In Our Time - Comets

Episode Date: January 17, 2013

Melvyn Bragg and his guests discuss comets, the 'dirty snowballs' of the Solar System. In the early 18th century the Astronomer Royal Sir Edmond Halley compiled a list of appearances of comets, bright objects like stars with long tails which are occasionally visible in the night sky. He concluded that many of these apparitions were in fact the same comet, which returns to our skies around every 75 years, and whose reappearance he correctly predicted. Halley's Comet is today the best known example of a comet, a body of ice and dust which orbits the Sun. Since they contain materials from the time when the Solar System was formed, comets are regarded by scientists as frozen time capsules, with the potential to reveal important information about the early history of our planet and others.With:Monica Grady Professor of Planetary and Space Sciences at the Open UniversityPaul Murdin Senior Fellow at the Institute of Astronomy at the University of CambridgeDon Pollacco Professor of Astronomy at the University of WarwickProducer: Thomas Morris.

This BBC podcast is supported by ads outside the UK. Thank you for downloading this episode of In Our Time. For more details about In Our Time and for our terms of use, please go to BBC.co.com. UK slash Radio 4. I hope you enjoy the programme. Hello, one evening in April 837, a strange new star was spotted in the skies above Northern Europe. It was exceptionally bright and had a mysterious long tale. Louis the Pius, king of the Franks, became convinced that it was a portent of his own death.

He wasn't the only one to be worried. A scholar in Iraq recorded that this fiery apparition caused widespread terror, while in China the emperor summoned his astronomer royal to demand an explanation. The object that caused so much concernation was Halley's comet, a small body of dust and ice which orbits the sun and is visible from Earth about every 75 years. Famously it appeared in 1066 just before the Norman invasion, and it was most recently seen in 1986. Comets are some of the most spectacular objects in the night sky,

but are rarely visible to the naked eye, although two unusually bright comets are forecast for later this year. So where do comets come from? What do they consist of? Why are they important? And why do they exert such a powerful hold in our imaginations? With me to discuss comets are Monica Grady,

Professor of Planetary and Space Sciences at the Open University. Paul Mirdin, senior fellow at the Institute of Astronomy at the University of Cambridge, and Don Palacko, Professor of Astronomy at the University of Warwick. Paul Merton, can you ever be? tell us about comets, there's been just attempts to describe them for over 2,000 years, but can you tell us exactly what a comet is? Well, comets are akin to planets, as you said, in that they orbit around the sun through

the solar system. They're solid bodies. They're typically lumps of ice, maybe in the range a few meters in diameter up to perhaps 100 kilometres in diameter. and they have orbits which are distinctly non-planetary. The planets all march in an orderly fashion around the sun, all going around in the same direction, going in orbits that are more or less circular and going in orbits which are all in the same plane. But comets come from every direction in the sky,

and their orbits are long and thin, very eccentric ellipses. They appear somewhere out in the outer parts of the solar system, coming from goodness knows where. We'll find out later. And they zoom around the sun and go back again. They may take 30 years to do that. They may take a thousand years.

They may only visit the sun and the solar system once, at least most of them. So they're akin to planets, but not planets. And they're there all the time. They're busy all the time. They're in the sky all the time. They come sporadically, and that's what gives them their superstitious power.

You can't predict the appearance. You usually can't predict the appearance of a... Well, honey did, didn't you? Yeah. Nowadays, there are maybe 150 that you can predict the recurrence of over and over again. But of the 4,000 comets that are known, most appear just once and comes sporadically. And if they come sporadically and shine very brightly and appear, goodness knows where in the sky, then the thought was that astrologically the heavens are. are a mirror of human activity, they must signify something really important and possibly catastrophic, like the death of a prince, the future death of a king, somebody changing government from the Saxon administration to the Norman administration. You said we know about 4,000 of them.

Is that the tip of the cometberg? I mean, are there probably 4 billion or something? Are we just beginning to know? So what we might know in the future might change our idea of them substantially? What we do know is that most of them are a long way away. and so they're very dim, very difficult to see, very difficult to find. So we know only 4,000, but there are millions, billions, perhaps. What's the difference between a comet in an asteroid?

