Planetary Radio: Space Exploration, Astronomy and Science - Studying the Rings With Imke de Pater

Episode Date: May 14, 2007

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Rings around the planets, and especially Uranus, this week on Planetary Radio. Hi everyone, welcome to Public Radio's travel show that takes you to the final frontier. I'm Matt Kaplan. Imke de Potter is a renowned UC Berkeley astronomer and planetary scientist. She studies those complex yet comely collections of cosmic crumbs that surround the gas giants in our solar system. We'll chat with her about some of the most interesting things we've recently learned about planetary rings,

including those surrounding crazily tilted Uranus. Later, Bruce Batts will resolve our space trivia question about planetary magnetic fields. We hope you won't find the answer repellent. And Emily is just a minute or so away from this week's Q&A segment. Let's check the headlines from around this best of all possible universes. Did you hear about the new supernova? It is by far the biggest ever detected and can't be a welcome sight to anyone with the misfortune to live nearby. Fortunately for us, SN 2006 GY is 240 million light years away in another galaxy.

On the other hand, Eta Carinae is just 7,500 light-years from our neighborhood, and scientists think it may blow soon. That's soon in the cosmic sense, by the way. Is there no end to wonders in the cosmos? No, Virginia, there isn't. How about the Jupiter-sized planet circling a star just 60 light-years away? Astronomers have used the Spitzer Infrared Space Telescope to determine that winds on that globe whirl at 4,500 miles per hour, or about 2 kilometers per second. Compare that to a measly 340 miles per hour tops at Jupiter. Get the details at planetary.org. What does asteroid Itokawa have in common with a sea otter? That's an answer you can get from Emily Lakdawalla's blog, also at planetary.org.

In the meantime, we'll bring her in to talk about other asteroids that should soon be welcoming a visitor from Earth. I'll be right back with Imke de Potter. Imke de Potter. Hi, I'm Emily Lakdawalla with questions and answers. A listener asked, Will Dawn be able to visit any asteroids besides Ceres and Vesta? Mission planners aren't yet sure whether there will be any bonus asteroid flybys for Dawn. Dawn is scheduled to launch on June 30th and will use ion propulsion to take it to an orbital rendezvous

with the two largest asteroids, first Vesta and then Ceres. Its ion propulsion means that it will have a very different cruise phase than traditional chemically powered spacecraft. Chemically powered spacecraft spend very long periods simply coasting, punctuated by an occasional course correction with their thrusters. So the trajectories of chemically powered spacecraft are laid out in great detail very far in advance, which means that bonus encounters like asteroid flybys can easily be planned ahead of time. This detailed advanced planning just doesn't work for an ion-propelled spacecraft.

Why not? Stay tuned to Planetary Radio to find out. Thank Cassini. Not just the spacecraft circling Saturn, but the 17th century astronomer who was so fascinated by those beautiful rings. Scientists have been dying of curiosity about them from his time and even further back to Galileo's cup handles. Imke de Potter is their spiritual descendant. The UC Berkeley professor of astronomy and of earth and planetary science is one of those who has been using huge earth-based telescopes in conjunction with the Hubble and the Cassini probe to fill the huge gaps in our knowledge. We decided it was a great time to talk rings with Imca, partly because of the so-called ring crossing at Uranus just a few days ago, when that planet's rings could be seen edge-on for the first time in decades.

Imca, thanks very much for joining us on Planetary Radio. Well, it's a pleasure. And we are speaking to you, I guess, in your office there at UC Berkeley. Yeah, I'm still in my office. This is a big time for people who keep their eye on the outer planets of our solar system. That ring crossing, as we speak, the first of two ring crossings that are taking place this year, I guess just took place about a week ago. Yeah, that's right.

