September 23rd, 2017
PASADENA, Calif. — When Cassini took its final bow into the upper atmosphere of the gas giant Saturn, a good many people who had labored on the nearly 20-year-long mission were forced to say goodbye to a machine that had become all but a member of the family. One thing that was not lost that day was the wealth of knowledge that Cassini had sent back to those it left behind on Earth.
At the end of the day, the 27 nations who had participated in Cassini’s mission would have a rather substantial legacy from the spacecraft to steward. SpaceFlight Insider had the opportunity to speak with Linda Spilker, the Cassini mission project scientist.
SFI: Was that from mission design as well as launch?
Spilker: “I started [working] on Cassini in 1988 and that was the phase where they were doing the science definition, and [they] use the science definition to find out what the instruments would be on your payload. I started on that early, early phase. Actually, I started working on [the] Voyager [program] in 1977.”
Spilker: “And so, I knew that was—I was working on one of the optical sensing [instruments] and I knew that Neptune, rather would Voyager  would be wrapping down and this opportunity came to work on this new mission which wasn’t even a mission yet, and didn’t have a name yet, but wanted to go back to Saturn, and since I was working on ring data and had written my thesis on the rings, I thought that Saturn was perfect. I thought: I want to go back…“.
SFI: And Titan, I imagine.
Spilker: “Yeah, that was the big reason we were going back; [it] was because Voyager  couldn’t see through to the surface, and so we needed Cassini, and I worked on Cassini from the beginning.
“Actually, I was on an instrument team that wrote a proposal [for a] composite infrared spectrometer, very similar to the one on [the twin] Voyager [probes] – I worked on that Voyager [program] team and worked on that proposal for an infrared spectrometer.
“And then, as well as project scientist, I kind of have two hats. So I’m like, ‘Whoa boy! Look at all this great data!’ From all of the instruments, so it’s good all the way around.”
SFI: In the press kit, they said that there were over 3,800 publications from Cassini – that just kind of blows my mind!
Spilker: “It is kind of staggering. Yeah.”
SFI: How many of them have you even read?
Spilker: “I know a lot about the ones that got published in like Science or Nature, or some of the other journals, but what’s nice is that we’re having a project science group meeting now and so we started out Monday meeting, four of the disciplines meeting Monday, one is meeting today, and each discipline has all kinds of science talks.
“They’re sort of a chance to catch up on all the science, then there are other science meetings – DPS Division of Planetary Science, AGU, and other meetings to go to catch up – but I probably haven’t read a fraction of those papers, because sometimes those papers spawn papers that spawn papers.”
SFI: And I’m sure there’s another 1–2,000 more that have yet to be generated.
Spilker: “Over the next decade or so, new discoveries—you know, we sort of have this firehose of data that we skim the cream the cream off the top of, and when you really dig down, there’s probably more Ph.D. theses in there. There’s going to be a lot more coming out.”
SFI: I can imagine.
Spilker: “It might slow down once the funding slows down, though. That’s the only thing. It will be nice to keep funding an outer planets research program at a reasonable level. But that’s a tough thing.”
SFI: I know Juno’s out there.
Spilker: “Yeah, Juno’s out there and Europa Clipper is the next one up. But we don’t have any flagships going anyplace, and nothing [is] in place. Nothing with the scope of Cassini. Everything else is much more focused.”
SFI: Cassini or Galileo. Something that’s orbiting there for years.
Spilker: “That’s right. I think we need like a Cassini 1 and a Cassini 2. Launch them together and fly them out to Jupiter – send it out to Uranus and the other out to Neptune. And just like you flew [the] Voyager[s], and even though they are orbiters, they would arrive at very different times, so you could probably do the operations.
SFI: Lots to learn about Neptune and Uranus.
Spilker: “Absolutely! Just one flyby is just so different. Yeah, yeah… absolutely.”
SFI: Phenomenal. So my editor did have some ring questions.
Spilker: “My favorite kind of questions. I love ring questions. I love Enceladus questions, too.”
SFI: We’ll get there, too. So diving between the rings is giving you a lot of data on the mass of Saturn and the mass of the rings, is there any preliminary data out on what the refined masses might be?
Spilker: “Very, very preliminary data seems to indicate less massive; if you take all the ring particles and scoop them up, it would be about the mass of Mimas, we think.”