If you'd ask me that question 10 years ago, I'd give you a very simple answer, which is that an asteroid is a rock in orbit around the sun between the planet Mars and the planet Jupiter, and a comet is ice and comes from a long way away, in one of these eccentric orbits that I've just described. Nowadays we see more of a continuum of between asteroids and comets. So you could say that asteroids were icy rocks and comets are rocky bits of ice.

Monica Grady, can we be more specific. Paul Merton hinted at it but left the answer to you. I think rather politely. Where did they come? You seem to know where they come from, so can you tell our listeners where that is in the solar system? So we don't know about the solar system. 5,000 million years ago,

and this is what we're in that system. We know we're ours at the moment. Yes. 4,500 million years ago, there wasn't a solar system. There was just a cloud of gas and dust. And that collapsed, and most of the gas went into the centre and formed the sun, which got very, very hot.

And the further away you go from the sun, the cooler it gets, and you get to a certain point called the snowline where ice can condense. And it's thought that that's where comets would have started to form, and they then got scattered outwards. And now all the comets live somewhere called the aught cloud. That's 00R-R-T after an astronomer. And we've never seen the ought cloud. We just think it's there. and it's the shell that surrounds the solar system.

So if you think of the solar system as a ball, then the outside surface of the ball is the aught cloud. And the ought cloud is about 50,000 times far away from the sun as the Earth is. And that's where all these billions of comets live for most of their lifetime. And they jarred into activity by gravitational pools following the dynamic of the solar system itself. Is that right? That's right. The solar system is not,

a static place. It's moving. It's within our galaxy, the Milky Way. It's moving. Stars around us move and the gravitational tugs that are exerted on the solar system, both of our own star, the sun and the other stars, cause waves, if you like, or perturbations in the surrounding interstellar medium and some of these might occasionally push a comet or a shower of comets into the inner part of the solar system,

where they get captured by Jupiter and they come in, you get these short period comets, like Hallis comet, which come round much more frequently. Insofar as anybody can count, I mean, given the availability of technology at different stages in our civilizations, it seems that at the beginning of the solar system, more comets came, landed here.

Does that, bringing life to this planet, does that mean that life from outer space began elsewhere? And does, I'm too big a question, but so I'll ask it, and then we can skip it. And does that mean that Fred Hoyle had was sort of, before, although poor chap was discredited, was right, actually? Well, that's an interesting question,

a very courageous one for you to ask. And there are seeds. of truth in it. Perhaps comets didn't bring life to the earth, but they certainly could have brought the building blocks because as Paul said, they're dominantly made of ice, they're made of water, they've got rock dust in them, and the things that are necessary for life to get going are water and other chemicals like carbon and nitrogen, and these are very abundant in comets. When the solar system was

forming, all those billions of years ago, we know it a more dynamic environment. There were more sort of loose cannons bombarded the Earth. Lose comets, absolutely. Bombarding the Earth and the moon and all the planets. And these were carrying water, carbon,

nitrogen, phosphorus and so on through the solar system. So water was brought to the Earth by comets, but we don't know whether it was all cometary water that makes up the Earth. So building blocks of life, not necessarily life itself. But given how

key water is and how we're always told can we find water on this planet? Can we find water on the planet? Meaning water is the key to life as we recognise it. It's intrigued me reading about this that it might have come from out there because the idea

people will know but Fred Hall was a great astronomer who was discredited for, although he did some people say he should have had a Nobel Prize, let's leave all that aside from him but he said it was to evolution, this the short span of four and a half billion years wasn't enough for evolution

to happen the way it had happened on this planet, stuff must have come in from outer space. And your sort of saying, we'll pass on this, it's a bit of a thing of mine, your sort of saying he may be, he was right. No, I'm not at all. He had this theory called panspermia, which is bringing bacteria.

And he famously matched the spectrum of a bacterium with a spectrum of intercellar space. And it was, you know, he was wrong. in that. So bacteria we think probably didn't come from comets, but the building blocks did

maybe, but the earth had some of its own building blocks still preserved in it. And we know a lot more about evolution now than we did when Fred Hoyle was proposing this. And it doesn't take that long for stuff to get going.

And all he's doing by saying it came from space is putting back the origin to some else. Don Palaka, the solid part of the comet is called its nucleus unless you want to pick up what Monica said.

