Planetary Radio • Nov 16, 2022

Voyager Mission Project Scientist Linda Spilker

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Linda Spilker

Voyager Mission Project Scientist at NASA's Jet Propulsion Laboratory

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Sarah Al-Ahmed

Planetary Radio Host and Producer for The Planetary Society

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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

She has been heard on Planetary Radio more than any other guest. Linda Spilker returns in her new role as the Voyager project scientist, following the legendary Ed Stone’s half-century in that job. Linda provides an update on the interstellar journey. The Planetary Society’s LightSail 2 may be only hours from the end of its very successful mission. LightSail Program Manager Bruce Betts makes a special up-front appearance to prepare us for this milestone. And incoming Planetary Radio host Sarah Al-Ahmed introduces The Planetary Academy, a terrific new opportunity for young explorers.

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.
Voyager 2 in the solar wind
Voyager 2 in the solar wind This artist's concept shows the venerable Voyager 2 spacecraft journeying out of the solar system at 15 kilometers per second (34,000 miles per hour) with the solar wind streaming past it four times faster.Image: NASA / GSFC Conceptual Image Lab
Voyager 1 and 2 Outside the Heliosphere
Voyager 1 and 2 Outside the Heliosphere This illustration shows the position of NASA's Voyager 1 and Voyager 2 probes, outside of the heliosphere, a protective bubble created by the Sun that extends well past the orbit of Pluto. Voyager 1 crossed the heliopause, or the edge of the heliosphere, in August 2012. Heading in a different direction, Voyager 2 crossed another part of the heliopause in November 2018.Image: NASA/JPL-Caltech

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Carl Sagan on Voyager 2: "There's only one time in history when this happens." Carl Sagan speaks at a 1990 NASA press conference about Voyager 2.

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Mat Kaplan: The new Voyager mission project scientist is an old friend and you'll hear from her 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. Linda Spilker of NASA's Jet Propulsion Lab has been named only the second project scientist in the more than half century history of Voyager. Our most frequent guest arrives to tell us about how honored she is to succeed the great Ed Stone, and what those distant twin spacecraft are up to in interstellar space. Before we check in with Linda, you'll want to hear our own Bruce Betts prepare us for the end of the planetary society's fabulously successful LightSail 2 mission. It could happen any moment now as the little cube set with the big wings reenters Earth's atmosphere. We'll also hear from Sarah Al-Ahmed about the society's brand new and pretty darn cool program for kids called The Planetary Academy. The latest edition of our free weekly newsletter, The Down Link is waiting for you at But the real elephant in the solar system this week is the Artemis 1 mission. I'm hitting this week's Planetary Radio deadline just a few hours before NASA attempts again to launch that giant space launch system rocket toward the moon. In addition to the uncrewed Orion capsule, it carries a herd of CubeSats, including the near earth asteroid or neo scout solar sail that will be released shortly after liftoff. I'm sorry I'm not at the cape this time for what should be a spectacular night launch if it happens. My colleague Jason Davis has updated his comprehensive Artemis 1 launch guide. You'll find it at Godspeed Artemis. What a long road it was to LightSail 2. I was around for The Planetary Society's attempt to become the first to fly a solar sail. That was our Cosmos 1 back in 2005. That big spacecraft plunged into the sea when its Russian booster failed. We came back 10 years later with a radically different approach. A CubeSat called LightSail that has famously been described as being the size of a loaf of bread. The brief test mission of LightSail 1 was difficult but ultimately successful in showing that we were on the right path. Finally, in 2019, LightSail 2 rocketed into mid earth orbit atop a SpaceX Falcon Heavy. It spread its sails a few weeks later, becoming the first solar sail to maintain and raise its orbit propelled by nothing but the light of the sun. Now, nearly three and a half years later, long after anyone thought it would still be sailing, our little spacecraft, paid for by 50,000 individual donors, is nearing the end. My What's Up colleague Bruce Betts leads the LightSail program. Hey, chief scientist and LightSail program manager. You look kind of tired. Is there a reason for that?

Bruce Betts: I feel kind of tired. That would be the reason. Because our spacecraft, it's burning up in the atmosphere really in the next few days and it's been kind of crazy monitoring that and reporting out on it and the weeping keeps me awake as well.

Mat Kaplan: I'm glad you haven't lost your sense of humor. It's a big deal. Three and a half years, 18,000 orbits, 8 million kilometers, 5 million miles. That's right out of the first sentence of your new article at LightSail 2 is About to Burn Up. And so I refer people there if you want more detail, but give it to us in a nutshell, which actually is one of your subheadings.

Bruce Betts: Well, that was actually reviewing the whole mission in a nutshell. Here, I will give you the burn up in a nutshell. Basically, we always knew that this was going to be the end of the mission. We weren't launched high enough with efficient enough or large enough sail to escape the earth and keep going higher or even stay where we were. There's enough atmosphere. We think of space as being a vacuum, but it's not. Even 700 kilometers up, there are enough atoms, ions that you run into at 30,000 kilometers per hour that it slows you down. And so we gradually, we fought it with solar sailing for a while but then we dropped lower and then solar activity kicked up and that inflated the atmosphere and there was more atmosphere so we dropped lower and it's quite the snowball effect. I mean, we're seeing huge drops now of 20 kilometers a day, whereas usually we were in the tens of meters per day a while ago. But we're seeing these huge drops because as you get lower in the atmosphere, of course, it drags more and then that drags you down faster and you're in denser atmosphere and et cetera, et cetera.

