Planetary Radio • Aug 18, 2021

Cassini, Voyager and beyond with Linda Spilker

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Linda Spilker

Voyager Mission Project Scientist at NASA's Jet Propulsion Laboratory

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Bruce Betts

Chief Scientist / LightSail Program Manager for The Planetary Society

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Mat Kaplan

Senior Communications Adviser and former Host of Planetary Radio for The Planetary Society

Cassini project scientist Linda Spilker is back to describe how data from the Saturn mission that ended four years ago is behind new, trailblazing science. Linda has also rejoined the team behind NASA'S Voyager mission that is celebrating important anniversaries. She closes with convincing arguments for missions to Saturn’s moon Enceladus and the ice giants Uranus and Neptune. Bruce Betts gets on the Cassini train with this week’s space trivia contest.

Mat Kaplan and Linda Spilker with Enceladus model
Mat Kaplan and Linda Spilker with Enceladus model Plumes from a model of Enceladus at the Jet Propulsion Laboratory.
Color view of the plumes of Enceladus
Color view of the plumes of Enceladus A true-color view of Enceladus' southern plumes.Image: NASA / JPL-Caltech / SSI / Kevin M. Gill
Neptune in natural color from Voyager 2
Neptune in natural color from Voyager 2 This Voyager 2 Narrow Angle Camera image of Neptune was taken on August 20, 1989 as the spacecraft approached the planet for a flyby on August 25. The Great Dark Spot, flanked by cirrus clouds, is at center. A smaller dark storm, Dark Spot Jr., is rotating into view at bottom left. Additionally, a patch of white cirrus clouds to its north, named "Scooter" for its rapid motion relative to other features, is visible. This image was constructed using orange, green and synthetic violet (50/50 blend of green filter and UV filter images) taken between 626 and 643 UT.Image: NASA / JPL-Caltech / Justin Cowart

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Trivia Contest

This Week’s Question:

How many orbits of Saturn did the Cassini spacecraft complete?

This Week’s Prize:

The ChopShopStore robotic spacecraft poster of your choice, including the three new posters for the Juno, Pioneer and Viking missions currently in development as part of a Kickstarter campaign.

To submit your answer:

Complete the contest entry form at or write to us at [email protected] no later than Wednesday, August 25 at 8am Pacific Time. Be sure to include your name and mailing address.

Last week's question:

What is the tallest mountain on Venus?


The winner will be revealed next week.

Question from the Aug. 4, 2021 space trivia contest:

Name all the Olympics for which an Olympic torch was flown in space.


Olympic torches have flown in space for three Olympic games: Atlanta in 1996, Sydney in 2000, and the 2014 Winter Games in Sochi.


Mat Kaplan: Another great conversation with Linda Spilker, this week on Planetary Radio. Welcome I'm Mat Kaplan of The Planetary Society with more of the human adventure across our solar system and beyond. The Cassini missions project scientist is back, with more news from a mission that officially ended nearly four years ago. Data gathered as the great orbiter flew through the plumes above Saturn's moon Enceladus has now been shown to be consistent with biological activity below the tiny world's icy exterior. Linda has also rejoined the Voyager team, as deputy project scientist. So some of her thoughts are in the space between the stars. There's much more in our latest conversation with Planetary Radio's most frequent guest. Bruce Betts is also thinking about Cassini, and he'll ask you about the spacecraft in his new What's Up Space Trivia contest.

Mat Kaplan: We're all getting tired of living in the virtual world, aren't we? Fortunately, as we stay safe, there are a couple of great upcoming events that will let you set sail with LightSail 2, and join Earth's smartest Martians. First up at 1:00 PM eastern, 1700 UTC on Saturday, August 28th is the premiere of Sailing the Light, The Planetary Society's beautiful new documentary about our LightSail. I hope you'll join Bill Nye, Bruce Betts, Jennifer Vaughn, and me, right after the online screening for a conversation about the project. Check out the trailer at, where you can also set a reminder. We'll head for the red planet next with our friends at Explore Mars. Their Humans to Mars summit is free this year, but you'll need to register. I'll be co-hosting the stream and talking with a few of the outstanding guests and speakers. This one is September 13 through 15. You can learn more and register on the Explore Mars website.

Mat Kaplan: The link along with many others is on this week's episode page Scroll down a bit on that same page, and you'll be able to sign up for my free monthly newsletter. The August edition has just been published. Great space headlines await you in the August 13 edition of the Downlink at, Beginning with an explanation of why Perseverance came up empty handed after its first attempt to collect a sample on Mars. The fault was in the crumbly rock, not the spacecraft. Two Venus flybys by separate spacecraft within 24 hours? Yes, it happened just as Bruce Betts said it would last week, on August 9th and 10th. The traffic jam included NASA and ISAS's solar orbiter, followed by ISAS and Jackson's BepiColombo mission to Mercury. And NASA researchers have used data from the OSIRIS-REx mission to provide a nearly 300 year margin of safety regarding near earth object Bennu.

