About science; About comets

This is about science produced by the California Institute of Technology and originally broadcast by station KPCC in Pasadena California. The programs are made available to the station by national educational radio. This program is about comets meeting to discuss this topic. Dr. Albert Hibbs chief of space sciences at Cal Tech's Jet Propulsion Laboratory and Dr. Raymond Lyttleton of Cambridge University now working at Cal Tech. Here now is Dr. Hibbs. About 400 years ago Leonardo da Vinci who said many things about many fields of science talked about comets and said Why this Comet seems variable in shape so that at one time it is round and another long at another divided in two or three parts and at another United sometimes invisible and sometimes becoming visible again. Well it's been 400 years now since Leonardo were raised all of these observational questions about comets and surely by now after 400 your years

really it must be that we know all about comets is that right. Or are we still as confused as Leonardo seemed. I think we are more confused than they are not else. There are so many people thinking about comets these days that it tends more to confuse the issue and then clarified. How come. You know we have telescopes now in order didn't have telescopes we can get a closer look at them. There's more people available we can we have computing machines and so on. Well there are numerous theories of the structure and origin of these things and it's round these theories that the controversy is run. And of course the very fact that we've got more evidence tends to make there is more controversial until we can absolutely find out on the spot as it were what these things are made of. The least where we've gotten away from a lot of the superstitions around and comments these old days when we thought that every time the earth went near the tail of a comet we'd all die of poison gas or something like that see around Halley's Comet. You might say that but I think there are still plenty of superstitions going. You mean the ones that are now called theories. Yeah. Well people who disagree with my theories that you know are the ones suspicions

are really isn't it. But there was in the ancient time quite a bit of fear connected with comets. And now at least in general this seems to be less. This last one in the case Sackey that came down. Everybody was watching which was certainly cause of less fear than before. Mystery interest excitement. Well this is the great thing that advancing science can do for you. You see in early days when there were no street lighting and this sort of thing the comments must have seemed to the ancients the most marvelous the most brilliant the most important object in the heavens and they get quite bright then I think apart from the sun comets can even be brighter than the moon. But until we got the low of gravitation and could see how much mass that was in these you imagine that stretch right across the sky from one horizon to the other brilliantly luminous light came unexpectedly they still do of course. And so there was always something going on that they could correlated with was always some catastrophe some political catastrophe just as it

is today. But you get a real close correlation and until you've got the law of gravitation there is no way of deciding that these things were of almost negligible mass. This is from an astronomical point of view. Can still be millions of tons millions and millions of tons but compared to a planet compared to a planet negligible. They're seen however in spite of all of the mystery there. They've caused an interest not very many it's only every now and then we see a comet or they are as rare as they seem in the solar system. They're not ready numerically. Something like 75 percent of them remain telescopic objects. This is simply because the solar system is so big. These comets move. They go out mainly to 100 times distance with suns a thousand times even more and remain there most of the time as it orbits very slow at these great great distances are very slow then they plunge in towards the sun. And if we happen to be suitably situated then we get a view of one of these and this but this is rare because they have the ones we've seen three

quarters of them are only in a telescope. Yes I see about once every three two or three years will be one bright enough to be seen somewhere at night and four or five times a century. There will be one bright enough to be seen in broad daylight. This is what we had last November. Yes I remember standing looking around the corner of a building so I could shield out the sun and seen it just in. But of course it's far more luminous it looks because of course you got it in the brilliant sky from the sun. There was a daylight comment in. And that's a very interesting one in a way because it came just as unexpectedly and just before Halley's Comet was Jew. And I think all these stories you hear of old granddad seeing Halley's Comet it was in fact the great day like when I'm 19 or 10. And if you read the literature of Halley's Comet it was in fact a very very poorly seen thing in 1910 a great surprise a great disappointment but there was this great daylight just by coincidence can after McNally's his periodic dizzy and can be predicted. 77 years or something like that.

It depends slightly on how near it goes to the other planets but it will be back again in 1986. Well out of these now 75 percent of the telescope and 25 percent of that we see by eye does is account for all of them are there must be still more we never see at all. Well a lot must come to the sun in the daylight sky and never be seen. And we know this because three or four times during eclipses. And this is just a few minutes. Say the totality of the when the moon has come directly between us and the sun. A brilliant Comet just stood out quite close to the sun just for a few minutes and then gone and never seen it not only on those occasions it gets comments. There are three or four of these recorded now and this means you'll see that in all this the time when the sun is not eclipse there must be lots of them get in and out without even being seen. And you see 50 percent of the sky practically is in the bright daylight sky. So we may lose half of them this way. I do not see harm in that maybe I will never come sufficiently close to the sun to be observed.