I think I've been comprehensively crushed by that, but that's fine. Sorry. It's fine. It goes with the territory. Do you want to talk about the nucleus of the comet and develop that? Yes, so the nucleus of the comet, I think we really only began to understand with

Haley's Comet and the first space prose. And we were quite surprised. because although they were icy, we knew they were icy bodies, we were quite surprised when we saw one for the first time, because they're really quite small. They're just a case of a few miles or kilometres across, not circular, often quite irregular and shaped, and surprisingly dark, black, as black as coal, in fact.

And as the comet comes in... Why is that surprising? Well, because you think of an icy snowball. You know, that was the current model, and ice is nice and shiny, and this is not like... that at all. It's because of the layers of dust and other things on the surface. Whipple called it the dirtiest now more, didn't he?

Well, there's a lot of dirt there, that's for certain. But as a comic gets closer to the sun and it starts to heat on the outside and the heat moves inside, then material inside starts sublimating. So it moves from being a solid straight into being

a gas. And that breaks out of the crust of the asteroid. And so we start seeing material moving away in jets from the nucleus. Are we seeing stuff inside that nucleus that gives us a view of what this solar system was like in its very inception? Is it, I said in my direction,

that it was, the comments were useful as fossils. Is that accurate? Well, sort of and sort of not. And the reason for that is that at first glance, you think it would be. But in the whole process, there are reactions that go on. You know, the heat will cause different reactions to occur.

So it's sort of right, except it's changed. The whole process has changed it slightly. Even the surface of the asteroid itself will be weathered by time. So it's not telling you directly. But being sort of right is pretty good in your territory, isn't it? Well, yes, as good as it gets, I'm afraid. You say that

as he gets near the sun, as he gets near the sun, it hurdles in being jarred into life or jarred it to speed, jarring the movement, to be more sensible about it. It has a small atmosphere known as a coma around it. Can you tell us about that? Yeah, so what happens is as it moves in and this activity starts, it forms almost like an atmosphere, I guess you could say, but it's a nebulous patch that surrounds the nucleus.

This is where the heaviest concentration of material is as it comes out. Surprisingly enough, as it crosses about the orbit of Mars, the coma gets a bit smaller. And that's because that's the time where you start seeing the impact of the solar wind on it, and it blows the coma behind the comet. So this very, very long tail sometimes? How long are those tails? Well, as long as you want, anything from nothing to the orbit of the Earth, or they can stretch across the solar system, not all the way out to Jupiter,

but a large part of the solar system. They're big. And as we speak, they're whizzing around, heading for the sun, and never making it because they melt before they get there. Well, not all of them. Some of them make it several times.

But not to the sun, just near the sun. The ones that get closest to the sun tend not to make it past. So these so-called sun-grazing comets tend to come in once, and they tend not to make it past the sun. But that's not too surprising. The radiation that they're experiencing

is quite significant and it disrupts the nucleus completely often, not always, but often. But other comets will continue round several times. So they'll have a standoff from the sun. They'll be at a distance and they'll start, and they'll start, what do they do? They're circled like to planets, or what do they do? Well, these are the short period comets. So these comets are not, they're not going so close to the sun that they evaporate away.

So each time they come in, they lose, well, 100 tons of material or a couple hundred tons of material per second. So they are losing a lot of material, but they've got enough material in them that they can survive for a good number of orbits. Paul Merton, let's talk a bit more about this, the tale of the comment, which takes people's imagination, this long trailing tale. Can you tell us more about that and why it is so effective to spur to imagination, really? Well, you're right, the tale of a comet is the really distinctive knock-your-ey-ey-out, stand-out feature. and in some comet, some so-called great comets, perhaps we'll get one later this year,

the tail can stretch across half the sky from the horizon to the zenith and will be by far the most prominent thing that you see from the comet, which is one of the reasons I think why it's so surprising that the real heart of the comet, the nucleus, is so small and black. I mean, it's a paradox,

compared with the bright, shining, luminescent tail. Where does the stuff come from to make the tail so long? If we're talking about a comparatively small thing, as Don was outlined in giving us almost measurements, and this thing goes for that long, why is the stuff coming from? Well, it comes off the surface of the comet. As Don said, as the comet approaches the sun and gets warmer, it lets loose material, which flows off the comet into the space around it. and then this material is pushed back by the power of the sun.

