Note: Where available, the PDF/Word icon below is provided to view the complete and fully formatted document
Disclaimer: The Parliamentary Library does not warrant the accuracy of closed captions. These are derived automatically from the broadcaster's signal.
Asteroids: The Good The Bad And The Ugly -

View in ParlView

(generated from captions) for generations to come. have created problems science does have solutions. As we have seen,

to ensure our future. We are in the race biology and the power of life. AARATHI PRASAD: Next time - How bacteria is creating fuel. produced from E. coli. This is the biodiesel The power of regeneration.

That's pretty impressive. I see it moving, now. of a long and healthy life. And unlocking the secrets Just by tweaking a few genes, the aging process. we can really influence Captions (c) SBS Australia 2012 High above us out in space there are millions of very strange but very special chunks of rock tumbling between the planets. a different story to tell Each one has to understanding the story of, and those stories are important of the solar system. from an extraordinary event - These are the asteroids, debris the birth of our solar system, four and a half billion years ago. solar system. Asteroids are fossils of the early from the starting materials They were accumulated from which everything else was made. to present a threat But asteroids continue of our planet. to the very future and child on the planet could die. If one hits, every man, woman about far more than destruction. Yet asteroids are scientists working to uncover Around the world, there are from the solar system tell us what these messengers about our place in the universe. the solar system we live in They essentially created and the planet that we live on. while asteroids may not be beautiful, And what they're finding is that over life and death on our planet. they do hold a surprising power Are we alone in the universe? (Man's voice echoes) in the outer solar system? Are there other unknown planets the same everywhere? Are the laws of nature Is the solar system stable?

(Laughs) High above the clouds, to challenge forces of the unknown. Professor Dave Jewitt dares

I like mysteries, I like to think about things that are really not understood something to address a problem, and if I can see some way to do really followed through with, you know, that other people haven't then that's what I want to do. And right now, more intriguing and mysterious he believes there is nothing of the solar system. than the tumbling rocks I'd be studying asteroids, If somebody told me 30 years ago I would have said "You're nuts, "all the hot science is elsewhere." spend my time on an object You know, why would I

to go anywhere in my lifetime? that basically is not going So how wrong can you be! And Dave Jewitt is not alone. Oh gosh, I just love asteroids. I suppose it makes me geeky, right? I love the way all that works. I love the equations of motion, I love the motion, up close We've seen very few of them we learn something new, but every time we see a new one, we weren't expecting. we find something left over from the solar nebula. Asteroids are the debris never quite made it to form a planet. They contain the raw material that of the early solar system. In a way, asteroids are fossils

we see have been processed On the Earth, all the materials or blown out of volcanoes by being sucked into the mantle on the Earth and so there's no material when the Earth still formed. which remembers what it was like that contain information Asteroids are time capsules in solar system history, about the earliest times from the other planets, information that's been lost lost from the Moon. that's been lost from the Earth, and they've, they've seen it all. Asteroids have been around into the earliest moments Asteroids offer tantalising clues of our solar system, the problem is - but for these scientists, how do you get at them? For the vast majority of asteroids, we have no information at all and a guess as to how big it is. except the existence of the object helps explain the problem. This fuzzy image This is what an asteroid looks like

optical telescope on Earth. through the most powerful an object you can hardly see? So how do you begin to study Well, one way is to study these - fallen to Earth and broken apart tiny fragments of asteroids that have called meteorites. you can hold in your hand, This is the oldest thing really a piece of history the Earth was formed, back to the time even before we were ever formed. even way, way before asteroids are actually made of Almost everything we know about what these kind of fragments. comes from studying of the solar system. Each one has its own history that we're trying to understand. They're like a puzzle some asteroids are made of iron In fact, we think that when they formed, or at least they were so large they heated and melted. to their core They could get all the iron has an iron core, like the Earth a big iron meteorite like this and when we take and slice it open, is really quite amazing. the quality of the metal It's a very pure metal. (METAL CHIMES) metal in the solar system Nickel iron, some of the oldest more than 4.5 billion years old. in fact, gigantic boulders ranging in size So, out there in space, there are to just a few metres, from 900 kilometres metal and dust. and made of primordial the remains of asteroids, But the more scientists have examined the stranger they get. It's probably a complete zoo and we find the meteorites of types and compositions, have a huge variety that the asteroids and it's telling us of compositions as well. must have a wide variety