Mat Kaplan: The worst odds come in the year 2182, when there will be a one in 2,700 chance of a bump in the interplanetary night. So you can tell your great-great-great-great grandchildren to breathe easy. Let's go directly to my conversation with Linda Spilker. We talked a few days ago. Linda, welcome back once again to the show. I have no idea how many times you have been my guests on Planetary Radio, but I do know I'm absolutely confident that you have been heard more than any other guest, probably a record that's going to stand for a long time. So once again, welcome back.

Linda Spilker: It's great to be back, Mat, thank you very much.

Mat Kaplan: We've spent virtually all of these previous sessions talking about Cassini, for which you continue to serve as project scientist. We're going to go in a slightly different direction this time, although we'll, don't worry, we'll come back to Cassini everybody because it's going to be integral to some of the things that we are going to be talking about. But as you pointed out to me, when we were talking about pulling this together once again, it's a big month, August of 2021 for the Voyager mission, particularly Voyager 1, a mission that you spent 12 years on before moving to Cassini. And you are affiliated once again. Why is this a special time in the grand tour of the solar system? Well, I guess they finished the solar system now. Now it's a grand tour of interstellar space.

Linda Spilker: Well, this is a special month because 40 years ago on August 25th, Voyager 2 had its closest flyby of Saturn. And that's the same planet that Cassini circled for 13 years. So just a nice overlapping in the studying of this very interesting world. And so Voyager 2 was the second flyby of Saturn and we learned a lot more about the Saturn system and for Voyager 2, that was the time when the scan platform got stuck. And we were behind the planet right at closest approach. And so in the intervening years between Saturn and Uranus, we figured out how to get Voyager's scan platform working again, to take really fascinating data at both Uranus and at Neptune.

Mat Kaplan: And there's an anniversary to celebrate regarding that last big world in our solar system, right?

Linda Spilker: That's right on August 26th, we flew by Neptune. And so that's another anniversary and that was in 1989. So two big by anniversaries for Voyager in August.

Mat Kaplan: I think I was correct in saying that you are once again affiliated with the Voyager missions that you worked on so many years ago. Am I right?

Linda Spilker: That's right, Mat, I'm now the deputy project scientist on Voyager. I have the privilege and pleasure to work with Ed Stone, who has been the project scientist throughout the mission. And what a wonderful feeling. It's almost like coming home, coming back to this mission where I first launched my career at JPL and literally went to my first launch, watching Voyager start its journey, Voyager 2 in particular into the solar system. And so it's really great to be back and to see just what Voyager is up to these days now that both spacecraft are in interstellar space. It turns out that Voyager 1 crossed the heliopause into interstellar space in 2012, and Voyager 2 followed in 2018.

Mat Kaplan: I think it's absolutely charming that now 44 years after you got to JPL, just in time for the launch of the spacecraft, that you are back in your old job, right? Deputy project scientist, and working with Ed Stone, one of the great planetary scientists and human beings that we have. One of the greatest, I should say that we've had the honor of hosting on Planetary Radio. We need to bring him back again pretty soon as well. I can certainly see how this would be very satisfying for you.

Linda Spilker: Oh, it is Mat. I wasn't deputy project scientist when I first started, I was just fresh out of college. I actually started working with one of the instrument teams, the infrared instrument team, IRIS was that the infrared radiometer team, and then came back as deputy project scientist. And what's interesting is that Voyager team went on to propose a Cassini infrared instrument as well. That was the composite infrared spectrometer. And I had enough years under my belt and education at that time that I actually was a co-investigator with the composite infrared spectrometer team. So I have a tie to Voyager through the infrared instruments as well.

Mat Kaplan: That would have been impressive if you had gone directly from college to deputy project scientist on the grand tour of the solar system.

Linda Spilker: It sure would have.

Mat Kaplan: So let's talk about what Voyager is up to now, because you also pointed me to a pretty significant piece of research that has surfaced recently because Voyager, both Voyager spacecraft, which we hope we're going to continue to be in touch with for a long time, right, are still doing great science.

Linda Spilker: That's right. Met. Both of them are still operating, but each year with a little bit less power, we're having to be more and more careful starting to turn off certain heaters on the spacecraft and on the instruments to keep both Voyagers going. And if you think about it, the two Voyager spacecraft is the first time we've had in-situ study, studying the local environment around the spacecraft in what we call the very local interstellar medium. And so now that they're outside the heliopause, we're studying the interactions going on the plasma density, that very fine mist of ions and electrons. That's an interstellar space called the plasma and what the effects of the sun still have out in the very local interstellar medium. It turns out that the solar wind, sometimes there are shocks from the sun within the solar wind that actually go through the heliopause and we can see these shocks and they understand their medium. And by looking at those, we can actually get the density of that plasma around Voyager.

