Astronomy for the layman; Origins of asteroids

The National Association of educational broadcasters with the cooperation of the California Academy of Sciences and radio station KPFA in Berkeley presents a program of the recorded series astronomy for the layman. On today's program Leoni Solomon a lecturer in astronomy at the Morrison Planetarium of the academy and John Hopkins an inquisitive layman discuss the origin of asteroids. Some recent theories Mr Hopkins begins the discussion recently at a seminar meeting at the licensure Observatory of the University of California in Berkeley. Dr. Kiper Yorkies observatory director discussed his hypothesis of a possible method of formation of the asteroids. You were there early on as I was and I'd like to review with you his remarks. Both to help me clinch my memories and just to help other people who might not have been able to be there I think perhaps we should first discuss a little bit about the asteroids themselves.

Yes I think it would be a good idea to fill in the basic background on what are these asteroids and where are they located. What do they represent and then talk about some of the theories of origin in particular of course the remarks that Dr Qype are made at the recent meeting. I think of the asteroids as being sort of miniature planets is a good description. Yes there are miniature planets or extremely large meteorites and there is in fact thought to be no essential difference between me here rights and asteroids. And there are again asteroids compare in size to some of the satellites. If you go around the planets and the our moon is only slightly infere Ferrier in size to Mercury and Jupiter has some satellites which aren't as big as the planet Mercury. So we can find representatives all the way along in terms of size the designation asteroid is largely made in terms of the orbit that

the paths that these bodies follow. When you mention them as being similar to meteors I presume by that you mean their shapes are irregular then they are just chunks. Yes the irregular shapes of the asteroids are best evidence by the variable light that they reflect from the sun. And we should point out of course at first that the asteroids are small the biggest one called Ceres is just under five hundred miles in diameter and it accounts for about half of the of all the asteroidal material. The other thousand that are known that we all make up something somewhat less than Ceres which by the way was discovered at the turn of the 19th century and located in the gap between the orbits of Mars and Jupiter which was predicted in a sense by boat is lost. Bode's Law is a sort of human rights numeral logical thing. If I didn't trick too badly on that by which you set down a series of a number of

four added the first zero to the next to three and start doubling the added figures 6 12 and so on and then set the decimal point over. So you have a point four point seven one point oh one point five and then two point eight and so on. Well the point four corresponds to the distance from the sun to mercury in terms of the distance from the sun of the earth which is unit point seven for Venus one point five for Mars and two point eight for blank at least as far as was known in the law was first set forth in any other part of the eighteenth century. I bowed and five point two units of distance corresponds to Jupiter so there is something in between Mars and Jupiter according to the relationship and nothing was known until the Italian astronomer Piaf he found little. I asked the Little Planet series to define it visually I was going to say visually or

photographically but at the time you mentioned I guess there was no real no photography at that time so it was found as a extra star quarterback in a field of view through a telescope. Star seemed to move therefore it was not a star it was a plane. Well even Ceres is not large enough to show any appreciable disk and some of the very close approaching asteroids which exist in orbit so elliptical that they come in fairly close to the Earth's orbit. They may show a little bit of a shape so to speak. I'm thinking particularly of the famous Eros which was quite close to us and 133 I believe it was. And on this close approach of sixteen or seventeen million miles it showed an irregular shape which was interpreted as representing a chunk of material about 14 miles long. Pardon me that's about eight miles long and three miles wide or something like that. Well this along with the fact that the light of certain asteroids varies in a

periodic way indicates that they are irregular in size just fragments. Well it's very. Very surprising that these pieces should be floating around in space that way. When you spoke of them as being between Mars and Jupiter do they cover all of that space or are they pretty well peaked at the point where the boat is lost so they should be as they're peaked at that point. But they are spread throughout the belt between Mars and Jupiter with certain exceptional regions where there are no asteroids found in this kind of work we use what's called the mean distance. After all the paths are not circles. In fact some of the asteroid orbits are very elliptical almost approaching a cometary type. Well all we can do is take the average distance of the so-called semi-major axis of the elliptical orbit

and we group the asteroids. And this is been done for about 600 of them. I group them according to the lengths of these the semi-major axes. This is a sort of statistical proposition. It's done with distribution diagrams. Well it's found that there are notable exceptions for example. There are very few if any asteroids. Just a little way outside the distance cars behind a series. When one studies the motion that an asteroid would have in that orbit one finds that it would take about half as long to get around the sun as Jupiter does. In other words it would go around the sun in about six years where Jupiter takes 12 years to go around the sun. This means that there's a simple fractional relationship between the the periodic revolution of Jupiter and of this hypothetical asteroid.

