"Jupiter's not quite as big as we thought."

Hello there, thanks for joining us yet again. This is Space Nuts. My name is Andrew Dunkley. We're here to talk astronomy and space science. And on today's program, we're going to look at a tiny weeni itsy bitsy Jupiter. Yes, it's not nearly as big as they thought it was. In fact, it could lose status as a consequence of this. Maybe not, but we'll talk about that. We're also going to look at a flipping interesting comet and solar gravitational lens focal points. Could we visit them and what will that mean? We'll find out on this episode of Space Nuts. Fifteen second in channel ten nine ignition sequence Space Nuts or three two. Space Nuts, as can I report it. Bills good and joining us again to talk about all of that stuff and probably a lot more as Professor Fred Watson, Astronomer at Large, Hello Fred. Hello Andrew, good to see you again, to see you. Always love the hoopy shirt. Oh yeah, sorry, it's very tutty old on this shirt. This one's nasty, tatty, but it's white and it's got more food on it than I've ever put in his stomach. So has he got the has he got the space not? His logo on it. Hang on, I've got one here somewhere, a spacent logo to know I have, but it's not on not on this shirt. It's on the other wide shirt. Where did that logo go? I've lost my logo. Anyway, we'll fight losing losing mojo. That's true too. Yes, indeed, let's begin because we've got a lot to talk about. This first story looks at Jupiter, the biggest planet in our solar system until we find planet nine. And this is a story that's suggesting that Jupiter is not quite as big as we thought it was. Yeah, it's it's shrunk by well eight kilometers at the equator and twenty four kilometers at the poles. So what this is all about? And I should just give you the numbers. So the revised radius of Jupiter at its equator is seventy four hundred and eighty eight kilometers, which is actually I think four kilometers less than we thought before, which doubles up to up to eight kilometers when you're talking about diameter. But it's polar radius, which is sixty six thousand, eight hundred and forty two, And those two numbers are quite different, which is why Jupiter's flattened at its poles, just as Saturn is. But that's actually twenty four kilometers less than the previous estimations. So for the diameter, so it's not a huge, huge amount, but it's. Not when you're talking about the size of the planet. That's exactly that's what I mean. Yeah, seventy one thousand and four. So it's what one one hundred and forty thousand kilometers or thereabouts in diameter, which is eleven Earth diameters, which is what we always say. So why, well, first of all, how have these measurements been made? And the answer is that the old measurements actually go back a long way. They come from the voyager and pioneer era of the exploration of the outer planets, and that goes back to the seventies and eighties. They So what led to the diameter or the size of Jupiter that we've been using since then is what's called a radio occultation. So the spacecraft is behind As it passes behind Jupiter, its signals get refracted actually by the atmosphere of the planet, probably scattered as well, but you can time it very accurately in time when the spacecraft disappears behind the planet, and you know it's trajectory. You can then time when it reappears, and from that you can calculate the and knowing about Jupiter's motion and the spacecraft's motion, you can calculate what the diameter is. So that those are the values that we've been using ever since. I think I know where all of this went wrong. They didn't a layer for it, stopping for gas. The planet or the spacecraft. Well, it's a gas giant, that's right, Yeah, the planet's a gas giant. So yes, it's a good point. Anyway, I let that one pass. So moving off wasn't very good. It was all right for the start of the show. They usually get better, as we call it's the new measurements come, of course from the spacecraft that is currently in orbit and working away very hard at Jupiter in orbit around the planet, and that is JUNO, that says JUNO Mission, which has been orbiting Jupiter since twenty sixteen and doing pretty well. It's yes, for the decades since we've had Juno, which gosh it, time flies, doesn't it anyway, So that's allowed much more accurate measurements because the space that JUNO spacecraft. Its orbit is very well understood. It's fairly close to Jupiter. But you might think, you know, well, why are we so keen to know the damage of the planet to such a high degree of accuracy, And the answer is to do with our model because it is. Yeah, that's right, it's to do with our modeling of the planet's interior because a small difference like that can make a big difference to what we imagine the interior of the planet is like. And remember, of course, everybody that Jupiter all wes see is it's cloud belts. When we look at the planet, we don't see any surface or any hint of a surface. So the internal structure of Jupiter is something we have to deduce from other measurements, and the an accurate measurement of the diameter of the planet comes into that. So that's the reason it Also, you know, one of the other things that's of interest in Jupiter is the behavior of the atmosphere itself and the winds that blow in Jupiter's atmosphere, and that also needs an accurate understanding of the diameter