Mat Kaplan: The heliopause is now we know, apparently, not really the end of the sun's influence over the space that surrounds us?

Linda Spilker: That's right, Mat. It turns out that that influence continues on and we'd like to see for just how far that influence continues right now. Voyager one is 150 AAU from the sun a little bit more than that. And Voyager 2 not far behind, is about 130 AAU from the sun. And to put that in perspective, the outer edge of the Kuiper Belt is at about 50 AU. So Voyager has gone all the way across Kuiper Belt, on out into interstellar space. And yet interstellar space, that's huge, the distance between the stars.

Mat Kaplan: Still got a ways to go before it gets out there beyond the Oort Cloud, right?

Linda Spilker: That's right. Yeah. You're talking light years, light years away past the edge of that.

Mat Kaplan: Just, I just remembered something. Another charming fact that was brought up by Alan Stern, principal investigator for the New Horizons mission, of course. Who talked on this show a couple of weeks ago, about how they pointed New Horizons and took an image looking out from where New Horizons is now considerably behind where Voyager is, the two Voyager spacecraft. Looking toward the section of space where I think it's where Voyager 1 is, of course was far too small to see, but I still thought that was a fairly romantic notion. Did you hear about that?

Linda Spilker: No, I didn't hear about that, Mat. What a nice tribute, in fact, Voyager uses information from new horizons to give us a warning of some of these shocks coming out from the sun, new horizons, being closer to the sun would see them first. And that gives us a few years notice that something might be coming our way. And so it's nice to have that interaction with new horizons.

Mat Kaplan: That's fascinating. And I had no idea that New Horizons was sort of acting as an early warning satellite for the Voyager spacecraft.

Linda Spilker: That's right. That it does.

Mat Kaplan: There is some other work that has been done just recently where, and I wasn't aware of any of this. So I hope you'll take us a little bit farther into talking about this interstellar medium, the plasma that you already mentioned. I got the idea from what you sent me in the abstract. I read that to detect the changes in this plasma, they had to wait for these sort of shock waves to come out from the sun, but now someone has come up with a better technique?

Linda Spilker: Yes, Mat, it turns out that on Voyager 1, the plasma wave spectrometer, basically that's two antennas pointing out that measure the radio waves that are generated from the oscillations in the plasma. It turns out that it's taking data all the time. Voyager 1 and Voyager 2 each have this plasma wave spectrometer or PWS. But on Voyager 1, we're still able to take high rate data with PWS and that's the key. And that story in and of itself is fascinating. It turns out that the PI of PWS, Don Granit when he put his instrument on board Voyager and he really wanted to get some what he called high rate data. And so they figured out a way where he could actually in between where we would take an image with Voyager, we'd take 48 seconds to read that image in. And then there'd be a 48 second pause to refresh the camera.

Linda Spilker: And in that 48 second pause, it would be difficult to downlink the data. Those would sort of be zeros coming back. And so Don grenade for PWS said, "How about if I just stick my data in just like an imaging frame, writing it, it's bits, just like lines of an image. And now you'll have a continuous rate to play back the data from the two Voyagers off the tape recorder. We have a digital tape recorder on Voyager that holds a hundred pictures." And so throughout the Voyager mission, the flybys of the planets, we got lots of this high rate PWS data. And now, three times a week, we actually put high rate frames, 48 seconds long onto the tape recorder, still working on Voyager 1. And then every few months we play back the tape recorder. We have to array the 70 meter and three 34 meter stations at the BSN to get back the signal coming back now from the tape recorder on Voyager one.

Linda Spilker: And it turns out you get spectrograms, basically information with frequency. And if you add the 48 second frame together and put all of those side-by-side over several months, then you can start to look for patterns. Are there places where there's a frequency that stands out? And sure enough, with very careful analysis of these data ,and that's in a new paper by Stella Ochre, you can actually start to see this very, very faint plasma signature, these sort of the oscillations in the plasma itself. And you don't need the shocks anymore, but can continuously measure the plasma density in these very faint, very faint little signals. And so what an interesting story that we can now see this nearly continuously, as long as the Voyager tape recorder keeps working. And as long as we can download those data. And PWS is healthy and doing quite well, it's a very low power instrument, and it might be one of the very last instruments that we turn off on Voyager, potentially lasting out until the late 2020s, maybe even to 2030.