I see what you mean that on one trip of Jupiter around the sun it would catch up. Or rather the asteroid if there were such an asteroid would catch up with Jupiter twice it would keep going by Jupiter just twice and in the same relative position on the orbits. And so. I should say the same relative position in the orbit of the asteroid vs. the orbit of Jupiter as certain conditions would be repeated periodically by conditions I mean directions and the forces of attraction. This is what we call perturbations periodic perturbations of the little asteroid would be produced by the attraction of Jupiter and there would be a sort of what we call it a resonance a sort of vibration. If we use that term a set up which will disturb the motion of the asteroid so that if it did it into that particular orbit it would soon be knocked out of course means in astronomical terms maybe. If you a few thousand years.

But. The upshot of all this is that there are certain belts in the asteroid region which are not occupied by any objects and when we study the periodic revolutions of hypothetical asteroids in those regions we find that they would all be simple fractions of the Revolution period of Jupiter. These are known as the Kirkwood gaps after an astronomer over pointed them out and they are important in understanding why the asteroids avoid or prefer a certain average distance from the sun. By the way as the asteroids go around the sun they all move in the same sense that the planets do. True some of the orbits are highly tilted but none of them are such that the planet is going. If the asteroid is going in the contrary direction this is something important to realize when theorizing on the possible origin of the asteroids. That again is a factor which makes them more like planets in that they're all going. Same way it seems planets are.

Yes love of course even in the case of the meteorites with which fall on the earth. We can find by tracing back their emotions how they were travelling before they hit the earth and they seemed to be following what we would call asteroid orbit Zorbas which were so disturbed however as to intersect the orbit of the earth not keeping the average of the distance between Mars and Saturn. The Mars and Jupiter from the sun the meteorites which flash in the sky and call cause meteor showers do not move always in the direct your action when they come in any direction and that links and more with comets. But the asteroids proper do move in the same sense with the planets do so called direct motion. Same the earth goes around the sun. You mentioned. The what was it Series.

Series was about half of the total mass of all of them yes and all of the asteroids together on a very liberal estimate might constitute 1 500 part of the mass of the earth. Which would mean that the series would be just half of that amount or a thousandth of the mass of the earth and the other thousands would be everything else best and powerless and some of the other large ones going on down to the very tiny ones like Eros and even its mark 1 500 the mass of the earth is of course still a considerable amount of material of the Earth's mass is measured as 66 quadrillions of tons. So if we divide this by 500 we get something like thirteen quadrillions tons so there's a good deal of material stretched around but the volume of space is so enormous that the density of the asteroid in the asteroid belt is very low chances of being hit by one as

we travel through our future space tripped on me and quite negligible. Of course this is an important figure to recall. The total amount of matter that makes up the known asteroids would not make say the moon for the moon is an eighty second as massive as the earth which is far far above the maximum permissible estimate for the total mass of all the asteroids. This makes it against the use of boat wasn't it wasn't really panning out here. Because if it had predicted a planet I assume that since there is a sort of a grid ation of sizes one goes out from the sun that a planet in that particular position should be well at least as large as Mars or the Earth and perhaps on up toward the size of Jupiter where as you say these masses are way below the mass of the earth. Yes this is a clear dividing line between the inner trus trail type planets and the outer such as Jupiter Saturn Uranus and Neptune. Pluto doesn't fall in either of

Pluto. Is this is a size which is estimated to be somewhere between that of Mars and the earth. But there's an interesting theory on that that Pluto is actually a castoff satellite of Neptune and so it is more a satellite in type rather than. Typical of the outer planet. But that's another matter which doesn't bear particularly on the origin of the asteroids. Well if we can't ascribe the asteroids then to boaties I would to suppose also them that there are going to be the same as the planets whose spaces are predicted course not scientifically I suppose. Well there's a lot I fear you're probably trying to put a little too much weight in Bode's Law there is some justification for the fairly geometrical spacing of the planetary orbits especially on Main and recent theories which make out the planets I think condensed from from the