There's two kinds of material that are coming off. There's gaseous stuff, melted ice, and there's little bits of dust. And these respond to the pressure of radiation from the sun. The dust is pushed back by the actual impact of light particles from the sun, the radiative radiation from the sun, what we feel as heat and see as light. And the gas is affected by the solar wind,

which is a flow of outgoing material from the sun, which sweeps past the comet and takes it back. So these two forces push material back from where the comet is, back and back and back behind it, generally away from the sun's direction and leave this, the comet leaves a great tail of material behind it, often in fact two tails, one the dust tail and one the so-called ion tail.

Can I turn to Monica Grady to it about that? Yeah, the material that's making the dust tail, they're tiny, tiny grains of dust, tiny grains, so much, much smaller than a grain of sand. and we know quite a lot about them because we can observe the spectra of the tails using telescopes on Earth and in space. And also there was a mission called Stardust which flew through the coma of a comet and brought some of the dust grains back. So we know their size, we know their composition. We don't know anything about the ice because it didn't bring any of the ice back or any of the gas,

but it brought the solid lumps of rock back and those lumps of rock were one micron across, so a millionth of a millimetre, no, a thousandth of a millimetre. Was there anything like the rocks that we have on Earth? Yeah, made of the same stuff, made of silicon and oxygen and magnesium and iron. So made of the same minerals that make the rocks on Earth and that make the rocks that are the asteroids and make up the moon. So it shows that the dust that made all the planets and the moons and the comets and the asteroid

came from the same place when the solar system formed. But the dust that made the comets, the stuff that we have from this stardust mission, is rich in carbon. It's not been quite as altered as the grains that made the Earth. Paul Morden referred to something called an ion trail. What's that? Well, this is more, this is iron, I-O-N, not iron the metal. And this is a complementary tail to the dust, which again is caused by the sun's radiative pressure.

Sometimes it's known as a plasma tail. And it's, we haven't actually managed to collect any of that because there's no physical material associated with that. John Polaka? Yeah, so the tails are different. They're made of different things. So we have a dust tail that Monica talked about. The iron tail is made from basically the reaction of the solar wind,

the material from the sun, and the water that is coming out of the comet. And so when you see them, and hopefully we'll see them this year, you will see two tails. One will be curved, which will be a bit yellower, actually. That's a dust tail, reflecting the sunlight. and the iron tail will be a blue colour, white colour, and it will be very straight, pointing directly away from the sun.

What's the relationship between these tails of comets and meteor showers? Well, as these comets are orbiting, they're leaving these trails of dust. And the dust, if the earth crosses the orbit where the dust is, then that dust has got to go somewhere and it goes into the Earth's atmosphere. The larger parts will appear as meteors. And so if you look at the astronomical diary for the year, you'll see maybe 20 or 30 times of the year where there's enhanced meteor activities,

there's more shooting stars. And those will correspond to orbits of comets, maybe dead comets now, but comets that were once active. I believe it was you and your own actually refer to the comments as cosmic debris. But they're emitting cosmic debris on a very large scale for millions of years. know, what happens to all that stuff?

Well, it just floats around, isn't it? So inside all the darkness, there's all that cosmic debris? Yeah, well, the universe, space isn't space, space is full of stuff, low-density stuff. I think we need to understand when we say debris, there's probably a

tiny fraction of an atom per cubic centimeter of material, so it's very low density. Yeah, yes. Very tiny things are supposed to start very big things in your world, don't they? So we were very tiny things. Paul Mirden, comics can be divided into two broad classes, I understand. Can you develop that?

Yes, well, let me just say to start with. In astronomy, you take what you're given by the universe. You've got to make the best of what it is on offer. So perhaps you'd like to know the inherent structure of comets and what they're really like, but what you can actually see are things like what orbits they've got. so comets are divided up into these two classes of short period and long period comets where long can be maybe more than a couple hundred years

could be thousands of years millions of years in terms of period in terms of when we see them and how they follow that trajectory yes do they go do any of them go back to the earth cloud to have this nice life there is yes there are long period can mean infinity so a comet will come

will come in once and go back to where it came from. It takes such a long time on its return journey that we don't know how long it's going to take to go and come back again. But there are other comets with periods which are shorter than that, 100 years, 30 years, 15 years, 5 years, and they're the short period comets. So the long period comets tend to come in

from any direction in the sky, which is the reason why Yanort, followed by his disciple Monica Grady just now, said that the Ork Cloud is spherical. It's a sphere around the solar system, and comets can drop in from any part of that sphere. And they come in over a very long distance and go back. It takes a long time, their long period. The short period comets tend to lie in the plane of the solar system.

