In Our Time - Asteroids

Episode Date: November 3, 2005

Melvyn Bragg and guests discuss the unique properties of asteroids. They used to be regarded as the 'vermin of the solar system', irritating rubble that got in the way of astronomers trying to study more interesting phenomena. It was difficult or even impossible for an observer of asteroids to book time using the world's best telescopes, because they were regarded as unspectacular objects that could tell us little about the origins of the universe. However, that has all changed. It is now thought that asteroids are the unused building blocks of planets, 'pristine material' that has remained chemically unchanged since the creation of the solar system; a snapshot of matter at the beginning of time. At the moment the Japanese probe Hayabusa is 180 million miles away, pinned to the back of the asteroid Itokawa, attempting to gain our first samples of the chemical composition of an asteroid. Why did asteroids fail to form planets? How do they differ from their celestial cousins, the comets? And are either of them likely to create another impact on planet Earth? With Monica Grady, Professor of Planetary and Space Sciences, Open University; Carolin Crawford, Royal Society Research Fellow, University of Cambridge; John Zarnecki, Professor of Space Science, Open University.

This BBC podcast is supported by ads outside the UK. Thanks for downloading the In Our Time podcast. For more details about In Our Time and for our terms of use, please go to BBC.co.com.uk forward slash radio four. I hope you enjoy the program. Hello, comets and asteroids used to be regarded as the vermin of the solar system, irritating rubble that got in the way of astronomers trying to study more interesting phenomena. It was difficult or even impossible for an observer of asteroids

to book time using the world's best telescopes because they were regarded as unspectacular objects that could tell us little about the origins of the universe. However, this has changed. It's now thought that asteroids are the unused building blocks of planets, pristine material that has remained chemically unchanged since the creation of the solar system,

a snapshot of matter at the beginning of time. At the present moment, the Japanese probe Hayabusa is 180 million miles away, pinned to the back of the asteroid Itokawa, attempting to gain our first samples of the chemical composition of an asteroid. Why did asteroids fail to form planets? How do they differ from their celestial cousins

the comets and are either of them likely to create another impact on planet Earth? With me to discuss asteroids are Monica Grady, Professor of Planetary and Space Sciences at the Open University. Carolyn Crawford, Royal Society Research Fellow at the University of Cambridge and John Zanecki, Professor of Space Science at the Open University. Monica Grady,

asteroids were discovered in the 18th century when they were looking for something else. Can you tell us about how the asteroid belt was discovered? Yes, Bode's law was, well, it was Titious Bode and also by Kepler, understood or was studying the gravitational morphology or shape of the solar system. And from Newton's laws,

they thought that there should be a planet, from the way that things went around the sun, they thought there should be a planet between Mars and Jupiter. Now we can see Mars in the sky, we can see Jupiter in the sky, naked eye objects, and so they thought

there must be something in between them. And so astronomers looked and looked and looked, and they just couldn't find a big planet between Mars and Jupiter. So instead of a planet, what did they find? Well, what they found was lots and lots and lots of very small planets, which we now call the asteroids,

and that was right at the start of the 19th century, when an astronomer called Piazzi was looking for this, missing planet. And he'd got some star charts and he found a spot of light where there shouldn't have been one. And he thought it was a very, very dim star. And he looked again the next night and it had moved quite a lot. Now, stars don't move from night to night when you look through a telescope, but things like asteroids and comets do. So he tracked this for several nights and saw that it was moving very rapidly. And what you've got to remember is this is the day. before there were artificial satellites and things.

And so he realised it couldn't be a star. It was too small to be a big planet. But then unfortunately, the weather closed in and he couldn't observe it. Then he died. And it was lost sight of. And so there were these observations and people accepted that it was a small planet.

It was an asteroid. But it was lost for several more years, for about six more years, until it was discovered again, a second time. And we now call this series, C-E-R-E-S, after the goddess of harvests. And that was the first of the asteroids, the minor planets. And then they discovered an asteroid belt, which even though it's an asteroid belt, it's not a planet, it's about a 20th, altogether.

They're about the 20th of the moon, as I understand it. That's right. I mean, we have this sort of picture of an asteroid belt, which maybe comes from watching too many science fiction movies, where we see, say, the USS Enterprise, weaving its way through the asteroids. But they're all miles, kilometres and kilometres apart from each. One of the problems I have with the whole of this, and it'll go on for the rest of this discussion,

is the problem with distance. I simply can't get my head around it. They're astronomical, aren't they? Thank you very much. Getting me out of that one, Monica. Carolyn Crawford, can you give us a definition of an asteroid, this series, which is quite a big asteroid, still I think the biggest asteroid? Most of them are quite small.