Mat Kaplan: So we are talking on the 15th. People aren't going to hear this until at the earliest the morning of Wednesday the 16th. Is this so imminent that I mean it could have already happened by the time this show is published?

Bruce Betts: It could, but I don't think so. But then I'm not an expert in deorbiting, although I've been learning a lot lately. But it could within the same day that this comes out. The different predictions that are out there from others and from our own predictions kind of say the 16th or the 17th are kind of most probable, which is really impressive considering we're still at over 350 kilometers up, and like space stations at 400 kilometers. But we have a giant area and a little tiny mass and they've got a giant area but a really big mass. So they don't get as affected by drag but even they have to boost every few weeks to stay in orbit.

Mat Kaplan: Please share how you know all this stuff. Where are these numbers coming from? Because it's a great answer.

Bruce Betts: Data on the orbit is coming from the 18th Space Defense Squadron of the US Space Force. They track all the stuff they can in orbit and they put out the information on a lot of this stuff publicly and then they've done predictions and Aerospace Corporation has, and our team has, that are models. But it's tricky because it's hard anytime to predict this, but it's very hard because we have this unusual shape. So no one else has done from that altitude something that's really big and really light. It's hard to get it right, but at least it's close and the more it drops, the more obviously the predictions will improve, so it's imminent.

Mat Kaplan: So I would dare say we and others are still learning from LightSail 2 because this is such a unique reentry.

Bruce Betts: Yeah. We certainly at the very least are contributing fairly unique data point to how something with a sail will deorbit. That's actually useful because a lot of, a lot, a few groups out there, including some of the members of our team working separately from The Planetary Society, are looking at not solar sails but drag sails. So they never tried a solar sail, but at the end of a normal, so to speak, satellite's lifetime, they deploy one of these sails and then it increases the drag and it deorbits much faster. So it's trying to address orbital debris and get stuff out of orbit faster. So we at least will contribute something to that. We're also trying to still get information down on the orientation of this spacecraft and how it's responding to going lower and lower. I'm still hoping that we get at least some thumbnails of additional pictures down. We're running out of time and it's getting trickier to track because the orbit's changing so fast that it's even harder to communicate than usual, which is usually fairly hard.

Mat Kaplan: But you have gotten some really beautiful shots just in the last few days LightSail's been able to send down and I hope people will take a look at those.

Bruce Betts: Yeah. And we've got a new one up in the article and also on the pictures page of LightSail.

Mat Kaplan: Do we need to worry? It's just going to burn up, I hope?

Bruce Betts: I wasted so much of our team's time trying to figure out if we could target you, Mat, but they were like, Bruce, you're an idiot, which they say fairly frequently. Actually, they don't say it. I just think they think it. And so no, we're, one, we live out of the latitude band that LightSail is going in. We're in a 24 degree inclination orbit, which means when it comes through the atmosphere, it will be somewhere between 24 degrees north and 24 degrees south. But also it's small enough that it will completely burn up, vaporize long before anything hits the ground. So helmets not required, at least not for our spacecraft.

Mat Kaplan: Is it possible, I mean you mentioned this in your article, that there might not be a gap in having a solar sail mission underway.

Bruce Betts: Yeah, it's quite the intriguing coincidence that we've been there three and a half years. And Neo Scout, NASA's asteroid exploring solar sail mission that will take the next steps out with solar sailing, we're coming down and they're going up. And so as we've discussed various times, they're trying to launch on SLS as part of Artemis 1. And when they deploy, they will have a little bit of overlap. We'll see. They're trying to launch tonight, again the night of the 15th to the 16th, and we're not trying but are succeeding in coming down in the next few days. So while we'll pass the torch so to speak as we're burning up in a fireball.

Mat Kaplan: Hang in there guy. Les Johnson, by the way, the leader of the Neo Scout mission, will be back on the show very soon.

Bruce Betts: Yeah, that's great.

Mat Kaplan: We will continue to track this. Again, your article is at, easy to find and that's where you can also track almost in real time, right, the actual numbers and see the graphs as LightSail proceeds toward its impending doom. Honorable doom.

Bruce Betts: Honorable doom. Oh it is. It's lasted much longer than we had hoped. Hey Mat? Hasta la vista baby. I'll be back.

Mat Kaplan: He will be back because he's not only the program manager for the LightSail project. He is our LightSail program. He is the chief scientist of The Planetary Society and that means he's going to join us in a few minutes after we hear from Linda Spilker and Sarah who's coming up in just a moment. But Bruce will be back for What's Up. Sarah, I don't think you were fortunate enough to be one of our colleagues at the Society when LightSail was launched, but you certainly have been around for the majority of this three and a half year mission. What has it meant to you?

