Cassini: Epic Journey at Saturn (live public talk)

(upbeat music) – [Narrator] NASA’s Jet Propulsion Laboratory presents The Von Karman Lecture, a series of talks by scientists and engineers who are exploring our planet, our solar system, and all that lies beyond – Wow, we packed the house tonight How’s everybody doing? Excellent, well thank you all very much for, again, coming to attend these wonderful lectures We very much appreciate them The Cassini Mission, a cooperative undertaking by NASA and the European and Italian Space Agencies has revolutionized our understanding of Saturn, it’s rings, and amazing assortment of moons and the planets dynamic, dynamic? Magnetic environment The astonishing discoveries continue to this day and we can’t wait to see what happens when Cassini repeatedly dives between the inner-most ring and the top of Saturn’s atmosphere during it’s final six months, starting in April 2017, before finally plunging into Saturn’s atmosphere in September Tonight we have two guests who will present highlights, expectations, challenges, and the promise of Cassini’s final year Dr. Earl Maze is the manager of the Cassini program A veteran of 32 years at JPL, he began his career working on the navigation and engineering teams for the Galileo mission to Jupiter After Galileo’s final earth flyby, he transferred to Cassini as the spacecraft operations manager and then deputy project manager He left the project for eight years to hold management positions in guidance, navigation, and control in avionics, then return to Cassini as the program manager in January 2013 Dr. Linda Spilker is the Cassini project scientist and the co-investigator on the Cassini composite infrared spectrometer team and has worked on Cassini since 1988 Since joining JPL almost 40 years ago, her first and only out of college job, by the way, she has worked on the Voyager project, the Cassini project, and conducted independent research on the origin and evolution of planetary ring systems She also supports proposals and concept studies for new missions to the outer planets She enjoys yoga and hiking, especially through her favorite park, Yosemite, and is married with three daughters and five grandchildren So, up first tonight, perhaps one of the coolest grandmothers ever, Dr. Linda Spilker (applause) – Thanks Marc, that was a great introduction And, as Marc indicated, Cassini has truly re-written whole textbooks on the Saturn system From the planet itself, to the complex ring system, to these just amazing and astonishing moons that come in all shapes and sizes, and then the great magnetic field that surrounds the planet itself Now I’m going to cover some of the highlights of Cassini’s journey in the Saturn system, her 12 year voyage around Saturn And Earl is going to talk about the grand finale Those last precious orbits of Cassini with truly unique science Essentially like a brand new mission And then those final moments with Cassini Now, if you look at the picture behind me, this is one of my very favorite montages from Cassini And as a ring scientist, you can probably guess why In this you can see all of the major rings of the Saturn system And it’s a unique geometry The planet itself is covering up the sun, allowing Cassini’s sensitive cameras and detectors to mosaic this back-lit view It’s kind of like looking through, you know, a dusty windshield or something and these particles brighten up and you can see them So what you see is the planet itself then the main ring system That faint ring just outside the main ring system is the G ring, and that beautiful blue ring is Saturn’s E ring And it’s created by tiny icy particles that come from the south pole of Enceladus that go on to form a ring that fills Enceladus’ orbit These particles even go all the way out to the orbit of Titan, one of the distant moons at Saturn Now, if you look closely at Saturn, you’ll notice that there’s a white ring around the planet And this is where the sunlight is refracted through the top of the atmosphere into your eyes And it’s so beautiful because when you look at this ring around Saturn, you’re seeing every sunrise and sunset on the planet at the very same time And you’re looking at the dark side of Saturn, and yet, something is lighting up the night side And what’s lighting up the night side is actually light coming from Saturn’s rings

So the sunlight hits the rings on one side and it then reflects onto the night side of Saturn So, just one of the many incredible images that have come back from the Cassini mission Now, I’m often asked, “Why do we explore space? “Why do we send robotic emissaries out like Cassini? “What are some of the grand questions we hope to answer?” And Cassini addresses two of those These are something that were in a survey for planetary science, we do these once ever 10 years So the first grand question is, are we alone in the universe? Has life originated somewhere other than Earth? Perhaps in our own solar system? And how did life originate on the earth? Another grand question is how did the solar system and the earth within it come to be? How is it evolving and where is it headed? By studying the planets in our solar system we can learn about how our solar system formed, how the planets may have migrated as the system evolved and where we might be headed And it’s a good analogy for other systems around other stars Now, here’s the, I just want to go back briefly here and show you these are the eight planets in our solar system Saturn is the sixth planet out from the sun, it’s the second largest planet and it takes 30 years to circle the sun a single time Now, Saturn is indeed huge It’s the second largest planet This shows the earth and the moon to scale and the distance in between them So you can see that Saturn would just fit in between the earth and the moon And if the earth were a tiny marble it would take 764 earths to fill up the volume of Saturn So truly a giant planet And what you’re seeing are just cloud tops Saturn doesn’t have a solid surface like the earth It’s all clouds, mostly hydrogen and helium, and maybe a tiny rocky core about the size of the earth in the center Here’s an overview of the Cassini mission Cassini was launched from the earth in 1997 We used gravity assists, two of Venus, one flyby of the Earth, one of Jupiter, and arriving at Saturn in July of 2004 Now, originally Cassini was funded for a four year prime mission And by the end of the prime mission we found we had enough fuel and a healthy spacecraft that we actually had two extended mission The Equinox Mission where the sun was shining right on Saturn’s equator edge onto the rings, and then a seven year Solstice mission And norther summer Solstice at Saturn will be in May of 2017 and the mission will last just past that, ending in September of 2017 And you can see there at the end in the green box what we call the proximal, so grand finale orbits, and they’re shown above highlighted in this box And this whole mission is shown against the 30 year orbital period for Saturn So by the end of the Cassini mission, at the end of 13 years we’ll have been in orbit in the Saturn system for almost two seasons They change very very slowly at Saturn And right now Cassini is almost to that green box We’re going up in inclination and we’re getting ready for out final set of orbits This is another view of the Cassini mission by year You can look across the top bar, shows the number of orbits and the shapes of those orbits Then you can see that by the end of the mission we’ll have 127 flybys of the giant moon Titan And Titan is like a giant rocket engine Every time we flyby Titan, it’s like expending almost as much fuel as we spent to go into orbit for Saturn orbit insertion And we get great views of this very interesting body, as well We’ve had 23 flybys of Enceladus, and the prime mission, the first four years we had three We discovered Enceladus was so interesting that it reshaped our thinking for the extended mission and we added 20 more flybys of Enceladus We have 15 flybys of the other IC satellites, and then you can see the seasons changing from northern winter to northern summer over the course of the Cassini mission And then, of course, those proximal or grand final orbits at the end, and Earl will be talking about those in more detail This is a Cassini Orbiter and the Hoygens Probe You can see a great model, a quarter scale model over in the corner of the Cassini spacecraft Cassini, she’s about 22 feet tall That antenna at the top is about 13 feet in diameter, it’s comparable to the voyager antenna You can see over in this other spacecraft here You can see people for reference And, fully fueled, Cassini weighed six tons And about half of that was fuel that we spent about a third of that just to go into orbit around Saturn