Can you tell us what an asteroid is? Well, series itself, it's the largest asteroid, and that's just over 900 kilometres across. Some of the next biggest ones are about 500 kilometres across. And as you go down in size, you just get more and more asteroids. And current estimates are there are several million within the solar system. And these, if you like, they're just lumps of rock and stone. They've formed within the inner solar system.

And we think they're fragments, first of all, left over from the formation of the planets and also fragments which didn't actually form a planet themselves. So there were two views. One was that they'd split off from planets and they were just sort of floating around, and then the superseded by a view that they had tried to become a planet and failed to become a planet because of the gravitational pull that was going on,

that was pulling them apart. Is that right? Well, the original idea, and this stems from what Monika was talking about, in that astronomers were originally looking for a missing planet. Now, when they didn't find the missing planet in the gap between Mars and Jupiter, and instead they found all these, these small chunks of rock, these small minor planets,

they thought maybe there had been a planet there that had exploded in some way. We now tend to subscribe more to the view that that didn't happen, but in fact it was just lumps of rock that never yet coalesced to form a planet in that place. Why should we be interested in asteroids then? Well, asteroids are fragments of the very early planetesimals that went on to form the planets we see today.

If you think back to the very early stages of the solar sister formation, you've got the young sun at the core of this big nebula of cloud and dust. Now, within this dust particles start sticking to each other to form clumps, clumps stick together, they attract other clumps. And soon, well, within about 100 million years, you start having the proto-planets to go and form the planets we see today. But there's still a lot of leftover stuff. Now, some of this gets cleared out with collisions with other planets,

but some of it was left between Mars and Jupiter in relatively stable orbits. Now these are all fragments, I call them planetesples, of rock and iron that didn't go on to build a planet. And the reason that there's this asteroid belt of these fragments between Mars and Jupiter is that we think there would have been a fifth rocky planet if it wasn't for the fact that you had Jupiter sitting right next to this position.

Now, Jupiter is massive. It's over 300 times the mass of the Earth, and it's second only to the sun in terms of the gravitational influence within the solar system. So you've got all these. these planetesimals, but the gravitational perturbations from Jupiter keep stirring them up, keep accelerating, it stopped them from coalescing to form

a planet. But in terms of, finally, before I go over to John, in terms of the interest that is now being aroused, I've been around for a little while recently, is that they are outside the Earth and other parts of the solar system, and therefore in pristine form, because when things came together to form the Earth, for example, the chemical forces, the volcanic forces, the nature of what stuff was, but this is pristine stuff from 4.5 billion years ago.

And therefore, when examined, might give us answers to what happened here 4.5 billion years ago, which you can't get on our planet because our planet's changed so much since then. That's right. These are so small, they haven't undergone the geological processes that we, such as volcanism and the weathering and things that go on within the rocky planets. So if you can look at the chemical makeup, the composition of what of these asteroids, it gives you an idea of what our planets were built up of and what the original solar nebula that went on to form the whole solar system was made of.

John Zanecki, can you, are there asteroids just in the asteroid belt or are the asteroids elsewhere? Indeed, there are asteroids elsewhere. The asteroid belt that's been referred to already, most of the objects in that belt move in roughly circular orbits around the sun, so they're constrained to within this belt. but we find some now that have rather different orbits, elongated orbits, in such a way that their orbits pass much closer to the sun.

They come closer in and then travel further out. And of course what is interesting about those is that some of them have orbits which cross the orbit of the Earth. So that means they have the potential to impact perhaps with the Earth. Now, it's very unlikely because we don't all orbit exactly in a flat plane. That plane we call the ecliptic. Some of them are inclined to that plane. So you have to get, you know, everything has to be just right to have the two orbits intersect.

So everything has to be set up properly before we can be bombed again. But we were here, 65, we were talking about this on the programme a few weeks ago, 65 million years ago. So how likely are we to be hit again by an asteroid? Well, I think the answer to that question is easy. I mean, it's absolutely certain that we will be hit again. The question is when will we be hit again? People want to know if it's sort of next week. Well, I mean, I can't give you any predictions here and now

or with any certainty. That's the problem. And the question is, will it be in five years' time? Will it be in 5,000, 5 million years' time? I mean, we just have to look around us in the solar system at objects like the moon, or in fact any solid body, and we see the surface just peppered with impact craters. And on the astronomical time scale, you know, these happen relatively frequently. Well, I mean, you know, spaced by hundreds of thousands or millions of years.

