Planetary Radio: Space Exploration, Astronomy and Science - Cassini at Saturn: The Final Year

Episode Date: September 20, 2016

The great Cassini spacecraft has a year to go before it plunges into the ringed planet. Project Scientist Linda Spilker returns with more amazing mission science.Learn more about your ad choices. Visit megaphone.fm/adchoicesSee omnystudio.com/listener for privacy information.See omnystudio.com/listener for privacy information.

One year to go at Saturn, this week on Planetary Radio. Welcome, I'm Matt Kaplan of the Planetary Society with more of the human adventure across our solar system and beyond. Our most frequent guest, Linda Spilker of the Cassini mission, returns with more news from the queen of planets, its rings and moons. Bill Nye is away this week, but we'll hear from senior editor Emily Lakdawalla in a moment. And Bruce Betts will drop by in virtual fashion with night sky and other goodies, including a new space trivia contest. Emily, good as always to talk to you this time about a mission that I think a lot of people, or at least a lot of Americans, are not as aware of as they probably should be.

Yeah, this is a European Space Agency mission called Gaia. And Gaia is not a planetary mission. It's a mission that's simply designed to survey the positions of stars in our galaxy. It is returning some truly stunning results. Just the numbers alone tell the story. Yes, we're talking about literally billions of stars in our galaxy. And what it's doing is it's mapping the density, the position, and eventually the motions of all of those stars so that we'll be able to intimately understand how our galaxy evolves with time as the stars move through their own individual orbits around the galaxy center. Now, this is all well and good, but it has a surprising amount of significance for people who maybe don't care about other stars, but do care a

lot about planetary science. I think that most of us do care about stars, even if we only care about planets most of the time. But yeah, so planets are by definition wanderers. They are the points of light that wander against the background stars. And so in order to measure the motions of the planets, we measure them against what we think of as the fixed positions of the stars in the background. Those stars in the background, we need to measure their positions very carefully in order to be able to have precise measures of the motions of the planets and the asteroids and the centaurs and the Kuiper belt objects. We also need to know how the background stars are moving if we want to get really precise

information on the motions of our planetary neighbors. So this stellar catalog that Gaia is putting together of both the positions and motions of stars is going to help make much more precise orbits for all of the stuff within our own solar system. You also mentioned in this September 14th blog entry at planetary.org, of course, that the New Horizons mission is benefiting from these observations. For sure, because New Horizons is right now navigating toward a very distant, very faint, very recently discovered small object. And again, we only know its position with respect to background stars. So the better we know the positions of the stars in the same field,

the more precisely we can navigate new horizons to it. And ultimately, the closer new horizons can fly past this object and its encounter in a few years. So this stellar catalog is really going to help our most distant planetary mission bring better science. There's even more to this story, like the discovery of exoplanets that is expected from the Gaia spacecraft. So congratulations to the people behind it and the European Space Agency. And Emily, thank you for bringing it to our attention. You're welcome, Matt. There's also

a beautiful, beautiful image of this galaxy map developed by Gaia that you can see in that September 14th blog entry. She is Emily Lakdawalla, our senior editor, the planetary evangelist for the Planetary Society, and a contributing editor to Sky and Telescope magazine. Now we'll check in as we do very regularly with Linda Spilker of the Cassini mission. In less than a year, it will all be over. The great Cassini spacecraft, its fuel tanks nearly empty, will plunge into the beautiful ring planet that it will have studied from up close for more than 13 years.

Project scientist Linda Spilker has been part of the Cassini team for all of those years and many before, going back to the planning of the mission at the Jet Propulsion Lab near Pasadena, California. She has returned to our show yet again with yet another update and will look forward at what may be the most exciting year for the mission since its arrival at Saturn in 2004. Linda, yet again, it is a great pleasure to welcome you back to Planetary Radio. It's great to be here, Matt.

And in your office here, right behind Mission Control at JPL. So as we were walking in, I felt like I really had a bow or something because that's where the magic happens. Right. That's where all the commanding of all the spacecraft happen. We have a lot to talk about. We'll talk moons. We'll talk rings. We'll talk the planet and some other interesting tidbits that you've given me. But first of all, this week, as we speak, marks a very interesting and significant anniversary. Right. It's the anniversary of Voyager 2's flyby of Saturn. Actually, it happened on August 25th,

and it's amazing to think it's been that long since we had the two Voyager spacecraft in the Saturn system. And what an incredible legacy. Voyager left questions for Cassini to answer. For instance, we couldn't see through the clouds to the surface of Titan. That haze blocked it. And so that was one of the things that Voyager left for Cassini to do. And so immediately when we saw Titan, we saw the incredible structure in the rings. We saw this bright moon Enceladus with very

few craters. Right away we knew we have to go back. And very shortly after that, a group of European and American scientists got together and started to plan what would ultimately be the Cassini mission. And of course, as you know, as part of that mission, we carried a radar to pierce through the haze on Titan, carried a probe named Huygens that ESA provided that landed on the surface, gave us our first close-up glimpse of a moon in the outer solar system. Just absolutely amazing. And gone on to find the plumes and geysers on Enceladus, reveal more about the rings and the planet. Watch the seasons change.

