10:57scientists have been able to piece together a picture of how they form
0:10:57 > 0:11:00and these studies have revealed some of the most remarkable rocks
0:11:00 > 0:11:02in the solar system.
0:11:03 > 0:11:06Few places in the world have got as many
0:11:06 > 0:11:10meteorites as the Natural History Museum, meteorites like this.
0:11:10 > 0:11:12It is a cracker, isn't it?
0:11:12 > 0:11:16The ones that are out here on display are just a fraction.
0:11:16 > 0:11:18The bulk of the collection is behind the scenes
0:11:18 > 0:11:21and that is where the science goes on.
0:11:26 > 0:11:29So all these are meteorites in some shape or form?
0:11:29 > 0:11:32They are all either meteorites.
0:11:32 > 0:11:36'Professor Sara Russell is expert at decoding the messages hidden
0:11:36 > 0:11:38'within these fragments of space rock.'
0:11:38 > 0:11:41This looks quite rocky, what about that one?
0:11:41 > 0:11:42It looks like a humble rock but if you hold it -
0:11:42 > 0:11:45be very careful of this one, this is older than the Earth, it is
0:11:45 > 0:11:46the oldest thing you will ever hold.
0:11:46 > 0:11:50- This is older than what, 4.6 billion years?- Yes.
0:11:50 > 0:11:54We have this number of 4.6 billion years of the age of the solar system,
0:11:54 > 0:11:56we know that from meteorites like this one,
0:11:56 > 0:12:00and from looking at the age of the components within it.
0:12:00 > 0:12:03If you can see, it has these rounded objects in it, which are 1mm to 1cm
0:12:03 > 0:12:08in size, these are called chondrules, and these were once free-floating.
0:12:08 > 0:12:11Before there were planets, these were free-floating in the solar system
0:12:11 > 0:12:16around the very young sun and then they slowly coalesced
0:12:16 > 0:12:18to make asteroids and larger and larger objects,
0:12:18 > 0:12:20until eventually planets were formed.
0:12:20 > 0:12:23These were the building blocks of planets.
0:12:23 > 0:12:25So the Russian meteorite, any news on what kind it is?
0:12:25 > 0:12:29Well, the early reports are that it is an ordinary chondrite
0:12:29 > 0:12:33and that means it will be similar to this one, so this is really exciting
0:12:33 > 0:12:37for us as scientists because we want to know how the planet is formed,
0:12:37 > 0:12:40what was around before the planets, what the environment was like
0:12:40 > 0:12:44and how the material that made up the planets first came together,
0:12:44 > 0:12:48and the chondrites are the best way of finding that out.
0:12:48 > 0:12:51Is this the most common in the solar system?
0:12:51 > 0:12:53It is the most common type to fall down to Earth.
0:12:53 > 0:12:56There is almost certainly a bias that the only material that we get
0:12:56 > 0:13:00to Earth is stuff that happens to cross the Earth's orbit.
0:13:00 > 0:13:02It has to be going in a slightly odd direction to cross the Earth
0:13:02 > 0:13:06anyway, so there is some kind of selection bias.
0:13:06 > 0:13:10This is a really special thing for you to kind of have in your career.
0:13:10 > 0:13:13Yes, if only something like this would happen in Britain
0:13:13 > 0:13:14so we could go and get it.
0:13:14 > 0:13:16I don't think there's too many people watching this programme
0:13:16 > 0:13:19that will be saying, "I wish it happened in the UK."
0:13:19 > 0:13:21- Obviously somewhere uninhabited.- OK.
0:13:21 > 0:13:24How much of this stuff comes to us every year?
0:13:24 > 0:13:26Actually, huge amounts. The Earth is growing by
0:13:26 > 0:13:31at least 40,000 tonnes a year, so a huge amount of material is falling
0:13:31 > 0:13:33to Earth but we don't really notice most of it
0:13:33 > 0:13:36because the vast majority of it comes in the form of dust.
0:13:36 > 0:13:40Although several thousand meteorites actually land on Earth every year,
0:13:40 > 0:13:43most of those actually go unnoticed.
0:13:43 > 0:13:46They fall just too far away from people.
0:13:46 > 0:13:49If a meteorite falls maybe 15 feet away from you,
0:13:49 > 0:13:51you probably won't notice it.
0:13:51 > 0:13:54It will make a dull thud and that will be it, unless it is very large.
0:13:54 > 0:13:57This event is special because it was so large.
0:13:57 > 0:14:01There was no way you could not notice this meteorite falling.
0:14:01 > 0:14:06It really wanted to get noticed. It said, "Ta-da! I am here."
0:14:06 > 0:14:11Those events are spectacular and they give us scientists these
0:14:11 > 0:14:15important pieces of rock from which we can learn about the solar system.
0:14:15 > 0:14:19It is remarkable how we are able to build up this picture of what
0:14:19 > 0:14:22is going on millions of miles away in the solar system.
0:14:22 > 0:14:27It is one of the joys of science really, almost like a detective
0:14:27 > 0:14:31picking up on those tiny clues to tell a bigger story.
0:14:31 > 0:14:35So that the big question, the one that really needs answering,
0:14:35 > 0:14:39is why do some of these asteroids suddenly head straight towards us?
0:14:42 > 0:14:46Over 95% of asteroids are found in an orbit between Jupiter
0:14:46 > 0:14:49and Mars, called the main belt.
0:14:49 > 0:14:52It's almost 200,000,000 kilometres across
0:14:52 > 0:14:55and home to millions of these orbiting rocks.
0:14:56 > 0:15:00These asteroids have been following the same path for millions of years.
0:15:02 > 0:15:06So long as they remain here, they pose no threat to Earth
0:15:06 > 0:15:09but occasionally, one goes astray.
0:15:11 > 0:15:14Collisions are one of the reasons why this might happen.
0:15:14 > 0:15:16But in the last decade,
0:15:16 > 0:15:21we have learned that just a few rays of light are enough, because one
0:15:21 > 0:15:25scientist has tracked the orbit of just one of these millions of rocks.
0:15:35 > 0:15:39Steve Chesley of NASA's Jet Propulsion Lab in California
0:15:39 > 0:15:45has made a study of 200,000,000 tonne asteroid called Golevka.
0:15:45 > 0:15:46This is a model of Golevka.