Now, on the earth, we see, I don't know, 100 or two impact craters. We have a very active earth with weather and wind and rain and so on, and these gradually eradicate signs of impacts. But we live in what is a very dynamic solar system. It's not a quiet, gentle place. So it's a matter of when. And of course that is the difficult aspect of the question from a practical point of view. Should we be taking, should we be planning and looking for ways of maybe deflecting an

asteroid when we don't know if this is an event that's going to happen, as I say, in five years' time or five million years time. Okay, well, we'll come back to asteroid research in a few minutes, but I think let's bring in comets at this stage. Carolyn, in antiquity comets and mystical associations, they disturbed the idea of a fixed heaven, and they were always portents. And is it since Halle's comet that they began to, they were included in something which could be scientifically measured? They were domesticated by science in that sense. I think it stems probably from about Taika Brahe's time because for many, many years, the Aristotelian view that comets weren't something to do with the solar system,

but something that happened in the upper atmosphere held sway. And it wasn't until Taka Brahe made measurements and estimated how far away a comet was that they began to be recognized as solar system bodies. The key thing about Halle's work was that he showed that these are objects which do orbit the sun, they are part of our solar system.

In particular, the comet that bears his name, he was stimulated to actually look at comets and plot their orbits by the arrival of a very spectacular comet in 1682. And he went back and he plotted the orbits of 20-so comets that had visited over the past 300 years. And he noticed that there were a couple of comets in the past which shared the same orbital parameters as this recent comet. And that's when he made this leap of intuition,

that it was the same comet going on a 75-76-year orbit around the sun. and he predicted its return. So he brought in this idea of them being objects which come back, they reappear, and that they're in orbit around the sun. And John, Nicky, Fred Whipple, as I in the Senate, in the 1950s, gave us our modern definition of comets, what they're made of. Can you tell us what they're made of? Yes, well, the Whipple model of a comet is commonly referred to as the dirty snowball model of a comet,

which is, I am quite graphic. We basically believe, and this is what he postulated in the 50s, that a comet is in a way nothing more than a lump of snow and ice. Dirty because this snowball contains dust particles, solid grains of rocky material. And these orbit, these are typically he postulated 5, 10, 20 kilometres in size. So tiny on the astronomical scale. And these mostly are in very elongated elliptical orbit.

So they spend most of their time deep in the outer reaches of the solar system, you know, freezing cold, inert, doing almost nothing. So preserving their pristine nature. And then every now and again, they come in close to the sun or closer to the sun. The sun shines on them, the solar radiation, the energy from the sun, evaporates the outer layers and produces gas and some of the dust particles get blown off as well. and these then get blown by sunlight away from the sun to form the characteristic tail.

Great tail, there must be thousands of millions miles long. Oh, absolutely, yes, but incredibly tenuous. But the dust particles in the tail, for example, they reflect the sunlight. And so that's how we see them. And as I say, as with asteroids, this is other material left over from the formation of the solar system. But once again, because these objects are so small,

when they came together, when they coalesced, there really wasn't enough energy for anything to happen for high temperatures and pressures to develop. So this, again, is pretty pristine material left over from the era of when the solar system formed. Comets and asteroids seem to be the two building blocks, one, the asteroids, solid, rocket, collards, gaseous, volatile and so on. Why do you think they're...

Sorry. I was just going to say they do overlap in some way. We think, as John has said, the comets are made of ice with dust in, the asteroids are made of rock and metal. There is another belt of objects called the Kuiper Belt, which is out beyond Neptune, which is made of perhaps mixtures of rock and ice, which we don't fully understand.

Now, a colleague of ours described this very graphically, I think, and he said that we used to regard asteroids and comets as animals which we kept in a zoo in separate cages. Now we regard them as perhaps the solar system more as a safari park where these animals mix together. And so we don't keep them separately anymore. And I think that's a lovely description of the way that these things perhaps overlap in some ways.

Clearly now, energy and research is going into comets and you're involved in the European Space Agency's Rosetta Mission, which is going to try to land on a comet, and bring some samples back. What do you expect to find from the analysis of these samples? Well, first of all, we should say that Rosetta is not going to bring material back to the Earth for analysis. It's going to do its analysis in situ actually at the comet and on the comet. I mean, it's really still, to me, even though I've been involved with it for some years,

still sounds like science fiction, but the spacecraft is already on its way. It was launched last year. So this is going to arrive at its target comet in 2010. It's going to travel along with it. It's going to rendezvous. It will travel in towards the sun as the comet approaches the sun. As I said before, the heat from the sun will gradually build up as the comet gets nearer

and we'll observe that the activities start to switch on. And then in 2014, I still find it difficult to. think in these timescales. A small craft will detach and will land on the surface of the comet and it will, the instrument I'm involved with

will try and measure physical properties. You know, how hard is the surface? How is the comet put together? Is it really, you know, a tough, hard surface or is it a loose collection of material? And there is a remarkable essentially a chemistry laboratory

built by my colleagues at the Open University. that will actually attempt to do a chemical analysis of the material in the comet. And they're really exciting. Well, I mean, I'm semi-involved in that instrument. And that will analyse the water in the ice, and it'll tell us, and we've all got a cup of water here in front of us to drink from. Well, that is commissary water in here. I mean, all the water on the earth has possibly come from, partly from comet.