Sarah Al-Ahmed: LightSail just mechanically as an example of new technologies is really impressive. But I remember the day that LightSail launched. I wasn't with all of you in person, but I was at home watching the broadcast online because I was going to be producing a segment about it for Griffith Observatory's All Space Considered. So watching the way that any Falcon Heavy or any SpaceX rocket relands those boosters is always a great moment. But I remember early on when I was kind of learning about light and relativity and all of the practical applications, I remember reading about solar sailing and about the potential for it to take us to new star systems. I just remember being completely mind blown by it and thinking, how could we ever make that happen? I might be a little old lady by the time that becomes a reality. So when I learned that LightSail was going to be a real thing, and not just that it was a real mission that was getting produced, but that it was backed by 50,000 people around the world as a passion project, it just makes it feel so much more important and like a pivotal moment in history.

Mat Kaplan: So you were there?

Sarah Al-Ahmed: Kind of in spirit.

Mat Kaplan: Yeah. You were there for the beginning at least, and it's so fun to hear. I didn't know Griffith, that you were producing a special segment about LightSail for that. Well, it looks like you're going to be around for the big finish, which who knows, as I said to Bruce, might even happen by the time people hear this program.

Sarah Al-Ahmed: People I know can't see me on the broadcast right now, but I'm actually wearing my LightSail necklace. It was given to me as a gift by my partner for my last birthday because LightSail's been a big part of my life for the last few years. I get excited every time a new image comes down and it's sad that it's going to burn up, but it's just the beginning of a whole new generation of solar sailing spacecraft. So I'm happy that I get to keep this necklace with me so I can just remember LightSail forever.

Mat Kaplan: What a great guy. What a great gift. There is something else that we're excited about at The Planetary Society. It's not an ending, it's a beginning. Tell us about the Planetary Academy.

Sarah Al-Ahmed: This is so exciting because I know that outreach to children and fostering their love of space has been something that our founders wanted for our organizations since the very beginning. It's wonderful to finally be able to launch the Planetary Academy. And for those who haven't been following this effort, the Planetary Academy is our membership program for kids aged nine years or younger. For a lot of people, including me, our love of space science and exploration started when we were just children. So this is an opportunity to get the kids in your life a membership to the Planetary Academy and begin their journey toward understanding more about our place in space. Children who are going to be enrolled in this program are going to receive quarterly adventure packs, and these are going to teach them all about our solar system, our planet, and its relationship with the moon, and then ultimately about the inner and outer solar system, so they have a good picture of what's going on in our place in space and beyond. These adventure packs, at least for the first one, will include a welcome letter from our CEO Bill Nye, so that would be really exciting for kids, I hope. Also, an official membership card for them, and then all kinds of fun little activities, so stickers, activity books, trading cards, games and things like that to really get them involved in the adventure through space.

Mat Kaplan: I have been waiting for something like this for young people. Get them while they're young. Like Bill says, you got to get them interested in science by the time they're 10. And that's what this is all about. It realizes a dream of the society and of Bill's that we have had for a long time. I am so impressed. I have seen the materials. I can't wait to sign up my grandson. And now is the time to do it, I guess, because it's a little bit cheaper right now, right?

Sarah Al-Ahmed: Yeah, we have a special launch introductory price. So anybody who's joining into this program before November 28th will be able to purchase the program at $89 a year. Now, you could still join after that, but if you do, it's going to go up to $99 a year. Still a great price for what we're offering to kids. This program will only be available to children that live in the United States, Canada, and Mexico. We're really hoping that if this program goes well, we can expand it to other countries in the future, but for now it's limited to those countries.

Mat Kaplan: Yeah, we're sorry about that. We would love to expand this across the world. And if it's as successful as I expect it will be, I hope that that will not take too long for us to achieve. Sarah, all they have to do is go to It's right there, it pops right up right now. Thank you very much for the introduction and I look forward to talking again next week.

Sarah Al-Ahmed: Thanks Mat.

Mat Kaplan: You can get to the Planetary Academy page directly at Sarah Al-Ahmed is The Planetary Society's digital community manager. She'll become the host of this show on January 4th, 2023. Our toll-free line is still open to anyone in North America who would like to leave Sarah or me a message. 844-Plan-Rad. And you've got till November 30 to vote for the best space events and more of 2022. Your ballot awaits at I'll be back for a wonderful conversation with Linda Spilker after this message from Q himself, actor, director, writer and producer, John de Lancie.

John de Lancie: Star Trek has always represented the hope for a better future. I don't think you can have that without pushing boundaries. And in the case of space, that is all that we're doing is pushing those boundaries and finding out more, always finding out more. And I think it's really important as a human being, as a society to be able to do something like that. And this is where we do it. 200, 300 years ago we did it on sailing ships across the ocean. Space is important to me because it's kind of a metaphor for risk taking, tremendous rewards, possible rewards, being more expansive in one's thinking and opening one's self up to the infinite possibilities. Probably the biggest thing that differentiates Star Trek from almost everything else is the community in which you enter. Well, The Planetary Society is that type of a community. If you share like me the need to expand into infinite possibilities, as my character does in Star Trek, and as I have said to Picard on more than one occasion, then certainly joining The Planetary Society is a good way to go. Join The Planetary Society.