0:15:46 > 0:15:48It is actually about 500 metres across,
0:15:48 > 0:15:51say the size of a football stadium.
0:15:51 > 0:15:54It rotates in this direction.
0:15:54 > 0:15:57As you can see, it has a very angular shape to it.
0:15:59 > 0:16:02He set out to investigate a 100-year-old theory that said
0:16:02 > 0:16:06asteroids were powered by the sun itself.
0:16:06 > 0:16:09It was called the Yarkovsky effect.
0:16:09 > 0:16:13The Yarkovsky effect is a very small acceleration and acts on asteroids
0:16:13 > 0:16:14and what it is is,
0:16:14 > 0:16:17if you take a model, the sun is hitting the asteroid,
0:16:17 > 0:16:19warming the surface, and as the asteroid rotates,
0:16:19 > 0:16:22that hot surface radiates the heat out in a different
0:16:22 > 0:16:26direction into space and that causes an acceleration, a very slight
0:16:26 > 0:16:29acceleration coming from the photons that are emitted from the asteroid.
0:16:29 > 0:16:31The idea is that this acceleration,
0:16:31 > 0:16:34slight as it is, can have significant effects upon
0:16:34 > 0:16:37the orbits of asteroids over millions of years.
0:16:39 > 0:16:42It was an intriguing idea.
0:16:42 > 0:16:45What sent asteroids out of their orbit and on a path towards
0:16:45 > 0:16:50Earth was photon propulsion, but what was lacking was proof.
0:17:00 > 0:17:04The Arecibo telescope is over 300 metres in diameter.
0:17:04 > 0:17:08It is one of the most powerful telescopes in the world and it
0:17:08 > 0:17:12uses radar to mark the precise position of objects in deep space.
0:17:16 > 0:17:20It was this telescope that would allow Steve Chesley to detect any
0:17:20 > 0:17:23tiny alterations in the orbit of asteroid Golevka,
0:17:23 > 0:17:27more than 15,000,000 kilometres out in space.
0:17:29 > 0:17:31We knew that it would be in one place
0:17:31 > 0:17:35if the Yarkovsky effect wasn't acting on it, and it would be over here
0:17:35 > 0:17:38if it was acting and our models were correct.
0:17:38 > 0:17:42When Steve and his team studied the data, the results were unequivocal.
0:17:44 > 0:17:48We knew from the radar measurements where Golevka was within a few
0:17:48 > 0:17:53tens of metres and yet it was actually 12 or 15 kilometres
0:17:53 > 0:17:56away from where it was predicted to be without Yarkovsky effect,
0:17:56 > 0:18:00so these very precise radar observations allowed us to see
0:18:00 > 0:18:04the 12-kilometre displacement caused by the Yarkovsky effect.
0:18:06 > 0:18:10So photons, those elementary mass-less particles of light,
0:18:10 > 0:18:13really can create a tiny force.
0:18:15 > 0:18:17The force is about one ounce on earth,
0:18:17 > 0:18:20say that the weight of a shot glass, that is
0:18:20 > 0:18:24the force on this huge asteroid, the size of a football stadium.
0:18:26 > 0:18:29Even for me it is truly remarkable, it is dramatic that a force
0:18:29 > 0:18:34so slight can have such dramatic changes on individual asteroids'
0:18:34 > 0:18:37orbit over millions of years.
0:18:37 > 0:18:40The Yarkovsky effect is subtle.
0:18:40 > 0:18:44It takes many millions of years to gently nudge an asteroid
0:18:44 > 0:18:46out of its regular orbit.
0:18:46 > 0:18:49But once that orbit has been disturbed,
0:18:49 > 0:18:51the consequences can be profound.
0:18:53 > 0:18:56Now it can come increasingly under the influence of the solar system's
0:18:56 > 0:19:00largest planet - Jupiter.
0:19:01 > 0:19:05Jupiter has a mass 300 times bigger than Earth's
0:19:05 > 0:19:07so there is a huge gravitational field.
0:19:10 > 0:19:13Often that works to our benefit.
0:19:13 > 0:19:17Stray objects can be swept up in Jupiter's gravity,
0:19:17 > 0:19:19drawing them into the planet.
0:19:19 > 0:19:23We've actually observed Jupiter acting as a shield in this way.
0:19:25 > 0:19:28This photograph, from the Hubble Space Telescope,
0:19:28 > 0:19:34shows the fragments of a comet torn apart by Jupiter's gravity...
0:19:34 > 0:19:37as the pieces were drawn to the planet's atmosphere
0:19:37 > 0:19:41the impacts left blast scars - some as big as the Earth...
0:19:43 > 0:19:47..but there is a downside to Jupiter...
0:19:47 > 0:19:51it can also deflect asteroids into orbits that cross the Earth's path.
0:19:54 > 0:19:58The Chelyabinsk meteor appears to be one of these typical
0:19:58 > 0:19:59Earth-crossing events.
0:20:00 > 0:20:04The likelihood is that it was thrown out of its regular orbit
0:20:04 > 0:20:07by either one or a combination of the known causes -
0:20:07 > 0:20:13collision, the Yarkovsky effect, Jupiter's gravity.
0:20:13 > 0:20:17It continued its new orbit for hundreds, thousands,
0:20:17 > 0:20:22even millions of years before meeting its fateful end.
0:20:22 > 0:20:26We can even begin to trace the exact path that the Chelyabinsk meteor
0:20:26 > 0:20:29took on its collision course with Earth.
0:20:30 > 0:20:35Within the 16 seconds of action are all the clues we need.
0:20:35 > 0:20:41Now, from just one vantage point it's not clear...
0:20:41 > 0:20:45exactly how far up it is or how far away it is
0:20:45 > 0:20:49but that's what we get from looking at other vantage points.
0:20:49 > 0:20:52So, here we are, again, at a different angle.
0:20:52 > 0:20:54The object is coming in, almost out of the sun, there,
0:20:54 > 0:20:58and by combining this video clip with the other video clips,
0:20:58 > 0:21:00what we can do is trigonometry.
0:21:00 > 0:21:04Basically, you can figure out how high up the object was
0:21:04 > 0:21:06and how far away it was.