The Hoygens probe was provided by the European Space Agency and it was specifically designed with the goal of being thrust into Saturn and Titan’s atmosphere, parachuting down, and landing on the surface of Titan Now, Cassini isn’t just a spacecraft that’s made up of metal and bolts and bits and pieces, but this is kind of my view of Cassini I see Cassini as made up of all the people that are on her team The scientists, the engineers, the support staff, and in a way, Cassini represents all of their hopes and dreams, all of the things that we want to accomplish There are times when I almost picture myself there with Cassini in the Saturn system as we get back some of these wonderful images or spectra or data of these incredible places I almost feel like I’m right there looking through Cassini’s eyes and watching as she collects her data And I feel very proud to be a part of this incredible mission Now, onto some of the science This is the tiny moon, Enceladus Enceladus is only 300 miles across Enceladus would fit between Los Angeles and San Francisco so it’s a very tiny moon And yet a very interesting one When we saw it with Voyager we saw a very bright icy surface Generally in the solar system something bright means that it’s young You haven’t had a chance to build up the pollution from the micro-meteorite bombardment Also you’ll notice as you go south, there are very few craters, in fact there are no craters at the south pole of Enceladus And you can see four tiger stripe fractures, that’s our nickname for those bluish features there Alexandria, Baghdad, Cairo, and Damascus Very interesting names and those fractures were something that were in darkness when the voyager spacecraft flew through the Saturn system So we didn’t know they were there until we had the Cassini spacecraft Now, our first flyby in July of 2005, our magnetometer team said there’s something interesting going on with Enceladus The magnetic field lines from Saturn don’t go down to the icy surface like they normally would for a body frozen solid Instead it kind of reminds us of a comet Those field lines are standing off, there’s something going on in the southern hemisphere And so they encouraged us, we had 1,000 kilometer flyby the first time They said, “Go closer, we can really get a lot better “data.” So we went closer and also trained our other instruments on Enceladus and we found, here this is with a composite infrared spectrometer, the team that I work with, they found that the Enceladus south pole was hot It was about 100 degrees hotter than the rest of Enceladus And if Enceladus were frozen solid it was much hotter than it should be And in looking more closely, that heat lined up with those tiger stripe like fractures So this excess heat was a puzzle We had an auscultation of a star going behind this region, we looked at these tiger stripes in more detail on the various flybys Here’s a tiger stripe, it’s about a mile or so across Typically about 100 miles wide, and it’s just this large gash Four of them in the south pole You can almost see what looks like a frosted side on the left hand side there We wondered what could be going on with these tiger stripes We also had images and the answer, it was very clear There are jets of material, water vapor, water ice particles shooting out of these tiger stripe like fractures Here’s another view of those jets coming out, just going all different directions, continuously going off ever since Cassini arrived at Saturn and we’ve been watching Enceladus So not only do you get water vapor and water ice coming out, you have things like ammonia, methane, carbon dioxide You have many of the key ingredients that you might need to find life, coming out of these jets on Enceladus And part of the goal of our flybys is actually to fly through this material And in October we came within 50 kilometers of Enceladus’ surface, right under the south pole And it gave us a chance to, essentially, taste and smell those particles, figure out what they were made of, and try and figure out the activity inside of Enceladus Here’s another view of those icy jets This is a back-lit view, similar to what you saw earlier in the Saturn image You can see the sunlight shining through each of these jets And we found in the particles that some of those were salty It says that there’s a global ocean underneath Enceladus’ icy crust and it’s as though they were frozen sea spray and they contained sodium and potassium salts And we know the PH of the ocean, very similar to the oceans here on the earth So very interesting finding in the particle data This is an interesting view, this is another Enceladus This is a fountain at Versailles in their gardens there and this particular Enceladus is a Greek giant and

he had a run-in with his grand niece, Athena, and he lost And so his fate was to be forever buried under Mount Etna So I think here he’s protesting a bit with this giant 82 foot high geyser of water Who knew in the 1670s that Enceladus would actually kind of be doing something like this? Now here’s an artists concept of what might be going on You have the liquid water ocean underneath the icy crust and that carbon dioxide might be sort of like shaking up a champagne bottle You pop the cork and perhaps that’s the energy that’s there to raise that water vapor and icy particles to send them continuously into space Now, most of the material falls back onto the surface of Enceladus The particles are too large and they just, they fall back, and it’s like it’s snowing If you could stand near a tiger stripe underneath it you could put out your hands and it would be like it would be snowing on Enceladus Maybe a future vacation destination, who knows? But some of the tiniest grains escape into space And they’re what go on to form that very beautiful blue E ring that you saw in the first image If you look carefully in this image you can see this tiny black dot, that’s Enceladus Underneath it is the bright plume of material coming out And you can see wisps and tendrils of those icy particles going out to form the E ring Now, the E ring particles are so tiny that they spread throughout the system and if you turned off Enceladus’ jets it might only take 100 or 200 years until the E ring is gone completely So that’s sort of a clue, we see the E ring we know the jets are going off at Enceladus This is just an artists concept of the inside of Enceladus We know it’s differentiated, that just means it’s separated into a rocky core, a global ocean, and an icy crust We’ve also found that in looking at some of our particle data there are tiny grains of silica We call these nano-silica grains What’s unique about them is these nano-silica grains can only form in water that’s near boiling So what we think happens is that the water goes into the rocky core of Enceladus, it’s heated up there, Enceladus is kept warm by a resonance with another moon, Dione, that’s essentially just pumping heat energy into it And once that water is heated up it absorbs these minerals, in particular silica When the silica comes back out through these hydrothermal vents, hits the cold water, those minerals condense into tiny particles Then those particles are frozen into the particles that go out into space that Cassini can measure So this is an indication that there’s a possibility of hydro-thermal vents on the sea floor of Enceladus Now, if we look at our own planet, we have the same kind of hydro-thermal vents on the sea floor of the earth This is along the mid-oceanic ridge in the Atlantic Ocean, it’s very very deep No sunlight penetrates to that depth This is illuminated from basically the headlight of the submarine that’s looking at this particular event And here you have silica and potassium and other minerals that condense in the cold water on the earth’s sea floor, forming what looks like smoke And these are what is known as white smokers on the earth There’s also something along the sea, depending on the composition, there are black smokers as well They’re more iron rich, so a different composition What’s interesting is here in the deep cold ocean where you have no sunlight, the only heat energy and nutrients are what’s coming out of these vents, you find an amazing array of life You find tiny crabs, you find tube worms, you find little tiny animals, all sorts of life in an island around these hydro-thermal vents So we wonder if we can find life in our own ocean, perhaps might there be life in the ocean of Enceladus? So some of the factors that life might exist there include a global salty ocean PH very similar to our own We know it’s long-lived, a global ocean probably formed at the same time as Enceladus There’s organics coming from the ocean to the limits of the instruments we have to detect them Carbon chains up to C6, C7, they’re probably even longer but that’s the cutoff of the instruments, what we can measure Heat energy coming from the hydro-thermal vents on the sea floor, and best of all for Enceladus, it’s giving us free samples And it turns out when we launched Cassini we had no idea that there’d be these jets, or vents, coming out of Enceladus so we didn’t carry the instruments that we would’ve needed to look for amino acids and fatty acids and long chain molecules that could tell us that life is there So this just means that this is a wonderful destination, this ocean world, to go back to Enceladus and to keep exploring and answer the question, are we alone in the universe or perhaps might there