Spilker: “And so if they are less massive than that, which we will directly measure from the gravity data, then they must be young. If the mass goes down too much, then they couldn’t have survived the micrometeoroid bombardment, and they are just probably young. If they are more massive, they may be as old as Saturn or they could be older they could be older. The data is sort of trending toward less massive rings, but the error bars are several Mimas masses. We have this data point…”.
SFI: So that’s huge at this point.
Spilker: “Huge error bars, and that’s just because Saturn’s gravity is turning out to be much more complicated than we thought. Our simple model of what the J2, J4, J6, what those coefficients should be, gravity coefficients – nope. Not right. Very different from Jupiter. Very different. And so they don’t know why. Why? What’s causing the models to be wrong and what do they need to do to fix that in agreement – three types of models – and it helps, but… they’re getting closer, but we’re going to think about this some more and bring in more people to think about it and make more models, and let’s try this model or that model, let’s try these two models together.”
SFI: What are the drivers…? Where are the errors…? Is there any crossover with the Juno team? I know that Juno is trying to…
Spilker: “Absolutely. What’s really nice about the Cassini gravity team is that you have many of the same players on Cassini and on Juno.”
Spilker: “And they’re looking at the data sets together, which is exactly the right way to do it. And so they have all of the models and things that they did for Jupiter, and, of course, you apply them to Saturn and ‘boom’ – the models are not working… not working. Let’s see what’s going on… what’s different.”
SFI: So perhaps a completely different kind of core or…
Spilker: “It could have something to do with the core or some kind of oscillations happening on a larger scale than we’re used to happening on Jupiter. Maybe you’ve got cylinders of rotation, maybe you have differential winds, how do the winds go inside, how does that affect the mass distribution? Because the mass distribution that they expected is not right.”
SFI: I know there was some uncertainty with the orbitals – or the rotation period of Saturn.
Spilker: “And that’s still a mystery. The way that we would find that is if the magnetic dipole is tilted then it would appear to wobble. That’s what Jupiter does, the Earth does. The magnetic dipole’s tilted [with] respect to the spin axis of the planet, and then you can get that wobble. The wobble gives you the length of the day.”
Spilker: “In the case of Saturn, we’re looking for that wobble, but now the two axes are aligned within 0.06 degrees, maybe even better than that. They are so perfectly aligned – it’s like a perfectly coaligned dipole. And they’re just shaking their heads because that tilted dipole also provides current flow in the metallic hydrogen or whatever the layer is, and without the current flow the magnetic field would go away. And if you take away the dipole tilt, then the magnetic field should go away.”
SFI: But it’s not.
Spilker: “It’s not. It’s a nice, powerful, booming magnetic field. So, length of the day, unless we figure something else out. They’re looking at the B5 component – all the little oscillations they see. They think, they think – they’re not sure yet – [the oscillations] are probably not anything to do with Saturn. It’s something else.”
SFI: That’s what makes it so exciting, right?
Spilker: “That’s why you’ve got so many happy scientists. You have all of these scientists down at Caltech meeting and they’re all completely happy because all of their models are wrong.”
Spilker: “And this means – this is exciting – it means you’ve got to fine tune your model, write more science papers, and discover something you didn’t know. If all of the data we had [had] agreed with the models, that would be kind of boring. Okay… check… check… it’s not boring. It’s not boring at all.”
SFI: Eventually, it’s good to be right, I imagine.
Spilker: “Yeah, [studying] the composition of the rings for the first time. To make in-situ measurements, use the Cosmic Dust Analyzer, that’s tremendously exciting. The tiny little dust-sized particles – the nanograins – are making it harder because you need a little bit of mass to get a mass spectrum. But when the mass gets down to nanograin size, there’s not much mass there, so they’re looking for ‘bigger’ particles like 20 or 30 nanograins to come in to create a spectrum. But that’s a big puzzle – what ground up the ring particles?
“We expected that particle size distribution, just because the particles become fewer and fewer, but the distribution we thought, naively, would remain unchanged. So, something is neatly chopping up and grinding up the ring particles to dust. Where does it happen? Why does it happen? How does it happen? Happy scientists.”
SFI: Do you see any differences between the E ring and the inner ring?
Spilker: “This innermost region, the particles of the E ring are like micro-sized, or 10-micron sized. So they’re huge compared to the particles nanograins that are in that gap.”
SFI: Oh, okay.