Mat Kaplan: Just amazing. So many other things come to mind as you mention this. I mean, one, nature abhors a vacuum, because even out there between the stars, there are still some material for us to measure some energy as well. But also, we're not just using tape recorder here as a euphemism, taping the data, we're talking about a mechanical tape recorder, right? Two of them that are still functioning after now, close enough to half a century.

Linda Spilker: Well, it turns out just the Voyager 1 tape recorder is still being used. We aren't using the tape recorder on Voyager 2 at this point. So just on Voyager 1, but Voyager 1 is the furthest from the sun and now measuring this sort of, I think of it as a background hum from the plasma that's sort of out there in interstellar space, from the energy that it contains. And it's just so exciting to think that we can do this with Voyager. We're still measuring the magnetic field direction, and the strength of the magnetic field, measuring the composition, not only of the electrons, but the ions, which could be atoms or molecules that make up this plasma.

Linda Spilker: Plasma is really very fascinating. It's one of the most abundant forms of matter, ordinary matter in the universe, you figure you have four phases. Gas, liquid, solid, and then plasma. And the plasma is associated with the stars. It fills the space in between the planets in our solar system and goes on into interstellar space, filling the space between the stars. It's very rarefied, and it's charged, which means it can be influenced by the magnetic field. High energy, and high temperature generally, high enough to have taken an electron or two off of an atom or molecule to create then that plasma.

Mat Kaplan: Let's pull it back into the solar system a bit, as you said, we're also celebrating that flyby of Neptune by Voyager 2. Still no missions planned to those twins out there, those ice giants, Uranus and Neptune. I'm sure you were following when the most recent interplanetary or planetary science missions were announced by NASA. We've got two spacecraft now being planned for a visit to Venus. And we had both of those principal investigators on the show recently. Nothing against them, of course, but you are still one of those who very much wants to see a mission headed in the other direction, aren't you?

Linda Spilker: Oh, absolutely. After having seen the Uranus and Neptune data returned by Voyager, it only seems natural at some point to visit one or perhaps both of these worlds to go back with a mission like Cassini and Orbiter, perhaps with probes for the atmosphere, or maybe a probe to study Triton in more detail, Neptune's moon Triton, but definitely to go back and to understand these ice giants and be able to compare them then to Jupiter and Saturn, the moons, the quick glimpses we got of the moons around these planets were fascinating, especially the large moon Triton at Neptune, and to go back and understand not only the moons, but the rings systems that each world has, and the planets themselves, because each one is so different. Neptune is releasing more energy than it receives from the sun. Uranus is not, Uranus is tipped on its side, looking like a giant bulls-eye with its rings sometimes pointed toward the sun.

Linda Spilker: That was the configuration at the time of the Voyager flyby. So to go back would be so exciting and so interesting. We're in the midst of something called the Decadal Survey. And one of the outcomes or outputs of the Decadal Survey will be what should our next flagship missions be? We have Europa Clipper, and its stage instruments being built, the spacecraft put together. So after Europa Clipper, what's the next big planetary flagship mission? And I'm hoping it could be a mission to one of the ice giants, either Uranus or Neptune. I have a slight preference for Neptune, just because Neptune has this fascinating moon Triton, with geysers that we saw with Voyager on its surface. And some hints that perhaps maybe Triton might have a subsurface liquid water ocean, very similar to Saturn's moon Enceladus, and Jupiter's moon Europa.

Mat Kaplan: There's another reason that has come up many times on the show, probably with you and others as well. And that is that as we look out across the galaxy and find other worlds, thousands of them now, so many of them look like either twins or brothers, sisters of Uranus and Neptune. And so it would seem that this is an opportunity for us to learn more about planets all over the place.

Linda Spilker: Yes, I think that would be a great opportunity. And hopefully in the next decade, we'll take advantage of it.

Mat Kaplan: Going to come back to this consideration of the next big mission from NASA, a so-called flagship mission. But before we leave this, I want to give more evidence of how deeply you're involved in working with the outer planets, because you co-chair something called the OPAG. Tell us about that.

Linda Spilker: OPAG is the Outer Planets Assessment Group. We get the outer planets or community together twice per year, hold meetings and discuss and get updates from currently flying missions, updates about ideas or proposed missions. And then we put together a white paper for the Decadal Survey with OPEG's priorities. And we really think that an ice giant mission should be a top flagship mission priority, followed maybe five years or so later with a mission to an ocean world like Enceladus, or perhaps Europa to try and have a mission that could make measurements perhaps, in the Enceladus's case you could fly through the plumes or directly sample the plumes, maybe land on the surface and get directly from these products coming from the ocean what the ocean might be like with Europa. Perhaps there are events on Europa as well, and to perhaps take a Europa Lander to land on the surface of Europa and make similar measurements.