0:21:06 > 0:21:08And if you catch the object early enough
0:21:08 > 0:21:11then you actually know where it was in the atmosphere
0:21:11 > 0:21:13the first time you saw it.
0:21:13 > 0:21:15So, you're kind of, triangulating to get that fixed position
0:21:15 > 0:21:19- and it changes over time so you get the trajectory?- That's right.
0:21:19 > 0:21:23In the first part of the trajectory, what you've got there
0:21:23 > 0:21:26is a path that is relatively unaffected by the Earth's atmosphere.
0:21:26 > 0:21:30So, we can use that part of the video footage to track back
0:21:30 > 0:21:33and figure out where this object came from in the solar system.
0:21:33 > 0:21:35I love watching this because I now know where it is going to come
0:21:35 > 0:21:38and you see it just hitting the edge of the atmosphere.
0:21:38 > 0:21:40It's going to be...just...
0:21:40 > 0:21:43Come on, come on...
0:21:43 > 0:21:44- There it is!- Yup.
0:21:44 > 0:21:47It's about 90 kilometres up, at that stage,
0:21:47 > 0:21:51travelling at 17.5 kilometres per second.
0:21:52 > 0:21:55Using the different camera positions,
0:21:55 > 0:21:57scientists have pinpointed the exact position
0:21:57 > 0:22:01at which the meteor entered the atmosphere...
0:22:01 > 0:22:04and, by tracking the speed and angle of the shadows
0:22:04 > 0:22:05that the meteor casts,
0:22:05 > 0:22:09they've calculated its velocity.
0:22:09 > 0:22:13Together this is enough to track back the asteroid's path
0:22:13 > 0:22:14from deep space.
0:22:16 > 0:22:20Although the asteroid and Earth orbits are different durations
0:22:20 > 0:22:23and at angles to one another
0:22:23 > 0:22:27their clockwork regularity means that we were bound to collide.
0:22:29 > 0:22:33So, this shows, speeded up, obviously, three and a half hours,
0:22:33 > 0:22:36the last three and a half hours of the life of this little rascal.
0:22:36 > 0:22:39Yeah, it's nice to see it from the asteroid's point of view.
0:22:39 > 0:22:42The thing to remember is that this asteroid has been in its orbit,
0:22:42 > 0:22:46going around the sun, roughly once every two years, we believe...
0:22:46 > 0:22:49- Minding its own business. - Absolutely.
0:22:49 > 0:22:53..and, unfortunately, on February 15 it found a planet in the way.
0:22:58 > 0:23:00Sure enough,
0:23:00 > 0:23:06at 09:20 hours the neat yet entered our atmosphere above Siberia.
0:23:06 > 0:23:09On this path and at time
0:23:09 > 0:23:12it was Chelyabinsk that took the full impact...
0:23:16 > 0:23:20..but could there have been another scenario?
0:23:22 > 0:23:27The meteorite landed at a latitude of 55 degrees north,
0:23:27 > 0:23:30had it arrived just a few hours later
0:23:30 > 0:23:33we would have been directly in its flight path.
0:23:33 > 0:23:36So, was this a near miss for us?
0:23:36 > 0:23:40If the asteroid had been in a different part of its orbit,
0:23:40 > 0:23:43so it didn't hit this year but it hit next year,
0:23:43 > 0:23:45it would have still hit us on February 15th
0:23:45 > 0:23:47but instead of coming in over Russia
0:23:47 > 0:23:49it would have come in over the UK and Ireland
0:23:49 > 0:23:52and would have entered the Earth's atmosphere,
0:23:52 > 0:23:56in fact, entered the North Atlantic Ocean.
0:23:56 > 0:23:59In order for the meteorite to strike anywhere near Britain
0:23:59 > 0:24:03our paths through space would have had to be fundamentally different.
0:24:08 > 0:24:10So we know where asteroids come from
0:24:10 > 0:24:14and the forces that shape their date with destiny...
0:24:14 > 0:24:16but what exactly happens next?
0:24:16 > 0:24:19The moment that a meteor strikes?
0:24:19 > 0:24:23And what determines just how devastating that strike will be?
0:24:25 > 0:24:28When the Chelyabinsk meteor reached our atmosphere
0:24:28 > 0:24:33it was travelling at more than 65,000 kilometres per hour...
0:24:33 > 0:24:37and measured more than 15 metres across.
0:24:37 > 0:24:39Apart from some unconfirmed reports
0:24:39 > 0:24:41of craters at the bottom of Lake Chebarkul
0:24:41 > 0:24:44there's surprisingly few signs of an impact.
0:24:46 > 0:24:50Little of the 7,000 tonnes of space rock that entered the atmosphere
0:24:50 > 0:24:51have been recovered...
0:24:53 > 0:24:55..perhaps 300 fragments...
0:24:59 > 0:25:05..and yet, the effects were felt over 3,000 square kilometres.
0:25:05 > 0:25:10The question is how can apparently so little do so much harm?
0:25:14 > 0:25:16There's a clue from the last time
0:25:16 > 0:25:19Earth experienced a meteor strike on this scale.
0:25:31 > 0:25:34On June 30th, 1908,
0:25:34 > 0:25:41a huge explosion tore through the forest of Tunguska, Siberia.
0:25:41 > 0:25:46It was 20 years before the Russians mounted an expedition to the site.
0:25:46 > 0:25:48What they found astonished them...
0:25:53 > 0:25:58..60 million trees across an area the size of London
0:25:58 > 0:25:59had been levelled.
0:26:02 > 0:26:06Scientists thought it has been caused by a meteorite strike...
0:26:09 > 0:26:12..but then why was there no sign of any kind of impact crater?
0:26:15 > 0:26:20The answer is that the devastation had to be caused by a meteor attack
0:26:20 > 0:26:22of a very particular kind.
0:26:24 > 0:26:27Physicist Mark Boslough has been fascinated
0:26:27 > 0:26:29by how so much destruction can be caused
0:26:29 > 0:26:34without any apparent direct contact.
0:26:34 > 0:26:36The explosion at Tunguska was caused by an asteroid
0:26:36 > 0:26:39that entered the atmosphere, got close to the surface
0:26:39 > 0:26:41and exploded before it hit the ground.
0:26:41 > 0:26:46And that explosion created a blast wave with hurricane force winds
0:26:46 > 0:26:50that knocked trees over for thousands of square miles.