that asteroids inhabit, and unfamiliar world in the coldness of space. out there are probably like this - We think that most asteroids very stony-like, we'd find on Earth the kinds of things different chemistry but they have a completely than Earth rocks. little bits of rocks all reheated They're put together like and glued together. and remelted and, And it tells us that the asteroid belt is a place with an incredible impact history, asteroids colliding into each other, breaking apart, reforming, and so when we see these meteorite samples, it's telling us about that amazing collision history in the asteroid belt. VOICEOVER: The place you grew up in

has a lot to do with who you become. Things you learn along the way stick with you, like a job well done is its own reward. We found it takes strength, pride, tenacity to make your mark, to dare to be different. And while a place shapes people, it's the people that make a place what it is, who we are. You see, we grew up in Newcastle and for nearly 110 years,

we've been developing smarter ways to grow your wealth, easier ways to do your banking, more affordable ways to own your own home,

and keeping the banks honest along the way. Now, everywhere we go, around the state, the country, around the world, we find people want the same things. When it comes to banking, you just want a fair go. And that's what we're here for. Over 90% of asteroids are found in an orbit between Jupiter and Mars called the main belt. Almost 200 million kilometres across, it is home to millions of these orbiting rocks

but perhaps the most pressing question is whether any of them are on a collision course with planet Earth. Arizona's arid desert air makes it the perfect place for a very special kind of job.

This is where people come to hunt asteroids. I started out hunting asteroids about 12 years ago, as an amateur.

I'd read an article in a popular magazine and that got me really interested in the field because not too many people were working there. It might not seem it, but Richard Kowalski is in the front line, defending planet Earth. Every night when I come up to the telescope,

I have it in the back of my head that every person on the planet

does have a vested interest in what I'm doing, and if one hits, there's the potential that every man, woman and child on the planet could die. Richard wants to discover any asteroids that could be on a collision course with Earth. This is our largest telescope. It's a 60-inch or 1.5-metres F-2. It's the telescope that we've been using for approximately five years now. We discover as many as 3000 new asteroids every night. But what Richard fears is that one of them could create destruction like this, or worse. Barringer Crater is just a few hundred kilometres from Richard's telescope.

It is one kilometre across and 200 metres deep.

It was made when a 300,000-ton asteroid smashed into Earth 50,000 years ago.

The Earth has been hit in the past and will be hit again in the future. What we'd like is to be able to discover these objects before they hit the Earth. So one of the great challenges for scientists is to understand what would happen if an asteroid were to strike planet Earth now. Pete Schultz wants to understand the unique nature of the explosion caused

if an asteroid were to impact with the Earth's surface. It takes a truly odd piece of equipment. Okay, we're getting close. So this was serial number 1. It was built during the Apollo time, I guess because they thought there would be several made, but this is the first one and the last one. It's the only one like it in the world. This is NASA's Vertical Gun Range. It was built to study how impacts affected the Moon as the astronauts prepared to make the first lunar landing. We are armed, gated and re-set.

Today, Professor Pete Schultz uses it to model precisely the dynamics

of an asteroid impact. We know that these asteroid impacts are bad but you want to understand really how bad. Schultz uses the NASA gun to fire projectiles at very high speed, to simulate an asteroid hitting the Earth. So for this experiment, we're going to fire this tiny quarter-inch aluminium sphere at very high speeds, up around five kilometres per second, and then we'll see what type of crater it produces. The target it will hit is made of sand. So we use sand because it records the shock effects very clearly. Outside of the impact chamber are special super-high-speed cameras that can film at up to one million frames per second, capturing every detail of the impact and the aftermath for analysis. Okay, lights out! Everything good? Yeah. Okay, we're out of here. We have high voltage, the panel is in, the warning lights - and rolling! (BUZZ AT ONE-SECOND INTERVALS) The ball travels 15 times faster than the speed of sound, and it incinerates exactly like some asteroids would. Perfect, perfect, perfect! Now we're seeing the fireball come in. It's brighter than the Sun and then kapow! It hits the surface. This whole region down-range would have been incinerated. It would have been incinerated just by this plasma, this exploding vapour plume engulfing everything.