nebula cloud of dust and gas around the primordial sun. The asteroid specifically may be thought of as being derived from an exploding planet. Or just be a lot of unconsolidated stuff from this process of forming proto planets that is clots of gas and dust which for some reason never did gather together into even the satellite which into the small planet which would be a 530 large as the as the earth. Well this is where Dr Kiper entered into the discussion. When the heard him as you recall in a very fine article that he contributed to a symposium on astrophysics published some years ago he had presumed that the asteroids are simply fragments of fragments of a proto planet. That is to say one which had condensed out of danger. Or I should say circle solar dust and gas a long time ago in precisely the

same way as the other planets did. But I gather from his remarks he has had to revise that according to his calculations and the minimum mass that a proto planet condensing from circum solar dust and gas would be about one ten thousandth of a solar mass that is say one ten thousandth the mass of the sun. And after certain evaporation and losses that taken place the mass for the final object would be about 30 millions of the mass of the sun. Well now the mass of Ceres is something like 40 billions. If I got the figure right. So it is way way below the minimum mass for a typical proto planet and so some other approach has to be made here to the problem of how did the stuff get together in the first place

which later became the asteroids. Well I remember that remark you put quite a bit of stress on the fact that there was a certain critical density with which the material in space had to achieve before you would get any even proto planet. Yes that's the big problem we know certain things about the nature of interstellar material. We see now and. Space we know that is very very low in density. We do feel however that in some conditions it can reach a higher density and under these conditions unease begin to clock together. Now just exactly how that happens is still I think being worked out but they are gathering again from my Dr. copper's remarks. They're placing increased emphasis on the process the cause accretion of interplanetary grains. That is to say several molecules of iron ore silicate come

together to produce a little grain which then grows by accretion. The glue being frozen water vapor and ammonia. Literally. That's the word they use and they think then of they can bomb ration of interplanetary dust grains being glued together with frozen ices of water and ammonia. And it is out of this sort of a process that it is thought that the asteroid of material might have been formed but of course everything depends on the density being high enough so that this process can take place. Of course he also felt as you say that the accretion is an important process but accretion as such as I recollect it even was the glue. I still wouldn't form a truly solid planetary mass. Know it. At first of course but as the objects began to grow they would

gather together providing their relative velocities were not too great and so to say stick. Now in this matter the relative velocity apparently is quite interesting. I noted that he remarked that after copper remarked that if an object got as big as 80 kilometers in diameter. It's about 50 miles in diameter. There would be a sufficient radioactive heat from radioactivity trapped within to cause melting and so any object such as these which might form would take on the characteristics which we are which we know are typical of meteorites either silica iron or iron nickel combinations anything less than that would simply remain as a sort of. What did he say just a calm aeration I forget the word he used but oh rebel rebel once they use the phrase they have some very character very quaint characteristic terms in this discussion

apparently. And this would be a granular in type and not achieve the hard Blassie of parents we associate with an easy reading material. I remember when he was talking about this. This melding process here marked it having a certain minimum size was important in that as the size gets greater the mass becomes greater rapidly but the outer surface from which radiation from which the heat being generated internally will be lost. Doesn't get great very rapidly and therefore you could have these bodies which were going to melt and have a truly planetary characteristics. They would form then yes. The thing you're getting at there of course is that as you. Increase the diameter of an object the mass goes up as the cube of the surface was only as the square doubling the diameter. I would give in general a ton of the

volume and for the same density material eight times the mass while the surface area would go up only four times. That means that the object would be more efficient in holding the heat that it had. Well in this way. Objects of fairly solid constitution might be built up according to this view. But then we have to explain where the very irregular objects such as Eros might have come from these have been described as the splinters and if our connection with Ben and meteorites is meaningful they are metallic splitters. Well now you know order to form this highly compact in material there had to be melting and melted object would assume of course a spherical form. The harder to get back into the frag mental state. Well of course you're going to say I started small enough that it would never get to a spherical state. No but that's the point that we have to keep in mind this 50 mile diameter which is thought to be critical in terms of internal heating and consolidation

so presumably something larger than the gradually formed and then there was fragmentation. This is thought to have been due far as I can get out of the present view is from a collision. A collision might have occurred where say the relative velocity was a mile or two a second. Well two objects let's guess let's say each one hundred miles in diameter more or less colliding in a kilometer per second or a couple of miles per second or a couple of kilometers you can take anything you want would cause quite a crash. Now one of the interesting theoretical thing that's been done is to ask this question well can't we take the orbits of the present asteroids and trace this all backward. Study the motions in the past and find they all had a common origin and in time and space of Japanese astronomers tackle this problem.