0:26:53 > 0:26:57Scientists call it an airburst -
0:26:57 > 0:26:59a massive explosion in the atmosphere,
0:26:59 > 0:27:01rather than on the ground.
0:27:03 > 0:27:08As it enters the atmosphere at speeds of up to 24 metres per second
0:27:08 > 0:27:12the air resistance decelerates the asteroid so fast
0:27:12 > 0:27:14it breaks apart in a huge explosion.
0:27:22 > 0:27:26Most of the damage from an explosion like this is actually the blast wave,
0:27:26 > 0:27:27it's the very high winds.
0:27:29 > 0:27:32Mark created a simulation to see what size
0:27:32 > 0:27:38an asteroid would need to be to generate such destructive power.
0:27:38 > 0:27:42In this simulation I include more of the physics to be more realistic.
0:27:42 > 0:27:44We can see that the main shockwave
0:27:44 > 0:27:46doesn't come out of the point of the explosion
0:27:46 > 0:27:49but it comes out of the point where the fireball descends to.
0:27:49 > 0:27:52So, by the time the shockwave gets to the ground
0:27:52 > 0:27:54it's much stronger than it would otherwise be
0:27:54 > 0:27:56and there's more damage on the ground
0:27:56 > 0:27:59because the destructive power was carried downward.
0:28:03 > 0:28:07Based on Mark's calculations, the devastation at Tunguska
0:28:07 > 0:28:09could have been caused by an asteroid,
0:28:09 > 0:28:12perhaps as small as 30 to 50 metres in diameter...
0:28:14 > 0:28:17..and this carries a worrying implication.
0:28:19 > 0:28:22Smaller asteroids are more dangerous than we used to think
0:28:22 > 0:28:25and because there are so many more smaller asteroids
0:28:25 > 0:28:26than bigger asteroids
0:28:26 > 0:28:30we need to take that risk more seriously than we used to.
0:28:32 > 0:28:37The lesson of Tunguska helps explain why in Chelyabinsk
0:28:37 > 0:28:41there's so much damage but very little meteorite to be found.
0:28:41 > 0:28:46If we go back to the video footage and we see the object coming in,
0:28:46 > 0:28:51when it's in the high atmosphere it suffers very little effect
0:28:51 > 0:28:53but just here you get this huge flare-up
0:28:53 > 0:28:57and that's because the atmosphere has become so dense
0:28:57 > 0:29:01that it's almost impossible for it to push through any more.
0:29:01 > 0:29:04And, basically, something's got to give, and the asteroid gives,
0:29:04 > 0:29:07and it, basically, just breaks apart
0:29:07 > 0:29:12in a huge catastrophic fragmentation effect,
0:29:12 > 0:29:15and that is what creates a shockwave,
0:29:17 > 0:29:18which we hear as this sonic boom.
0:29:18 > 0:29:21EXPLOSION
0:29:21 > 0:29:26Really it's a balance between the size of the object,
0:29:26 > 0:29:29its speed into the atmosphere and, critically,
0:29:29 > 0:29:31the altitude at which it explodes.
0:29:31 > 0:29:35Too high, if it's too small and it explodes too high
0:29:35 > 0:29:38the shockwave has little effect on the ground.
0:29:38 > 0:29:43If it's...quite low in the atmosphere, it's a large object,
0:29:43 > 0:29:45then that shockwave is completely devastating.
0:29:45 > 0:29:48Actually seeing it in real life really brings home to you
0:29:48 > 0:29:50the energy that these things carry
0:29:50 > 0:29:55and, even though it exploded tens of kilometres, perhaps, up in the air,
0:29:55 > 0:29:58so, quite a long way from the ground, the force of the explosion,
0:29:58 > 0:30:03the shockwave, was able to damage buildings over a huge area and injure people,
0:30:03 > 0:30:06and that was quite a shocking thing to see.
0:30:06 > 0:30:09The destructive power of an air blast is immense
0:30:09 > 0:30:13but, in a way, the people of Chelyabinsk are lucky
0:30:13 > 0:30:17because out there in the cosmos is a different kind of asteroid,
0:30:17 > 0:30:20one that poses an even greater threat.
0:30:20 > 0:30:23I've seen the evidence of what one of those can do,
0:30:23 > 0:30:25the damage that it leaves behind,
0:30:25 > 0:30:28and what you realise is the Earth's own destructive forces -
0:30:28 > 0:30:31you know, the great earthquakes, the volcanic eruptions -
0:30:31 > 0:30:34seem trivial in comparison.
0:30:41 > 0:30:44This is Barringer Crater, Arizona...
0:30:46 > 0:30:50..the 50,000-year-old remnant of a massive meteorite impact.
0:30:52 > 0:30:55'This place really gives you a sense of the destructive power'
0:30:55 > 0:30:57of incoming meteorites.
0:30:57 > 0:31:02The blast here would have vaporised a city larger than London
0:31:02 > 0:31:07but the lump of rock that did it measured barely 15 metres across.
0:31:19 > 0:31:24Down on the ground the scale of the impact is even more breathtaking...
0:31:29 > 0:31:32..the crater is more than a kilometre across
0:31:32 > 0:31:35and nearly 200 metres deep.
0:31:48 > 0:31:51The forces here were enormous,
0:31:51 > 0:31:53the impact turned this solid rock
0:31:53 > 0:31:56into this pulverised mush.
0:31:56 > 0:31:58It just...bursts out in your hand.
0:31:58 > 0:32:00I mean, look at that.
0:32:00 > 0:32:04They started out as the same kind of rock.
0:32:07 > 0:32:09The meteor that struck here was about the same size
0:32:09 > 0:32:12as the one that flattened Tunguska
0:32:12 > 0:32:16but there is a critical difference...
0:32:16 > 0:32:20at Barringer the meteor didn't explode in the atmosphere,
0:32:20 > 0:32:22it struck ground.
0:32:22 > 0:32:26So, this is just a fragment of the true devastation unleashed here.
0:32:35 > 0:32:39Fortunately, to understand exactly why ground strikes
0:32:39 > 0:32:41are so very destructive
0:32:41 > 0:32:45we don't have to wait for another Barringer to happen...
0:32:45 > 0:32:50because today we can simulate this kind of impact.