Linda Spilker: The key there is to figure out, do these ocean worlds potentially harbor life? There are some indications from Enceladus, from Cassini flying seven times through and directly sampling the gas and the particles coming from the plume that we have organics, we have evidence for hydrogen and methane and salts and some of the key ingredients for life. Some of the discoveries by Cassini led us to know that Enceladus has a global liquid water ocean beneath its icy crust. There are five tiger stripe fractures is what we nicknamed them at the south pole. And over a hundred individual jets of material are shooting out into space, carrying not only water vapor and water particles, but all of these other very interesting key ingredients. Some of those, the excess of hydrogen, we saw methane also, some salts that could only form at very high temperatures, perhaps deep on the sea floor of Enceladus point to potentially hydrothermal vents.

Linda Spilker: And these hydrothermal vents would be a source of energy that could be used by any form of life that could be potentially on the sea floor. We know deep in the Earth's oceans where sunlight doesn't penetrate, we find around Earth's hydrothermal vents teaming with little islands of life provided with the nutrients and the energy to thrive in an environment very deep in the ocean. So we wonder, could something similar be happening on Enceladus perhaps other ocean worlds. And so it's just very intriguing to think about a mission to go back. A lot of modeling and a lot of studies are being done. We're still going through the Cassini data and doing lab work and trying to see how you could create some of the spectra that we saw, both in the particle and in the gas data looking for evidence when we go back long chain hydrocarbons that could perhaps point to life.

Mat Kaplan: More of my conversation with Linda Spilker is coming up right after a short message.

Bruce Betts: Hi, again, everyone, it's Bruce. Many of you know that I'm the program manager for The Planetary Society's LightSail program. LightSail 2 made history with its launch and deployment in 2019, and it's still sailing. It will soon be featured in the Smithsonian's new Futures exhibition. Your support made this happen. LightSail still has much to teach us. Will you help us sail on into our extended mission? Your gift will sustain daily operations and help us inform future solar sailing missions. Like NASA's NEA Scout. When you give today, your contribution will be matched up to $25,000 by a generous society member. Plus, when you give a hundred dollars or more, we will send you the official LightSail 2 extended mission patch to wear with pride. Make your contribution to science and history at That's Thanks.

Mat Kaplan: There are a couple of articles along these lines. One that you pointed me to, and another one that I showed to you. First, the one that you came up with, and this is by, I guess it's led I guess by Morgan Cable, a paper called the Science Case for Return to Enceladus. And you recommended talking to Morgan on some future Planetary Radio episode. We'll put up links to at least the abstracts for all of these things we talk about on this week's show page at I assume that this is a paper that you've read and Morgan and her colleagues make a good case.

Linda Spilker: Right. Yes, I'm actually a coauthor on the paper by Morgan Cable.

Mat Kaplan: I didn't see your name. I was surprised when I didn't see it. I guess I missed it. Sorry, Linda. I'm not surprised at all.

Linda Spilker: That's okay. A very long list of coauthors. Lots of people very interested in this topic. And it just goes on to describe and make the case for potential life on Enceladus. By looking at the fact that you have the liquid water, you have the energy source potentially from the hydrothermal fence, as well as a whole host of organics and salts and other key ingredients that provide an environment in which life might thrive. And with Enceladus, what's nice is you have direct access to fresh material coming from the ocean itself through these jets that are spewing out into space. And so with all of these conditions being right, we really wonder, could you perhaps have life on Enceladus? And the second paper you pointed me to talks about a process called methanogenesis, where you could have biotic production of methane, and that could explain the escape rates of methane in hydrogen that we saw in the Cassini data.

Linda Spilker: And you have methanogenic colonies of life around the alkaline hydrothermal vents that are present on earth's sea floor. And so we wonder if you could have a similar kind of process going on in Enceladus. And as I said, a lot of lab work is going on, a lot of looking at earth analogies with Earth's hydrothermal vents, and basically getting ready for the next kinds of instruments that we'd want to send on a mission. They would go back to Enceladus. And they're just a lot of work going on in this area. Or what kinds of instruments would you send to look for long chain hydrocarbons and what would be the best way to make those samples? Even down to perhaps maybe one way is to go perhaps collect a sample of the ocean someday, and maybe even bring it back to earth, or we could do all kinds of experiments here on earth as well. And that might be the flagship after the next flagship kind of mission, but there's always that possibility to bring back a sample from one of the oceans of these worlds.

Mat Kaplan: That paper about methanogenesis. I hope I got that right. There is an article written by my colleague at The Planetary Society, Jatan Mehta. It's a July 28th article at that goes into this a little bit and talks about how they modeled this fascinating work that was done by these researchers, they've set out a 50,000 simulated environment runs, because they basically built the model, and then put different variables into it. About a third of those would have been habitable for the kinds of earth creatures you just mentioned, those critters that live down around hydrothermal vents here on the home world.