But we didn't really look at that one in particular because it's a bit hard to observe from the Earth. Really? So, yeah, depending on which telescope you use, it's still pretty low in the sky. Now, you do a lot of work, I know, with the Keck out in Hawaii, those incredible instruments. Yes, I do, and in particular of Uranus and this ring plane crossing. of work i know with the uh the kek uh out in hawaii those incredible instruments yes i do and in particular of uranus and this ring plane crossing so we we hope to get a lot of observing time to do really great work really try to observe these rings in in a lot of detail but the the data

from this first ring crossing even though it may be of lower quality, is not available yet. Well, we will get some Hubble Space Telescope images later this month. And so those data are not available yet. And so we did not observe last week. When is the next ring crossing? So the next one is on the 16th of August. But we will observe several times in July. Well, there is a crossing, which is the exact point of crossing,

but even before and after that time, the rings are more or less edge-on, basically edge-on as we see it from Earth. So we can really get a lot of observing already in the time before and also after crossing. a lot of observing already in the time before and also after crossing. Remind us of why this is such an important time to be observing these ring systems. When Voyager flew by in 86, the south pole of Uranus was more or less pointing towards the sun. And let me just remind you that Uranus is tipped about 90 degrees over. Right.

So it's a very different planet than, at least the view and geometry is very different than for the other planets. So when Voyager flew by, it saw the planet South Pole, and you could then really nicely see the rings they projected on the sky. That is really a perfect configuration to look at the rings if the rings are bright enough to actually see them very well. So right now, the rings will be almost edge-on. And the interesting thing then is that the sort of optically thick, the rings which are normally bright will become very, very dark, while very faint dust, things that are hardly visible in the normal circumstances, will

bright up, will lit up tremendously. So that's why we are really looking forward to this event, because that gives us an opportunity to really look at the very faint, dusty component of the rings. So these two ring crossings and another major event at Uranus, which several months ago we heard a little bit about from your colleague Heidi Hamel, who was trying to build enthusiasm for a quite amazing thing that will be happening as we observe Uranus. Yeah, that's the equinox. So on the 7th of December, the as we observe Uranus. Yeah, that's the equinox. So on the

7th of December, the Sun will cross Uranus' rings. Well, there are a couple of interesting things here, and that is that during certain times, and that is, for example, between the first and the second ring plane crossing, so between last week and August 16, the sun and the earth are on the opposite sides of the rings. That may sort of seem kind of difficult to imagine, but essentially we see the so-called dark side of the rings. So we essentially can look at sunlight shining through the rings but of course these geometries are all kind of hard to imagine

the rings are almost edge on so the sun is just at one side and we are just at the other side and so that means that rings which are normally very bright will be especially dark, and that really helps a lot to see the very, very faint, dusty component of the rings. And one of the reasons that you can see these dusty rings so well is that essentially all the ring particles are kind of lined up along the line of sight, and they all scatter sunlight towards us, and that really makes them very very bright since normally if they're not edge on you get light scattered from these dust

particles and it sort of is so faint that you you can't see it and when they're all lined up they sort of amplify each other so you'll really see them all nicely in a line. You'll really see the light scattered from all these dust particles in a line, which really makes them much, much brighter than normally. Are you expecting surprises from these observations? Well, expecting and hoping, of course, is different. Not words, not language you're supposed to use with a good scientist. It's always hard to say what you expect.

I mean, what we expect and also hope to see is these very faint outer rings. And I don't know if you have heard about these, but I sort of refer to them as the red and the blue ring. I found that, of course, on your website, which I strongly encourage listeners to take a look at. And we will put a link up to your website there at UC Berkeley at planetary.org, where some people may be listening to this program. Of course, I was intrigued by the Dr. Seuss title, One Ring, Two Ring, Red Ring, Blue Ring. two-ring, red-ring, blue-ring. Yes. Mark Showalter discovered this outer ring system of Uranus with the Hubble Space Telescope. Sort of around the same time, we actually had data and we could see

this inner of the two outer rings. And then later on, we observed again and tried very hard to find the outer ring. To our surprise, I mean that was a surprise, we did not see anything. We did not see this outer ring. We could easily see the inner ring. So when we put the data together in the form of a spectrum using the Hubble Space Telescope and the Keck infrared data, we noticed that the inner of the two rings was very red, which is kind of normal for dusty rings. But the outer ring was extremely blue. We could not see it in the infrared. It was easily seen in the visible. And then we compared these colors of the rings with the colors of Saturn's outer rings, the outer G and E ring. And they're just the same. The G ring is very red, just like

the inner of the two outer rings of Uranus. And the outer ring of Saturn, the E ring, is very, very blue, which was always very, very peculiar. And so the colors are just the same as far as we can see. Also, our color in the infrared is based on upper limit. So we don't know if we will see it when they're at John or not. We hope so, but who knows? I'll be right back with UC Berkeley astronomer Imke de Potter. This is Planetary Radio. I'm Sally Ride.