be life in Enceladus’ ocean? Now, another very interesting moon is Saturn’s moon, Titan Titan is about 10 times bigger than Enceladus and, in fact, Titan is about the size of the planet Mercury If Titan had formed anywhere else in the solar system, Titan would be a planet instead of a moon Now, this was a Voyager view of Titan And we just saw this hazy world and we couldn’t see through to the surface So after the Voyager flybys in the 1980s, a group of scientists got together and said, “You know, we really need to start thinking about “going back.” And it was both US and European scientists and that was basically the birth of the idea for what became the Cassini mission Now, Titan has a very dense atmosphere, it’s made mostly of Nitrogen, very similar to the Earth’s atmosphere No oxygen, but it has methane in it’s atmosphere And methane is really the key at Titan Because, you see, methane plays the role at Titan that water plays here on the earth The methane can be a gas, it can be a liquid, it can form clouds, it can rain onto the surface of Titan That the temperature of Titan’s surface is just right to be at the triple point where you could have a liquid, a solid, or a gas for methane Now, the methane is also part of the problem with the smog on Titan Because you see some of the methane goes high up in the atmosphere, the solar photons, the UV breaks the methane apart, they grow into larger and larger chains of molecules, and that forms haze very similar to the smog that we have here on the earth When the particles grow large enough they actually fall down onto the surface of Titan Now, the Hoygens probe was built specifically to go through the atmosphere, land on the surface, and reveal the surface for the first time So this is an artists concept of the Hoygens probe You can see it coming in It was released from Cassini on December 25, 2004 and entered into the atmosphere and landed on Titan on January 15 of 2005 So the heat shield basically ablated away, carrying away the heat energy And once the probe had slowed down enough, then the parachute could come out and for the next two and a half hours the Hoygens probe floated gently down to the surface of Titan, softly landed on the surface, and returned data for another half hour Cassini was the relay so as the Hoygens probe was floating down Cassini was flying overhead collecting the data then to send back to the earth for the Hoygens probe So really an amazing mission With Hoygens we didn’t know what we’d find Would we land in an ocean? Global ocean of methane? That was a possibility so we built the Hoygens probe to float, at least for a few minutes But it turns that we didn’t have to worry about landing in an ocean Instead, here’s the view that we had with the cameras We measured not only the pressure, temperature, and composition of Titan’s atmosphere on the way down, but the cameras took these pictures At about 60 kilometers above the surface the haze finally started to clear and we got a view of the surface And we started to see what looked like mountains as we went on our way down And, in fact, the Hoygens probe became the very first object to land in the outer solar system, land on a body the furthest away from anything we’ve had previously Here’s a view of the surface You can see on the leftmost panel these are rounded icy pebbles That tells us that fluid has flowed in this region Probably we landed in what was the equivalent of a dry lake bed We had a lamp, you can see the spot for the lamp here to give us an idea of what the color might be of the surface, and you can see the icy pebbles here And here’s a really neat comparison This is from our own moon, here’s a footprint of one of the Apollo astronauts You can see the astronaut and the little flag up here so this is sort of the same perspective view that we had on Titan And we also could see all of these channels, indicating that, indeed, methane was flowing We found a world that was remarkably like the earth in so many ways In fact, there were lakes and seas at Titans north pole Lakes of methane In fact, this lake by Geomare is about 50% larger than Lake Superior It’s about 500 feet deep, which is about the depth of the great lakes, as well So there’s a tremendous volume of methane on the surface of Titan And, in fact, if you could gather up all of that methane knowing the depth of this sea is a typical depth, you’d have 10 times more hydracarbons than all of the reservoirs we have here on the earth So if only we could build a pipeline big enough to go from Titan all the way back to the earth, our problems would be solved But there’s just a tremendous amount of hydracarbons on the surface And you can see the channels flowing into that particular sea Dunes, those particles that form high in the atmosphere

fall down, form these long dark linear dunes that wrap around the equator of Titan So those long dark linear features There’s also mountains This is a mountain color-coded with height Mountains can be as high as a kilometer or so on titan and we think, perhaps in this case, you look at it it might’ve even been an ancient cryovolcano Perhaps water mixed with ammonia flowed out on the surface of Titan And perhaps with that water perhaps came the methane There’s not enough methane in Titan’s atmosphere to have lasted from the time Titan formed So there needs to be some internal source periodically releasing methane Otherwise, once the methane gets divided up in the upper atmosphere, the atmosphere would collapse So there’s some source of that methane Clouds, we’ve seen lots of clouds This is a colorized cloud We’ve seen lots of clouds and weather on Titan We even saw a rainstorm, a methane rainstorm on Titan that darkened the surface and then we watched with time as the surface slowly dried up Then here’s a view of the dry river beds Now, in looking at these images, what you see here, the lakes and the dunes are taken at radar wavelengths Radar wavelengths are very good at penetrating through the haze and so we really have gotten tremendous views of a large portion of Titan’s surface This view is what you would see with the cameras You can see hints of the lakes In the north polar region what we did is we carried near infrared filters specifically designed to go through and penetrate the haze and look at those One of the things in the beginning we didn’t know for sure is in those lakes was that truly a liquid or some kind of a goo or something? What really was it? And we were trying to figure out, how do we find out if it’s a liquid without going there, landing in the lake, and finding out? And it turns out we have another instrument, the visual and infrared mapping spectrometer Looking at near infrared wavelengths And at five microns it found a bright spot called a specular reflection If you have sunlight coming at an angle reflecting off a liquid surface, it comes out at the same angle, and if Cassini is looking at that angle you’ll see a bright spot over the lake If you’ve ever been on an airplane, sometimes if you’re looking out the afternoon window as you go across a lake or a river you might notice there’s this bright spot that pops up when you go over a liquid And that’s a specular reflection I just wanted to say a little bit about the rings The rings have very simple names, A through G We keep naming them with other letters as more of the rings are discovered The main rings of Saturn, Saturn is off to your left, the main rings that you would see through a telescope are the A ring, the Cassini division, which is astronomer that discovered the Cassini division and for which our mission is named The B ring, which is the most optically thick ring, and then the C ring And there are also additional rings just shown in the bottom panel Here’s the inner-most D ring, it’s very very faint You’ve got the tenuous very narrow F ring just outside Then here you have the E ring going all the way out to Titan And it turns out there’s one more ring in the Saturn system And this ring wasn’t discovered by Cassini, but it was discovered by ground-based observers and it has created by Phoebe So there’s the Phoebe ring that actually comes in to the Saturn system, as well Here’s a Cassini view of the rings of Saturn They’re made mostly of water ice and on average they’re only 30 feet thick So incredibly narrow for the hundreds of thousands of kilometers that they span from end to end There’s tremendous amount of detailed structure there Some of it we understand is the interactions with the tiny moons just outside But so much of that structure we still have no idea what’s causing that incredible structure We do know that there are two moons that actually orbit in the rings There’s one that orbits in the Yankee gap named Daphnis, another one named Pan These two moons keep their gaps open So we know that information about the rings And here’s a nice view of the very dark, very very tenuous D ring Now this is the lit side of the rings, what you would see through a telescope But there’s also another side to the rings and this movie was taken by Cassini Basically you’re riding along as Cassini is plunging down through the ring plane You can see the A ring Cassini division and B ring Every once in a while you’ll see a tiny moon go by There’s Titan, you can see it’s much larger Now you get to see the other side, the dark side of the rings The side where the sun isn’t shining In this case, the B ring blocks out all the sunlight The Cassini division is very bright, the A ring is bright, and you can just see a hint of the bright C ring So the rings look very different And that’s the advantage If you go to a place like Saturn you can see the rings on both their lit and their unlit sides Now, Cassini also had a rare opportunity at Equinox In fact, we just had our autumn Equinox and just,

