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 TX Jet Propulsion Laboratory and Dr. Raymond Littleton of Cambridge University now working at Cal Tech. Here now is Dr. Hitz. About 400 years ago Leonardo DaVinci who said many things about many fields of science talked about comets and said Why this comets seems variable in shape so that at one time it is round at another long at another divide it in two or three parts and at another United sometimes invisible and sometimes becoming visible again. Well it's been 400 years now since L.A. NRO raised all
of these observational questions about comets and surely by now after four hundred your years writing it must be that we know all about commas as I write. Or are we still as confused as Leonato seems I think we're more confused than they are not so many people thinking about comets these days is that it tends more to confuse the issue than how can you know we have telescopes now Inara didn't have telescopes and 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 around these theories that the controversies 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 thought the least we're good we've gotten away from a lot of the superstitions around comets the 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 do not surround Halley's Comet once you. 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 the people who disagree with my theories that's the only other one suspicions are related but there was an ancient time quite a bit of fear connected with comets and now at least in the in general seems to be less This last one that came down everybody was watching. Will certainly cause of lost fear. Before our mystery interest excitement. Well this is the great thing that advancing science can do for you. You see in early days where there were no street lighting and the sort of thing the comics must have seemed to the ancients the most marvelous the most brilliant of the most important objects in the heavens as they could quite break I think apart from the sun. Comets can even be bright in the moment. But until we got the law of gravitation and could see how much mass there was in these you imagine they stretched right across the sky from one arising to the other. Yeah brilliantly luminous they came unexpectedly they still do of course. And so they say. And there was always something going on that they could correlate it with was always some catastrophe some political
catastrophe just as those today have a real close correlation. And until you got the law of gravitation there was no way of deciding that these things were of almost negligible mass. This is from an astronomical point of view. They can still be millions of tons millions of millions of tons but compared to a planet compared to a planet. There seemed however in spite of all of the mystery there there cause an interest not very many. It's only every now and then we see a comet our way I think is rare is they seen in the solar system. They're not really 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 two hundred times our distance when the sun's on them a thousand times more and remain there most of the time. Life is orbits very slow at its greatest at the rate this is a 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 is but this is rare because even of 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 seen it just but of course it's far more luminous look because of course you got it in the brilliant. Sky sun there was a daylight comedy night 10 and that's a very interesting one in a way because it came just as unexpectedly and just before Halley's Comet was due and I think all these stories you hear about Grandad seeing how his comedy was in fact the great daylight one of nineteen ten and if you read the literature of Halley's Comet it was in fact a very very poorly seen thing in 1910 a great discipline his great disappointment. But there was this great delight was just by coincidence but I can have an account Alice is periodic you see and can be predicted every Seventy seven years or something
like that it depends slightly on how how near it goes to the other planets. But it will be back again in 1986. Well out of these 75 percent of the telescope and 25 percent that we see by IDers as a camper 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 you see the totality of this when the moon is come directly between us and the sun. A brilliant Comet has stood out quite close to the sun just for a few minutes and then gone and so the moon. Never seen it before only on those occasions it gets comments there are three or four of these recorded now. This means you say that in all this the time when the sun is not eclipse the most blots often get in and out without ever 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. Yeah I do not see how often then the maybe I was that never comes efficiently close to the sun to be observed.