of the planet. Yeah, I actually I was just looking at that diagram that shows the different potential diameter situations based on the behavior of the planet. And yeah, without wind it loses another what fourteen kilometers. Yes, that's right, it does. If you if you imagine the winds aren't there, it does. It shrinks, so by fourteen kilometers exactly. That's the radius the not the diameter. So we we have, you know, a tiny figure that looks minuscule compared with the diameters of the planet itself, but it is important in understanding the upper atmosphere. It's if there were no winds, then what we will be seeing will be fourteen kilometers smaller. I'm surprised that it's taken us a decade to figure it out, and Juno as they have been there nearly ten years. Yeah, but maybe you know, the accuracy that we're getting with this relies on many passages of Juno around Jupiter. There were and because you're always you know that the cord that of the Jupiter's disc that the planet that the spacecraft flies behind is different every time, and so we you probably need to build up a statistically significant sample of entry and egress times when you're looking at you know, the object disappearing by behind the planet. Occultation is what we call it an occultation, is when one object hides another, and that's how you're measuring these diameters. So yeah, it's probably it's probably taken ten years, partly to a mass the data to give us this kind of level of accuracy. So, okay, how accurate do you think it is now compared to those early flybys with Voyager and Pioneer. That's a really good question. Actually, I haven't seen any error estimates on and as you know in physics and certainly in astronomy too, you always need a plus or minus an error estimate as to you know what the likelihood of your measurement being that number is, and I'm seen it for these so I don't know the answer to that. But my guess is that we're talking about in the region of a kilometer, which is pretty impressive for something that diameter, and something that's that far away have a billion kilometers away, So. Does this mean that air estimations of other planets in the Solar System are probably a bit off as well? When you consider that Neptune, for example, I think we've only visited once, would that be right? Yeah, Yeah, that's right. So yes, I think you're right, You know you certainly the estimates of the planets beyond Jupiter and Saturn in terms of their diameter and physical characteristics will have much bigger error limits on them, just because we can't make the measurements as accurate as you can when you've got a spacecraft in orbits around. One of them. Okay, so that's that's work in progress. Whenever we go back, we might be able to fix that. But yeah, they've got any missions plan and for Nepturing and Urinus or anything. There's always calls for them because they're such interesting worlds. Yeah, but I don't think I mean, I think there are there are lots of proposals, but I don't think there's anything FUNDED might be wrong about that. Maybe our listeners can tell me if I'm wrong about. They may well, because a lot of it. Actually, We've got one fellow on Facebook who regularly researches some of the things we talk about, and he publishes his findings on the Facebook podcast group. Yeah, and I think it's great. I've read a few of his explanations and they're very good. So we're probably going to get sacked, but. It's surprise them sectors or. Well they can afford us. That's why we're still here. Well, that's true. Yes, that's true, very true. No, it's a really good discussion point. So it sort of keeps the momentum going when we discuss these things. So I'm sure it'll work on our tiny Jupiter story, which good, yes, which you can read about at the Daily Galaxy dot com website, or you can read the paper at Nature Astronomy. This is space Nuts with Andrew Dunkley and Professor Fred Watson. That's a that's agreement, is that if the goodness say, I'm really sorry, Okay, he gets very enthusiastic. Space nuts. I couldn't help it. Turn that into a link. It's it's yeah, brilliant. I'll tell him, gosh, it's just too good. It's just too good. He was going off his nut that day, wasn't. He He was? Yes, he's very highly strung. Well, that that's how he That's how he greeted us when we visited you late last year. He came tearing down. The stairs doing his rooster impersonation. No one could ever rob your Fred. The one good thing about it, yes, is the one good thing. They don't have to be big aggressive dogs they just have to be loud. Or even you know, a brush turkey going past the window in the middle of the night. That's enough as well. Yes, that's it's all that takes sometimes. Now let's move on to our next story. And this is a story that's got scientists really well. The headline says, scientists stunned. We're talking about a comet that has done something really, really unusual. Unusual. We're talking about comet forty one P. What's it done this time? Because it keeps making the news this one, Yes. It does. Tuttle. Jacobini cressak is its full name, better known as forty one P. It's an object probably a kilometer across a flying iceberg like basically like comets are, and it orbits the I think about every five and a half years, so it's in what we would call a short period comet orbit, and it's when it passes near the Sun. Of course, it does what comets