I think, very early this morning And that’s when the sun shines directly on the equator And in this case it shines on the rings edge on And that’s important because with the sun edge on to the rings, essentially, you’ve turned the sunlight off for the rings And in this mosaic taken by Cassini what we’ve had to do here is increase the brightness of the rings by about a factor of 20 so you could even see them because they’re only now illuminated by Saturn shine And around on the dark side of the rings where it’s dark before Saturn’s shadow, you had to increase the contrast by about a factor of 60 Here you can see the narrow F ring, but it’s slightly tilted so it can still catch the sunlight, even around Equinox Now, with 30 foot thick rings what’s unique is you can look for anything that sticks up above or below the rings So if you’re bigger than 30 feet in size, there’s a chance you’ll cast a shadow and we can see you So we’re looking for objects with Cassini that would be larger and would cast shadows And so I’m just going to show you an image now This is the outer edge of this ring, the B ring And it’s stretched out And low and behold we found shadows, lots of them Turns out that the outer edge of the B ring is held in place by a resonance with one of Saturn’s moon And it looks like some of the largest particles, or maybe they form and grow right there at the edge of the B ring Now some of these are probably a kilometer or two in size casting very long shadows But they’re hundreds of them across the B ring Almost looking like little mountains along the B ring And a good analogy is if you wanted to, say, find the pyramids if you’re looking out from a space station, if you looked around noon they’d be hard to see against their sandy background But if you looked near dawn or dusk, the equivalent of Equinox, they would cast long shadows, making them much easier to pick out against the sandy background So in the same way, Cassini used this to look for structures and we found a number of different structures like this that would cast shadows in the ring So, as a ring scientist, a very exciting time to be looking at Saturn’s rings And, finally, here’s a very interesting discovery for the rings It turns out that there is a feature, this feature is about 1,200 kilometers long, 10 kilometers or so wide, indicating that there’s a tiny object two or three kilometers in size creating this feature This feature is right at the edge of the A ring So it was discovered in 2013, it’s discoverer Carl Murray discovered it on his mother-in-law’s birthday so he nicknamed it Peggy after her So this tiny object that’s here creating this feature, Peggy, we’ve been watching for her ever since She comes and goes, we’re wondering will she break free of the rings and become a moon in her own right? Or will she be torn apart and jostled by the other particles in the rings and disappear? So, so far she’s still there and we’re going to keep watching for her through the end of the Cassini mission We’re kind of rooting for her by now ’cause she’s been around for a few years Moving on to Saturn, one very interesting event happened at Saturn A giant storm developed toward the end of 2010 This storm grew so huge it was a giant vortex and that vortex swirled off this huge tail The tail of the storm wrapped itself around the planet There was another vortex on the other end, kind of like a hurricane When these two vortices merged, that marked the end of the storm A tremendous amount of energy was released in this storm at Saturn And typically these storms happen about once every 30 years So this was the fifth time we’ve seen a giant storm like this at Saturn But what was unique is this storm was early It had only been 20 years since the last storm and so it came early so Cassini could get a good view of it and watch it And so we watched it, as did ground-based observers It lasted about nine months and started to fade This in the visible, if you look toward the near infrared you see deeper into the atmosphere The colors in this view, if it’s white or yellow that’s high up in the atmosphere, green is also high up, that’s the center of the storm Then the oranges and the reds are looking deeper So we’re basically getting a profile of what that storm looked like and how those clouds behaved And we can model that and perhaps use it as an analogy to storms in the earths atmosphere Looking at some of the longest infrared wavelengths, the thermal infrared, turns out that the storm was in the lower atmosphere of Saturn, the troposphere But when those two spots merged it released a tremendous amount of energy, kind of like a giant burp And here up in the stratosphere there is a large, very hot, feature And this feature persisted for a couple of years and has slowly cooled So a very dynamic and active Saturn, at least in that time period Now, Saturn has a very interesting feature at it’s north pole Here’s that feature, you’re looking right down at the