Mat Kaplan: Linda Spilker, welcome back to Planetary Radio. Our most frequent guest here for one more visit at least while I'm the host and who knows, I hope we'll have more chances to talk in the future. Mostly it's good to see you again. Welcome back.

Linda Spilker: It's a pleasure to be here, Mat. Thank you.

Mat Kaplan: Someone needs to update your JPL bio page. It leaves off with Cassini Project scientist 2010 to the present, which is fine. You got a new job. What does it feel like to be the Voyager Project Scientist?

Linda Spilker: It's a tremendous opportunity. I'm really grateful and happy to have it. Ed Stone retired and I was asked to step in for him. Of course, Ed Stone is truly the heart and soul of Voyager and will continue to be so. He's following the Voyager science. And so I feel as I'm just following in his footsteps, following that dream for Voyager, going into interstellar space and we're just going to go as far as we can go. As long as those two spacecrafts stay healthy and keep working, we're going to keep exploring.

Mat Kaplan: What is it, second star on the right and straight on toward morning?

Linda Spilker: Sounds good.

Mat Kaplan: We had that event that we covered here on Planetary Radio, that wonderful anniversary event, and it was so delightful. I mean you had a great turnout, first of all, so many great people who have served the mission over the years. At that point you had already come back as deputy project scientist of course, and Ed was there as well and it was just delightful that after all these years, he was still project scientist and able to participate in that celebration. It was very gratifying.

Linda Spilker: Yes, it was wonderful just to see all of the people, many of whom I hadn't seen probably for a decade or more come back for Voyager's 45th anniversary. And already we're thinking ahead to what the 50th anniversary might be like.

Mat Kaplan: That's great. There is another connection that I read about, which I hope you can say something about. It's the person that you are training to take on the job of deputy project scientist, the one that you just moved up from, and she has a connection to Ed.

Linda Spilker: That's right. That's right. Her name is Jamie Rankin. She's currently at Princeton University and she was Ed Stone's last graduate student. In fact, she started with him in 2012 just after Voyager 1 had crossed into interstellar space. And then she graduated in 2018 and she worked with the Cosmic Ray team analyzing their data and that was part of her thesis. So I'm very excited. I'd gotten to know her earlier on in the mission and I'm very excited to be able to train her and have her carry Voyager forward because everything goes well, it could be the 2030s until we keep sending back those last data.

Mat Kaplan: Okay, so that's my next question. As I asked Ed every time he was on, what's the health of the spacecraft, how much more juice are we going to get out of those RTGs?

Linda Spilker: Well, right now, Mat, both spacecraft continued to operate successfully and they're sending back data just about every day on interstellar space. Both are now flying in the local interstellar medium, Voyager 1 headed north of the ecliptic, Voyager 2 headed south based on the little nudges they got with their last flybys of the planets. Voyager 1 is currently at 158 AU. If you can imagine, that's the distance between the earth and the sun, so far surpassing its original intention to fly by Neptune, but much, much further than that. And Voyager 2 is at 132 AU, and each one is going a little over, Voyager 2 is going about 3.1 AU per year, Voyager 1 about 3.6. And so they're just gradually getting further and further away from the sun. In fact, it takes about 22 hours one way to communicate with Voyager for the data to come back from Voyager to these deep space network stations to the earth. And if you think about that, that's almost a light day. 22 hours for the data to come back. And so then it's another 22 hours for a signal to travel from Earth back to Voyager. So you have to be patient to communicate with Voyager these days. It just takes a little bit of time. Well, recently, Voyager 1 had some trouble with its attitude control system, and what happened is the attitude control system information suddenly became garbled. The engineering information that tells us the status of that data, we just couldn't make sense of it. But the spacecraft continued to operate normally and send back science data as though nothing were wrong. We just couldn't communicate with it or understand what it was telling us. And through some very careful detective work, we figured out that what had happened is, for a reason we don't know why it happened, it switched over to the backup flight data system computer, which we knew was not working correctly. And so when we commanded it back to the system that was working, all of a sudden, all of the attitude control data suddenly cleared up and the problem was solved. But we're still trying to figure out why it happened. Clearly we don't want that to happen again or something else to happen, but right now, both spacecraft are operating and successfully returning data.

Mat Kaplan: There are so many, I don't want to call them miracles, because this is the work of humanity, but it is near miraculous to think of what is happening here. It also strikes me that this ought to be in the Guinness Book of World Records as the most distant repair job ever accomplished.

Linda Spilker: That's absolutely true, Mat. The most distant spacecraft and the most distant, very careful understanding of what's wrong and be able to fix it with the distances we're at. And if you think about it, those Voyager computers are tiny by comparison to the computers that now fly on the spacecraft or even have in your iPhone.

Mat Kaplan: Yeah, I'm sure my iPhone is vastly smarter than either of the Voyager spacecraft, but much less adventurous. I go back to those RTGs, the radio isotope thermal generators gradually ticking down as they work their way through multiple half-lifes. You talk about into the 2030s, I know you're having to turn stuff off as we lose power. How much longer, what are the estimates of how much longer can we keep things going?