So we're really and truly standing on the shoulders of a voyager on those giants. a voyager on those giants. A few weeks ago, I had Alex Hayes, part of the team on the show, talking about those Grand Canyons of Titan, liquid-filled Grand Canyons. What are your thoughts about that? Well, it's absolutely incredible. Who would have guessed we could use the radar not only to find the seas of liquid methane, but also to measure their depth. So we use the same kind of idea to measure the depths in these canyons that flow into Lygeomare.

We found they were very steep, you know, maybe 40 degrees on a side, hundreds of feet deep because we could measure the surface, as well as the bottom of the canyons. And as we got further away from Lygeomare, the liquid was higher, at a higher level, clearly flowing into that giant sea. And we think perhaps maybe the land was uplifted a little bit, or else that's very soft, to carve such steep canyons. And it reminds me of the Grand Canyon. I was there not that long ago, and you have the Colorado River, a plain that was uplifted, and that river has just been busy cutting away deeper and deeper, creating the Grand Canyon. And now we have Grand Canyons on Titan.

All right, let's jump from the Grand Canyons of Titan to the old faithfuls of Enceladus. What's new there? Well, with Enceladus, we discovered not only do you have these geysers and jets coming out, but that the amount of material, the icy particles coming out, varies with where Enceladus is in its orbit. Its orbit isn't perfectly circular, and so when it's furthest away from Saturn, we get more icy particles, about a factor of three. And so recently we had a chance to go and look to see if the amount of gas, which we know has to lift the particles,

what's coming out of those jets is about 90% gas. What was the gas doing? Was it also mimicking the particles, or what role did it play? And so we actually had a very distant occultation. It was the center star in Orion's belt. Oh, not the sun. Okay. No, this was a star, actually. Orion's belt. Oh, not the sun. Okay. No, this was a star actually. It went behind the jets and we found that that plume overall only had about 20% more water vapor coming out when it was close to

its most distant point from Saturn. But what we did find is when that star went behind one of the jets, tremendous amount of material, about a factor of four or more coming out of the jet. So it looks like the answer is that we have a lot more flow out of these sort of discrete sources that lift the particles coming up. But overall, if you look at the plume by itself, it's only about maybe 20% more gas than would be when it was closest to Saturn, say. I've got to throw this in there. to Saturn, say. I've got to throw this in there. We had a listener who sent in an entry in our Solar System Olympics competition proposing events for around the solar system. Titan was the most popular location for those events, by the way. But this other person said, I want to have people jump into the jets on Enceladus and see how high they can get. And that would be the event, gold, silver, and bronze. So it might be a while before we attempt one of those.

Yeah, those particles are coming out pretty fast near the surface. So this could be very interesting. Might be quite a ride. Yes. The rings, you continue to make wonderful discoveries there. We're certainly not done. We'll get to what is still coming up for Cassini. And in this last year that we've entered into of its mission in the Saturnian system, what's come out recently in the science? Well, we had a surprise with the rings. One of the puzzles left to us by Voyager, and even one that Cassini hasn't yet solved, is how much mass, how much material do we have in Saturn's rings?

And this is key to their age. If there's less material in the rings than we think, then the rings might be young. Perhaps a comet or something came close, was torn apart. If the rings have more material, they might be old, perhaps as old as Saturn itself. So the recent result, we were actually able, by adding together a number of these stellar occultations,

to see these what we call spiral density waves created by resonances with the moons in Saturn's B ring. And if you can watch how the wave damps, it can tell you how much material is in the ring. And so while the B ring itself is maybe 10 times more opaque than Saturn's A ring, it looks like it's maybe only two or three times more massive. So a very interesting puzzle. A good analogy is if you look across a meadow on a foggy day, a lot of the trees and things are blocked by the fog. And yet if you look in a swimming pool, you can see through to the bottom. And the water in the swimming pool is obviously much denser than the fog that you see

in the meadow. So perhaps there's something about the particles themselves that are a little bit different that we don't quite understand that caused this difference. And if this holds up, if the B ring really is a lot less massive, then that points to young rings, maybe a hundred million, a few hundred million years old. Now we'll have the definitive answer with Cassini. When we're at the end of the mission, we dive in between the rings and the planet. That will allow us to get the mass of the rings by themselves. And that'll help answer the question, are these waves and unique spots in the B ring? Because you're really only probing a few very small regions. Or is it true? The B ring is really less massive. Interesting puzzle.