There would have been winds that would have been going so fast, they could pick up houses and spread them hundreds of kilometres away. This would have been Armageddon. Experiments like this reveal several important things. One is that it's not just the impact, it's all that vapour that runs down-range. In fact you can see areas here where there was so much wind, it actually carved out pieces of this landscape. So what these experiments help us do, they actually allow us to witness the event, see it in real time and try to understand the processes that are going on. It's really complex but we have to see it to understand it. So asteroid impacts unleash a trail of destruction far greater than suggested simply by the footprint of the crater alone. It means they are far more complex and dangerous than many had previously thought.

But with a threat like this, what can you do? SONG: # I've seen the demons # But they didn't make a sound... # Well, for now, there's really only one thing you can do - and that's to keep an eye out for them.

# So miles and miles of squares # Where's the feeling there? # Still nobody cares # For miles and miles of squares... #

Watching for asteroids is what Richard Kowalski does, night after night, at his observatory here in the Arizona Desert. What you can see on this screen is that we've divided the sky into thousands of areas. We then choose a number of these areas into a single block which will then tell the telescope to observe each individual area in succession. Once it's gotten to the last area, it then goes back to the first area and repeats the process.

(EERIE MUSIC) Over the course of an hour, the telescope repeatedly scans the same areas of the sky. While the stars appear stationary, the telescope can spot any other objects that change position which could be asteroids. As you can see on this screen, is the sequence of four images that came from the telescope. These objects around the screen are not moving so we know that they're stars but this object in the centre is moving and thus we know that it's an asteroid. The importance of surveying for near-Earth asteroids is an asteroid impact on the Earth is truly the only natural disaster that we can actually predict before it happens. So whenever Richard finds an asteroid he thinks could be on a collision course with Earth, he immediately files a report... It goes to the central body whose job is to monitor possible asteroid impacts. Just outside Boston is the home of the Minor Planet Center. Its director is Tim Spahr, and his job is to keep track of every asteroid in the solar system. This is the nerve centre of the entire asteroid field. If somebody discovers something, it has to come through here. Our job is to then distribute that to the rest of the world. Asteroids' elusiveness is part of the thrill. In some cases, if you're studying an asteroid that's moving extremely fast, we ambush it. We go ahead of where we think it'll be and set the telescope up in that area, and hope it comes through the field, so it's actually ambushing. When you get the asteroid, then you chase it down and follow it. Not surprisingly, keeping track of thousands of objects in the sky

isn't something that you can do in your head. Thankfully, help is at hand. This is really the brains of the Minor Planet Center, right in here. This computer system has all the information

about where asteroids are, where they will be in the future. All the observations, all the software is in here. We definitely need this machine to be running all the time, we need it to be safe. We need everything working here. Do you feel a sense of responsibility? I definitely feel a sense of responsibility for keeping track of the asteroids.

I feel like it's our duty, it's our task to do that and I do feel personally responsible for it. And the task facing Tim is growing rapidly. In 1999, only 10,000 asteroids were known of. Since then, hundreds of thousands more of all shapes and sizes have been discovered. Tim has developed a map to visualise their location. And on that map, there's one class of asteroid he's concerned with above all - those near-Earth asteroids closest to our planet. On the screen here is a map of the solar system and I've got the Sun in the centre and the third planet out here would be that of the Earth. The red dots in here are actually near-Earth asteroids. The green ones are the regular main-belt asteroids. There are over 7000 near-Earth asteroids but there's one type of them they are particularly concerned to locate, those asteroids that are over one kilometre in diameter. These are the monsters of the skies. An Earth impact with one of these would spell catastrophe for the planet. If a one-kilometre diameter asteroid were to hit say, New York City, that would very likely affect people in, you know, 100 miles away, it might kill people 100 miles away, so you're talking really a catastrophe instantaneously as soon as it hits. Tim's data reveals that there are 900 asteroids bigger than a kilometre in those dangerous near-Earth orbits. But the big question -

are any of them on a collision course with Earth?