do. It out gases, produces basically plumes of gas leaving its surface. It's usually water ice being converted directly to water vapor by the process known as sublimation. But what has been recorded in in fact, in quite a while ago actually, I think this is eight years ago by a NASA spacecraft observations made by NASA Swift spacecraft measuring its rotation and basically over sixty days, what's that sort of nine weeks or something like that, it slowed down from rotating once every twenty hours to once every fifty three hours. So that is a you know, it's almost a three factor of three in the level of spin that this commet has got. Ye, and it's there's suggestion that maybe it's now rotating in the other direction from what it was before that there has been some sort of reverse. It hasn't slided down to about one third, it's it's reversed. So it's it's slowed down well five times. Yeah, if the other way it could be, I mean, part of it could be due to how you measure the rotation, because it could be tumbling as well, so you might be seeing it going the other way around. But it does seem to be I think you're right. It's I think what you've just said is correct that it's a it's a reversal and genuine reversal of its rotation direction. So yes, it's it's got much more than a factor of three. That's right. That kill my theory because my first thought was, well, this must just be an observational era. But an observational era wouldn't get it the wrong way around. Less, of course you're talking about the color of the universe, but we won't get there. But it was so, yeah, what else could be causing this change behind? Well, I think if it was anything other than a comet, you know, if it was an asteroid doing this, or a planetismal, or a distant one of the distant Couiper Belt objects or something like that. If it was any of those, we would be utterly gobsmacked because there's no physical mechanism to do that other than an interaction with another body. You know, if you had two bodies gravitating close together, it could have an effect on the rotation, but in fact, more especially a collision that would do it as well. But with a comet, you've got this process that outgases what I was saying earlier, as it gets near the sun there they're basically the ices start to vaporize and you get a thrust from the from the outgasing material, which is what we call a non gravitational perturbation. It's when when you know, the outgasing material is acting like a rocket engine and it's changing the dynamics of the object as it's orbiting the Sun. And you can imagine that if there was a formation of ice on the on the surface of the comet that essentially tilted the blast of the of the escaping material as as it's sublimated as the as the the material that water mostly went straight from a solid to a gas. It's like having a you know, a sort of Vernia thruster. It's like where you've got a thrust that is changing the rotation of a spacecraft because it's not going the line of the of the thrust is not going through the center of gravity of the comet. If it's off the center of gravity, then it's going to impart a rotation on it, and if it's strong enough, then it might be enough to slow it down and perhaps even reverse its reverse its rotation. So that's what the thinking is. But it's never been seen before. No, like retro rockets. Yeah, that's right, it's a retro rocket, but one that's not slowing it down in its orbit. It's changing its rotation because if the angle that the rocket, if you want to call it that the rocket exhaust is coming out. At the moment, it's seven hundred and seventy four million kilometers from Earth by point one out astronomical units, And as you mentioned, this unusual behavior was checked back in twenty seventeen and they've only just sort of put a paper together to try and explain it. It's got a five point four year orbit, so it comes back quite often. Yeah, that's right. It's it's it's captured, basically captured by Jupiter, so its orbit is dictated. It would have been in its early history, it would have been a comet coming into the Inner Solar System from the Oort Cloud, this spherical sort of reservoir of comets, but would have had its orbit modified maybe several times by the influence of Jupiter, which is why it's now in this really short, such short period orbit five point four years. There has been a suggestion that if you've got these sort of oblique out gassing that we've just been talking about that would change the rotation of the object, that that might also signal that there might be weaknesses in the comet structure, and it may even be a precursor to it breaking up, which is something that I think will be observed with great interest as to how it progresses since since this change of spin. Yeah maybe, and we will find We could find out as late as or as soon as late twenty twenty eight, I think, is its next appearance near the Sun or near us. Or whatever you like. So I'll keep an eye on forty one P. I guess you P. That's right. There was a proposal long and long ago to send a spacecraft to it because it's a short period comet, so it's always in the inner Solar System, and that was what was then called EZRA, the European Space Research Organization, the precursor of ISSA, the European Space Agency. This is in the nineteen sixties. They looked at sending a probe to that comet, but they changed their mind so it