And Cassini hopes to solve it with the end of the mission. And what an event that's going to be. Cassini project scientist Linda Spilker. She'll have more exciting news from Saturn when Planetary Radio continues in a minute. Hello, I'm Robert Picardo, Planetary Society board member and now the host of the Society's

Planetary Post video newsletter. There's a new edition every month. We've already gone behind the scenes at JPL, partied at Yuri's Night, and visited with CEO Bill Nye. We've also got the month's top headlines from around the solar system. You can sign up at planetary.org forward slash connect. When you do, you'll be among the first to see each new show. I hope you'll join us.

Hi, I'm Kate. And I'm Whitney. We've been building a youth education program here at the Planetary Society. We want to get space science in all classrooms to engage young people around the world in science learning. But Kate, are you a science teacher? No. Are you? Nope. We're going to need help. We want to involve teachers and education experts from the beginning to make sure that what we produce is useful in your classroom. As a first step, we're building the STEAM team. That's science, technology, engineering, arts, and mathematics.

So teachers, to learn more about how you can help guide this effort, check out planetary.org slash STEAM team. That's planetary.org slash STEAM team. That's planetary.org slash STEAM team and help us spread the word. Thanks. Bye. Welcome back to Planetary Radio. I'm Matt Kaplan.

My guest this week has joined us on the show more often than anyone else. Linda Spilker has been the Cassini Project Scientist since 2010. She joined the mission team 28 years ago. Our September 6 conversation in her JPL office was only a few days before the beginning of Cassini's last year orbiting Saturn. And what a year it will be. First, though, we return to the feature that most prominently sets the sixth planet apart from its sisters. What else about the rings?

I mean, you had sent me a note about something to do with the seasonal changes that you've already mentioned we've been able to watch because we've been there so long. Right, right. Seasons have been changing. We've been there almost half a Saturn year. When Cassini first arrived, the ring shadow went all the way out across all of the rings. And it was a huge shadow. And now as we get close to summer solstice, that shadow is pulling in,

and it's just outside the Cassini division. By the end of the mission at solstice, that ring shadow will only go out to about the middle of the B ring. So sort of one way to watch the ring shadow to know the seasons really are changing on Saturn. World's largest sundial that we've got out there. I'm being followed by a ring shadow.

Good analogy. The planet itself, can the rings reveal anything to us about what's happening down there on the surface and below? Well, much to our surprise, we saw some very interesting waves in the rings in Saturn's C ring. And these waves looked kind of like a spiral density wave, but the problem was rather than damping outward toward the tiny moons that create them, these waves were damping inward. It was almost as though there was a tiny moon inside of Saturn creating these very special waves. But what we know instead is it's telling

us that there's some asymmetry, some unevenness in how the mass is distributed in Saturn. We've seen these waves for m equals 2, m equals 3, m equals 4, different modes of waves, different types of oscillations, two-lobed, three- or four-lobed oscillations. We originally expected just one wave for each mode, but in some cases there's three waves per mode. So something is causing mode splitting, and it's just so tantalizing to think,

what could be going on inside, deep inside the planet? And again, when we get our gravity measurements in those very special end-of-mission orbits, our grand finale orbits, we'll hope to answer those questions about this very unusual distribution of material inside Saturn itself. Cassini is doing, it turns out, much more than just teaching us more about the Saturnian system. Is it helping us to learn about what else is out there between the stars? Yes, it is, Matt. It turns out that we have a cosmic dust analyzer that can measure particles on Cassini. It's measured the composition of millions of particles. Most of those are icy grains coming from Enceladus, but there were 36 unique grains. 36 out of? So it's like looking for a needle in a haystack. And it took the many years that Cassini had at Saturn to accumulate these grains and start to then compare their compositions.