system and their comets which have got sort of caught up in the orderly traffic of the solar system of the planets around the sun principally Jupiter they come in from the oort cloud and they happen to pass close to Jupiter and maybe Saturn on Uranus and they get sort of given a kick which brings them eventually into an orbit which is much more akin to a planet. Accentric, perhaps an eccentric ellipse, not a circular path, but something which is kind of planetary-like has a period of, let's say, 30 years or something of that sort. Monica Grady, the best known example that our listeners know about of a short period

comment is the one named after the English astronomer, Royal Edmund Halley. Can you just refresh our memories about when and how he discovered that and what significance it had? Well, he was working in the 17th century and he observed this comet and made some calculations about where he thought it was coming from and how long it was going to be in the sky.

And he looked back at records and he very famously made a prediction that it would come back again in between 75 and 76 years, which of course he was going to be long dead. But it actually did come back when he said said it was going to. And so if you like, this was the start of, I don't know, mathematical cometology and the prediction of cometary orbits and a better understanding of how they relate to the solar system. His comet famously came back at the start of the 20th century when there was

an understanding that comets released gases and there was a thought that perhaps some of these gases were going to be rich in cyanide and thought that maybe the whole of the atmosphere would be poisoned by this, but of course that wasn't the case. But it was all part of the mythology around comets that some of their returns can be predicted and that they were harbingers of great change.

Paul Murdoin. I see the Halley's comet and Halley's prediction and its fulfillment as quite a pivotal moment intellectual history actually, of relevance far beyond science and comets, especially in France. I mean, the emotive power of somebody predicting a celestial event 76 years into the future

and of that prediction coming true after he was gone, although his intellectual thought and his intellectual achievement lived on after he was dead is something which really resonated among intellectuals at that time and the popular imagination. And I think that was part of the reason

why science became so respected as a motive force in human history, in economics, in the Enlightenment movement, in rational thought. That one event? That one event was significant, yeah. Don Fulacca.

Yeah, I mean, there was another important issue as regards comets at that time. Prior to that, a lot of people thought comets were an atmospheric phenomenon. And this proved beyond doubt that they were solar system bodies. So it was an important observation. While you are talking, is it possible people will still be predicting how this comet, I presume. Can they predict when it'll die? I mean, do, we talked before about heading for the sun and giving up or backing off and going around the sun for a while.

Is there any predictive force at work there? I think the thing about comets that you have to understand is that they're inherently unpredictable. Which means that... So far? Well, so far it's their complicated systems although they seem quite simple but we don't know for instance how much material will come off

we can say roughly how quickly it's going to evaporate as it comes into the sun towards a sun. But we can't say when it's going to break apart and things like that. So we don't really know. That's the answer. A big comet, for instance, that gets close to the sun will often be, will disintegrate close to the sun. Monica Grady. I was just going to pick up on that and say you've not just got the coming close to the sun, but you've also got, as the comets evaporate and become smaller,

they become more subject to Jupiter's influence as well. And so Jupiter can also tear a comet apart, as has been seen several times in the past. So you've got the influence of Jupiter perturbing an orbit and also possibly wrecking comets. So very unpredictable. Sorry. Paul, can I be more specific about that.

In 1994, a comet was observed crashing into Jupiter. which seemed to eat it, big though it was. Yes, this is a comet Shoemaker-Levy-9. Shoemaker and Levy were a group of, in fact, three astronomers, two of them, two of them married with the same name, and the ninth comet which they had discovered. They discovered it in, I think it's 1993 the year before, and curiously it was a comet which was in orbit around Jupiter,

not around the sun. It had been presumably in orbit around the Earth, the sun in the past, but Jupiter had captured it. And the circumstances of the capture was such that it became very close to Jupiter and that brought it

closer and closer. It spiraled in towards Jupiter and very quickly it was realized soon after its discovery that actually in 1994 it was going to plunge into Jupiter's atmosphere, which it did. And it was quite a size of this comment, wasn't it? Can you have some idea of the size

compared to, say, Earth? Oh, tiny, tiny compared with Earth. Monica's shaking her head. Moni shaking her head. Well, what I'm saying, and sorry, yes, the comet itself is tiny, tiny compared to the Earth. Yeah, you're right. But once it fell into Jupiter, some of the biggest pieces made scars on Jupiter, which were like 80% of the size of the Earth.