Linda Spilker: Yes, it's fascinating. It's now starting to make these models actually do statistical runs where you can look at if you produce conditions that are very similar and tweak them a little bit and make all of these runs, what do the statistics start to look like?

Mat Kaplan: The data behind this paper provides proof once again, of something that we have talked about, you and I many times that Cassini is the gift that keeps on giving, because this was based on that data that Cassini gathered largely when it flew through those plumes. You're still the project scientist for the mission. It has to be rewarding to see this kind of work going on.

Linda Spilker: Oh, oh, it is Mat. Absolutely. To see it continue. And September 15th is going to be the four year anniversary of the Cassini end of mission. It's just amazing how quickly that time flies. And it's very likely that over the next several decades, we'll continue to look at the Cassini data, build models to best match those data, and continue to try to understand that very interesting tiny little world, only 500 kilometers in diameter, so small and yet so very interesting.

Mat Kaplan: What a night and what a morning that was for end of mission. And then we followed it with that standing room only crowd at Caltech, which we'll put up a link to that show. Well, those several shows actually, because they were shows that we based on the work we did during the actual end of mission and then the celebration of the Cassini mission that you were part of at Caltech. So great memories. And it sure doesn't seem like four years ago.

Linda Spilker: No, it doesn't. It's amazing how fast that time has gone by

Mat Kaplan: And how. As we get close to maybe the end of this conversation, I just wonder, I mean, you have so much still going on at JPL, this return to the Voyager mission, you're a JPL fellow. You recently got the title senior research scientist. So congratulations on that, because that's up there in the science stratosphere at JPL and really any place. You've been at this for a very long time. Is it just as fascinating as ever for you?

Linda Spilker: Yes, Mat, it really is, because it's like just around the next corner, who knows what we might find. And that's so true now, especially with Voyager, with the Bose spacecraft out in the interstellar medium, and making new discoveries in the Voyager data to this day. And I know when I first started on Cassini, I would go back and look at Voyager data from Saturn, and use that to build the kinds of signal to noise or whatever I was looking for in the Cassini data. So I'm sure with Voyager and with Cassini that will continue on for years to come. And it's just wonderful to be back on Voyager. And what's fascinating is that Voyager's power is degrading by about four watts per year. And so that means we're having to start to very carefully consider what to turn off. One are the first things that they did with Voyagers to turn off the heaters to some of the instruments. Here, the temperatures would drop 40 or 50 degrees centigrade, and yet the instruments continued to work.

Linda Spilker: And so we've now turned off several heaters on the cosmic ray spectrometer and the low energy charge particle instrument. The instruments keep working, a little stepping motor on LACP keeps stepping, and just a little bit of recalibration and they continue to return good data. And they're so far outside their engineering specs, and yet a spacecraft that was so well-built and crafted that we can continue to get data even with those instrument heaters off, that the power for running the instrument is just enough to keep everything working on the Voyager spacecraft. So what an incredible mission.

Mat Kaplan: They know how to build hardware at that place where you work. And when you consider that it's sending that signal across all those many, many, many kilometers or miles, it's all the more amazing. Here's the other thing that I meant to ask you earlier, as we look forward to the launch and arrival at Titan of Dragonfly, I'm sure that you're very much looking forward to that, but it's also the development of the instruments that will be on Dragonfly, like spectrometers that can find those long polymers, those long hydrocarbons that you would like to look for at Enceladus. Does seeing that mission Dragonfly also make you feel good about heading somewhere else like Enceladus?

Linda Spilker: Oh, absolutely. Dragonfly is an incredible mission, and the chance to go back to Titan and you're right, building instruments, because they're going to collect samples out of the front two legs of Dragonfly and then perform experiments on those samples with a very sophisticated spectrometer. And perhaps we could fly some of those instruments to another ocean world. The major difference is Titan has an atmosphere. So they're having to design the instruments to work with an atmosphere. Whereas many of these other ocean worlds, especially on the surface, you wouldn't have an atmosphere. So there are some differences, but what a great mission to think about actually flying this huge octocopter across the surface of Titan and scouting out landing sites, and then coming back and then flying to the next landing site and continuing the scouting and exploration, an absolutely incredible mission.

Mat Kaplan: And that mission of course, led by the Johns Hopkins University Applied Physics lab. Do you ever get across the hall or to another building and talk to your colleagues who even sooner than Dragonfly are hoping to send Europa Clipper off to that moon of Jupiter?

Linda Spilker: Well, it's a little hard to walk across the hall at this point, though, Mat, though are all kinds of all kinds of telecoms and other ways in which we keep in touch.

Mat Kaplan: A virtual walk across the hall.