Spilker: “So nanograin – like 10–9 compared to 10–6.”
SFI: So tiny.
Spilker: “Three orders of magnitude smaller than the typical particle in the E ring.”
SFI: That gives you some indication of fresh particles versus.
Spilker: “Or just grounded up. The ice is all gone. Well, almost all gone. Even the water ice particles are tiny nanograins. So it’s like something is deconstructed the ring particles because we expected to see if you look further out in the C ring – we were thinking things would be millimeter, half-millimeter and bigger, and then something smaller. But where did all the microsized particles go? Where did all the 100 microsized particles go?!”
SFI: So you’re getting no big ones at all, even in the areas…?
Spilker: “Maybe one or two hits in a pass in the micron range.”
SFI: Okay. Does that include the areas where you get the topography along the edges of the rings?
Spilker: “I think when we’re in the gap we’re so far away from the D ring edge that – we even flew what we thought was inside the edge of the D ring, and we were tilted and using the high-gain antenna as a shield and that was what we had planned, and when they were so small we decided to tip so the instrument can get some bigger particles. Not much there. Oh, maybe we got maybe 40-nanograin, 50-nanograin. They aren’t a whole lot bigger there.”
SFI: I guess at least there was no indication of damage to the spacecraft.
Spilker: “No. That tiny, it’s like getting hit with smoke particles even at 76,000 miles per hour. Which is good. This is good news for Cassini. It was great. The first dive was on my birthday. So I was here at night at JPL, and I was waiting for the data to come back, of course, because, as good scientists, the best science is at closest approach, so what we did wasn’t just fly through the gap and then turn back to Earth and say ‘we’re okay’.
“We’re saying we’re flying through the gap and for the next 12 or 14 hours we’re taking data, and then we’ll get back in touch with you. So it’s the first one, we don’t care. So then it got back in touch and it was like we’re here, it’s night, and we’re waiting for the radio signal to come back up. Did Cassini survive, is it in good shape, is it safe-ing, or whatever. The signal popped back up, everything looked great, and I thought: great birthday present.”
SFI: (Laughs) Yeah, much relief.
Spilker: “Yeah, and then great pictures came not too long after that.”
SFI: The pictures have been absolutely amazing.
Spilker: “Yeah, of the rings, of the planet. Feel like taking a magnifying glass to the rings.”
SFI: Every time I think I’ve seen the coolest one, you guys release more.
Spilker: “Yeah, great color ring picture came out just a couple of days ago. It was really cool. And the thing is, we thought as we got closer and closer [that] maybe the rings would get kind of fuzzier and spread out, like their one size would get spread out, but instead, there are places in the B ring where they get closer and closer and closer. There’s just more and more ringlets.”
SFI: How interesting!
Spilker: “Some places they’re more spread out, but why and how. Anyway… happy scientist.”
SFI: That’s a hard gravity model to put together, I guess.
Spilker: “Yeah yeah… it’s very hard to figure out how you can maintain that over a long time.”
SFI: For the ring particles, because you mentioned that they’re getting so stratifies, does it appear that they stay in the same orbits or do you see migration between the rings?
Spilker: “When they get small enough, they can get charged up, and they end up turning into ring rain. That’s some of what we’re seeing, too, when we’re crossing a little further away sampling with the gases, we’re seeing basically a mixture of ring particles and atmosphere. As you get closer, they mix together. Evidence of the rings interacting probably through getting charged and brought in to water the planet, they call it ring rain.”
SFI: Yeah, so electrostatic charges and then it rains down on the planet, that’s a pretty cool concept. Would that be visible do you think?
Spilker: “You might be able to sense it with the mass spectrometers. It’s so small, and if it’s a gas, we’ll see how that all works out.”
SFI: You’ve done spectroscopic analysis of the rings, too, as well, right?
Spilker: “Right, right. That gives you hints about the composition, by the reddening or blue, but to directly sample, that’s the way to get the composition of the non-icy part.”
SFI: Do you think future missions is actually worth sending something that’s designed to go straight through the B ring or the C ring or…?
Spilker: “It would be great to have a mission that would sort of hover over the rings and, you know, maybe be able to go closer in some places, and maybe send off little – what are those called? – CubeSats, you know, to communicate to the mother ship and go down with your little cameras or whatever and get really close to the rings and look at ‘em and sample one and bring it back or whatever, you know? Whatever CubeSats.”