Name a name of here Yama. He studied the long term changes in the known asteroid orbits. He knew for instance that the mean distances and the time of the periodic revolutions would remain practically unchanged but the degree of eccentricity of the orbits and the inclinations to the general plane of the solar system would be changed. He studied these changes and found that there were several groups of asteroids groups which it would seem all start together at a certain point in time and in space. This gives some credence to the idea that the asteroids originated as fragments of something which occurred as a collision. But an interesting result of the calculation was that these objects left each other with only a relative velocity of something like a tenth of a mile a second more or less. This means in the terms used by physicists that the collision must have been very elastic

like two clouds of wet earth smashing together and a certain amount of bouncing taking place but very little. Just enough to make a general scattering. It may be then that this was the sort of process which took place a collision between two objects which did not actually rebound but just sort of interpenetrated scattered rather gently comparatively speaking sending a smaller object often all directions which subsequently then became the asteroids. That certainly sounds like rubble. You speak with that I think that's a very suggestive term. When I when this sort of thing happened. If there were splinters of that size. Is this a possible source of some still smaller splitters. Yes assume this thing couldn't you couldn't do succumb also from meteors. This would lead to other more fragmentation as the number of objects

increased chances for a collision would increase and there would be a reduction in size. And as I hinted before and as Dr. Cooper So clearly stated. President view is that all media rights are fragments of asteroids. Remember He also remarked that to you could see the results of their having been part of the asteroid of the meteorites having been part of some small body where the mass was too small or to melt. But as he put it they would look as if they'd been heat treated. Yes if they program someone I was full of phrases and very suggestive analogies. If there are there are certain you're right specimens which when subjected to an out cut polished and analyzed look like lumps of stuff that have just been pressed and warmed up a little bit but not quite enough to cause them to fuse. And then on the other extreme and we have

got samples saved from the Great Bear. Meteorite in Arizona where the etched metal surface shows a crystalline structure. Well all this wooden steps and figures which are peculiar to certain types of me here at iron. Well I know that quite great distances on the Great several miles from the Meteor Crater in Arizona I've picked up materials which were just sort of looked like baked rock when he spoke of this heat treatment I thought of these pieces of real looked like rocks that had been in a fire. Well that may have been the subsequent oxidation here on the surface of the earth I don't know exactly I've seen some of those things and they do look somewhat as you describe but I'm thinking particularly of the meteorite specimens which are picked up in other places where there is no. Overall melting together apparent but it just clumps of metallic and then it

clumps of stony material in the corporate view of course is to be the difference. Coming from an object which had been large enough to produce internal melting through heat liberated by radioactivity or otherwise if smaller. Another interesting fact that we have brought out. I think we should mention perhaps is the fact the length of time it takes for this melting to take place we normally think of when we melt something we put in a fire in a furnace and it comes up to melting the melting time. As he remarked I recollect that some millions of years required to get up there were 400 million years or something like that. Yes the amount of heat liberated by the trapped or radioactive elements and me and the primordial material is quite small for a given volume and space is quite cold so there is a tendency for the heat to leak out light if the amount of material can be gathered enough and sufficient mass and then the building up gradually takes place and the melting temperature is reached. But of course

in all these things we have plenty of time to operate in it. Of course there again if you know what kind of. When you speak of plenty of time if you know what kind of radioactive material it is I suppose this is another way of getting at the age of the earth really. If these asteroids have been through this process and are now solidified you can calculate how long it was to take for that to happen. Yes you can gather together information that will give the time that this material has been consolidated and I know Dr. Karp mentioned the new determination which puts it at four billion years minimum. So that's something longer than that. Well that's nice it puts him in line with the rest of the estimates of the age of the earth. You've been listening to a discussion between Leoni Solomon a lecturer in astronomy at the Morrison Planetarium in San Francisco John Hopkins and inquisitive layman on the origin of asteroids. Some recent theories the series astronomy for the layman was recorded at radio station KPFA in

Berkeley with the cooperation of the California Academy of Sciences. This is the end of the network.