Linda Spilker: A virtual walk across the hall. I mean, even our OPPAG meetings are virtual at this point, but I am looking forward to the time when we do get back together, both at OPAG meetings and planetary science conferences. And to see my colleagues, they're slowly starting to open up JPL on a voluntary basis. If you want to go back a day or two, and I'm sure there'll be more plans to continue to open up in the future.

Mat Kaplan: Linda, it has never been less than wonderful to talk with you on Planetary Radio. I'm so glad we had the chance to do this again. And with your permission, we'll do it yet again sometime in the near future.

Linda Spilker: Oh, absolutely. Mat, it's always wonderful to talk with you and chat about all these interesting topics that come along the way, whether it's Voyager or Cassini.

Mat Kaplan: Linda Spilker of JPL is the project scientist for the Cassini mission. The deputy project scientist for the Voyager mission that has now been underway for 44 years, and co-chair of NASA's Outer Planets Assessment Group time for what's Up on Planetary Radio. Here is the chief scientist of the planetary society who joins us every week at about this point in the show. It's Bruce Betts. Welcome back. Come on. I need a little more enthusiasm in that.

Bruce Betts: Hi Mat. I'm really excited about doing Planetary Radio this week. We got some cool stuff.

Mat Kaplan: Well, there's always cool stuff. Prove it to me.

Bruce Betts: All right. How about you look in the west in the early evening and see super bright Venus. And if that's not good enough for you, look over in the other part of the sky over in that east direction. And you'll see a really bright Jupiter and above it to its right, yellowish Saturn. But wait, don't order yet. I can also offer you the moon hanging out near Saturn on August 20th and near Jupiter on August 21st.

Mat Kaplan: Is this a package deal?

Bruce Betts: It is, it's package deal. You get all the planets for one low price of awesomeness.

Mat Kaplan: All right, we move on though. We move on to this week in space history. It was 55 years ago this week and the fine year of 1966, Lunar Orbiter took the first image of earth from hanging out around the moon.

Bruce Betts: You know, this came up when we talked with Andy Chaiken because we talked about the Earthrise image that everybody knows so much better from Apollo 8, and how yeah, Lunar Orbiter had already done it, but it meant so much more to know that a human was actually seeing that site and capturing it with his Nikon or whatever, Hasselblad, whatever they gave those guys.

Bruce Betts: That's a prettier nice color picture too. 1976, 45 years ago, Luna 24 returned samples from the moon, robotic sample collection. Last of the three flown by the Soviets.

Mat Kaplan: Yeah. And the last until China did it what, last year? Or was it early this year? I forget. But anyway, we're back in the business of sample return, we earthlings.

Bruce Betts: Yeah. All right. We move on to random space fact.

Mat Kaplan: That was lovely.

Bruce Betts: Thank you. Uranus's moon Miranda. It's a weird looking thing. But listen to this. It's got big old canyons from massive faulting that are as much as 12 times deeper than the Grand Canyon. But wait, once again, don't order yet. Here's the amazing, amazing fact. Miranda's low gravity and huge large cliffs. As you would do Mat, if you dropped a rock off the edge of the highest cliff, it would take about 10 minutes to reach the foot of the cliff.

Mat Kaplan: I would do that, but a rock? That's so boring, I would drop a piece of pie, a piece of cherry pie.

Bruce Betts: A bowling ball?

Mat Kaplan: A bowling ball. Hell, why not?

Bruce Betts: A bowling ball and cherry pie. We can do the classic Galileo, Dave Scott on the moon experiment, but instead of a hammer and a feather, we'll use a bowling ball and cherry pie off Miranda's cliffs. What a plan, party on.

Mat Kaplan: You beat me to mentioning the feather. I really wanted to get that in.

Bruce Betts: Well, you edit the show. I mean, you can still do it.

Mat Kaplan: That's true. I could change everything. I have the power.

Bruce Betts: Shall we move on to the trivia contest, oh, powerful one?

Mat Kaplan: Yeah. It's entirely up to you, and the editor.

Bruce Betts: All right. So I asked you to name all the Olympics for which an Olympic torch was flown in space. How'd we do Mat?

Mat Kaplan: Dun dun dun dun dun dun dun, dun dun dun dun dun dun dun dun da de dun da dun.

Bruce Betts: Sorry. I didn't know we were really going to keep going through the whole thing.

Mat Kaplan: Yeah, sorry. I didn't either actually. That's the best part. It's really fascinating. A lot of listeners found this fascinating. I'm going to let David Mustarde of California provide the beginning of the answer with a little Olympic haiku. One small step for torch, Atlanta, Sydney, Sochi, giant leap for sport.

Bruce Betts: Nice.