north pole of Saturn That feature is a six sided jet stream, called the hexagon The Voyager spacecraft first saw this feature in the 1980s and it was still here when Cassini arrived You can see the pinkish clouds, this is a false color view, rotating around And they go faster the closer you get directly to the north pole And at the north pole there’s a giant hurricane And this hurricane is about 50 times larger than a typical earth hurricane, blowing about 340 miles and hour And, finally, before I pass it over to Earl, this is a view of the changing seasons In fact, Saturn’s shadow on the rings you can think of as a giant sun dial And this picture taken back in April of 2016 you can see that the shadow of Saturn goes out just past the Cassini division At Solstice, that shadow will pull in until it’s about in the middle of the B ring And so as that shadow pulls in, so will Cassini’s time shorter at Saturn With that, I’d like to turn it over to Earl to talk about the grand finale (applause) – So how does Cassini follow that? How do I follow that? I want to go first next time You know, one of the things about Cassini is it always trumps itself As we keep finding, one year we announce a sub-surface ocean the next we announce a global ocean So it keeps building and building and building and you look at the last 12 years and how do we do something even more spectacular in our final year? Well I’m going to tell you at least the potential for that Before we do that I want just a little bit of a backup here I’m going to go to the end This is September 14, 2017, it’s about two o’clock in the afternoon here in Pasadena and Cassini has just wrapped up a 30 hour or so observing session The recorders are packed full of images and fuels and particles data and so now it’s time for Cassini to turn back to Earth and begin to play those back So this is a high speed slow speed version of the last of these periods So Cassini’s going to be working with this for about 10 hours DSN’s going to be receiving all this data, we’re going to be streaming these back and as soon as we see them, you’ll see them Some spectacular images of the poles and of the rings as we come in And then when the SSR’s, the solid state records are empty, be about 10 hours, Cassini’s going to reconfigure for the periapsis here at Saturn So we’ve done that 292 times over the last seven years Periapsis at Saturn is pretty routine, but not this one This one is absolutely unique Because it’s Cassini’s last Three days before this event Cassini had a close encounter with Titan Titan gave it a little gravitational nudge and that nudge has pretty much sealed Cassini’s fate As a matter of fact, it’s not coming out of this periapsis It’s moved the periapsis the closest approach distance inside the capture radius of the Saturn’s atmosphere And so, Cassini, this one is going to reconfigure itself so that it doesn’t put the data on the recorder, it’s going to put everything out on the pipe as quick as it can So the minute you see, it’s going to start to turn colors here as Cassini reconfigures Cassini’s going to go into the atmosphere and every second of this data is going to be coming back to the earth And, unfortunately, Cassini is going to be going 77,000 miles per hour You can get around the earth in about 20 minutes at that speed So what’s going to happen, it’s going to happen very, very fast We are going to have every piece of data streaming back down We’re going to be sampling the atmosphere and trying to answer some of the fundamental questions about Saturn’s atmosphere But it’s not going to be very long At 77,000 miles per hour, Cassini is going to be going in with it’s antenna pointing to the earth, but the atmosphere is going to quickly overpower it’s ability to point It just doesn’t have that kind of control It’s going to push it off and then we’ll lose com It essentially will disappear from our monitors and about three or four minutes later that speed and the density of Saturn’s atmosphere will vaporize Cassini and it is over One of the most spectacular missions ever to leave earth A discovery machine like you will never see and it’s going to be done So why are you doing that? First thing Did you guys ask anybody’s permission to take something that has rewritten science programs, redirected NASA programs and re-contoured missions, you’re just going to

destroy it? Let me give you, try to explain why we think that’s a good idea In order to do that I’ve got to go back a little ways So back to 2009, Linda told you about the prime mission and the extended missions We got to Saturn in 2004, right, we had a four year mission, but we didn’t have any end There was no end-game planned But we got to the end of that mission, realized we had an incredibly good spacecraft, lots of propellant, so we went for another two years About midway through that second mission, 2009, geeze, all sub-systems are great, this system is wonderful, we’ve got a lot of gas in the tank, let’s do something else So what? What’s next? And we actually got a lot of studies done There’s a lot of opportunities at this point with all the sub-systems going We could’ve left Saturn We could’ve gone off the Centaur Asteroids and turn ourselves into, re-configured, re-purposed Cassini as an asteroid mission We could’ve, believe it or not, left Saturn and gone to Jupiter, or gone out to Uranus, or gone out to Neptune Now, I gotta say this was a 40 year cruise So it would’ve been a long cruise, but look where Voyager is It was possible We could’ve gone back to Jupiter That actually is an image we took on our way out and we could’ve gone back and spent the same set of resources on Jupiter as we did on Saturn Uranus is also a possibility Or more Saturn Well, this is kind of a no-brainer I mean, we had barely scratched the surface Saturn is just incredible You couldn’t have asked for a more dynamic environment You’ve got the rings, you’ve got the planet, you’ve got the icy satellites, you’ve got Titan and Enceladus little pre-biotic worlds on their own and Cassini’s still unwrapping this So it’s really not hard to figure, “Okay, we gotta stay.” So I’ll jump to the chase real quick Nah, we’re not going there Nah, we’re not going there, we’re going to stay But there’s a catch If you want to stay at Saturn, there’s some rules And they are, we call it planetary protection, but the real essence of this is you’ve got to protect Saturn’s ocean worlds Cassini is essentially a victim of her own discoveries My apologies to the Oakridge boys, but you can visit, but you can’t stay So you’ve got to make sure that if you stay in the Saturn system, there is no possibility of a crash landing on Enceladus or Titan Cassini is room temperature inside If there are little microbes in there that don’t mind a vacuum, they could last forever We are running essentially at about 72 degrees inside this spacecraft So going and taking some of our earth microbes or spores onto Enceladus in particular where we know there’s water, warm water, would just be absolutely unacceptable So you guys can stay, but you’ve got to be careful about what you do about Titan and Enceladus So, with that in mind you’ll say, “How are we going to do that?” We could stay and go big long orbits, stay way outside the orbits of Enceladus, way outside orbits of Titan, but guess where all the science is? It’s down there with Titan an Enceladus So, we want to explore these guys but we want, at the same time, to remain safe So we go to the trajectory designers, got fair bit of propellant, got good subsystems, what can you guys do for us? We want to be able to stay inside Saturn’s system, we want to explore all the icy satellites, we want to explore the rings of Saturn, but we want to still remain safe for these two incredible worlds So what these guys gave us was the Solstice mission trajectory This started in 2010 and this is a trajectory designer’s masterpiece A lot of squiggly lines, but each of those is an orbit and every time that orbit changes shape, it’s because Titan moved us As Linda said, we get essentially a Saturn orbit insertion velocity change every time flyby Titan So we want to go up, you fly under Titan and it pulls you up You want to go in, you go to the right, go to the left And it can take you all over this system So what we did, what the trajectory designers did, they took these orbits that stayed very flat in Saturn’s plane By the way, that’s Saturn there in the middle Very flat and then you could stay and do all the satellite interrogation you want You could go very inclined, very looping orbits and do magnetic fields and to do the poles and the rings, and as a bonus to all of that, this is the first six years of this mission, the mission that Linda just reported, you also get this