SFI: How fast are they orbiting? Are they all orbiting in the same direction?
Spilker: “They all orbit in the same direction – prograde. And depending, just like the planets, periods are dependent on the distance away, so you get 14 or more hours further out and then you [halve] that as you get closer. So just like the planets.”
SFI: And I guess the orbital mechanics team has gotten good enough playing around with Titan that you could probably get whatever orbital velocity that you need.
Spilker: “Right, right. In fact, this was unfortunately after we had designed the Solstice tour, we found that you could actually get into a very low inclination. Titan’s orbit is slightly inclined, so if you do it just right, you could really almost have gotten probably within tens or a hundred kilometers over the rings with a very, very low inclination.”
SFI: That would’ve been a really wild ride.
Spilker: “With the ring plane crossing on the outside, but we had already designed the tour and we had already had everything in place, so it’s all written up in a paper and a future mission can go back.”
SFI: I have to go look that up. That will be a hell of a ride.
Spilker: “They’d have to wrap up pretty quick.”
SFI: Do you have any thoughts to Enceladus’ geysers and how old they might be?
Spilker: “I don’t think there’s a really good – until we understand the mechanism for the geysers and what that could be, and the age of the ocean, too. We think that the global ocean would be much harder if Enceladus were frozen to create an ocean.
“It’s much easier to sustain an ocean there, so maybe the ocean has been there for a long time, and then this resonance with Dione is the tidal [force] by making its orbit slightly eccentric, that gives you the tidal flexing that you need to maintain the ocean. It’s easier to maintain it than it is to create it.”
SFI: We believe you.
Spilker: “As far as the tiger stripes and how long the geysers have been active and how long have they been more or less active, I don’t know.”
SFI: Is there anywhere on Earth for an analogous terrain to Enceladus or Titan that you think is a good – since we can’t go visit there yet, you know? Where’s the closest place on Earth?
Spilker: “Yeah… An analogy for Enceladus’ geysers – we call them Cold Faithful, so you can think of the Yellowstone geysers. For Titan, if you just substitute liquid water there, you could say that the rivers and seas and things on the Earth are very similar to the terrain on Titan. It’s getting down to those cold temperatures to see what it could be like to have methane as a liquid. Ice would be the rock.”
SFI: That’s very, very cool.
Spilker: “So yeah, Titan looks a lot like Earth in many respects.”
SFI: Amazed. You’ve seen weather; you’ve seen rain…
Spilker: “Seen weather, seen rain, dunes along the equator – all of these things that we see here.”
SFI: And the dunes are hydrocarbon remnants or something?
Spilker: “Yes, we think that formed high up into large particles that kind of fall down, and then it’s very dry in the equatorial regions and [that] can create dunes. There’s a lot of questions about that, though. It would be grand to figure that out.”
SFI: It would, again.
Spilker: “Take a helicopter or one of these quadcopters and then you can go.”
SFI: Do you have researchers working on them or the gliders for Mars. It’s very exciting and I can’t remember the equation – the one for guessing how many life forms might be out in the universe.
Spilker: “Oh, the Drake Equation.”
SFI: Yeah, the Drake Equation! I mean certainly the discoveries you’ve made at Enceladus and Titan showing that [there are] ocean worlds.
Spilker: “It’s a paradigm change – life doesn’t have to be just in this narrow zone with liquid water on the surface; it could be in ocean worlds and could be spread anywhere in the Solar System.”
SFI: Completely changes that equation and makes all the exoplanets – what is it? – 3000 or so…
Spilker: “Definitely implications for the exoplanets, too.”
SFI: Thanks for taking the time to speak with us today!
Spilker: “Sure thing, I hope you got everything you needed.”
Tune in to SpaceFlight Insider tomorrow for an interview with Hunter Waite, the INMS team lead at the Southwest Research Institute, about his experiences working on the Cassini mission and what it means for future NASA missions.
Matthew Kuhns is an aerospace engineer living in California and enjoys capturing the beauty of the aerospace world with his camera. As an engineer he specializes in fuel & propulsion systems and as a photographer his internationally award-winning images are published in magazines and books. Kuhns was introduced to the founder of SpaceFlight Insider during the pre launch activities for SpaceX’s CRS-4 mission and was promptly brought on to the team as SFI’s California photographer.