Right now, there's no information

that any of those large objects will hit the Earth in the next 100 years, so we're safe from impacts of those objects for at least 100 years. But there's still smaller asteroids than one kilometre that we have not yet discovered, so I can't say we're safe from them because we don't know where they are just yet. So for now, we are safe from a catastrophic asteroid impact -

even if the thousands of smaller asteroids might still pose a threat.

However, another group of scientists have a very different mystery about asteroids to investigate,

one that may help solve one of the greatest quandaries about life on Earth. In me is an excitement that doesn't waver. A sense of curiosity. Of purpose. Of pride. It builds in me, contained and focused. What will today bring? Some distant challenge beyond my knowing? I'm guardian. Protector. Halo of the fleet. I'm a Naval Aviator. I'm part of the team. This is Mauna Kea in Hawaii. It is home to some of the most powerful telescopes in the world. For 30 years, Professor Dave Jewitt has used them to probe deep into the solar system, and once Dave interrogates deep space, it's rarely ever the same again. 1992, I discovered the first objects found beyond Neptune since Pluto, the biggest discovery in the solar system since the discovery of the asteroids. It was a discovery that led to Pluto losing its status as a planet, something the world had taken for granted for over 60 years.

It established Dave's reputation as a pioneering astronomer. It's most important not to work on things other people are working on because you'll get the same result as everybody else and you won't make any discoveries. You'll just confirm what's already known. Dave's desire to journey where others fear to tread has led him to this. This bright dot with a long, hazy tail is called Elst-Pizarro. It was found alongside all the other asteroids in the asteroid belt. The problem was it just didn't look like an asteroid.

So why did it seem so out of place? It didn't look like the other asteroids, so it was a freak, and it got a lot of attention straight away because it was such a remarkable object. Nobody had seen anything like that before. What had got them excited was that, to astronomers, asteroids normally look like this, just a point of light, no dust cloud and definitely no tail. For years, Elst-Pizarro with its orbit of an asteroid but strange fuzzy appearance, left scientists baffled, until finally someone suggested an explanation. Finally, a paper came out saying it must be due to the collision between two asteroids, so two asteroids slammed into each other with high speed, and shattered, and produced a cloud of dust. So the strange tail was thought to be the debris from a collision between Elst-Pizarro and another asteroid.

And very quickly most of the scientific world forgot about Elst-Pizarro. But Dave didn't. He had a hunch that there was more to this puzzling little light in the sky than at first appeared, a hunch that, if proved correct, might help solve one of the great mysteries of life here on Earth.

Dave decided to investigate and began looking for someone to work with, somebody with a head for the challenge. When Dave suggested that I look at this object, I didn't actually know anything about it. Nothing had really been said about it in the last six years since it's been discovered and so I decided "It's just an interesting thing to look at." Dave and Henry knew that if Elst-Pizarro's fuzzy tail really had been caused by a collision, the debris should have dispersed by now and Elst-Pizarro should look like a normal asteroid again. But when they looked again, what they saw was that the tail was still there. It was strong evidence the collision theory was wrong. Collisions are very, very rare, so either Elst-Pizarro is the unluckiest asteroid in the solar system that keeps getting whacked and producing dust in that way, which doesn't make any sense, or there's another mechanism for producing the dust. Elst-Pizarro's appearance remained an anomaly. Dave and Henry realised that if they were going to make any real sense of it, they needed to find another example of an asteroid behaving in the same strange way. Dave and Henry's problem was that in the 200 years since asteroids were discovered,