Linda Spilker: Well, it turns out, Mat, that each Voyager is losing about four watts of power per year. And so we've been slowly turning off the redundant systems, and even in the last couple of years turned off the heaters to the particle instruments. They're on the boom that holds the scan platform about halfway out. And so one by one we've turned off the heaters on those instruments and they dropped some 60 or 70 degrees in temperature and yet all of them continue to work.

Mat Kaplan: Yeah, I'm laughing because I remember you saying this, I think, when we last talked at that celebration and how amazed everybody was after all these years, they're working at temperatures far below what they were designed for.

Linda Spilker: Absolutely. And in fact, in some ways the seal to noise has gotten better. Some of these detectors like being colder and so it's been some recalibration effort, but every single one of them, even one of them, the low energy charge park instrument has a little stepper motor that steps it through eight different quadrants to look in different directions. And that little stepper motor is happily still stepping and still sending back data on all of its channels. So it really is an amazing story.

Mat Kaplan: I want to make sure people understand what we're talking about here. Moving parts, a tiny electric motor working in space, radiation, cold, vacuum for over 45 years.

Linda Spilker: It is incredible and that's true across the whole spacecraft, that those systems all keep working. And I know in the era that Voyager was built and the engineers were so proud of what they were doing. And they knew it had to last out to Neptune for Voyager 2. And so if they had a chance to put in a slightly better part or whatever, they would do that. And so just very carefully built up the best spacecraft they could.

Mat Kaplan: The other, at least semi miracle that I want to bring up here is the one we've probably talked about in every conversation that has mentioned Voyager on this program for 20 years, and that is what you said, the deep space network, still talking to these spacecraft on a regular basis, trying to pick out that tiny stream of photons coming from so far away with those dishes here on earth. I mean, hats off to the DSN.

Linda Spilker: Absolutely, Mat, absolutely. At 160 bits per second, the data are not coming back very quickly. Although on Voyager 1, we are still using the tape recorder. The tape is still moving across the heads. We record data from the plasma wave spectrometer just a frame or two a week and then play back those data. And that's at a much higher rate. We can't really control, we are at the lowest rate we can get with that tape recorder. And so we have to start a four and sometimes five stations, the 70 meter with multiple 34 meters to catch that very faint signal from space. And we're hoping out to about 2026 or so to keep getting those data back, because that was one of the more interesting, more recent discoveries from Voyager One, that there's a very faint signal in their data that, actually it was a graduate student, Stella Ochre, figured out how to process the data, pick up that faint signal and provide the plasma density essentially continuously by looking at those recorded frames and very carefully processing them in a new way. And so we're hoping to continue to be able to play back those data for another several years. But slowly we're having to turn things off and get more and more creative about what we turn off next and how we manage the data. And so it's just very interesting time actually to think about how to carefully manage these spacecraft well beyond the warranty much further than they were initially expected to go and to manage that. It's possible Voyager 1 might make it out to 200 AU and that would be 2030s. And it would only probably have two instruments still working. Probably the magnetometer and the plasma waste spectrometer because we will over the next several years have to slowly start turning off the science instruments for their power. We hope maybe another three years or so we can wait, but at some point, that's the next step.

Mat Kaplan: All right. Well, I look forward to celebrating the 55th anniversary, I suppose of the mission. I hope to be around for that just as much as I hope the Voyager spacecraft are. Linda, you stay one step ahead of me. I wanted to go next to what are we continuing to learn from the Voyager that are telling us about this region of interstellar space that we've never visited before?

Linda Spilker: Well, when we get out to now pass the heliopause, we can suddenly start to test some of the ideas and theories we had about interstellar space. That's the boundary where the sun's influence stops. It's basically a balance between the sun, the solar wind pushing outward in interstellar space. And at that boundary, once we crossed it, we're making measurements. And I think one of the biggest surprises is that the sun's influence continues out past that boundary. If there's a big event, a coronal mass ejection, something on the sun, we actually are seeing shocks and pressure ramps that are propagating out quite some distance into interstellar space. We're measuring the energetic particles that are of solar origin, but also of the origin from other stars in interstellar space. The electron densities, they were expected to plateau once we got out across the heliopause, but they haven't. In some cases, they've continued to pile up and increase, maybe piling up ahead of the heliopause. Cosmic rays have both the density and the composition. And what's interesting is the cosmic rays themselves display unexpected periodicities. They were not sure what's out there modulating the cosmic rays. The magnetic field for interstellar space, we were expecting it, once we crossed out of the sun's influence, it would rotate into the interstellar direction. And that just hasn't happened yet. For both Voyagers, it's still predominantly dominated by that solar direction and we're not sure when we'll see that rotation happen and why it's persisting and influenced so much by the sun. And of course there's a lot of questions about the shape of the Heliosphere. Is it like a giant bubble? Does it look more like a comet with a long tail? Is it maybe as some people think twisted like a croissant with two lobes to it? There are lots of ideas and as the Voyager mission continues, we hope to be able to provide more information to the modelers to help better understand the actual shape of the heliosphere. If you think about it, the sun has a heliosphere, but other stars have the equivalent, we call them astrospheres. Studying our own heliosphere, we're now going to be learning more about those other stars. And Voyager's gone from planetary to heliophysics and now into astrophysics. So covering many different types of science along the way.