This is the last year of the mission And this is, unfortunately, at the end, the demise of Cassini as I described just a few moments ago But on the way in, we have an entirely new mission, something we have never done before We’re going to flirt with the outside of the rings and then we’re going to go diving deep in between Saturn and the rings And I’ll show you a little bit more about that just to show off some here These things are just phenomenal The key orbital characteristics of this final set of orbits, which we call the F ring approximals, you’ll hear some of the flight team call them FERPPO, which sounds more like a dog food than a real acronym But really the F ring orbits and what we’re now calling the grand finale 42 short period orbits Each of these orbits lasts about a week The flight team is going to be running around like you’ve never seen 20 of them are going to be, oops I’ve hit the wrong button, sorry I hit it again There we go 20 of them are going to be just outside the F ring This is the outer-most ring here Titan is going to be, both of these essentially run into Titan right out here 20 of these outside with great coverage of the poles and the rings, and then another Titan flyby is going to move us in to the gap between the inner most D ring and the outer most edges of Saturn’s atmosphere 22 of those The periapsis is going to be what we call the 2,400 kilometer clear zone between the, essentially (mumbling) We’ve got their dust on the left and we’ve got the Saturn on the right We’ve got to navigate in between Next slide is, I think, a look at the view from Earth Not only are these things phenomenal from their proximity to the system, the geometry is also phenomenal because, if you look at what happens here, these orbits go behind the rings and behind Saturn, almost every one of them from a view from earth, provides what we call (mumbles) Not only do we have instruments that can photograph and sample, we also have instruments that can send a very very precisely tuned radio signal to earth And passing that signal through the rings and the atmosphere can tell us a tremendous amount about their internal structure The opportunities here are absolutely phenomenal and we, by the way Saturn has obliged by never, if you recall back to Linda’s picture, never opening up the rings more than they are right now So essentially be passing these waves right through It’s an absolutely unique and spectacular opportunity This, again, is just to show a little bit of what happens at the periapses You can see, you don’t quite see the F ring on this illustration, but it is right– Oops, I done it again There we go We have these rings here, the F ring is actually coming out right here We have about an 8,000 kilometer gap there, but there’s extended dust And if you look at the F ring, I’m actually more terrified about that than I am about the gap because of these tendrils that keep coming off of the F ring But nevertheless, that’s where we’re going And then again you see the proximity of these periapses here inside the, what we call the proximals, the grand finale This is, again, just kind of showing off, but here is a flattened out version These are the rings of Saturn, here’s the F ring up here, and here is Saturn’s atmosphere down here Saturn atmosphere Saturn doesn’t have a surface, or if it does it’s way down in there So what we call the surface is essentially one bar level Essentially the pressure at sea level So that’s what we’re calling the surface of Saturn So here are, graphically, each of our periapses So we’re flirting around in this safe little gap between the F ring dust– I’ve done it again We’ll get to that (laughs) Between the the rings here Incredibly precise navigation to stay between the dust hazards between the F ring and the (mumbles) rings here Then the Titan flyby that brings us down in here And then we stay very carefully and very precisely within the gap between Saturn’s atmosphere and the dust until our final flyby here I should point out there are a couple that are actually flirting with this and we’re going to do some things here to keep ourselves safe because they’re a little bit more dicey than the others Okay, so what’s it like to be on Cassini when we’re doing this? I’ve already stolen my thunder on this slide a couple of times Imagine you’re sitting on the prow of Cassini going through This is seven seconds of terror every seven days for seven, plus two months

(chuckles) So, Mars has got their seven minutes, we’ve got seven seconds every seven weeks And this is exactly, this is the white knuckle time for us Now, we won’t know, because most of the time going through here we don’t want to have the spacecraft talking to us, we want it to be doing science So we’ll find out if we’ve survived these ring plane crossings much later in the game But that’s the way it goes You want to get the science, you don’t want to find out if you’re going to make it So this is going to happen 22 times, every Tuesday, I believe But I could be wrong about that So the flight team, there are many of them here, we are going to be very busy So, the science I love the engineering of all this, but really, the engineering is all because of the science And this is, this is just some of the unique things You’ve seen what Linda showed already and it is phenomenal and we’re going to continue to do some more of that even while we’re here But these are opportunities that we will never ever get any other time Saturn internal structure, magnetic fields, and gravity We’ll actually be able to determine for the first time the mass of the rings by flying in between the rings and Saturn we can get a sense of which one’s which And that tells us something very fundamental Believe it or not, we don’t know how old the rings are They could be a couple hundred million years, they could be a billion years There’s a big argument about that and very, very intelligent people on both sides of the case We think we can help with some of these measurements Saturn’s atmosphere and the inner-most ring particles and the highest resolution ever ring observations themselves We went into orbit in 2004 we went over the rings, but they were not lit, we got the dark side So now we can finally see these rings fully illuminated by the sun And as I showed in that picture earlier, Saturn’s cooperating by providing an incredibly good phase angle at the sun Also, we’re going to radar the rings You saw the radar images of Titan, we’re going to try to do the same thing with the rings Pole observations and aurora of Saturn And then, finally, as I mentioned in my first slide, we are actually going to sample Saturn’s atmosphere Every ounce of Cassini’s last effort will be made in sampling the atmosphere and trying to understand and answer some of the fundamental issues about the constituents of the hydrogen helium ratios and things like that So we’ll see Let me just quickly run through this November 30, right after Thanksgiving, this whole thing starts And this is just to show you that not only, sometimes you get your good and sometimes you’re lucky The longitudinal coverage of the F rings is absolutely phenomenal, we’re going to get the whole planet covered with the F ring timeline, 20 orbits April 22 is our first targeted flyby, last targeted flyby And this is the one from Titan’s going to push us in So I’m going to try to not hit the go button and show you Titan’s going to come in from over here, here’s the F ring, final F ring orbit We’re going to come out back around and then here comes Titan and watch what happens to this orbit Boom It’s about a couple thousand kilometers, so it’s pretty close But now, rather than going outside, in we go And that’s going to happen for 22 times And so there’s that and I won’t show you 22 more April 23 the grand finale begins and we have a lot of Titan flybys pushing us around I won’t show you a whole lot of those But the first dive through the gap, and here’s our longitudinal coverage with the proximals Again, it’s almost a perfect grid all the way around the planet Absolutely phenomenal First dive through the gap is on April 26 and then September 11 out last flyby of Titan And I mentioned that before We call it T292 It’s a distance flyby, about 100,000 kilometers, but it doesn’t take much to push us in to an impacting trajectory And September 15, boom, we’re in The end of mission and the end of a very spectacular set of investigations, etc So, I want to share a cartoon with you that we at the flight team like to pass around “Hey Cassini, I hear you’re retiring “How about that, congrats Do you want to celebrate? “Maybe lunch with me and my moons.” How about that? “Nah, I’m just going to go barreling straight into “your atmosphere, learning as much as I can before “I’m crushed to death and vaporized to spectacular “whirling inferno beneath your mysterious stormy clouds.” So you can imagine Saturn’s reaction to that It’s the same