But what is it we really see when we look at a car we used to just see this huge tail. Now what is that make up and what the what is a bright material that will look at. Well you have to start at the beginning to see is that when a comment like Halley's first looms into view it just is almost indistinguishable from the background sky. In fact only very skilled observers can see it. The great American astronomer astronomer Barnard who had the most remarkable eyesight he could discover comets that other people couldn't see in the eyepiece when they said there's one and he would discover them and then they'd come into view and they'd get the orbit you see so it wasn't just saying that he could see them when he couldn't. Well when at this stage they shine entirely by reflected light. It's just the solar spectrum reflected off like an asteroid or the moon or something. Yes except that it is. It is much more tenuous as it were because a comet is not a continuous object it is it is a whole lot of particles these I think so and a lot of the older astronomers thought so in days gone by spread over an enormous volume. So you will see right through it when a comet passes

across a star. The star can be seen quite and in right through the head of the comet. When a comet goes in front of the disk of the sun it's completely invisible although they know exactly where it is. This comment did this in it makes no shadow no shadow at all. The light comes right through the particles are so small is that diffraction and skews you know that it's just not visible at all. So it's the whole comet and as it is a diffuse mass it's not just the tail is defeated. Now when this thing comes in towards the sun until it gets fairly close then the activity begins and the comments starts to give out gas. And then there's an emission spectrum as well. Then it becomes very complicated and becomes much more luminous than the mere distance of the sun in the gauge you see. So there's a little bit of florescence going on and since this is a great deal of it going on it it can brighten up no objects change brightness more than Comet a effect of a thousand million for example a comet will be brilliantly luminous like this one was last November brighter than the

moon it was when it was near the sun and then a few months later you couldn't see it in the biggest telescope. What range of magnitude is this huge changes. Thousand millions. And this is because not only reflected light but there was actually there is energy from the sun was stimulating the particles to give off. Well this is a controversial matter as to whether it is light energy from the sun or whether it is motion within the comet produced ultimately about the sun's gravitational field and then the particles of the comments of the comet collide at considerable speeds of the order of a kilometre a second this will not only produce a lot of dust which you as a result of the collisions but the impacts of sufficiently high speed are the order of a kilometer or second that they probably drive off a lot of gas as well simply by the impacts. Now this is this is controversial in the sense that it's very difficult to settle because when the current becomes really active like this it throws out all this dust. Actual it appears almost explosively the dust in it or at least expanding shells appear. It's like an

octopus when you approach it you see it gives out the zinc County. Now this is one of the great problems with comets that the very activity obscures the thing and makes it difficult to decide what's going on. And this is one of the reasons why it's so difficult to decide directly what is happening or why does material seem to stream up behind it it's not going through the atmosphere. And yet it appears to wave a flag behind it of something or other. The flag runs behind it as the comet comes in and it runs on ahead head as the comet goes out. Now it's believed that the reason for this is the pressure of the sun's light. The on line does exert pressure and the particles small enough this force can rise to enormous amounts because the area goes up you know more than the mass goes down as it were the the ordinary particles of a comet cannot be affected by the sun's radiation. We know this because you work out the orbit and this is completely consistent with ordinary Newtonian gravitation not being pushed around by by like what you see when these particles collide and then you get a whole lot of much tiny particles

emitted and then the radiation of the summer automatically pick out those that are in a sort of maximum range wavelength of the sun's light and then you can repel them quite strongly and blow them out into the tail. But it also blows gas out them. There must be a second process that can blow gas out selective absorption or something of that sort I can actually see these particular molecules and blow them out and you will have bands of intensity. The comet dust doesn't have just a single tail release. Comets got the record and there were nine different tails observable stretched out behind the comet. One of these must have been a sort of different material or physical shape or something different matyr with a different repulsive force and therefore it's describing a kind of different selectively pushed by sunlight. They are different. There are different trajectories and go make it all making details of the deal that is pushed away from the sun no matter which way the current details are being pushed away from the sun really away from it. There is a slight curvature due to orbital motion.