Mat Kaplan: It occurs to me that if you want to figure out the shape of this gigantic body that surrounds our solar system, it's pretty useful to have two different spacecraft going in two different directions.

Linda Spilker: That's exactly right. That's exactly right. Going above and below the ecliptic, above and below essentially the nose of the heliopause, and making measurements. And we can then compare those and we also use information from other spacecraft inside the heliosphere. New Horizons is one of them. It basically can provide us information about what's coming and what Voyager should expect to see. And so, we have sort of a synergy with the spacecraft that observed the sun and can let us know what we'll be seeing. What's interesting, there's a lag as you would expect, that Voyager 1 right now is seeing information from the solar minimum. And so a few more years it will start to see a change as we head back to the solar maximum as seen from Voyager 1.

Mat Kaplan: Wow. I thought of this question earlier when you said that we're seeing Voyager's able to detect these coronal mass ejections that we see as we observe the sun, but of course we're only one AU away by definition. How long does it take when something happens on the sun for the effect of that to get out that far? I mean, we're not quite talking about the speed of light obviously.

Linda Spilker: Right, right. It's quite a few years to propagate out to the distance of Voyager. And what it does is those shocks interact with the heliopause, and then from the heliopause, there's a shock that's generated from that that propagates into interstellar space. And so in the plasma waste spectrometer data and the magnetic field data, we see the effects of these shocks. And since we now have been looking more at solar minimum, the number there used to be a one per year or so have sort of tapered off and we'll see if they increase again as we start to see information from the sun that starts back up towards solar maximum.

Mat Kaplan: So theorists and those modelers you mentioned, they must be following this data pretty carefully. And I imagine it's affecting our models of the solar system and other solar systems.

Linda Spilker: That's right, Mat. We're getting information to better understand the interactions between the heliosphere and interstellar space. And what's really nice is there's a group, we call them guest investigators. Many of these are early career young scientists that are eagerly coming in and working with the Voyager teams and coming up with the models to try and better understand exactly what's happening in interstellar space. And even to tease out, there's still some questions about what exactly happened in the heliosheath, that region right before the heliopause, and how to put all of the pieces together into a more global model. So it's really wonderful to have these young scientists who are enthusiastic and excited and transferring that knowledge about the Voyager data to the next generation. And for future missions, if we ever have other missions that go out as far as Voyager's going or perhaps even further for interstellar probe mission, it would be really good to have as much information to inform that mission as we can.

Mat Kaplan: How big is the Voyager team now? It can't be more than a fraction of what it was 40 years ago.

Linda Spilker: Oh, Mat, that's absolutely true. On the engineering side, there's maybe between 15 and 20 people. They're not full-time and they're the ones that are sort of working and flying the spacecraft. Each of the Voyager teams is quite small. We have five instruments across, four on one spacecraft, five on the other. And those teams are pretty small, but they're really augmented by these guest investigators, the program that NASA is funding to provide additional resources and scientists to help us understand those data. But it's oh so much smaller than those days of flying by the planets where you just had a big team of scientists and a team of engineers because there was so much going on with both Voyagers.

Mat Kaplan: I'm going to turn away from Voyager, give you a chance to say anything if you would like about the Cassini mission, the one that first got us talking to each other years and years ago. Of course, it has sadly been over now for years, but really we've also talked about the fact that it's really not over. Is there anything that you want to call out that is still ongoing because of what Cassini and Huygens were able to tell us?

Linda Spilker: Yeah, there's a couple of new papers, Mat, that have come out recently from Cassini. One involved the ultraviolet spectrometer. They had a series of occultations of the rings using a star, but they also had about 40 occultations using the sun. And so they've now been able to carefully analyze and look at those and get some information about the particle size distribution of Saturn's rings. When you look in the ultraviolet, you're seeing really some of the very smallest particles. And so trying to understand Saturn's rings and their age and where they might come from, all of that's very important. And especially as we look at other missions, there's going to be a Uranus orbiter with probe flagship class mission. What we can learn about Saturn's rings we can then apply at Uranus and the other planets as well. So that paper just recently came out, a more complete analysis of all of the 41 solar occultations in the ultraviolet. And another one has to do with the ionic neutral mass spectrometer. They made a series of very interesting measurements in those grand finale orbits as we dove in between the planet and the rings. And they found a very complex, surprisingly complex spectrum during those final orbits. And it indicates there's a strong compositional interaction between the rings and the atmosphere of Saturn itself. And they think that maybe a large amount of the signal that they saw was perhaps coming from vaporized ices and organics from the rings that are now flowing into Saturn's atmosphere, and so, a paper came out recently going into more detail and trying to understand from those series of flybys that they took data to try and understand what exactly is happening between Saturn and its rings. And there's such a treasure trove of data on Cassini that there will be people looking at it, I think, for decades, trying to understand and put the pieces together that come up with a more complete picture of what we know about the Saturn system. And a final paper that's very interesting has to do with the detection of a new kind of radiation. They call it the anomalous myriametric radiation. They've nicknamed it SAM, so it's the Saturn anomalous myriametric radiation. And this came from the radio and plasma wave spectrometer onboard Cassini, and they saw a series of these events. It's not like the Saturn telemetric radiation. It's something slightly different. And in very carefully looking at this large suite of data, they found over 190 of these events. They seem to be stronger and more frequent and close to Saturn. And so this paper is just identifying them, pointing them out. And so a lot of work still remains to be done to try and understand exactly the connection to the magnetosphere and what the source of these new SAM, this new SAM radiation is. So those are some of the more recent papers from Cassini.