Elst-Pizarro was the only one like it. Finding another one could be a complete wild goose chase. Using the giant telescopes on Mauna Kea, Dave and Henry began to hunt through the asteroid belt. For four years, they scanned the skies. They studied 300 more asteroids. All of them looked identical - except for one. When we saw these images, I didn't know what to think, actually. Maybe it's what we've been looking for all this time but we just... maybe just a bit nervous, you know. We may be on the cusp of something big. What they'd seen was an asteroid

sporting a tiny, faint, fan-shaped tail. Just like with Elst-Pizarro,

they were convinced it was just impossible this tail was created by a collision between asteroids. They had another explanation that to many seemed unthinkable. This is an image of a comet. They are objects that are thought to have been born in the freezing outer reaches of the solar system. They have long elliptical orbits that bring them towards the Sun and the Earth, and in comets, the tail is a sign of something very special inside the centre -

ice. Their appearance is due to the vaporisation of the ice that blows material off to make a tail,

so they have this distinctive appearance basically of having a long tail of dust. For 200 years,

the asteroid belt was thought to be an orbiting collection

of dry lumps of rock and metal. Dave and Henry's new idea was that those asteroids they had observed might look fuzzy and have tails because they too actually had ice inside them. It was a radical suggestion because scientists had always thought asteroid orbits were far too close to the Sun for them to be icy. People were uncomfortable because of this prevailing idea that the asteroids are rocky, and the comets are icy and there should be nothing in between. The reason why ice in the asteroids mattered so much is that it could help explain something that makes our planet unique in the solar system. Our beautiful blue planet is the only one to have an abundant supply of liquid water. Around 70% of the Earth's surface is covered by the oceans but there has always been a mystery

as to where all this water actually came from. For a decade, Dave Jewitt has been investigating this problem because scientists have established that when Earth formed over 4.5 billion years ago, it used to be a very different kind of place. The early Earth was really hot and formed from hot material in orbit around the Sun, so hot that we think the entire surface of the Earth was covered by liquid lava for the first 100 million years, a bit like the land that we see behind us. Dave believes the searing heat of molten rock would have had a profound effect on the Earth's early climate. Because it was so hot, we also think the early Earth was very dry. It's like baking something in the oven for too long, it comes out bone dry. We think the Earth was bone dry when it formed. That would mean that the lush, wet climate that we enjoy today must be the result of some dramatic events long after the Earth was born. The Earth got its water some time after it had formed and cooled down, by being hit by objects that carried water from somewhere else in the solar system. If Dave and Henry were right, a constant stream of icy asteroids hitting the early Earth could have played a vital role in bringing our planet its water. But for all their observations, they hadn't actually seen ice on an asteroid. So the one problem with our observations is that they only told us what the object looked like, and with that information, we knew, we thought we could only explain it with the presence of ice but we couldn't actually prove that that was the case. The last piece of the jigsaw finally arrived early this year

with help from the mighty telescopes of Mauna Kea. Andy Rivkin makes the invisible visible by using a NASA telescope to look at objects using infrared light. The infrared part of the spectrum is useful because it contains information about the composition of asteroids and other objects, and so, by observing that you get a better handle on the composition than you would if you observed only in the visible. Andy studies the shape of the infrared spectrum reflected off the surface of asteroids because tiny differences in the peaks and troughs can reveal what the surface is made of. Andy became interested in an asteroid called 24 Themis. The shape of its spectrum meant something very odd must be happening at its surface. We started by comparing it to other materials and objects

that we thought might be similar. We compared it to other asteroids but it didn't look like any of the other asteroids. We tried comparing it to meteorites and it didn't look like any of the meteorites. So we knew we had to come up with some other explanation. Finally, in April this year, Andy and his team published their explanation as to why 24 Themis gives off such a strange kind of light. We'd found that water ice was actually the best choice, and that was really exciting because it was the first time certainly that we knew of that anyone had found water ice out in the asteroid belt, even though it had been suspected for some time that it could be out there, no one had ever seen it. Andy had finally proved an asteroid really could be icy.