(laughter) It’s the same one that we all have, maybe you all have when you see that we’re going to burn this thing up You think about that for a little bit and hopefully what I just told you might come to agree with all of us that it’s too bad, it’s a wonderful machine, it’s been an incredible discovery machine, but it’s awesome (applause) Okay, I think we’re– We’d be happy to entertain any questions you might have And if you do have a question, we appreciate you going up to the microphone – Thank you for a really awesome presentation So, I believe that the Juno mission is using highly elliptical orbits to explore the internal structure of Jupiter, and I assume, you mentioned that you’re going to be probing the magnetic and gravitational fields of Saturn So my question is, at Jupiter they expect to confirm the existence of metallic hydrogen inside of Jupiter Is Saturn having enough gravitational pressure to form metallic hydrogen, do you believe? – Yes, Saturn certainly has enough pressure inside to form metallic hydrogen We’re wondering if we can maybe also detect that boundary inside of Saturn I just want to point out one difference between Juno and Cassini, Juno is in a polar orbit, basically going over the poles, Cassini we’re only tipped at 63 degrees And that’s basically our optimum orbit to keep the periapses from precessing and putting us prematurely into the rings So very similar complementary science for the two missions, probing the interiors of two gas giants and then comparing the results – Thank you – Thank you for the great presentation, I wanted to ask about contingencies during this final year You’re on a risky pathway and if something were to happen to the spacecraft on one of these passes through the rings, what do you expect to become of the rest of the mission? Is there a chance that it can still have it’s crash into Saturn? – Yeah One of the things that’s pretty amazing about this trajectory, once we’ve flown by the final Titan flyby, if we lose the spacecraft, it’s still going in And as a matter of fact after T125 we require, which is the penultimate Titan flyby, very minimal trajectory maintenance We’re essentially on a ballistic trajectory to our entry Now, that being said, we’re still going to try to get, we’ve worked contingencies in case we find the dust is higher than we want, we can hide behind the high gain antenna If the atmosphere is thicker than we would like, although some of the scientists think that’s just great, we can move ourselves out a little bit So we have worked all the contingency plans to make sure the mission is as successful as possible, but if we are damaged, we still will be able to keep our promise to Enceladus and Titan – In fact, if the atmosphere shrinks, and that’s a possibility, we also have a plan we could go a little bit lower because we want to dip our toe, for sure, in that atmosphere of Saturn – Hi, this is more of a question about the capability of the spacecraft So I understand that the decision to de-orbit it is quite final, but it would it ever have been possible to attempt, is there sufficient delta V in the tanks to attempt a rocky or icy moon, smaller moon landing like a janky near style landing, use the low gain antenna, send another spacecraft later and have a passive station sitting in orbit around Saturn – I’m afraid, well that could’ve been, in that set of scenarios there may have been a landing scenario that we didn’t work, but now there absolutely is not When we designed the solstice mission we designed it, you don’t want to end a mission with a full tank In fact, you want to end the mission with a completely empty tank and right now we are almost completely empty So the possibility of a controlled landing on anything would be absolutely out of the question Again, those sort of things, most of the controlled landings that we see are really more like controlled crashes They’re low speed crashes and so really the realistic opportunity to create a beacon, I think you want to design something like Hoygens that actually was built to broadcast up But unfortunately it was on batteries and that was that

But now it’s, like you said, the decision is made and we have spent all our propellant doing what we’ve been doing Thanks for the question – Thank you both for that presentation, it was excellent You noted earlier your concern over contaminating the environments of Enceladus and Titan How were you able to prevent that when you landed Hoygens probe on the surface of Titan? – Ah, good question I think the key difference between those is that Cassini is powered by these radio isotope thermo-electric generators with plutonium on board And to access the ocean on Enceladus you’d probably have to melt through some ice And with the heat from that plutonium, that might be a possibility Hoygens probe had batteries and it has some small RHU heaters And also, when we landed on Titan, we didn’t know about the methane lakes, we didn’t know that Titan also had a global ocean, we didn’t know about Enceladus So a lot of things, as Earl said, Cassini is kind of a victim of her own discoveries – I see, thank you – Absolutely superb presentations, of course Quick questions, what’s the cause of the highlights that we see here at 12 o’clock and six o’clock on the outer most rings? – Excellent question, what is the cause of those bright spots? It turns out that those spots are actually, you can think of somewhat pulled in closer to the sun and so it’s sort of a phase angle effect If you can think of it that way, the ansa are further away from the sun than the points at the north and the south And so they are brighter, as many things brighten as you get toward that very low phase angle, or that distance between the sun and your target is small Good question, though, good catch – To start, thank you for a wonderful presentation, I really enjoyed it Knowing what we know now about Saturn, what’s next? And when do we get to go? – That’s yours – Well there’s a proposal cycle underway now within NASA called New Frontiers And there’s a fixed list of missions for New Frontiers, one of those is a Saturn probe Much as we had a probe in the Galileo’s atmosphere, we’d like to send a probe into Saturn’s atmosphere, in particular to measure the nobel gases that you can’t really measure any other way And there are a host of other things you could do with a probe There are also now, two targets that were added to the list for New Frontiers, those targets are Enceladus and Titan Basically, these new ocean worlds unveiled by Cassini And so there are missions to go to fly through the plumes of Enceladus with more capable instruments to, perhaps, look for those amino and fatty acids Missions to maybe land something in one of those seas on Titan and make measurements there So there’s a whole host of proposals, there’s probably 30 or 40 or who knows how many NASA will get sometime next spring and then they’ll get to pick one of those missions So we might go back as early, but it still is a long trip You’re talking about maybe a launch in the mid-2020s, ’25, ’26 and then maybe a decade or so to get back to Saturn It’s not a quick trip to get there – I can’t wait Thank you – And don’t forget Uranus and Neptune, I mean, they’re out there too and it would be great to send a flagship mission like Cassini on out to one of the ice giants, Uranus or Neptune We’ve just had tantalizing glimpses with Voyager and to go back to one of those places for a future flagship, maybe after Europa Maybe a flagship to Uranus or Neptune – Thank you so much – I’ll jump in for one more question How long does it take for an image to get from Cassini to earth? Like, to get the data here? My iPhone I think has eight megapixels, what’s Cassini have? – One megapixel, and it’s black and white (laughs) We colorize our images with filters and it takes anywhere from an hour to 90 minutes for the image to get from Cassini here to the earth – Whole megapixel image – The megapixel, let’s see, we do 140 kilobits per second So it’d take 10 seconds or so, roughly, let’s say 20 counting overhead to get an image down here once it starts But Saturn is an hour and a half light time away So when we start, when Cassini starts to send a signal, her bits don’t get to the ground for an hour and a half So when we want to send something to Cassini and have it answer, we have to wait anywhere from two to three hours – Wow, that was really great, thank you – It just means Cassini has to be very smart