0:32:50 > 0:32:53And that's thanks to the research of Pete Schultz...
0:32:53 > 0:32:56and one very special piece of equipment.
0:33:00 > 0:33:05So, so, this was serial number one, it was built during the Apollo time.
0:33:05 > 0:33:08I guess because they thought there would be several of them made
0:33:08 > 0:33:10but this is the first one and the last one.
0:33:10 > 0:33:13And is the only one like it, in the world.
0:33:16 > 0:33:19This is NASA's Vertical Gun Range.
0:33:19 > 0:33:22It was built to study how impacts affected the moon
0:33:22 > 0:33:26as the astronauts prepared to make the first lunar landing.
0:33:26 > 0:33:29We are armed, gated and reset.
0:33:32 > 0:33:36Today, Professor Pete Schultz uses it to model precisely
0:33:36 > 0:33:38the dynamics of an asteroid impact.
0:33:38 > 0:33:43We know that these...asteroid impacts are bad
0:33:43 > 0:33:46but you want to understand really how bad.
0:33:48 > 0:33:53Peter uses the NASA gun to fire projectiles at very high speed
0:33:53 > 0:33:56to simulate an asteroid hitting the Earth.
0:33:56 > 0:33:58So, for this experiment we're going to fire
0:33:58 > 0:34:02this tiny quarter-inch aluminium sphere at very high speeds,
0:34:02 > 0:34:04up to around five kilometres per second,
0:34:04 > 0:34:07and then we will see what type of crater it produces.
0:34:09 > 0:34:13The target it will hit is made of sand.
0:34:13 > 0:34:18So, we use sand because it records the shock affects very clearly.
0:34:21 > 0:34:25Outside of the impact chamber are super high-speed cameras
0:34:25 > 0:34:27that can film at up to 1,000,000 frames per second,
0:34:27 > 0:34:31capturing every detail of the impact and the aftermath.
0:34:33 > 0:34:38- OK, lights out. Everything good? - Yeah.- OK, we're out of here.
0:34:45 > 0:34:51We have high voltage, the paddle is in, the warning lights...
0:34:51 > 0:34:53and...rolling.
0:34:53 > 0:34:55ALARM BUZZES
0:35:13 > 0:35:16Oh, perfect. Perfect, perfect.
0:35:16 > 0:35:20Now we're seeing the fireball come in - it's brighter than the sun
0:35:20 > 0:35:23and then, "Kapow!", it hits the surface. Jeez!
0:35:23 > 0:35:29This whole region, downrange, would have been incinerated.
0:35:29 > 0:35:32It would have been incinerated just by this plasma,
0:35:32 > 0:35:37this exploding vapour plume engulfing everything.
0:35:37 > 0:35:40There would have been winds that would have been going so fast
0:35:40 > 0:35:45it could pick up houses and spread them hundreds of kilometres away.
0:35:47 > 0:35:49This would have been Armageddon.
0:35:55 > 0:35:59Experiments like this reveal several important things.
0:35:59 > 0:36:01One is that it's not just the impact,
0:36:01 > 0:36:04it's all that vapour that runs downrange.
0:36:04 > 0:36:06In fact, you can see areas, here,
0:36:06 > 0:36:11where there was so much wind it actually carved out
0:36:11 > 0:36:12pieces of this landscape.
0:36:12 > 0:36:15So, what these experiments help us do,
0:36:15 > 0:36:18they actually allow us to witness the event -
0:36:18 > 0:36:20see it in real time -
0:36:20 > 0:36:24and try to understand the processes that are going on.
0:36:24 > 0:36:28It's really complex but we have to see it to understand it.
0:36:30 > 0:36:34So, asteroid impacts unleash a trail of destruction far greater
0:36:34 > 0:36:38than suggested by the footprint of the crater alone.
0:36:44 > 0:36:49Comparing the effects of an airburst with a ground strike,
0:36:49 > 0:36:51it seems the Chelyabins got away lightly.
0:36:56 > 0:36:59It's estimated that the largest piece to hit the ground
0:36:59 > 0:37:00weighed 500 kilos,
0:37:00 > 0:37:05a fraction of the asteroid's original mass of 7,000 tonnes.
0:37:06 > 0:37:10Now if a piece of rock that big had hit that area of Russia,
0:37:10 > 0:37:12it would have produced a huge impact crater.
0:37:12 > 0:37:16Then that kinetic energy is then delivered into the ground
0:37:16 > 0:37:18and we see things like seismic shock.
0:37:18 > 0:37:22So, you get... People would feel earthquakes on the ground.
0:37:22 > 0:37:26So, the fact that it was an airburst actually limited the consequences
0:37:26 > 0:37:27for the people on the ground.
0:37:27 > 0:37:29So, yes, still quite dramatic,
0:37:29 > 0:37:32still, you know, obviously, causing injuries
0:37:32 > 0:37:36but it could have been a lot worse, had it survived down to ground.
0:37:47 > 0:37:50Ground strikes are amongst the most destructive
0:37:50 > 0:37:52natural hazards we know of.
0:37:53 > 0:37:55When viewed from space,
0:37:55 > 0:37:58Earth's encounters with giant asteroids in its deep history
0:37:58 > 0:38:01are revealed.
0:38:01 > 0:38:03And there is evidence from our planet's past
0:38:03 > 0:38:06of a truly devastating meteorite strike
0:38:06 > 0:38:09that decisively altered the course of life on Earth.
0:38:11 > 0:38:15Today, millions of years after the impact,
0:38:15 > 0:38:18the evidence for that crater is well hidden.
0:38:21 > 0:38:23SHE SHOUTS
0:38:26 > 0:38:30This is a gateway to the cenotes,
0:38:30 > 0:38:33the unique cave system of Mexico's Yucatan Peninsula.
0:38:37 > 0:38:38Wow!
0:38:40 > 0:38:42Look at the size of this!
0:38:43 > 0:38:44This is magnificent!
0:38:45 > 0:38:47That is beautiful.
0:38:49 > 0:38:52'This cave may be stunning,
0:38:52 > 0:38:55'but it provides the evidence for one of the greatest catastrophes
0:38:55 > 0:38:58'in the Earth's history.'
0:38:58 > 0:39:00And that water, it's so clear!
0:39:02 > 0:39:04Lower the gear, please!