and we can analyze this. We know water has got hydrogen and oxygen in it. Some of that hydrogen has the heavy isotope called deuterium. And the chemical laboratory on the comet for the Rosetta Mission will look at the hydrogen and the deuterium. And it will be able to tell from the comet then how much of the water on the earth because of its hydrogen to deuterium ratio, how much of this water has actually come from cometry sources

and how much has come from the earth itself. Can we just bring in meteorites before we go back to more research on asteroids? You have in your hand an extraordinary object. It's about half the size of a cricket ball. It's black. I'm looking across with not so very good. It's got sparkly bits. And it's 4.5 billion years old. It's about 4.5.5. It's 4,569.5 million years old,

if you want to be a bit more precise about it. And what I've got here is a chunk of an asteroid. It's a chunk of a rocky asteroid. And we were talking earlier about big impacts on the Earth or small impacts happen every day. Small impacts from meteorites, and these are fragments broken from the asteroid belts. It looks on the outside.

It just looks like a rock that you would find by the side of the road in your garden. It's when you break it open inside and you see the shiny bits, which is metal, and you don't get metal like that on Earth. One of the other ones that I've got here is a solid chunk of iron, nickel, metal. And we study these to learn about the original material that form the solar system,

these bits of... But I understand, I've got two views on meteorites from reading for this programme. One is that there are bits of asteroids coming in, giving us similar information to that which we can get from. The other, that actually they're debris from Mars. Oh, right, there are a few. I mean, there are 30,000 known meteorites, and 35... of those have come from Mars, all right? 35 have come from the moon, by coincidence,

and all the rest are from the asteroid belt. So there are 35 that have very specifically come, an asteroid has hit the surface of Mars, made a crater, blasted bits off, and they've fallen to Earth as meteorised. Can we go back to asteroids now, talk about asteroid research and what you're trying to get out of this research

and how important it is? Carolyn Crawford, there are these fly-past missions. There's one of these European probe, Galileo, which photographed two asteroids, I understand it, Gaspora and Eider. What did it find? Well, this was the first time that we were able to get close to an asteroid. Remember, if you're trying to observe them from Earth, they're very faint, they're very small, it's very difficult.

You have to be quite inventive to get details about the physical properties of the asteroid. There's no substitute for actually being there. And these first images from this flyby, Galileo was en route to Jupiter, and he just did two firepasses of these asteroids. And it's from these images that we get our very modern view of what an asteroid looks like and they're very irregular in shape. They're misshaper.

The analogy is often drawn to potato shapes almost. Remember they're small, they don't have enough gravity to pull them into a spherical shape. They're pot mark with craters. They've undergone collisions themselves. They're irregular. They're potmark with craters

and they have some kind of dusty surface layer, perhaps debris from these past collisions. And in fact, one of these one of these asteroids Ida was found to have its own little moon which was called dactyl orbiting around it and that's true we think now for several of the asteroids that they do have individual moons themselves

John? I mean isn't it surprising that as you say in some of these images we see boulders and perhaps layer of dust on the surface now I mean presumably the surface gravity of these objects is absolutely negligible so why does this material hang around

I mean, it's strange, isn't it? I don't know if maybe it's electrostatic attraction. I may say that both of your colleagues are shaking their head. We don't want to answer that question. I mean, we've got some wonderful close-up images of the asteroid Eros, which came from another mission called the Near Earth Asteroid Rondevous mission from about four years ago, where a satellite orbited the asteroid for a whole year

and took marvellous close-up images of it. I mean, we've got some spectacular view. But the problem as I understand it, Monica, is that taking photographs, taking images, takes you so far. Oh, absolutely. But you've got to get the stuff in order to find out what you people really want to find out. And John, can you tell us about the Japanese mission is on the way to do that, isn't it? It's the spacecraft called Hayabusa. It's currently gathering evidence from the asteroid Itokawa.