Mat Kaplan: And I believe he's correct, but let me read the winner to you as well. And then see if you confirm it. I am very pleased to say that chose Claude Plyymate. Claude, who is a past guest on this show. He and his lovely wife recently retired from the Big Bear Solar Observatory, where we did an interesting Planetary Radio episode a while back. Claude is also a regular listener to the show. He last won, he is a previous winner, won once before, three years ago, almost exactly three years ago. has quite a sense of timing. He said, they were the games of the 26th Olympiad, that was Atlanta 1996, Space Shuttle Columbia, STS 78, the games of the 27th Olympiad, AKA Sydney, 2000 Atlanta Space shuttle STS 101, and the 22nd Olympic Winter Games in Sochi, 2014 on Soyuz TMA 11M. Does that match what you discovered?

Bruce Betts: It did. And the last one went and hung out on the International Space Station.

Mat Kaplan: Yeah, and I read also went on an EVA. At least we heard that from many, many listeners actually brought the torch outside. It probably didn't stay lit. Actually, that was also addressed by a number of people that we'll get to here. Congratulations, Claude. And so we are going to be sending you that fabulous Planetary Society Kick Asteroid rubber asteroid. We did get some nice poems from our poet Laureate and elsewhere, but I'm afraid they're a little bit too long for us to read. There's just some other priceless stuff here. Like what we got from Vlad Bogdanove in British Columbia, the idea of sending an Olympic torch to space was initially conceived by Australian-born NASA, astronaut Dr. Andrew SW Thomas. This is interesting. Edwin King in the UK reported that in 1976, a signal from heat sensors in Athens was broadcast via satellite to Ottawa, where it was re-lit by laser beam for the Montreal games.

Bruce Betts: Interesting, weird, and doesn't count.

Mat Kaplan: No, definitely not. And I think he knew that too. Ben Drought in Iowa discovered that there was hope that the torch would be lit in a special container in 1996. But NASA didn't go for that. A little bit of mythology from Mark Little in Northern Ireland, back in the day, of course, Zeus stole fire from us mere mortals. The Olympic torch represents Prometheus's stealing it back from Zeus and returning it to humanity. We all know what a Prometheus got as a reward for that from Zeus. If he hadn't stolen it, we wouldn't have been able to ignite the rockets that take us into orbit and beyond in modern times. So we are endeavors in outer space to an act of theft.

Bruce Betts: Wow, nice job Prometheus.

Mat Kaplan: Finally this from Pierre Louis Fon in France, hopefully, one day the Olympic torch will be carried to Olympus Mons on Mars.

Bruce Betts: Cool. There would be some amazing, amazing Olympic sports you could do there.

Mat Kaplan: And it will be cool because of course there won't be enough oxygen in the atmosphere, at least not until after the terraforming is complete to light the torch on Mars. So don't hold your breath anyone.

Bruce Betts: Ha ha. Oh gosh. Let's move on. So, here's a new question for you. You've been hearing all about Cassini. Well, not all about it. It's a very complex mission. Tell me how many orbits of Saturn did the Cassini spacecraft complete. Go to

Mat Kaplan: Would you accept a bunch?

Bruce Betts: From you, yes. From anyone else? No.

Mat Kaplan: Well, there's some guidelines for you. At least we've given you a hint. You have...

Bruce Betts: Have we?

Mat Kaplan: No. You have until August 25th, that's Wednesday, August 25th at 8:00 AM Pacific time. And get a load of prize. Our friend Thomas Roemer, the guy behind, where you will find all of The Planetary Society merch, and a whole bunch of other good stuff, including the Planetary Radio T-shirt of course. He has a third round of his robotic spacecraft poster series underway. There's a Kickstarter for it that you can get in on at Bit.LY/spacecraft3. That's bit.LY/spacecraft3. If you win this one, then you in a couple of weeks will have your pick of any poster, any of the existing posters from the first two rounds. Or the three new ones, which we can announce will be Pioneer, Juneau, and Viking. Now those won't be ready until October roughly, but you can also pick one from inventory. It's a pretty good selection too. Voyager Mars science, because he's kind of lumped together now, Curiosity, Perseverance and Ingenuity, Sputnik, Mars exploration rovers opportunity and spirit, new horizons, Rosetta with filet, and Galileo. Get those entries in and you might just win yourself a very cool poster. They're beautiful.

Bruce Betts: All right, everybody, go out there, look up at the night sky and think about falling bowling balls and pie. Thank you, and good night.

Mat Kaplan: Cherry pie. That's the absolute favorite of everyone on Miranda. I'm sure you know why. He knows why, he's Bruce Betts, the chief scientist of 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 members who span the solar system and beyond. Join the grand tour at Mark Hilverda and Jason Davis are associate producers. Josh Doyle composed our theme, which is arranged and performed by Pieter Schlosser. Ad Astra.