0:39:06 > 0:39:09There's actually much more to this amazing cavern
0:39:09 > 0:39:11than first meets the eye.
0:39:12 > 0:39:15But to understand the scale of what happened here,
0:39:15 > 0:39:17you have to go deeper still.
0:39:17 > 0:39:18Underwater.
0:39:21 > 0:39:22OK?
0:39:23 > 0:39:25I'm not sure if I'm ready for this.
0:39:25 > 0:39:28I've got all the equipment, but...
0:39:28 > 0:39:30there's something about going down into water
0:39:30 > 0:39:33when you're not quite sure where your exit is...
0:39:33 > 0:39:36But I trust Bernadette completely here.
0:39:36 > 0:39:37HE CHUCKLES
0:39:37 > 0:39:38She knows what she's doing.
0:39:38 > 0:39:40So I'm as ready as I'll ever be.
0:39:40 > 0:39:41- Ready?- All right.
0:39:57 > 0:40:02Descending into the depths of the cenote is like entering a new world.
0:40:10 > 0:40:14Fewer people have visited some of these drowned caverns
0:40:14 > 0:40:16than the surface of the moon.
0:40:26 > 0:40:28As divers have explored further,
0:40:28 > 0:40:33they've discovered the cenotes are actually part of a huge complex
0:40:33 > 0:40:35of tunnels and caves.
0:40:43 > 0:40:45In fact, when you look from above,
0:40:45 > 0:40:47you can see there are cenotes
0:40:47 > 0:40:50scattered across hundreds of kilometres.
0:41:01 > 0:41:03And when they're mapped,
0:41:03 > 0:41:05it becomes clear that they follow
0:41:05 > 0:41:07a distinctive circular course through the jungle.
0:41:10 > 0:41:14They mark out the rim of a giant crater.
0:41:16 > 0:41:18Scientific instruments show
0:41:18 > 0:41:22the structure of the underlying rock has been deformed,
0:41:22 > 0:41:27revealing the boundaries of a colossal meteorite impact crater.
0:41:38 > 0:41:44This amazing cavern is part of a bigger story, a much bigger story.
0:41:44 > 0:41:4765 million years ago, THIS was the site
0:41:47 > 0:41:51of one of the most catastrophic impacts in Earth's history.
0:41:51 > 0:41:56What became known as the Chicxulub meteorite landed here.
0:41:56 > 0:42:00And THAT triggered the extinction of the dinosaurs.
0:42:03 > 0:42:07The meteorite was 15 kilometres across,
0:42:07 > 0:42:10enough to cause utter devastation
0:42:10 > 0:42:12across the whole planet.
0:42:14 > 0:42:19It exploded with a force of 100 million million tonnes of TNT.
0:42:22 > 0:42:27The blast sent a giant plume of vaporised rock out into space.
0:42:29 > 0:42:33A crater was punched 30 kilometres into the Earth's crust.
0:42:35 > 0:42:40It was above this rim of weakened rock that these cenotes formed,
0:42:40 > 0:42:41millions of years later.
0:42:44 > 0:42:47The blast would have been ferocious.
0:42:55 > 0:42:56But it was what happened next
0:42:56 > 0:43:00that made the impact a global catastrophe.
0:43:02 > 0:43:06The blast plume that shot into space fell back to Earth.
0:43:09 > 0:43:12Billions of molten particles superheated the air
0:43:12 > 0:43:15to a temperature of hundreds of degrees.
0:43:18 > 0:43:20Fires swept the planet,
0:43:20 > 0:43:23choking the atmosphere with soot and dust.
0:43:23 > 0:43:28The dinosaurs, and most other creatures, were doomed.
0:43:30 > 0:43:32That discovery, back in the 1980s,
0:43:32 > 0:43:34about what happened at Chicxulub,
0:43:34 > 0:43:36changed everything.
0:43:36 > 0:43:38Up until then, we thought
0:43:38 > 0:43:41that the Earth had changed only through grindingly slow processes,
0:43:41 > 0:43:45but now we knew that there was also sudden, violent catastrophes
0:43:45 > 0:43:48that made the Earth the way it was.
0:43:48 > 0:43:49Of course, what that meant
0:43:49 > 0:43:52was that something like this could happen again.
0:43:52 > 0:43:53At any moment.
0:43:56 > 0:44:01Luckily, the very biggest asteroids are few and far between.
0:44:02 > 0:44:05But there are still plenty of rocks out there
0:44:05 > 0:44:07that represent a significant danger to us.
0:44:13 > 0:44:16So, at the summit of an extinct Hawaiian volcano,
0:44:16 > 0:44:18Professor Nick Kaiser and his colleagues
0:44:18 > 0:44:21are searching the skies for killer asteroids.
0:44:25 > 0:44:30Each night, using a revolutionary billion-pixel sensor,
0:44:30 > 0:44:34the team scans a vast swathe of the sky.
0:44:34 > 0:44:36Follow me up to the next floor,
0:44:36 > 0:44:37you'll see a better view
0:44:37 > 0:44:39of the telescope itself.
0:44:43 > 0:44:45They are looking for any unidentified objects
0:44:45 > 0:44:48that could be heading our way.
0:44:48 > 0:44:51By capturing several images of the same patch of sky,
0:44:51 > 0:44:53separated by several minutes,
0:44:53 > 0:44:55the team can see if anything's changed
0:44:55 > 0:44:57against the background of stars.
0:45:00 > 0:45:05You can see that there's a dark thing and a white thing.
0:45:05 > 0:45:08What that means is, in these two exposures,
0:45:08 > 0:45:11there was an asteroid, which was here in the first exposure
0:45:11 > 0:45:13and there in the second one.
0:45:13 > 0:45:16It's kind of cute, here's another one in the same image.
0:45:16 > 0:45:20And, in fact, we'll detect hundreds of asteroids in a single exposure.
0:45:23 > 0:45:25Their observations are collated
0:45:25 > 0:45:28at the nerve centre of asteroid detection -
0:45:28 > 0:45:30the Minor Planet Centre,
0:45:30 > 0:45:32just outside Boston.
0:45:42 > 0:45:45Its director is Tim Spahr.
0:45:45 > 0:45:50And his job is to keep track of every asteroid in the solar system.