never went. Ah, yes, I'm sure that happens a lot in astronomy. I mean not an easy, not an easy thing to do to you know, set up a mission and actually execute it, and you're going to come up with the dollars and yeah, you know, it's only so many ten cent pieces confider on a jar on the mantelpiece. So yeah, that's right, Yeah, all right. You can read all about comment forty one p at it's at the Daily Galaxy dot com website. But you can also read the paper. I think it's just been pre published or pre there's a pre print available on the archive. This is Space Nuts Andrew Dunkley here with Professor Fred what's a bolt? If I'm promund. Piece Nuts, I've read to our final story on this episode. We've talked many times about gravitational lensing and some of the strain things that it does. You can watch something happen two, three, four times over the course of many years because of gravitational lensing, because the light is redirected and takes longer to get here, and so you can see something and go, oh, what was that? Hang on, I'll know again in a couple of years because not quite. But what we're talking about, what we're talking about in this particular case though, is actually going out to a solar gravitational lens focal point. Is that the crux of the story. It is, That's right. So this is a really interesting kind of assay really on the Universe Today website by Thomas Wick about about the solar gravitational lens and about how you'd get there. But the solar gravitational lens itself is really interesting. So the idea is exactly as you've said. If you've got an object of any mass, and it happens with planets as well stars, it's going to bend the light passing around it because it's distorting space under the protocols introduced by not sorry, as we understand it, by the protocols introduced by Einstein's general theory of relativity. That's what lets us do all the calculations about this sort of thing. So you've got an object in space, it's bending the light around it, which means it acts kind of like a lens. It's a very odd sort of lens, though. I've seen a glass kind of interpretation of a gravitational lens. There's a specialist who works on this stuff in Melbourne who showed me her glass version of a gravitational lens, and it resembles, you know, the bottom of a wineglass where the stalk comes up out of the middle of that. I know them very well for it. Yeah, Well, if you break the wine glass off and you're left with that sort of flared part at the bottom, that's more less the same as a gravitational lens in the way it would act on the light going around it. So it's not like a magnifying loss, which is what you'd like it to be. It's this very peculiar cusp shaped lens, and so it gives you a focus that is blurred, but it's because you know the properties of the object that's doing the lensing, and in this case we're talking about the Sun. Because you know the properties of the Sun, you can calculate what that blurring does to the image and you can essentially compensate it. So you could recreate the light coming from a very distant object and recreate the image that the Sun is forming as though it was a proper lens rather than a peculiar cusp lens. And that's what's sort of being proposed. Could we send a spacecraft to the solar gravitational lens focus where you could look directly back at a planet on the other side of the Sun around a distant star, so you're looking into another Solar system a long way away, but you're using the Sun's gravity to bend that light by relativity and focus it to a point. And if you put a spacecraft there with a camera and a fancy computer, you might be able to reveal continents on an exoplanet, for example, or even cities. That's the sort of thing that people are thinking of. So here's the snag though. That's a great idea, but a snag. I think I just read that exact paragraph as you're about to say it. I might blow the whistle. Yeah, well you can now you do it. It's the distance, isn't it. It's yeah, about somewhere between six hundred and fifty and nine hundred astronomical units is what's quoted in this in this article. In an astronomical unit is one hundred and fifty million, killer me. So it is a number with a lot of zeros after it in kilometers, and you know it's getting your spacecraft that is the issue we're talking about. Well, the estimate here is four times further than Voyager one has traveled, and that, as we know, is twenty three light hours away. They reckon that it would be more than another one hundred and thirty years to for Voyager one to get to the Sun's gravitational lens point. By my calculation, and this is probably way wrong. Ninety seven five hundred million kilometers. Sounds about right. Yeah, it sounds like a lot. Yep. I didn't think my calculator could fit that many numbers on it. Remember the old calculators when they first came out. If you gave it too big a problem and it would just give you a little EVA era. Now I can't do that. Sorry. Yeah, yeah, this computer says no. Really, that's right. Yeah, that's a long way away and very difficult to achieve. But I think one day maybe we could do yes. So that that's really the thrust of this article. How about you know what's the way to do it? Can you? Can you get to that point? And they the author's got some nice calculations which have checked these are correct, I should check them. But anyway, if you were trying