These grains are coming in very fast. They're coming in from one particular place outside of our solar system. We think it's a giant gas cloud that's the source of these particles. Turns out that both Galileo and Ulysses also saw these grains coming from that region of space. But the difference with Cassini is we had an instrument on board to measure their composition for the first time. So composition of interstellar grains, we found magnesium, iron, silicate, many things in the right kind of composition and distribution that

you would expect, you know, for a solar composition. Now, these interstellar grains are bits of star stuff. They were originated and formed inside of stars and now coming to and being measured by Cassini. So our interstellar visitors are actually came in, were measured by Cassini instruments. Welcome to the neighborhood. You also wanted to clear something up, some misunderstanding about something else that may be way out there, still very much a part of our solar system. It's a Planet 9-related mystery. Right, right. There was an erroneous report that Planet 9 gravity was somehow perturbing Cassini's orbit. And this just isn't true at all,

that Cassini's orbit hasn't been modified in any way by Planet 9. But what is unique for Cassini is we've been at Saturn long enough now that we have a good arc of Saturn's orbit, and we know it very, very precisely. And by looking at tiny changes or perturbations in Saturn's orbit, that helped the scientists figure out where in its orbit Planet 9 might be. Now, Planet 9 has a 15,000-year orbit. It's 10 times more massive than the Earth. And we think it's holding in place six of these trans-Neptunian objects, including Sedna, keeping their orbits in a certain alignment.

So we're still on the lookout for Planet 9. It isn't impacting Sedna, keeping their orbits in a certain alignment. So we're still on the lookout for Planet 9. It isn't impacting Cassini, but it is in a very subtle way impacting Saturn. Helping to point the way. You've mentioned a couple of mysteries that remain, which Cassini may be able to solve in these last few months of the mission. But I bet there are going to be plenty of other questions left open waiting for us to head out there again with some other mission. Absolutely. You know, Cassini will join the list of missions that have explored Saturn,

another giant in its own right, and we have to go back. There are so many interesting targets from Titan, the Titan seas, the jets coming from Enceladus to go back with a mission with the instruments to perhaps fly through those jets and tell us are there amino acids, fatty acids, anything that might point to life on Enceladus. There are many mysteries still about the rings and the planet itself. We've gotten tantalizing glimpses of the moons, Dione in particular. We wonder, could perhaps Dione be active, but at just a much lower level than Enceladus, and we just haven't been in the right place to see that activity. So, so many mysteries, so many reasons to go back.

Remind us of when the big finish takes place next year. Well, the end of the mission is September 15, 2017. We get a nudge from distant Titan that actually on that final orbit plunges us into Saturn's atmosphere. We're going to point the high-gain antenna at Earth, send back data for as long as we can, measuring the composition of Saturn's atmosphere as we go in. You can imagine a very fiery finish to Cassini. Finally, the spacecraft will tumble, and then the end will come very quickly. And we'll be sad for this loss of this tremendous mission, but celebrating a tremendous success. There'll be a lot of joy in all that Cassini has accomplished.

Just having you describe it that way, my pulse increases. It's going to be so amazingly exciting and such a mixture of both sadness and triumph. And you have a right to feel that as much as anybody else involved with this mission. It's been part of your life for so long. Absolutely. I've been with the Cassini mission, been very fortunate from when it was first being thought about not long after Voyager, to then being part of the ideas and the science definition team, to finally watching the spacecraft built, launched, and then arriving at Saturn. So it's really been a very wonderful journey. I mean, I started out on Voyager, and it just seems wonderful to be part of the team that went back to Saturn.

Let's finish as we began with Voyager. Big anniversary year for those still active spacecraft. Right. The two Voyager spacecraft were launched in 1977. And so next year will be the 40th anniversary of that launch. And I remember I was there for the Voyager 2 launch. I was one of those people with their mouths open, watching in awe as the spacecraft lifted off the pad and went on out to the planets. What an incredible journey. Linda, it's always a pleasure. Thank you so much. And

we'll talk again before that big day yet to come next year. Thanks, Matt. It was a pleasure to be here. Linda Spilker, she is the project scientist for the Cassini mission. Prior to that, quite a long time ago now, she was the deputy project scientist with a long history of involvement with other missions here at the Jet Propulsion Laboratory, where we've been talking in her office. Time for What's Up on Planetary Radio. Bruce Betts is the Director of Science and Technology for the Planetary Society, and he's ready to tell us about the night sky. And we'll learn a little bit looking back into the early history of human spaceflight. Welcome. It's just so much more fun to see you face to face, to sit across from you in the studio.

Yeah, but I imagine you with like an ice cream cone stuck to the top of your head. So it makes me, sometimes I enjoy it more when we're separated. How did you know? Oh, yes. Sometimes I enjoy it more when we're separated. How did you know? Oh, yes! The only other thing that makes me as happy as an ice cream cone is the evening sky. I don't know if that's true, but we're not going to analyze that now. The early evening is just spectacular. We got Venus over low in the west just after sunset, super bright in the south and throughout the early

evening and moving west as things will want to do, there's still the lovely triangle of Mars over to the left and then Saturn looking yellowish and then the reddish star Antares. We move on to this week in space history. It was two years ago that both MAVEN and India's MOM, or Mars Orbiter Mission, went into orbit around Mars. Still going strong. Indeed. We move on to... Wee-wee-wee-wee-wee-wee-wee-wee-wee.