It now seems certain the strange behaviour and tail seen by Dave and Henry on their asteroids was caused by ice too. I think any time you make a discovery, it's exciting. Any time you find a new thing, it's a big thrill, definitely a big thrill, yeah, because it's hard. It means that asteroids could have played one of the most important roles in creating the Earth we see today. We know that asteroids did hit the Earth for billions of years. The question is what the asteroids brought with them. We previously thought mostly rock and metal. Now we understand that the asteroids would also have brought with them a lot more water and ice than we had previously expected. These discoveries are starting to change our understanding of the solar system. Water and ice really are abundant in the asteroid belt.

And that maybe water and ice is more abundant

throughout the entire inner solar system so finding the water in the asteroid belt is the key to starting to change our thinking about where Earth's water may have come from. Astronomers still don't know how much of Earth's water came from asteroids, and how much from other sources of ice, such as comets. Without that water, of course, life on Earth could not exist, which provokes what is perhaps the most intriguing question of all. Did asteroids play a role in the creation of life? Not far from San Francisco, California, there are scientists pondering this very question. Scott Sandford wants to investigate whether the basic chemicals of life could have been formed in space, perhaps even on an asteroid, so he's created the conditions of deep space in a machine. This machine has been developed to allow us to simulate environments that are out in space, either in the interstellar medium, the dense clouds where stars form, or the environments, let's say, in the icy satellites of planets in the outer solar system, environments that have low temperatures, no air, so vacuum, and high radiation fields. He wants to see if the complex carbon molecules that are essential to life could be created from the much simpler chemicals found in space. In this chamber is a sample probe covered in a tiny layer of water,

methanol and pyrimidine that is frozen to just 20 degrees above absolute zero and exposed to intense ultraviolet light. In this particular experiment, we're looking at whether certain conditions will form one of the nucleobases, so one of the molecules that makes up our DNA. And from his analysis of samples from experiments like this, Scott has made a remarkable discovery. By processing ices of the type we see out in space, we can make some of the buildings blocks that we see in biology on the Earth today. We're making the building blocks of life, that's what we're finding. But just because you can create these building blocks of life in a lab, it doesn't mean it really happens on an asteroid. So Scott has carefully examined meteorite samples to see if they contain traces of these chemicals. In some classes of meteorites, which we think come from asteroids, we find a variety of organic compounds, these include things some people are familiar with, like amino acids, which are the building blocks of proteins in our bodies, but also materials like the nucleobases which are the building blocks of DNA. So, hidden within the rock could have been the materials that made possible the emergence of life on Earth. And that means that when asteroids struck Earth, billions of years ago, they could have completely transformed our planet. Asteroids could have played an important role in getting life started on Earth by delivering the raw starting materials that we need to get everything going, to get life started. It seems the story of life on Earth is inextricably linked to the story... of asteroids. While many scientists are excited about what asteroids might tell us about the beginnings of life on Earth, new research suggests that it is how asteroids might put an end to life that should really concern us. (COUGHS)

(ALL LAUGH) (WHEEZES) VOICEOVER: At any time, your smoker's cough... ..can become smoker with lung cancer's cough. On 6 October 2008, asteroid hunter Richard Kowalski saw something that would help change the assessment of the threat presented by asteroid impacts. The night was proceeding normally and up on the screen came another asteroid. As I continued to make observations throughout the night, it appeared to be moving slightly faster

and this indicates that the object is close to the Earth. As with any other asteroid, Richard reported what he'd found to the Minor Planet Center. I got up in the morning about seven o'clock and I had a message on the computer saying it could not compute an orbit for a particular object.