After becoming the first American woman in space, I dedicated myself to supporting space exploration and the education and inspiration of our youth. That's why I formed Sally Ride Science, and that's why I support the Planetary Society. The Society works with space agencies around the world and gets people directly involved with real space missions. It takes a lot to create exciting projects like the first solar sail, informative publications like an award-winning magazine, and many other outreach efforts like this radio show. Help make space exploration and inspiration happen. Here's how you can join us.

You can learn more about the Planetary Society at our website, planetary.org slash radio, or by calling 1-800-9-WORLDS. Planetary Radio listeners who aren't yet members can join and receive a Planetary Radio t-shirt. Members receive the internationally acclaimed Planetary Report magazine. That's planetary.org slash radio. The Planetary Society, exploring new worlds. Welcome back to Planetary Radio. I'm Matt Kaplan. We're talking rings with Imke de Potter,

UC Berkeley Professor of Astronomy and of Earth and Planetary Science. Imke was just telling us how much the ring systems at Saturn and Uranus have in common. Finding this similarity between the ring structure at two different planets, yes they're neighbors but quite different in many ways, I can see how that would have been surprising. Yes, that was very surprising. Of course, in particular, because with Saturn's earring, people have always said, this is just really odd. I mean, there's nothing else in our solar system or anywhere that is as blue as this ring. The ring is located around the satellite Enceladus in Saturn's system.

And so it was immediately sort of clear that Enceladus somehow produced this ring. And so early models showed that if you have indeed particles being knocked off by micrometeorite impacts, then the tiniest particles actually would, their orbits would evolve away from Enceladus, and those particles would basically survive in the system. And so people could kind of explain why only these tiniest particles in Saturn's system would survive. Well, then when Cassini came there and found these enormous geysers emanating from Enceladus, from the South Pole, it was quite clear how this enigmatic ring basically came into being. I mean, it's cryovolcanism from Enceladus. So that made the discovery of Uranus' blue ring even more strange.

So then Mark Showalter, at the same time that he announced the discovery of this outer ring, he also announced the discovery of a tiny moonlet embedded in that ring, Moonlet Matt. Well, that thing is only about 10 kilometers across, so there's just no way in the world that can be volcanically active. Of course, they said that about Enceladus. They said that about Enceladus, too. Also, very early models, you know, they always said, well, it might be micrometeorite impacts. But, you know, it's also possible that perhaps there are some geysers on Enceladus, although they, of course, never imagined that these geysers would be so large and energetic as they found.

I was mostly kidding because there's still quite a difference in size between this moon and Enceladus. Yeah, but still, I mean, Enceladus is 500 kilometers diameter. It's still very small, and one would not ordinarily expect volcanism there. And it's still puzzling. I mean, people still haven't really come up with a good theory to explain it. Even though our observation capabilities from Earth and near Earth continue to improve, and this is something that your research group has been very involved with, the incredible improvements in adaptive optics over

the last 10-15 years i would bet that you would agree with others that a mission out to uranus and if you could to neptune as well is something that you'd like to see oh yes no definitely and and in particular because we have been there only once with the Voyager flybys. And there are just amazing things that people have seen also on the satellites, on Miranda of Uranus and Triton of Neptune. These are still very difficult to observe in detail from Earth, even with adaptive optics. I mean, adaptive optics helps, but you can't get the detail as you can when you actually go there. So I indeed think that it would be fantastic to just go back with orbiters to Uranus and Neptune.