But what is it we really see when we look at a car we usually just see this huge tail. What we know is that made of what or what is a bright material that we look at well you have to start at the beginning to see this that when a comet lie 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 is one and he could discover them and then they come into view and they get the orbit you see so this 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 the bike. Let's get an asteroid or them on or something. Yes except that it isn't 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 those gone by spread over an enormous volume so you could see right through it when a comet passes
across a star the star can be seen quite Undine 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. Ellis comic did this in I make no shadow no shadow at all of the light comes right through the particles are so small it's a diffraction excuse that you know they're just not visible at all. So the whole comet and as it is a diffuse mass it's not just the tail is diffused. Now when this thing comes in towards the sun until it gets in fairly close then the activity begins and the and the comet starts to give out gas and then there's an emission spectrum as well. Then it becomes very complicated becomes much more luminous than the mere distance of the sun in the gauge. So there's a little bit of fluorescents going on SAT's this is there's a great deal of it going on and it can brighten up. No objects change brightness more than how much of a factor of a thousand million for example a comet would 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 change. Yes yes thousands millions and this is because not only reflected light but there was actually the energy from the sun was stimulating the particles to give off quite well. This is a controversial matter as to whether it is light energy from the sun or whether it is. Motion within the comic produced ultimately by the sun's gravitational field and then the particles of the comment of the comet collide at considerable speeds of the order of a kilometer a second. This will not only produce a lot of dust which is as a result of the collisions but the impacts are sufficiently high speed of the order of a kilometer a second that they probably drive off a lot of gas as well simply by the impact. Now this is this is controversial in the sense that it's very difficult to settle because when the comet becomes really active like this it throws out all this dust. Actually it appears almost explosively the dust or at least expanding shells appear. It's like an octopus when you approach it you see it gives out the sink. You can see it 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 and what is material seem to stream up behind it it's not going through any atmosphere and yet it appears to wave or fly behind it of something or other. The flag runs behind it as the comet comes in and it runs on ahead as the comet goes out. Now it's believed that the reason for this is the pressure of the sun's light. Your light goes exert pressure. Oh yes of the particle 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 light pressure. But you see when these particles collide then you get a whole lot of a much tinier particles emitted and then the radiation of the sun will automatically pick out those
that are in the sort of maximum range of wavelengths of the sun's light and then it can repel them quite strongly and blow them out into the tail. But it also blows gas out that there's a MUST be a second process that can blow gas out selective absorption or something of that sort can actually see that particular molecules and blow them out and you will have bands of intensity comet dust doesn't have just a single tell Briley ZCom it's got the record and the nine different tails observable stretched up behind the car each one of these must have been a sort of different material or dx is a color shape or something different material with a different repulsive force and therefore it's describing a kind of different selectively pushed by sunlight. Yes different words have different trajectories and all make it are making details of the deal that is pushed away from the sun to matter which way the come through and the tails being pushed away from the sun only away from it there is a slight curvature due to orbital motion. And inside and the part that you can that's lost in the confusion is that.
It's not a solid mass what is it made of the material in the middle of it. Well I think the whole thing is a huge swarm of these tiny part of rather like a gigantic swarm of nets. 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 the particle size of a golf ball can in fact get down to ground level without losing its mass. But no meteor ever win as a heavy meteor shower. Nothing comes through the ground connection between meteors and comets. Well when there are these intense meteor showers as they're called they can in fact work out the orbits and these correspond to the orbits of the comet. I see so this material flying around through the solar system the same path as a comet ahead of it and behind. And 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 sometimes as long as a week.
So shooting stars are pieces of comet tails and this seems almost certain that not of the tails of the comet itself comet itself yes particles that have come out of the head you see if you alter 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 it slower it will trail behind and Comet has particles both ahead of it. And behind it all going around the same moment the order changes only slightly with this and I know you know well that over the years what broadens it this is what brought us yes over the years. For some cases the stream has been detected all the way around the orbit but then it's usually a very thin but they intense showers are usually fairly close to the comet itself. 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 of very small particles. Yes I would think this is a fair conclusions we can be held together by what just
their own gravity. Well now this this is here again of course one's got to look at what is happening that a comment 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 inverse square to inverse cube thing and so at 100 times the size of a tidal forces a tidal force on a comet. Yes calling the two sides of it with different strings. This falls off the inverse cube so it a hundred times the earth distance. You're down to a millionth of this force and the comical weakly pull itself together but when it comes rushing in and gets to this sort of distance that we're at from the sun its gravitation is negligible compared to the sun and then all the particles in it describe their own orbits. I see there are now completely under the sun's gravitational force for all intents Yes yes and so they each are describing an orbit in a plane and they are unaware of their neighbors.