0:45:58 > 0:46:01Tim has developed a map to visualise their location.
0:46:01 > 0:46:06And, on that map, the most important are the Near-Earth Asteroids,
0:46:06 > 0:46:08the ones closest to the planet.
0:46:09 > 0:46:13On the screen here is a map of the solar system.
0:46:13 > 0:46:15And I've got the sun in the centre
0:46:15 > 0:46:18and the third planet out here would be that of the Earth.
0:46:18 > 0:46:21The red dots in here are actually Near-Earth Asteroids,
0:46:21 > 0:46:24the green ones are the regular Main-Belt Asteroids.
0:46:27 > 0:46:31There are over 9,000 Near-Earth Asteroids.
0:46:31 > 0:46:34But there's one type they're particularly concerned to locate...
0:46:36 > 0:46:39..those asteroids that are over one kilometre in diameter.
0:46:42 > 0:46:46An Earth impact with one of these would spell disaster.
0:46:53 > 0:46:55Tim's data reveals
0:46:55 > 0:46:58that there are 900 asteroids bigger than a kilometre
0:46:58 > 0:47:01in those dangerous near-Earth orbits.
0:47:02 > 0:47:04But he has some good news.
0:47:06 > 0:47:08Right now, there's no information
0:47:08 > 0:47:12that any of those large objects will hit the Earth in the next 100 years,
0:47:12 > 0:47:16so we're safe from impact of those objects for at least 100 years.
0:47:17 > 0:47:21So there are no catastrophic asteroid impacts on the horizon.
0:47:23 > 0:47:25But there are still dangers out there.
0:47:36 > 0:47:38On 6th October 2008,
0:47:38 > 0:47:42asteroid hunter Richard Kowalski saw something that would change
0:47:42 > 0:47:45the assessment of threats presented by asteroid impacts.
0:47:45 > 0:47:48The night was proceeding normally
0:47:48 > 0:47:52and up on the screen came another asteroid.
0:47:52 > 0:47:55As I continued to make observations throughout the night,
0:47:55 > 0:47:57it appeared to be moving slightly faster.
0:47:57 > 0:48:00And this indicates that the object is close to the Earth.
0:48:02 > 0:48:04As with any other asteroid,
0:48:04 > 0:48:07Richard reported what he'd found to the Minor Planet Centre.
0:48:12 > 0:48:14I got up in the morning about seven o'clock
0:48:14 > 0:48:16and I had a message on the computer saying,
0:48:16 > 0:48:20"Could not compute an orbit for a particular object."
0:48:20 > 0:48:24I grabbed the observations of this object and I computed an orbit
0:48:24 > 0:48:28and it was immediately apparent, right then,
0:48:28 > 0:48:30that that object was going to hit the Earth.
0:48:30 > 0:48:33And, sort of ominous fashion,
0:48:33 > 0:48:35it said it was in 19 hours.
0:48:38 > 0:48:40Following a strict written protocol,
0:48:40 > 0:48:44Tim quickly reported the findings to NASA's asteroid investigation team,
0:48:44 > 0:48:46in California.
0:48:47 > 0:48:49We got a call from Tim Spahr, at the Minor Planet Centre,
0:48:49 > 0:48:54saying we had an impacter coming in, in less than 24 hours.
0:48:54 > 0:48:55That woke me up.
0:48:57 > 0:49:01NASA's expert on asteroid orbits, Dr Steve Chesley,
0:49:01 > 0:49:03raced to verify the data.
0:49:03 > 0:49:06The first thing I saw was a 1.000,
0:49:06 > 0:49:08a 100% probability of impact
0:49:08 > 0:49:09in less than a day's time.
0:49:09 > 0:49:11I'd never seen anything like this
0:49:11 > 0:49:13outside of simulations and software testing.
0:49:15 > 0:49:19An asteroid strike would create a huge explosion.
0:49:19 > 0:49:22NASA feared this might even be mistaken for a nuclear bomb.
0:49:22 > 0:49:25We wanted folks to know this was a natural event,
0:49:25 > 0:49:28by Mother Nature rather than some sort of man-made
0:49:28 > 0:49:30event like a missile or something dreadful.
0:49:32 > 0:49:35Information passed rapidly up the chain of command.
0:49:35 > 0:49:39NASA headquarters notified the White House that this was coming.
0:49:40 > 0:49:43Everyone wanted to know where it would strike.
0:49:45 > 0:49:48NASA predicted a remote area of the Nubian Desert.
0:49:59 > 0:50:02At 2:45 in the morning, NASA were proved right.
0:50:04 > 0:50:08The explosion created a vast fireball burning as hot as the sun.
0:50:10 > 0:50:14It was so big and so hot this image was captured by a weather satellite.
0:50:17 > 0:50:20And yet the object that caused it was only four metres across.
0:50:20 > 0:50:25Smaller than the asteroid which exploded over Chelyabinsk.
0:50:25 > 0:50:29I definitely think the impact was a wake-up call.
0:50:29 > 0:50:32I have to admit I never thought I'd see that in my career,
0:50:32 > 0:50:35where we would discover something and it would hit the Earth later that day.
0:50:40 > 0:50:43What was worrying about that impact was that the asteroid was too
0:50:43 > 0:50:47small to detect until it was very, very close to the Earth.
0:50:47 > 0:50:51Of course, for Chebarkul, it wasn't even spotted until it was already here.
0:50:53 > 0:50:56But we are getting better at spotting smaller asteroids.
0:50:57 > 0:51:01On the same day that Chebarkul was hit, another asteroid,
0:51:01 > 0:51:04similar in size to the object that created the Barringer Crater,
0:51:04 > 0:51:08came within just 28,000 kilometres of the Earth.
0:51:10 > 0:51:15Approaching from beneath the planet, asteroid 2012 DA14,
0:51:15 > 0:51:18passed inside the orbit of our geostationary satellites
0:51:18 > 0:51:20before heading off to the north.
0:51:22 > 0:51:26This asteroid had been successfully tracked for a year.
0:51:26 > 0:51:30Despite its proximity, scientists knew that it posed a threat.
0:51:35 > 0:51:41So we know we are safe for at least 100 years from most near-Earth asteroids over a kilometre in size.
0:51:43 > 0:51:46We are better at detecting objects down to 50 metres across,
0:51:46 > 0:51:48like DA14.