Ha ha ha ha ha! You know, it's been a long, long time that we've been doing this. I didn't know you could do a Donald Duck. Yeah, well, as people just heard, I can't. Well, neither can Donald, so it's okay. There you go. If you can get water ice into a permanently shadowed crater at Mercury's poles, and I'm sure you have ways to do that, man. Sure, every day. It is theoretically stable in that vacuum environment at tens of kelvins in temperature, theoretically stable for many billions of years.

It just sits there. It's like a rock. You get no evaporation or what's that other process where it goes directly to gas? Sublimation. Yes, of course. Thank you. You get very, very little. It's interesting if you're into techie nerd things.

The water ice just, once you get it below about 100 kelvins, even in a vacuum environment, it just hangs out. It's just, well, let's say chilling. Can you hang on? Because I'm going to go downstairs and turn the ice maker on my freezer down as low as it'll go because I hate those little shrinking ice cubes. Yeah, I'll wait. I'll do it later.

Go ahead. Okay. Now we move on to the trivia contest. I asked you which Mercury program human missions landed in the Pacific Ocean. How'd we do, Matt? We did well. And so did the listeners. I got this entry from Stephen Harris.

And if he's right, he's a first-time winner. He's in Satellite Beach, Florida. He said the only two Mercury missions, Project Mercury, that came down in the Pacific Ocean were Sigma-7 and Faith-7, otherwise known as Mercury Atlas 8 and 9. Was he correct? That is correct. So it was the last two of the Mercury missions with Wally Schirra and Scott Cooper, respectively.

Gordo Cooper, yeah. Wally Schirra, probably the coolest, I think, the funniest, anyway, of the early astronauts, the first generation of astronauts. They knew how to have fun while they were making history. So congratulations, Stephen, you're going to get the Planetary Radio t-shirt, the Planetary Society rubber asteroid, and the 200-point itelescope.net astronomy account this week, as somebody else is going to win in the new contest that Bruce is going to announce in a moment.

We also got James Kerr. He mentioned that Sigma-7 is now on display at KSC, the Kennedy Space Center in Florida. If you want to see Faith-7, you go to Space Center in Houston, which is that big public facility attached to the Johnson Space Center. And not surprisingly, we got this entry from Dave Fairchild, our poet laureate on Planetary Radio. The Mercury program sent six into space, with two of them to the Pacific. The Kearsarge, I think, pulled them both from the drink that Sigma and Faith, if specific.

So I will point out for those who are wondering, all of the previous Mercury missions splashed down in the Atlantic Ocean. Yes, and as was pointed out by, oh, you know, now I'm going to get in trouble because I can't remember who said it, it would have been really difficult for the first two Mercury flights with humans to come down in the Pacific as they were suborbital. Yeah, that would have been a tough one. That one seemed pretty obvious once you were suborbital. Yeah, that would have been a tough one. That one seemed pretty obvious once you launched them from Florida. Yeah.

All right, now we're ready to move on. We take you to NASA's Deep Space Network, the places where NASA communicates with its spacecraft in deep space using big giant antennas. There are three NASA Deep Space Network antenna facilities. Where are they? Not too tough if you're a NASA aficionado. Where are the three NASA Deep Space Network antenna facilities? Go to planetary.org slash radio contest. at 8 a.m. Pacific time to get us this answer. This should be a pretty easy one. I bet at least half, probably more of our audience,

knows it off the top of their head where they keep their ice cream cones. Planetary Radio t-shirt, a rubber asteroid, and a 200-point itelescope.net account, that worldwide network, a nonprofit network of telescopes, waiting for you to point those telescopes at the universe will be yours if you get it right and you're chosen by random dot org. Alright everybody, go out there, look up the night sky, and well, I just can't stop thinking about it. Think about ice cream cones. Thank you, and good night. If I turn my freezer down to a few degrees Kelvin, I wonder

what that's going to do for the ice cream. Probably make it hard to scoop. He's Bruce Betts. He's the Director of Science and Technology for the Planetary Society, who joins us every week here for What's Up. Planetary Radio is produced by the Planetary Society in Pasadena, California, and is made possible by its awe-inspiring members. Josh Doyle composed our theme, which was arranged and performed by Peter Schlosser. in Pasadena, California, and is made possible by its awe-inspiring members. Josh Doyle composed our theme, which was arranged and performed by Peter Schlosser.

I'm Matt Kaplan. Clear skies.