She has to basically have commands on board to keep her going typically for 10 weeks at a time Where to point, where to look, when to send data back, and so very smart spacecraft – Actually, speaking of photos, I was wondering what’s the plan for the grand finale photo wise? Like, what are you expecting to see? If you’re expecting to take photos or expecting to see maybe some resolving some individual clumps of ice in the rings since you’re going so close, or looking at clouds of Saturn ’cause the periapses are going to be so close? Are you guys expecting to take a lot of photos from this mission? – We’ll be taking a lot of photos of both the rings and the planet Ring particles, on average, are millimeters to centimeters in size, even if they were 10s of meters, we still couldn’t resolve an individual ring particle But we certainly could resolve the structure that we see in the rings at much higher resolution SOIs are also just on the dark side of the rings, this is a chance to look at that resolution, but on the lighted side of the rings Radar of the rings, as well Also we’ll get close up views of the planet, of the poles of the atmosphere itself I think that the surprises might be the questions that we don’t yet know to ask When we look at those pictures, whether it’s the rings or the planet, what might we see? Also we have a detector that some of those tiny ring particles from the main rings charge up and the fill bines then will go into one of our sensors, the cosmic dust analyzer and we’ll get, for the first time, the direct composition of the rings We know they’re water ice, but we don’t know if the non-icy component is silicates, iron, tholens, we don’t know what it is So we’ll get the answer for that for sure for the first time – I might also add that, as we enter the atmosphere, everything is going to be focused on atmospheric construction and constituents The spectrometers, the fields and particles, they’re going to be pointing at the atmosphere, unfortunately that means the camera is going to be pointing someplace else And furthermore, in order to play all that data back as fast as we can, we have to narrow down the bandwidth and a megapixel is a megapixel We could get 10 or 20 mass spectrometer packets down for one image So the camera is not even going to be recorded and sent down during those final seconds – Thank you – You said in response to an earlier question that you’re getting pictures in black and white and then you’re coloring them with filters How does that work? Are you choosing or do you know what colors to use? – Our cameras have two filter wheels You know, essentially you can take a green, blue, and magenta, blue, green, and whatever three colors you pick and colorize them, right? It has infrared filters that penetrate the haze And so each of these filter wheels, you actually rotate that filter into the image path and take an image, then rotate another filter, take another image, and they’re combined and colorized on the ground – So the colors you end up with represent what you’re actually looking at or is it? – They can, or they can represent some of the false colors that you’ve seen like the red hurricanes and things like that that accentuate levels of elevation or of chemical constituents A lot of the pictures you’ve seen were natural, but some of the others were false colored to highlight whatever (mumbles) or chemical item you’re trying to look at – But you can get true color You take those filters and add them together in different ways and you get the true color that you would see with your eyes in those pictures – Thank you – Hi Thank you for your presentations I have two questions here I want to ask First is you guys said that Cassini satellite, I mean the Cassini drone, whatever, is the farthest in the solar system that we have ever gone – Oh no – No, what I said was that Hoygens probe landing on the surface on Titan is the furthest we’ve landed a probe on the surface But the furthest spacecraft, now, away from the sun would be the Voyager spacecraft They’re well past the orbits of Neptune, Pluto, they’re on out, even one of them into the interstellar winds – Even at light time, they’re a day and a half for a signal to get from the probe to Earth So they’re way out there – I see and the other question I have is that, I remember correctly, Saturn has five big moons, correct? And so why do you only land on two of those moons? – Titan is the very biggest moon and it’s the only moon in our solar system with a thick atmosphere And it was the one that had the most questions and puzzles about it So we really had the weight on Cassini to carry just

a single probe and so it was easiest to land on Titan You could land with a parachute, you didn’t need rockets or anything fancy And we wanted to see what that surface looked like So if we go back we could carry probes that could land on multiple moons and look at those, as well – Okay So you mean that you choose the moons you will land before– – Right, we chose Titan Before Cassini even launched we had chosen Titan – Okay, I see, thank you – Thank you for the very amazing talk I had a question on a radio (mumbles) For the three different frequency centers that you have available on the spacecraft, can you characterize a little bit on ring particles that are smaller than the shortest wavelength and larger than the longer wavelengths and the defraction patterns and how we would be able to ascertain the particle population – The three wavelengths of the radio science are very diagnostic in helping us understand the particle size distribution of the ring particles And what we’ve found in looking at those is they’re pretty much seeing all of the particles in that particular size range for that So they do a good job, the KA, X band, and S band, in looking through the rings And sometimes the S band signal is blocked out first because the rings are so optically thick – And how does the defraction or dispersion occur on particles that are outside those wavelength correlation? – The radio science actually there’s a fairly large field of view so it’s integrated particles all the way across that field of view And sometimes we see defraction patterns that tell us that the ring particles are lining up and are structured in a certain way forming these things we call self-gravity wakes We can actually do some work to detect those in radio science, as well If you want more detailed answer I can give those if you want to come up afterwards – Thank you – So this is more of an engineering than a science question, but for all these precise orbital maneuvers, how do you know you’re positioned accurately enough to perform these maneuvers? ‘Cause you can’t exactly open Google Maps and get your GPS, right? – I’ve got to brag a little bit because JPL is an absolute center of excellence for navigation What we do a couple of different things First of all, we track the spacecraft very carefully, we use doppler and ranging to measure it’s velocity and distance very precisely We fit that to an orbit and at the same time we’re solving for all the ephemeralities Essentially the positional points of all the satellites and Saturn And that’s a daily process As a matter of fact, we’re going to do a very tiny OTM tonight, over trim maneuver tonight based on latest observations Because we move, you know, we’re moving a kilometer or maybe a few hundred meters just being pushed around by our own shennanigans as well as smaller forces So we’re constantly tracking the spacecraft And over the decades we’ve hit comets, I gotta say the navigation at Saturn is one of the triumphs of modern interplanetary navigation because of the precision that we’re able to do this You could do the same thing with much coarser measurements, but you’d have to be carrying tremendous amounts of propellant Because every time you miss you’ve got to fix it to get back on track So I’d be happy to share a paper with you, or two, we’ve got a lot of papers about this (chuckles) – And one of the comments, the navigation is so good it’s allowed us to go closer and closer and closer to these targets until we came within just 50 kilometers of the south pole of Enceladus In fact, our closest flyby was 25 kilometers, but it just wasn’t under the pole So we have just gotten so good we can go close, know where we’re going to hit, and we don’t miss – Thanks – I have some online questions here Just want to go through a couple of those A question from Titan82 wants to know what is the temperature of the surface, sub-surface ocean of Enceladus? Well if it’s true that we have hydro-thermal vents, it might be as high as close to boiling point around those sub-surface vents But clearly if the water is a liquid, even though it’s under a little bit of pressure and perhaps with some ammonia, it must be very close, it must be above the freezing point of water So we know that, otherwise the ocean wouldn’t be a liquid The next question is will Cassini be able to photograph the vertical ring structures as it passes through the ring plane That’s a great question, unfortunately the answer is no We can’t photograph these vertical structures very well because they aren’t very big

We don’t think we’ll have the resolution to resolve something that’s a kilometer or less And we don’t think there’s vertical structure in the C ring and D ring where we’ll get the very closest to the rings So I’m sure we’ll be looking and in fact we have looked as we’ve gone through the ring plane crossings, I don’t think we’ll have the resolution to be able to do that And then if we really wanted to look for shadows, which is a really great way to look for structure, during this point in the mission there’ll be no shadows cast by the ring particles We’re not in equinox so Okay, are there any other questions? Okay, if not, thank you very much (applause) One down, one to go (upbeat music)