And yet, it is pretty amazing how our observational abilities have improved. And I'm really struck by how you've gone back and forth between what you've learned from the Hubble Space Telescope and from what you've learned from EarthBased at Keck and elsewhere. I mean, it really says something about how these instruments work best as a team. Yeah, no, that is definitely true true because the Keck telescope with adaptive optics has a very similar spatial resolution as the Hubble Space Telescope at the visible wavelength. So the instruments are really very complementary and certainly for this planetary work. I mean, it's just ideal to use them both and then compare the data and get spectra and, yeah, just really learn a lot more about the system than using either Hubble Space Telescope or Keck alone. We are just about out of time. I

wonder if you can tell us about your observation schedule, what you expect to be doing as these additional events at Uranus approach us this year. Well, we hope to observe with both the Keck telescope and actually also with the Very Large Telescope in Chile, where I work together with European collaborators. I mean, I did my PhD in Holland, so I'm still also Dutch. So we will use the very large telescope as well. And the nice thing is that they are in different parts of the world. So you can actually observe them at different times and put the whole string together. So we hope to observe in August, a few days before the ring plane crossing, through the ring plane crossing, and then afterwards, and see how the intensity of the various rings

change over time. So it promises to be a pretty exciting year for those of you who study not just Uranus, but its neighbor Neptune, but especially Uranus. not just Uranus, but its neighbor Neptune, but especially Uranus. Especially Uranus this year, because that is the planet that will go through the ring plane crossing, and you will see the rings edge on. Of course, the atmosphere is interesting, too,

but Heidi Hemel will probably talk to you about that. Someone. We hope to talk to her again about that sometime soon. But everyone is fascinated by rings, and we hope we can have you back. Yeah. So I hope to be able to come back and talk about what we actually found, and if there was something unexpected. We'll look forward to it. Thank you so much, Imke. Okay, thank you. Imke de Pater

is a professor of astronomy and professor of earth and planetary science at the University of California, Berkeley. We will be back with What's up, other than Uranus. I'm sure Bruce will tell us everything that's up in the night sky, at least the things within our solar system, right after this return visit from Emily. I'm Emily Lakdawalla, back with Q&A.

It's very difficult to plan the trajectory of an ion-propelled spacecraft like Dawn very far in advance, so mission planners don't yet know whether they'll be able to perform flybys of asteroids in addition to the planned rendezvous with Vesta and Ceres. Unlike chemical engines, ion engines are almost always on, providing a very small amount of thrust over a very long period of time. The problem is that Dawn's engineers won't really know how much electricity the solar panels will deliver or how well the ion engines will perform until after the launch. The uncertainty is large enough that Dawn's arrival date at Vesta, the launch. The uncertainty is large enough that Dawn's arrival date at Vesta, nominally planned for October 2011, could shift forward or backward in time by several months or even longer. With

that kind of uncertainty, it's impossible to know which other asteroids could be in the right place at the right time for Dawn to do flyby observations. Even after launch, it's not clear how many flybys may be planned for. Mission planners may want to save all of Dawn's energy for getting to Ceres and Vesta and spending as long as possible at each asteroid. Ceres and Vesta are nothing like the tiny rocks that have been visited by spacecraft to date. They are both big, round globes, the largest completely unexplored worlds inside the Kuiper Belt. Got a question about the universe?

Send it to us at planetaryradio at planetary.org. And now here's Matt with more Planetary Radio. Time for What's Up with Bruce Betts, the director of projects for the Planetary Society. He is here to tell us about the night sky, hand us a new space trivia contest, and resolve the old one, which is really interesting because it got into, you had to do some research, magnetic field research. Well, I was doing it to make sure I could answer every possible strange question that you usually ask. That's good.

Yes. I'll do my best. But people complain that it had been too easy lately, you said. You're right. They did. I was trying to compensate. Meantime, let's tell you what's up in the sky. Check out Venus, of course. Venus and Jupiter are just dominating the two extremes of the evening.

In the early evening, Venus is extremely bright over there in the west. And in the pre-dawn, the brightest-looking-like object is jupiter which is up high in the sky right before dawn you can also see jupiter now i've been seeing it uh if you're up reasonably high by 11 p.m it's rising around 10 or so we've also got saturn up in the early evening sky and depending on when you listen to this you still may be able to catch it looking close to the moon on may 22nd and to a lesser extent on may 21st it's the brightest star-like object near the moon on those dates we've also got jupiter going to be really near the moon although it's impossible to miss but it will look pretty on the 31st but But Venus really is incredibly bright. Is it just close to us right now? It's just incredibly bright, just pretty much all the time.

It's a magnitude minus 4.1, which is actually, it gets brighter. Not by much, but it does get brighter. And it is, if I haven't mentioned it before, the brightest natural object in the sky besides the sun and the moon. At least until Adekarone goes supernova. Yeah, at least until that point. Okay. For now. All right, enough already.