This is evidence of me going to see a lot of Japanese films in my time. And I just want that noted. And what are they looking for? And what are they going to do? How is you going to push this research forward? Well, this is an absolutely remarkable mission. I'm really surprised that it hasn't, you know, made the headlines. It's more than on its way to this asteroid.

It's actually circling it at this very moment. It's literally hovering kilometers above the surface. It's attempting to sort of scoop a sample of material from the asteroid and bring it back to Earth. But it's got some technical problems. It's the system which controls its attitude, which stabilises it, has broken down. So the spacecraft is tumbling, I think,

and the controllers are trying to stabilize it. But it really is, whether it works or not, I don't know. But, you know, really to understand these objects, we've got to get the material back to the earth. We can do fantastic things, you know, there in situ. But with the incredible analytical instruments that we have, you know, in the laboratory, is here on Earth, that's really, that's the gold standard really for this research.

And this is what Hayabusa is hoping to do. Once it stabilises its gyros, it's going to drop a small probe onto the asteroid. Now, you've already said there's not much gravity there. And so possibly if it's not careful, it can, you know, shove the asteroid out of the way. So this thing isn't going to be able to land in the way a spacecraft lands on the moon or on other planets. So it's actually going to roll and tumble across the world. surface and gradually it'll sort of come to a halt they hope and then it'll it'll shoot a laser into the surface and explode some material which will be sort of sucked or shot up into a cone

which will then be pushed back up to the sort of mothership and then brought back to earth bringing it's only a few grams five grams or something a tiny amount of material back to worth for us to analyse. But we can get so much information just out of a single grain, something that only weighs 50 micrograms. I mean, everybody listening to this frog will inevitably think see the world in a grain of sand. Absolutely, absolutely. William Blake, etc.

What you hope to get from these few grams is what? Well, to say we hope to get the history of the universe is a bit, pushing it a bit, but certainly... At least the history of the solar system. The history of the solar system. The universe is 13.7 billion years old. Our solar system's 4.5 billion, so the universe was going for 10 billion years before our sun formed. But looking at these objects, we're going to have samples absolutely of this pristine material, the original dust grains, that have not been changed as they come through the Earth's atmosphere.

We're going to be able to see perhaps grains from other stars, which have also seeded our solar system. and we will be able to say, right, okay, this happened then, and then it got heated and then perhaps it melted on its little planet, and put together a whole timeline. You know, we think it's very important to understand the history of the Earth, the history of the history of the whole solar system. Do asteroids still have water? We know they're water and very important metals, Caroline.

Are they going to be useful as sort of, little pump petrol stations, fuel stations on the way to future explorations of space? Well, yes, this is getting a little bit, again, into the realms of science fiction, but it's a very interesting possibility when you look at our dwindling mineral resources on Earth,

they haven't run out yet, but given a couple of centuries, they're going to be in short supply, and Monica's been telling us about these asteroids that are made of nickel and iron. If you could set up a mining camp on one of these asteroids, you could get billions of tons of high-grade metal ore.

There's also the war. water ice there to sustain the mining camp. Or if you want to be really challenging, there's the nice idea if we could tow an asteroid into orbit around the earth itself as a handy, as you say, petrol station. So it's a supply of mineral resources, and we're going to need these if we are going to explore and colonise the solar system, if we're building large space structures, if we're needing lots of rocket fuel, it doesn't make sense to keep launch them away from the Earth's gravity. Why don't use stuff that's already out there in space is easily extracted from small objects

they don't have a lot of gravity. I remember AC Clark saying once that yesterday's magic is tomorrow's reality. Do you think that's tomorrow's reality, Monica Crudy? Yes, I do. I mean, these metal meteorites are not just iron and nickel, but they are also, have got platinum, palladium, iridium in them, and platinum we use as a catalyst for catalytic converters. All these sort of elements that are actually in the core of the earth,

which we can't get at. It's easier in terms of technological terms to envisage, developing systems which will go out to the asteroid belt and mine them there, rather than penetrating to the great depths, pressures and heats of the Earth's core. I basically agree with my colleagues here that the trouble is our funding agencies might not look quite so positively. You know, if we put together a grant application in the next few weeks to study something like this, unfortunately their horizon tends to be a bit shorter. I mean, what I would like to see us doing though actually is being a lot more,

careful with the Earth's resources rather than saying, well, okay, if we run out of our own stuff, we'll go and get it from an asteroid. I'd rather... Towing an asteroid. Yes, I'd rather we're much more careful here. Well, thank you all very much. Thank you, Monica Grady, Carolyn Crawford, John Zaneke.

And thank you very much for listening. We hope you've enjoyed this Radio 4 podcast. You can find hundreds of other programmes about history, science and philosophy at BBC.com.com.com.