And inside then the part that you can that's lost in the confusion is that if it's not a solid mass what is it made of this material in the middle. Well I think the whole thing is a huge swarm of these tiny particles rather like a gigantic swarm of gnats that the particles are quite tiny. Now the reason we know that they're tiny is this that the meteors that into the high atmosphere nothing of those ever comes to ground level. Now we know from other things that a particle the size of a golf ball can in fact get down to ground level without losing its mass. But no meteor ever when as a heavy meteor shot. Nothing comes through the ground. The connection between meteors and comets. Well and when there are these intense meteor showers as they call they can in fact work out the orbits and these correspond to the orbits of the comet. I see so this material is flying around to the solar system the same path as a comet. Ahead of it and behind every now and then we run into it.

And that's a meteor shower in the few cases where the Earth's orbit happens to take us through this ice. Then you'll get a meteor shower lasting a day sometimes as long as a week. So shooting stars are pieces of comet tails and this seems almost certain not of the tails of the comet it's the comet itself. Yes particles that have come out of the head you see if you will to the velocity of a particle the least bit by a collision or something of that sort. Then you alter its period if it happens to be quicker it will move on ahead of its slower. It will trail behind and Comet has particles but the head of it and behind it all going around the same orbit it changes only slightly with this and that and go over the bones it is what brought Yes or for some cases the stream has been detected all the way around the orbit but then it's usually a very thin shower. But the intense showers are usually fairly close to the comet. So then this material since we've never seen anything big fall in one of these showers. If this is a sample of the center of the comet then what you're saying is that that implies material on a comet is made a very small particle.

Yes I would I would think this is as far a conclusion as we can get all together by what just their own gravity. Well now this here again of course one has got to look at what is happening that a comet like this one that came last November when that goes out to its furthest distance from the sun. It can weakly hold itself together by gravitation because the effect of the sun and pulling the thing apart is not the square inverse killed nothing. And so at 100 times the sort of tidal forces and tide of course on the comet yes calling the two sides of it with different strings. This falls off the ice cube. So it's 100 times the earth distance. You're down to a millionth of this force and the Comet the weekly pull itself together. But when it comes rushing in and gets to this sort of distance that we are at from the sun it's gravitation is negligible compared to the sun. And then all the particles in it describe their own orbits and I say they're all now completely under the sun's gravitational force for all intents and purposes. Yes.

And so they each are describing an orbit in a plane and they're unaware of their neighbors. And these planes of course converge because everything has to pass in a path in a plane through the center of the sun. And it's known that the size of a comet contracts as it comes from the sun. That's what Leonardo was saying. And then a great distance when Halley's Comet is first seen it maybe 200000 miles across and then it converges as it comes in the sun. I see and this is just the dynamics of the sun's gravitational field itself acting on a conglomeration of these particles. Yes. And then some astronomers are looking for complicated forces magnetic fields goodness knows what when it's there in the simplest dynamics. If you just follow the schoolboy dynamics then the comic starts doing all these wonderful things. And the comment as a result of this convention it turns inside out the stuff on the right hand side of the road as it were has to cross over to the left and vice versa. And so all these particles are trying to pass through a kind of median plane. Now most of them get through because the comment is so diffuse. But a lot of them

collide and then when they collide more collide because collisions produce collisions and a fragment each other the relative speeds of these collisions in the comet itself. Halles when it's near the sun you're doing about 60 kilometers a second and the relative speeds hit about half a kilometre a second that's enough to that's quite that's quite a speed that's enough for. You know a couple thousand miles an hour astronomically it's very small but from the point of view of the strength of materials it's Tuesday breaks up these any particles that hit each of them are said are immediately much smaller particles and made much smaller and also that the heating of the collisions will drive off the most volatile things. So after this has happened several times there must be nothing left but very tiny particles. The comet itself is so diffuse that I think this can happen scores and scores of times without any serious loss you see because. And yet comets do disappear. Some There have been records of a comet that goes around behind the sun has never seen of again or breaks into as Leonardo pointed out sometimes.