And these planes of course converge because everything has to pass in a path you know in a plane through the center of the sun and it's no that the size of the comet contracts as it comes to the sun. That's what Leonardo was saying that a great distance When Halley's Comet is first seen it may be two hundred thousand miles across and then it converges as it comes into the sun. I see and this is just the dynamics of the sun's gravitational bionics it's impacting on a conglomeration of loose particles. Yes and yes some astronomers are looking for complicated forces magnetic fields goodness knows what when it's there in the simplest dynamics. If you just follow it out. Schoolboy dynamics then the comic starts doing all these wonderful things and the comic 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 comet is so diffuse but a lot of them collide. And then when they collide more collide because collisions produce things
in the right marry each other the relative speeds of these collisions in the comet itself Halley's when as near as the sun is doing about 60 kilometers a second and the relative speeds here about half a kilometer a second. That's enough to matter. It's quite it's quite a speed. That's enough for your traffic. A couple thousand miles an hour astronomically it's very small but from the point of view of the strength of materials it's to take breaks up these tiny particles at each other then others are immediately much smaller particles and made much smaller and also that the heating of the collisions it will drive off the most volatile things. So after this is happened several times of 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 actually and yet comets do disappear some Durbin records of a comet that goes around behind the sun is never seen of again or breaks into as Leonarda pointed out sometimes I mean this is this is so you can do two pieces one's got to be sure which comics one is talking about this a second smaller class of
comets called a short period comet. These are much fainter than the sort of thing we had last November in the great daylight Comet's there was a big long period comets with a little short period comets like Winkies and billers was one of the ones that you're speaking of now Bill is comic 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 billies moving in the orbit of a bit of common use a short period long period what are the periods what how long does it take a long period comet to go around the sun. Want to look at the long period comets can be anything from a thousand years as far as you like a million if necessary but the when it's gets as long as that it is very difficult to calculate accurately in the short period comets they are in the 5 to 10 year bracket that's what they're called members of the solar system. They're just go around among the planets they're nearly all associated with Jupiter and it's it's pretty certain.
Dynamical studies the Jupiters deflected long comments into these little bits you can do that if one happens to pass near but where did they come from to begin with by the way you pretty flecking them into smaller bits what was original how. What is a good idea of the origin of Comet I mean that's a best way to ask a question. Well is it because of a personal question. Until comparatively recent times there were 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. We just had the stars like billiard balls working out their fates in the empty space there are a few calcium atoms and things of that soil and then it began to be suspected that the must be material between the star and now of course you would quantities of hydrogen have been discovered in the galaxy and dust the dust by the way was first suspected by
Barna there are great patches you know in the Milky Way where there were no way to diners. This wasn't understood for a long time you see because even the distribution of the stars is a compared to recent thing in the beginning of the century. They had no idea of how the stars in the galaxy would distribute it to see how far we've come. Now we now have this galaxy pretty well figured out with them and they get exactly what you see that's why they were called nebulae. Because the external galaxies were thought to belittle patches of gas was also our star clusters in our own galaxy until they got the hundred and in particular the 200 range that couldn't resolve these things and see that they were in fact formed of bright stars and therefore very distant. We getting away from them 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 and empty space between. Yes. So now we now there's theories of gas and dust in between but how does that connect to comets as it's 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. 10 percent of the volume of the galaxy is filled with this gas and that's when I say Phil patchouli distributed and it's all it's very very tenuous and diffuse. But it does exist in sort of clumps. So this thing wasn't even known about at the turn of the century now we find articles up 10 percent of the space. Yes. And in its career around the galaxy and all these objects are also going around the galaxy with slightly different spheres huge cartwheel motion 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 and have a really interesting time. It'd be very nice if this began to happen. Good display in the sky. But still now 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 that cloud cannot ignore the sun's attraction. OK. And as the dust streams by the sun's attraction focuses almost like Elaine's except that it's a gravitational focusing it bends the little the material in places or pulls it together around behind it pulls it into the actual line as it were just almost like a lens a gravitational lens and we can increase the concentration of this material it's it's extraordinary diffuse as interstellar dust 10 to minus twenty four is that the density of water if that figure means anything to you one over a number with 24 zeros and you can put this up by a factor of a million million and it will still be very tenuous. The dust will still be there even at that but very very much denser now and then it can we play pull itself together at these great distances and then fall in towards the sun.