Mat Kaplan: To clarify, when you say a new type of radiation, it's not like something that they invent for an episode of Star Trek. It's something coming from the planet and we just don't know what the source is yet?

Linda Spilker: That's right. I should just say it's a newly detected type of radiation that they've seen called the Saturn anomalous myriametric radiation. They are trying to characterize its distribution, frequency, duration, bandwidth. Some of these I guess last as much as 11 hours. And just carefully looking through the data, they're able to identify and find these. And it just shows you here, five years after, more than five years after the Cassini mission ended, still we're finding new things in the data and I think that's very exciting.

Mat Kaplan: It is. I appreciate the clarification. First of all, that'll keep my nightmare of some tabloid or awful website saying Saturn scientist says new form of radiation threatens Earth, which it happens. But also, what I like to say, planetary science missions, the gifts that keep on giving. You're still batting a thousand because I wanted to ask you about that potential Uranus orbiter as well, which as we know, we reported on the show, was the top recommendation in the Decadal study from the National Academies for a new mission at least last spring, that we need a mission to the outer planets, which is what I've been hearing from you and other people, including your husband, Tom Spilker, and so many other people for so many years. It sounds like, one, Cassini has played a role in encouraging the academies to make that a top recommendation, and two, that we can achieve it. We did it at Saturn, we can move one more planet out.

Linda Spilker: That's right, Mat. And what's so exciting for me about the Uranus orbiter with probe is that there's only been one spacecraft that's flown by Uranus and Neptune.

Mat Kaplan: Your spacecraft.

Linda Spilker: That's right. That was Voyager 2. And that was back, for Uranus, back in 1986. So it's been a while since we've been out there to take a look at what's going on at this planet. And so it would be an orbiter, similar to Cassini, probably full of all different science instruments, carry an atmospheric probe to actually go in and probe the atmosphere of an ice giant. These planets, Uranus and Neptune, are different from Jupiter and Saturn. And so a chance to really study the system in detail. There are some incredible moons in the Uranus system. Tiny Miranda, that looks like it's been torn apart and thrown back together. And some of the surfaces that look young. They're not heavily cratered, look young like some of the moons in the Saturn and in the Jupiter system. And of course, a very intriguing and interesting ring system. So really be exciting to go back and study Uranus in more detail. And so there's a lot of effort underway to start to identify what are the key science goals that you would have for a mission to go back to Uranus. And from there then you start to talk about what instruments do you want to take and then what kind of a spacecraft do you need to build to carry those instruments out to Uranus? So we have a ways to go, but it's very exciting to think about the fact that there now will be another mission to an outer planet, one that we've only visited a single time.

Mat Kaplan: And I have been told that there are teams already devoted to putting together these mission plans, laying out how it might work at JPL, and at some of the other centers like the Applied Physics Lab, APL, out on the east coast and elsewhere around the country. So as you say, we have a long ways to go, but it's nice to know that at least it's on the drawing boards.

Linda Spilker: I think it'd be great if we had the funds. I mean, that's always the problem. How much money do we have to actually send a pair of spacecraft, similar to what we did with Voyager, and send one to Uranus and one to Neptune?

Mat Kaplan: Oh gosh, that would be-

Linda Spilker: Only if we had enough money. That would be a great pair of missions.

Mat Kaplan: They're very different places. There was something I was going to mention when we were talking about Cassini and I'll bring it up now. It's my curiosity wondering whether if it had not been for Cassini, if we would now be seeing the Dragonfly mission coming together, because we would just not know that much about Titan. Probably, I'm guessing not enough to be able to confidently send an octocopter there to fly around.

Linda Spilker: Mat, I agree. For the Dragonfly mission, what Cassini found, especially with the Huygens probe parachuting to the surface, the combination of the orbiter plus the Huygens probe really provided enough detailed information to put together a mission like Dragonfly. And also some of the science questions that need to be answered. Prior to that, we really pretty much just had the Voyager information and we hadn't been able to pierce through the haze of Titan to even see what the surface looked like, much less understand the weather and the atmosphere in detail. So certainly in this case, starting with the Voyager information, we built the instruments for Cassini, including the radar and neutral mass spectrometer just basically to help study Titan. And now we've got Dragonfly to go back, land in multiple places, and really get down there and study the surface in detail. What an exciting, exciting mission.