So you could actually see the shadow of the reactions as the comets plunged. What happened was that the comet broke up into about 25 pieces on its earth. on its trajectory as it came in. Each piece plunged into the atmosphere, plunged in so fast 60 kilometres every second, that momentarily it made a hole, a channel in the atmosphere of Jupiter. And the heat that was released by that

at the base of the hole caused material to shoot up the hole. It came shooting out like water out of a water pistol and sprayed back over the upper surface of the atmosphere. And it was a big bow-shot. scattered out over the atmosphere. So each piece made a stain on the top of Jupiter, which, as Monica said, was very large indeed.

Don't for me. It was certainly an omen for me, because I got married that day. Good or bad, Omen. Good Omen, excellent. But, I mean, the unfortunate thing about that event was that all the impacts happened just beyond the side of the Jupiter that we could see. But it was a very impressive event.

that important because we couldn't see what it happened but you've spoke to, Paul and Monica have spoke about it quite graphically. So what couldn't you see that we should have seen might add to the story that we have heard so far? Well let me say what we did see. What we did see was we saw the plume of material that was thrown out of Jupiter

just like a mushroom cloud after the impact and we could see this superimposed against the side of the planet. And then as the planet rotated because it rotates very quickly Jupiter, 10 hours for a full rotation, we could see the scar as it came into view.

And so then there was the most bizarre sight of Jupiter with a whole pile of black holes, wrong word to use, a whole pile of black marks on its disc where you could see the scars from this comet impact. I was just going to say one of the descriptions of the comet was a string of pearls because they were strung out like beads on a necklace.

and they hit the comet one after the other and Shoemaker and Levy had predicted this and so there were lots of telescopes actually focused on Jupiter at the time and so as Jupiter turned as Don said you could see these stains one after the other across the face of Jupiter as each comet had plunged in

because we're talking about the time in the 1980s and so on when we're getting spacecraft just observing these comments. Which spacecraft observed this one? Was this the... Well, this was two people. Caroline and Eugene Shoemaker and David Levy. And they were observing it from the Earth.

Once the prediction had been made, I think nearly all the telescopes on Earth were watching the impact. Everybody trying... Hubble Space Telescope, all the ground-based telescope. I mean, it was such a new phenomenon that anything you could see would be useful information.

And I think it was quite... influential because once our eyes had been open to this phenomenon of the individual bits of the comic coming crashing onto the surface of onto the top of the clouds of Jupiter then people looked around at other planets and other satellites and they found chains of craters half a dozen 10 15 craters in a row straight line and the inference is that these were formed in a similar way by the breakup of of things and the bits are going, bom, bump, bump, bump, bump,

on the ground. Don, you want to come out, Don, but it's something I want to ask you as well. What I would say is that astronomy is very interesting. Most things in astronomy take billions of years to happen. Then occasionally you get something that happens really quick

and you see things change in a fraction. So this was an example to me where you saw really dynamic astronomy going on and an important event that really did demonstrate how bombardments could do things. Can we talk about the spacecraft, which I jumped a gun with a little bit earlier,

but they're going up there having a deeper look at space. Say the deep spacecraft deep space in 2001. What more is that telling us about comments? And I'm anxious before the program closes to get to terms of what are we really learning from these comments? What is it adding to, you're learning about comets, okay? But what are we learning about the Earth? Is it having an impact on the way that our knowledge about ourselves, and so on.

Yeah, so the deep space, deep space one, that was actually part of a series of missions that really were testing new technology. And a lot of the things we, new probes now, are only possible because of this particular probe. It used an iron drive,

which had never been demonstrated before. And it went to Comet Borrelli, I think it was, in 2003. I can't remember, I'm not very good with dates. And the interesting thing there was really, again, we haven't really seen many comet nuclei up close. And so again, this demonstrated, this black, this, how small they were. I think people originally expected these nucleuses to be a lot bigger than they actually were. And in that particular object, there was jets of material coming off it.

Some people postulate or think, at least, that the jets of material along the rotation axis of the, of the comet. Now the importance of that then is that you've got a material coming out of an axis that the thing is rotating on and conceivably this could lead to the breakup of the comet.