So what happens 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 bump under 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 there must be a loss of energy in order that the sun can capture. So why are members of the solar system somehow but perhaps very distant members. They must be very distant in order that that this these collisions can bring them together and form these things and then they will fall in practically along a line. And if the someone absolutely alone in space they would just fall plop into the middle of the sun and wind up. Well there's a little saving grace comes in here. Jupiter and Saturn and to some extent Eurus and Neptune pull 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 much realized but the 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 you can just get it can get out to just over to radio guy 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 a Sun's radius from the surface that is very close. So it was falling toward the center of the whole solar system that 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 save it. It won't save them all. But naturally the ones we see have probably been in before and got by Once you say then they'll get 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's moved around just a bit. It will be different every time. There have been even closer. Last one I think nine hundred ninety thousand miles from the surface is the 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 this picture you've just given this idea that tail of the sun gathers up in its tail a concentration of dust that it passes through and at this concentration then collects into the comets which have lost a little energy in the collection process and I'll 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 as I do of 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 need to believe in the else when they get like they were to unravel I was so sad that a good idea started as a heresy and ends as a superstition. Well I have my own ideas must be very near the beginning of that rather than the end. And there are other ideas all of comets.
Dr. Winkel now the Smithsonian believes that that a comet is a very small object but it's sort of icy snowball just a few miles in size dirty snowball. Yes and that this warms up as it comes to the sun. Now I can't believe in this because of the fact that you can see some of these things out beyond the orbit of Saturn and nothing that size could be seen out of that distance. Now how big does he picture these a mile or two will he yes he must 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 a planetary mass and no comet has ever deflected anything Brooks's comet went in amongst the satellites of Jupiter with no measurable it on its own orbit it 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 dirty snowball ideas. Well if it's not explained where these dirty snowballs come from but there is very strong evidence in the distribution of these comets in space that they are associated with the Galactic motion of the sun. Well Also I've heard that there is an idea of a shell of comets out around the whole solar system sort of sitting out there every now and then one comes in or something. Yes this is this is art's idea but here again it's very difficult to sustain in and the statistics are comets don't agree with the mean frequency of how often we do see them and when the door opens our arms only come from in the distribution of these orbits and I see 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 by the stars then the distribution would be different. We know how we were going to settle any of these things right. So if we can't see really the inside of a comet it makes no shadow when it crosses the sun.
And what we see is so we femoral as it swings by briefly and of most of these comments don't come back 1000 years how are we ever going to find out what's going on in the car when 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 there's any agreement is the precise nature of the lunar surface until we can actually get instruments there analyze it on the spot. And I think the same is really going to be true for the for the comets that if we could far and there are plenty of opportunities for this. When I say plenty you know every year or so there's an opportunity where even with present energy a probe could actually be fired to a comet. And then what I would like to see I would like to see built into that as it's instrumentation passed the capability of testing all these theories. For example we'd have to count the dust particles This is the techniques
already. It would be very nice actually if we could collect the dust particles because here we probably have a sample of interstellar matter. The matter has been thrown out just as a listener will not just leave the primordial dust cloud. Well it may not be primordial but there may have been thrown out of exploding stuff and then we would know what the Constitution of the elements were from these great nuclear explosions in space. But don't tell 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 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 that Ray I want to thank you very much for being with us tonight and telling us about comets. Great very good. This was about science with host Dr. Albert Hibbs of Cal Tech's Jet Propulsion Laboratory and his guest Dr. Raymond Littleton of Cambridge University.
- About science
- About comets
- Producing Organization
- California Institute of Technology
- KPCC-FM (Radio station : Pasadena, Calif.)
- Contributing Organization
- University of Maryland (College Park, Maryland)
- AAPB ID
- Episode Description
- This program focuses on the science of comets. The guest for this program is Dr. Raymond Littleton, Cambridge University and research associate at Jet Propulsion Laboratory.
- Other Description
- Interview series on variety of science-related subjects, produced by the California Institute of Technology. Features three Cal Tech faculty members: Dr. Peter Lissaman, Dr. Albert R. Hibbs, and Dr. Robert Meghreblian.
- Broadcast Date
- Media type
Guest: Littleton, Raymond
Host: Hibbs, Albert R.
Producing Organization: California Institute of Technology
Producing Organization: KPCC-FM (Radio station : Pasadena, Calif.)
- AAPB Contributor Holdings
University of Maryland
Identifier: 66-40-2 (National Association of Educational Broadcasters)
Format: 1/4 inch audio tape
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- MLA: “About science; About comets.” 1966-09-19. University of Maryland, American Archive of Public Broadcasting (GBH and the Library of Congress), Boston, MA and Washington, DC. Web. December 2, 2021. <http://americanarchive.org/catalog/cpb-aacip-500-599z4175>.
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