0:51:53 > 0:51:57But for asteroids smaller than that, like the one which exploded over
0:51:57 > 0:52:00Chelyabinsk, we still have little or no warning.
0:52:02 > 0:52:04There are still some we haven't found.
0:52:04 > 0:52:08So there's this unknown bit of the equation where we are still looking
0:52:08 > 0:52:11for some, we know they are there but we don't know where they are.
0:52:11 > 0:52:15So this is a threat, but hopefully as technology moves on,
0:52:15 > 0:52:19we'll always have a much better idea whether one's going to pose a risk to the Earth.
0:52:19 > 0:52:24We could see an event tomorrow or in 10 or 20 years time,
0:52:24 > 0:52:26that we hadn't previously detected.
0:52:26 > 0:52:28That is always the risk we face.
0:52:28 > 0:52:32Until we can catalogue and identify all the hazardous
0:52:32 > 0:52:36objects in the solar system, that risk will always remain.
0:52:39 > 0:52:44And there's one other factor that can make it particularly hard to spot an incoming object.
0:52:44 > 0:52:48It's the reason why no-one saw the asteroid that was hurtling towards Chelyabinsk.
0:52:51 > 0:52:54It came in in the daytime sky out of the sun.
0:52:54 > 0:52:55Right.
0:52:55 > 0:52:58We've got telescopes looking out there for these objects,
0:52:58 > 0:53:01but they only work at night.
0:53:01 > 0:53:04Radar doesn't help either, because to really use radar,
0:53:04 > 0:53:08to find these objects, you have to know exactly where to look.
0:53:08 > 0:53:12If you don't know what's coming in, you don't know where to look.
0:53:12 > 0:53:15Because of that then, this thing and objects like this,
0:53:15 > 0:53:18if they come in at that particular direction they're always going
0:53:18 > 0:53:21to take us by surprise at the moment with our current survey system.
0:53:24 > 0:53:27But even if we can spot an asteroid heading towards us
0:53:27 > 0:53:30and in good time to prepare,
0:53:30 > 0:53:33what if anything can we do?
0:53:34 > 0:53:40There's different options for deflecting asteroids and it is a bit sci-fi at the moment.
0:53:40 > 0:53:42The idea of shooting it out of
0:53:42 > 0:53:43the sky with a nuclear weapon
0:53:43 > 0:53:45would really be a dreadful idea.
0:53:45 > 0:53:49It would just shower us with radioactive debris,
0:53:49 > 0:53:53and it would just be... do more harm than good.
0:53:53 > 0:53:55What would be much better would be to push it, nudge it
0:53:55 > 0:53:59slightly off its course so that it wasn't then going to collide.
0:53:59 > 0:54:02So how do you gently nudge an asteroid?
0:54:02 > 0:54:04There's lots of different techniques to push it.
0:54:04 > 0:54:08So... The one I love is called a mass driver.
0:54:08 > 0:54:12There's a machine, which sits on the asteroid and throws off rocks,
0:54:12 > 0:54:15so it is accelerating rocks that way
0:54:15 > 0:54:20and that makes the asteroid gradually move in the opposite direction.
0:54:20 > 0:54:23You can paint one side of the asteroid white.
0:54:23 > 0:54:30That reflects the sun and there's this weird effect that makes the asteroid gradually drift across.
0:54:30 > 0:54:37We can launch a mission now, which is essentially can impact an asteroid
0:54:37 > 0:54:41and then deflect it - a bit like a billiard shot or a snooker shot.
0:54:41 > 0:54:45We just hit the asteroid extremely fast with a spacecraft
0:54:45 > 0:54:49and that small impact is sufficient to just alter its course
0:54:49 > 0:54:51so that it misses the Earth.
0:54:55 > 0:55:00When you consider Earth's history, stretching over billions of years,
0:55:00 > 0:55:04it's clear that meteorite impacts, far from being unexpected,
0:55:04 > 0:55:07are just a normal part of the life cycle of our planet.
0:55:16 > 0:55:18But that is not how they seem to us.
0:55:23 > 0:55:26The Chebarkul meteorite is a reminder of something
0:55:26 > 0:55:29we would probably rather not think about too often -
0:55:29 > 0:55:34how a sudden, apparently random event could have devastating consequences.
0:55:38 > 0:55:41EXPLOSION AND SCREAMING
0:55:43 > 0:55:45But this time we have been lucky.
0:55:45 > 0:55:49Although it was terrifying for those who witnessed it,
0:55:49 > 0:55:53this meteor struck without causing any fatalities.
0:55:53 > 0:55:56And close enough to be captured on multiple cameras.
0:55:56 > 0:55:59So it's given us a huge amount of information to help us
0:55:59 > 0:56:01prepare for the next one.
0:56:03 > 0:56:07I think perhaps the real lasting legacy of the Russian meteor
0:56:07 > 0:56:12will be the effect it has had on the popular consciousness and perhaps on politicians.
0:56:12 > 0:56:16Scientists have been saying for decades now that these things do happen from time to time,
0:56:16 > 0:56:20that they could be dangerous if they happened over populated area.
0:56:20 > 0:56:25But now we have actual proof, we have an event we can point to.
0:56:25 > 0:56:28We know it could've been worse than this.
0:56:28 > 0:56:32So, I think if this leads to more vigilance and perhaps,
0:56:32 > 0:56:36the detection of future impacting events, that'll be a good outcome.
0:56:36 > 0:56:41When a bit of an asteroid, comes through the atmosphere and lands on
0:56:41 > 0:56:45the Earth as a meteorite, it reminds us that the solar system is a dynamic place.
0:56:45 > 0:56:49It's... It's not finished.
0:56:49 > 0:56:54It's still working. It's still evolving and still changing.
0:56:54 > 0:56:58So next time you look up at the night sky, spare a thought
0:56:58 > 0:57:02for those thousands of rocky lumps whizzing across our path.
0:57:02 > 0:57:04A few of them have got our name on them,
0:57:04 > 0:57:08but the thing is by analysing in detail the data from the meteor,
0:57:08 > 0:57:11it means that next time, and there will be a next time,
0:57:11 > 0:57:14we will be much better prepared.
0:57:24 > 0:57:27Subtitles by Red Bee Media Ltd