The comet could split in half, more debris, more things to hit the earth, etc. Let's continue with this spacecraft, Monica Grady. There's a stardust and deep impact as it goes on. What more is being found out that's significant?

I mean, sticking to comets. They're doing all sorts of stuff. sticking to comets. Well, the mission star dust brought cometry dust back to the earth. Prior to this, we'd always thought that the dust was very primitive, if you like. It was the dust that had managed to accumulate beyond the snowline. But what we found from the grains that were brought back from star dust is that some of them had formed at very, very high temperatures indeed

and must have formed close to the sun. And so this cloud that I referred to that formed the whole of the solar system, it must have been really dynamic and turbulent, and material that formed close into the sun, must have been thrown outwards to be on the snowline so it could have got amalgamated with the ice that was forming the comets. And so what we learnt from stardust was that there was turbulence across the whole level of the disk that formed from the planets.

we've learnt enough about comets to know that each comet is different in subtle ways and that the distribution of the dust within the comets and the distribution of carbon and the volatile materials within the comets doesn't seem to be equal. I mean there seems to be material on the surface which we don't know whether that how representative is of the stuff that's below the surface. Paul Menden. Monica's put her finger on the major significance of comets to my mind now.

Let me give me an analogy. If you're an archaeologist, you would like to find a buried city like Troy and you'd like to study the palaces and the temples and the structures and it would all be orderly laid out, and you'd have a really great time and write a really great thesis on it. But you might have, as an archaeologist, to be content with excavating a rubbish heap. It's unstructured. It's full of discarded building material

and stuff that's happened in the lifetime of the city It's very difficult to sort it out But you get a different picture of the way The people have lived, the processes that have gone on And how life happened in that city And comets to some extent are the rubbish heap Of the solar system It's the sweepings from the building up

Of the planets in their early days A lot of things have happened to it since but if you study that collection of material, if you study how it's all made, if you can bring back bits to look at, then you've got clues to, first of all, how it all happened in the first place, how interstellar material came into the solar nebula.

You've got clues about what happened during the construction of the solar system, how this snowline developed, where it was, what happened to the material as a result, result of some bits being warm and some bits being colder, and also about the processes that have gone on since in the ageing of that material. Don, you want to come in here?

Well, I was actually just thinking about the all clouds, so it's slightly different. The orch clouds would refer to. These are why the comments live, basking around in a leisurely fashion, billions of them. Yeah, I mean, throughout history, we're always very human-centric. So we have flat earth, we have Earth at the centre of the universe, etc. and now we have an orc cloud. But what is it? It really is just a region

where the gravity from the sun is pretty low and it's balanced by other stars. So I wouldn't be surprised if there is no such thing as an or cloud. And in fact, it's just a place where debris from solar systems all in the whole of this galactic area is gathering. And so it's not a cloud as such. It's just a region where different comets from different stars will exist.

Before we're nearly at the end, Monica. Please say, but I've got one final question to ask which is on the... Anyway, all I was going to say is that we don't know as much about comets as we would like to do, but there is one big mission that's coming up, which arrives at a comet in spring next year, 2014. The Rosetta mission. The Rosetta mission. And it will rendezvous with a comet.

The missions that we've had so far have all swept past the comet. This will ronet. over with the comet and ride with the comet as it develops its tail. And then it will put a lander called Feli on the nucleus of the comet, drill into the cometary nucleus, and analyze some material from the comet. So although it won't bring anything back, it will be another piece of the jigsaw that is the comet. Can you tell us why, in your view, Paul, this causes so much excitement, the comet's coming. You say we're going to see a couple later.

It's caused excitement for a very long time now, hasn't it? still does by the sound of it. Yes, I mean, I think I hinted at that earlier on when I said that comets are first of all very spectacular, if they are in fact spectacular. And they come

sporadically. I mean, in a lifetime you might get a chance to see one or two or three great comets. You can go out and marvel at them. Everybody in the world can see and participate in the

in the experience. I think the two that are forthcoming, which have a prospect of being those sorts of comets, are going to capture people's imagination and capture people's wonder. Possible to see them with the naked eye?

Yes, in November. Hopefully. We have to go now, which is a shame. Thank you very much to Monica Grady, Paul Mirden and Don Palacca. Next week we'll be talking about Romulus and Remus and the Foundation Myth of Rome. Thank you for listening. There are many more Radio 4 arts and discussion programs to download for free.

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