This is going into two pieces. One's got to be sure which comments one is talking about. There's a second small class of comments called the short period comments. These are much fainter than this sort of thing we had last November and the great daylight comments there was a big long period comments about the little shop here Tom is lying. He's and Bilas was one of the ones that are speaking of now Bilas comet in 1846 did divide into two. And the two pieces were seen at the subsequent return much further apart and they've never been seen since. But there's an enormously intense meteor shower known as the Bilas moving in the orbit of the of them. Bill it's common when you say short period longer. What are the periods what. How long does it take a long period comet to go around the sun. The long period comets can be anything from a thousand years as far as you're like a million years necessarily but when it gets as long as that it is very difficult to calculate accurately. In the short period comets there in the five to 10 year bracket that's there close members of the solar system they go around among the

planets they're nearly all associated with Jupiter and it's pretty certain from the dynamical studies that Jupiter was deflected long period comets into these little orbits can do that if one happens to Bosnia. Where did they come from to begin with. By the way if dupers deflecting them into smaller orbits What was their original how what is. Is there a good idea of the origin of comets maybe that's the best way to ask the question. Well he's got a personal question isn't until comparatively recent times there are practically no theories of the origin of comets. And I think it was a fairly good reason for this that it's only in comparatively recent years that the existence of material between the stars has been appreciated. When I first started doing astronomy that isn't too terribly long ago had we just had the stars like billiard balls working out their fates in empty space. A few calcium atoms and things of that so. And then when it began to be suspected that there must be material between the stars. And now of course huge quantities of

hydrogen have been discovered in the galaxy and dust the dust by the way was first suspected by Bhana. There are great patches as you know in the Milky Way where there are no doctors at all. And this wasn't understood for a long time you see because even the distribution of the stars is a comparatively recent thing. In the beginning of the century they had no idea of how the stars in the galaxy were distributed. See how far we've come. Now we not only have this galaxy pretty well figured out exactly what you see. That's why they were called nebulae because the external galaxies were thought to be little patches of gas or our star clusters in our own galaxy and until they got the 100 inch and particularly the 200 range they couldn't resolve these things and see that they were in fact formed of bright stars and therefore very distant. We getting away from home comets right. You were saying that the thing that was the thing that we started with was a picture in which we had a universe with stars only in an empty space between yes now. Now there's theories of gas and dust in between. But how does that connect to comets is connected to the origin of comets you think.

Well I think there's a lot of evidence suggesting that the comets as it were something extra in the solar system produced as a result of the sun's motion in the galaxy we see 10 percent of the volume of the galaxy is filled with this gas and dust. When I say fill I mean it's patchouli distributed and it's all it's very very tenuous and diffuse but it does exist in sort of clumps. So this thing that wasn't even known about the turn of the century. Now we find ourselves up 10 percent of the space. Yes. And in its career around the galaxy and all these objects also going around the galaxy with slightly different spewed cartwheel motions every now and then one of the sun in particular must go through these clouds it must spend 10 percent of its life going through this at the present time we're probably not in such a cloud. And so the solar system is probably a fairly dull place if we went into a really dense one of these. You never know what might happen the sun might develop an enormous atmosphere huge quantities of meteors might come in with a really interesting time. It would be very nice if this began to happen.

A good display in the sky. But still what does this have to do with comets. Well when the sun goes into one of these dust clouds there's got to be a gravitational reaction between the two. The cloud cannot ignore the sun's attraction. OK. And as the dust streams by the sun's attraction focus is almost like a lens except that it's a gravitational focusing it bends the material in sort of pulls it together around behind as it pulls it into the actual line as it were just almost like a Layne's gravitational lens and it can increase the concentration of this material. It's extraordinarily diffuse as interstellar dust from the Manas 24 that the density of water as that figure means anything to you. One over a number was 24 zeroes and you can put this up by a factor of a million million and it'll still be very tenuous. The dust will still be even at that. But very very much denser now and then. Can we play pull itself

together at these great distances and then fall into the sun. So what happens then when the sun goes through a dust cloud it pulls this material of that of the dust interstellar dust around behind it by its own gravity. These particles now get together much more closely than they were before and perhaps bomp tend to stick together a little bit lose some of their energy and then stay around instead of just going on their way. They're not captured as this must be a loss of energy in order that the sun can capture them. So the other members of the solar system somehow but perhaps very distant members they must be very distant in order that this these collisions as it were can bring them together and form these things and then they will fall in practically along the line. And if the somewhat absolutely alone in space they would just fall plop into the middle of the sun and wind up well as a little saving grace comes in here. Jupiter and Saturn and to some extent Uranus and Neptune pool the center of attraction out a little bit from the sun and just give them the chance to get

by. And this is an extraordinary thing. It's not materialized but this is sort of center of attraction of the solar system as a whole doesn't lie at the center of the sun. It lies out at a distance and it can it can get out to just over to radiate from the center of the sun. And this just gives enough room for these fellows to get by. Now this one last November as you know is what's called a sun grazer. And it went within something like just under about three fifths of the sun's radius from the surface is very close. So it was falling toward the center of the whole solar system to begin with. And then when it gets inside the orbit of Jupiter then switches over a little bit but it's too late it's got out to the side and that's enough to say that it won't save them all. But naturally the ones we see had probably been in before and got by Once you see and then they'll get by again and each time they go past I suppose Jupiter may have a slightly different effect because it's in a different portion of its orbit so that this whole thing has moved around just a bit.