Last I checked the sky. On to this week in space history. In 1973, Skylab, the Earth-orbiting space station, was launched. Mm-hmm. Mm-hmm. Should have built on that. Should have done more with that. Shouldn't have let it plunge into Australia.

No. Speaking of which, Random Space Fact! Excellent breath control. Thank you. Could have gone longer, but I figured it was already annoying. Skylab, hey! Of course Skylab's crashing was an international media event

with T-shirts with target marks on them and all sorts of good things. There was actually one thing I found that I had not heard about. The San Francisco Examiner offered a $10,000 prize for the first piece of Skylab to be delivered to their offices. Really? Sure enough, a 17-year-old, Stan Thornton, scooped up a few pieces of Skylab off the roof of his home, caught a plane to San Francisco, and collected his $10,000. Not bad.

I wonder where he is today. I don't know. Maybe a future interview. Yeah, we'll have to look him up. That leads us to the trivia contest. a future interview. Yeah, we'll have to look them up. That leads us to the trivia contest. We asked you what planet in our

solar system has the second highest magnetic field, and here are the key words, at its equator? At its equator. Jupiter, number one, no matter pretty much anything you come up with in terms of magnetic fields. But in this case, the answer, somewhat surprisingly

perhaps, is Earth. And people got thrown off because Saturn has the second strongest inherent magnetic field strength. But magnetic fields drop off really quickly, one over radius to the third power. And so by the time you get out to what would be considered the equator of Saturn, either it's cloud tops, one bar level, one millibar, however you measure it, it's actually Saturn has the second strongest magnetic moment for those playing that game, which is roughly the field strength at the equator, what I asked you about, times the cube of the radius. We wanted you to explain that fully because so many people came up with Saturn. Although Earth did win, it had a plurality, I think.

And there were a few Uranuses, Uranuses, Urani, and there was even a Neptune. There was a vote for Neptune. But I'm sure that explanation satisfied everyone. I'm lying. I don't know. I bet we'll get some email. But you know what? We already got email from Justin, Justin Dildine,

or Dildine, who lives not far from us, San Diego, California. Justin said Earth has the second strongest magnetic field at 0.305 Gauss. Gauss. Gauss. Gauss, sorry. And so, Justin, guess what? You're getting a T-shirt.

Congratulations. Nice job. By the way, Saturn is about 0.22 Gauss at its equator, as opposed to 0.35 or 0.31, somewhere in that range with Earth. And Jupiter up at 4.28 because it just has this monster magnetic field. So let's ask a perhaps simpler question and I'll be chastised again. You can even guess at this one and have a good chance of getting it right. No, people wanted a hard one, like you said. I know, but we're going to go back and forth. I'm guessing there's both kinds. Now, this one actually isn't necessarily as easy to look up,

but it isn't that hard either. How many legs does the Phoenix Lander have? The Phoenix Lander, which just arrived in Florida, been delivered there, flown there in a C-17, and is getting prepared for its launch in early August. How many legs does the lander have? It doesn't seem like it should be difficult. I'm going to go check the website and see if I can count. They rarely say, and our lander has three legs, so you may have to look at pictures. It's not always taken from a weird angle. So, no, it's no magnetic field question, mind you.

But it does celebrate Phoenix getting to Florida, including the Planetary Society provided DVD with a quarter million names and visions of Mars, all sorts of great Mars literature and such. So anyway, if you want to answer that, go to planetary.org slash radio. Find out how to send us your entry and compete for a Planetary Radio t-shirt. But do that by May 21, Monday, May 21, 2 p.m. Pacific time, as you all know. You might win a T-shirt. Exactamundo. All right, we're done.

In fact, we've got to get out of here. All right, thank you, everyone. Go out there, look up at the night sky, and think about little bits of metal that you find on the asphalt, and you wonder, what the heck is this from when you're walking through the parking lot? Thank you, and good night. Man, I hope they're surprised, because I'm going to pick some of those up and take them to the San Francisco Examiner. He's Bruce Fetz, the Director of Projects for the Planetary Society. He's here every week for What's Up.

Planetary Radio is produced by the Planetary Society in Pasadena, California. Have a great week, everyone. Thank you.