to get to that solar gravitational lens point focal point in twenty years, then you need your spacecraft to travel at about one hundred and fifty kilometers per second. It's which is very hard when you're pointing away from the sum. The Parker solar probe, they point out, and we kind of know this because we've talked about it, has actually got to nearly two hundred kilometers per second but that's only when it's a what we call perihelium. It's at its closest point to the Sun where it's going fast. And what we're talking about here is something going in the opposite direction, going away from the Sun. For it to travel at that sort of speed, you need an extraordinary amount of thrust. I don't think you're talking about chemical rockets to get up tow one hundred and fifty kilometers per second. Lights, So the light sales, Yeah, that's one of the things that you and I've spoken about before. If you can beam out laser lights to a solar sale or gigantic piece of you know, something very thin like MYLA that's reflective, then the light itself pushes it along and you just keep going so that it just keeps up, building up speed. There are trouble is when you get there, how do you stop it? Yeah, you don't. You just keep going, That's right, unless it had something on board to like you turn off the light and reverse something. I don't know, you're never going to slow it down because even if you turn the light, it stops it accelerating, but it's still going at that speed. That's right. There's a a possibility that, you know, could you do the solar sale trick and basically make it successful. The problem with solar sales is you can only carry objects that are very light in weight or have low mass. And you might remember we've looked at this with what was it, the Breakthrough star Shot program, which I think has now ceased. Breakthrough star Shop looked at the feasibility of using a solar sale to send a spacecraft to proximate Centauri, which is only four light years away, and it could be done, but your spacecraft would basically consist of one what's it called printed circuit board and a detector. There's not really room for anything else. It will be so it'd have to be so light in weight it will be measured in grams rather than kilograms or tons. So that will be the problem with your you know, with sending a spacecraft to the solar gravitational lens using a solar sale. So you're talking then about nuclear sources and things of that sort. That this very nice article goes into some of the niceties of nuclear thermal propulsion and things of that sort. Even so, it's still a very tough ask to send a spacecraft to that interesting part of the Sun's environment where you've got the solar gravity forming a focus to get there. It's really to get there in you know, twenty years or so. You're talking about really new technologies that we simply don't have at the moment. Yeah, well one day it might be a long way off, but the time may come, and then again we might have figured everything out by then, so yeah. Yeah, I mean, you know, the other thing is you'd want to choose. So you've got to choose the direction that you go in to be in the opposite direction to the planet that you want to observe, the exoplanet. And if you get that wrong, if you choose a planet that's completely boring and has no surface features whatsoever, then you don't really contribute much to our knowledge, particularly our knowledge of whether we're alone or not, whether it's life and dowhere else. It's sort of like leaving the lens cap on the camera when you land on birdness. Yes, actually they didn't leave it on. It melted on. I think. Yeah, well one of them fell off as well, didn't it fall off some top of the on top of the scale that was going to give the you know, as a ruler that they jettison to give the camera something to look at so you could measure the size of things, and the lens cap landed right and top of it. I think that's what happens. They've had quite a few venus disasters over the years. But yeah, you're right, this would be very, very difficult to swallow if you buging it up, because you couldn't go and fix it, not. Like honey, that's right. You can't move it in any direction. You're stuck on one plate. Really. Indeed, it's food for thought though, but one day we'll figure out a way. If you'd like to read about that story, it is at Universe today dot com, as Fred said, and we're done, Fred, thank you very much. Oh yeah, that was great to talk about all those things. So I hope you do it again sometimes. One topic, so I'm sure we will. If you would like to visit us in the meantime, don't forget to visit our website, space nuts podcast dot com or space nuts dot io, or visit our social media platforms the official space nuts Facebook page or Instagram page or YouTube channel or whatever you like. Or if you want to talk to like minded space nuts, you can do that on the Space Nuts podcast group on Facebook, which is always a lot of fun. Thanks Red, We'll see you soon. Yes, I hope so Bill he said that. And thanks to HEU in the studio who couldn't be with us today. He was invited by a friend to see a comet. He couldn't wait, so he ran over there. It turned out to be Goldfish. Some people will get that and from me Andrew Dunkley, thanks for your company. See you on the next episode of Space Nuts. Bye Bye snus. 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