It will be different every time. There have been even closer shaves than this last one I think 19th ninety thousand miles from the surface is that is that is the record. This is about a fifth of a solar radius and that's that's pretty close. But tell me Ray maybe this is the wrong question to ask you but to this picture you've just given this idea that the tail of the sun gathers up in its tail a concentration of dust that it passes through and it does concentration then collect into the comets which have lost a little energy in the collection process and now stay around as members of the family. Is this an idea that is generally accepted now among astronomers who worry about comets. Well this is a very difficult question to answer. I don't keep us out of the interrogation going through the mails of all my colleagues but new ideas are seldom accepted you know it takes a generation of people to get used to them and they say they never believed in the else when you know like Bertrand Russell once said that a good idea starts as a

heresy and ends as a superstition. Yes well I have my own ideas must be very near the beginning of that rather than the end. And there are other ideas of comets. Dr. Wico now the Smithsonian believes that that Accomack is a very small object that it's sort of icy snowball just a few miles inside the dirty snowball. Yes and that this warms up as it comes to the sun. Now I can believe in this because of the fact that you can see some of these things out beyond the orbit of Saturn and then nothing that size could be seen at that distance. Now how big this picture these a mile or two. Yes. You mustn't have a very much bigger. Because otherwise you could you could see a solid object. And of course also if you have a solid object very much bigger it becomes of planetary mass and no comet has ever deflected anything. Brookes's comet went in amongst the satellites of Jupiter with no measurable and it on its own orbit

was tremendously changed as a result of this. And the satellites weren't deviated in the smallest degree. So then just on mass alone and size it would appear that that comets can't be a solid dirties nobody is. Well it's not explained where these dirty snowballs come from but there is very strong evidence in the distribution of these comments in space that they are associated with the Galactic motion of the sun. Well also I've heard that there is a body of a shell of comets out around the whole solar system just sort of sitting out there and every now and then one comes in or something. Yes this is Ott's idea. But here again it's very difficult to sustain and the statistics of comets don't agree with the frequency of how often we do see them and what they're doing because they come from and the distribution of these orbits is such that you could if you got them from the shell that have another distribution if there were such a

shell and they were being deflected in the stars then the distribution would be different. So how are we ever going to settle any of these things ray to see if we can't see really the inside of a comet and if it makes no shadow when it crosses the sun and what we see is so ephemeral as it swings by briefly and if most of these comments don't come back for a thousand years how are we ever going to find out what's going on in the car what it is really. Well it's going to be like a lot of problems in astronomy and this really is one of the reasons the space program is so important. Look at the question of the surface of the moon. Here's the thing that we can see we can photograph and there's no agreement as to the precise nature. I don't think we any agreement as to the precise nature of the lunar surface until we can actually get instruments that analyze it on the spot. And I think the same is really going to be true for the for the comments that if we could fire and there are plenty of opportunities for this. Well I said plenty you know every year or so is an opportunity where even with present energy a probe could actually be far to

a comet. And then what I would like to see I would like to see built into that is its instrumentation the possibly the capability of testing all these theories. For example we'd have to count the dust particles that are techniques already. It is very nice actually if we could collect the dust particles because here we probably have a sample of interstellar matter a matter has been thrown out to stars a little you know desolately of the primordial dust cloud. Well it may not be primordial but it may have been thrown out of exploding stars and then we would know what the Constitution of the elements were from these great nuclear explosions in space. But until we do get some sort of a probe that can fly into the somewhere in the middle of the middle of a comet or in the region in the center of the comet from all you've told us I would judge that the comets are still the chief bearers of mystery in the solar system. And Ray I want to thank you very much for being with us tonight and telling us about comets and great pleasure great pleasure.

This was about science with host Dr. Albert Hibbs of Caltex Jet Propulsion Laboratory and his guest Dr. Raymond Littleton of Cambridge University about science is produced by the California Institute of Technology and is originally broadcast by station KPCC in Pasadena California. The programs are made available to the station by national educational radio. This is the national educational radio network