I grabbed the observations of this object and I computed an orbit, and it was immediately apparent, right then, that that object was going to hit the Earth and in sort of ominous fashion, it said it was in 19 hours. Following a strict written protocol, Tim quickly reported the findings to NASA's asteroid investigation team in California. We got a call from Tim Spahr at the Minor Planet Center saying we had an impactor coming in, in less than 24 hours. That woke me up! NASA's expert on asteroid orbits, Steve Chesley, immediately started to verify the data. The first thing I saw was 1.000, which is 100% probability of impact in less than a day's time. I'd never seen anything like this outside of simulations and software testing. An asteroid strike would create a huge explosion. NASA feared this might even be mistaken for a nuclear bomb. We wanted folks to know that this was a natural event by Mother Nature, rather than some sort of a manmade event like a missile or something dreadful. Information passed rapidly up the chain of command. NASA headquarters notified the White House that this was coming. Everyone wanted to know where it would strike.

NASA predicted a remote area of the Nubian Desert. At quarter to three in the morning, NASA were proved right. The explosion created a vast fireball burning as hot as the sun. It was so big and so hot this image was captured by a weather satellite. As dawn broke, the smoke trail it left behind was still visible from the ground. I definitely think the impact was a wake-up call. I must admit, I never thought I'd see that in my career, where we'd discover something and it'd hit the Earth later that day. What makes this impact so worrying is that this asteroid was too small for anyone to see until it was very, very close to the Earth. For one scientist, it was a salutary reminder that we cannot afford to ignore the threat posed by small asteroids. Physicist Mark Boslough uses one of the world's most powerful supercomputers to study the hazards facing our planet from climate change to nuclear explosions. But for years he's been fascinated by a strange event at the beginning of the last century, and what it might tell us about the threat of asteroid impacts. On June 30, 1908, without warning, a massive explosion wiped out over 1,500 square kilometres of Siberian forest. Millions of trees were destroyed. Scientists thought it had been caused by an asteroid strike. But then, why was there no sign of any kind of impact crater?

The answer is that the devastation had to be caused by an asteroid attack of a very particular kind. The explosion at Tunguska was caused by an asteroid that entered the atmosphere, got close to the surface and exploded before it hit the ground, and that explosion created a blast wave with hurricane-force winds that knocked trees over for thousands of square miles. Scientists call it an air burst, a massive explosion in the atmosphere rather than on the ground. As it enters the atmosphere at speeds of up to 20 kilometres per second, the air resistance decelerates the asteroid so fast it breaks apart in a huge explosion. And crucially, it is small asteroids that are most likely to explode in this way. Most of the damage from an explosion like this

is actually the blast wave. It's the very high winds. Based on the physics of nuclear explosions, the original air burst model estimates the Tunguska explosion must have been 1000 times bigger

than the nuclear bombs at Hiroshima and Nagasaki. But crucially, the air burst model suggests the asteroid would have packed this huge destructive force even though it was as small as 100 metres in diameter. But Mark realised there was another problem. The model was ignoring a crucial difference

between nuclear bomb air bursts and asteroids. Asteroids are extremely heavy and move so fast that they carry huge momentum. He created a new simulation to investigate the effect this would have on their destructive power. In this simulation, I include more of the physics, to be more realistic. You can see the main shock wave doesn't come out

at the point of the explosion. It comes out at the point the fireball descends to, so by the time the shock wave reaches the ground it's much stronger than it'd otherwise be and there's more damage on the ground

because the destructive power was carried downward. Based on Mark's new calculations, the devastation at Tunguska could have been caused by an asteroid only one-third as large as previous estimates, perhaps as small as 30 to 50 metres in diameter, and for him, this carries a worrying implication. Smaller asteroids are more dangerous than we used to think and because there's so many more smaller asteroids than bigger asteroids, we need to take that risk more seriously than we used to. Mark's work means scientists may have to redraw the asteroid threat map. If a Tunguska-scale asteroid exploded over London or New York, it would be very destructive. It would be as destructive as a nuclear bomb exploding over one of those cities. Scientists estimate that there could be over a million of these kinds of asteroids up in space. But nobody knows where they are, or where they are headed. A 2010 report by the American National Academies of Sciences