Mat Kaplan: Absolutely. And I'm kicking myself now with apologies to our friends at the European Space Agency for not mentioning Huygens once again, because after all, it took us down to the surface of Titan. Linda, I hope that you have many, many more years ahead of you doing this exciting work. But of course, you've been at it for many years as well. And there is a question as we near the end here that I don't think I've ever thought to ask. I'm pretty sure I never asked Ed Stone, or anybody else with the Voyager mission? So I'll ask you now. Star Trek the motion picture came out in 1979. I think you were fairly new on the Voyager mission team in 1979. I got to think that a bunch of you went to see the Star Trek movie and there at the core, the center of the premise of that movie was something called V'Ger. What do you remember? What was the reaction to that?

Linda Spilker: Oh, I absolutely remember that, Mat. It was so exciting. Yeah, I had started on Voyager in 1977. I was fortunate enough to be actually at the pad, watched the launch. And so yes, we went to see the movie. And in fact, what had happened is several of the friends I went with had seen the movie already, and so they knew what was coming. But I didn't. I didn't figure out that V'Ger meant Voyager. And so we get to the end of the movie and here they are walking down and they're, and I'm all, it's Voyager. It's Voyager. Everybody's looking at me and not watching the movie. And it was just so wonderful to think that there on Star Trek, one of my favorite shows growing up, would watch it all the time. There was Voyager 6 completing its mission in a very amazing way, collecting all of this data and all of this information. So that was really a fun, really, really a fun time. Another one with Mimas, with the big crater on its surface, looking so much like the Death Star.

Mat Kaplan: Death Star.

Linda Spilker: On Star Wars and making that comparison and just really the fun times on Voyager for all of those planetary flybys. So happy to be part of that.

Mat Kaplan: I had a very similar experience in the theater. I was there with a bunch of friends, college friends, and V'Ger, V'Ger. And then you see it in the distance and I thought, oh, it's Voyager. Shh, quiet.

Linda Spilker: I know. I did the same thing. I just shouted out, it's Voyager.

Mat Kaplan: Well, we've now ruined the big reveal at the end of the movie. For all its flaw, I'm still a fan of that first Star Trek movie, so I do recommend that people see it. Linda, it has always been, and I hope will continue to be one of my favorite experiences doing this show to be able to talk to you as we have watched these missions develop over a couple of decades. Let's keep it up.

Linda Spilker: Oh, absolutely, Mat. Let's stay in touch. And just want to congratulate you on your retirement and a chance to do what you love the most and continue on and learning and keeping in touch with the missions and the projects as well. So congratulations on your retirement.

Mat Kaplan: Thank you so much Linda. And I will say to you, it means more for somebody working on Voyager than it does for most people. Ad Astra to the stars.

Linda Spilker: Yes, absolutely agree.

Mat Kaplan: As promised at the open of today's show, we are back with the chief scientist of The Planetary Society. It's Bruce Betts. It hasn't come down in the last 20 minutes, has it, since we recorded that first segment?

Bruce Betts: Oh shoot, I didn't check.

Mat Kaplan: Yeah, don't look, don't look. I don't want you to get more depressed. What's up? Other than LightSail 2, Bruce.

Bruce Betts: Nothing. Nothing is up, Mat.

Mat Kaplan: Nothing of significance.

Bruce Betts: Everything's down. Everything is down. All right. Just kidding. There's still all sorts of good stuff to see. Plus you can't really see LightSail 2 very well unless you, oh, never mind, won't be there to see anyway. So anyway, we've got Jupiter and Saturn in the evening sky up after sunset hanging out in the east or south, hanging out high overhead if you're in the southern hemisphere, Jupiter looking super bright, brighter than the brightest star. And Saturn looking kind of yellowish. But we've got reddish Mars. It's just spectacular. It's really approaching the brightness of Jupiter. It'll keep growing in brightness until its closest approach to Earth in their orbits in early December. And right now, if you look at it, it's hanging out between the horns, the tips of the horns of Taurus.

Mat Kaplan: Oh, be careful Mars.

Bruce Betts: It's right in the middle right now, roughly between the much dimmer than Mars stars LNAF and Tanwan. Mars is looking much brighter now than aldebaran the reddish star of Taurus, that it used to be similar in brightness a few months ago. Now it's much brighter. Also kind of near Mars, but a little ways away is the bright star Capella. So a whole Mars party. And I failed to mention that's in the evening within a couple hours after sunset over in the east, or later in the evening, higher up. And then one more thing, we've got a meteor shower. The Leonids, famous for huge meteor storms about every 33 years. And this is not one of those years. The last one was 2001. And so we expect average count shower, 10 to 15 meteors. That's peaking on the night of the 17th and the 18th of November. But also, we'll have extra meteors before and after that date. Moons coming up will interfere with visibility. But as long as you look elsewhere, you'll do okay. But we got the Geminids much a higher average rate coming up in December, so look forward to that. Onto this week in space history. It was this week in 1969 that Apollo 12 successfully landed humans on the moon for the second time.

Mat Kaplan: And think about that. Quite a delay from Apollo 11. And yeah, it went beautifully. Couldn't say that about Apollo 13, of course, but Apollo 12, just a perfect mission.

Bruce Betts: Well, they did have excitement at the launch when they got hit by lightning twice.

Mat Kaplan: Yeah, there's that, but they built that rocket well.

Bruce Betts: Yeah, it worked. They did good stuff. It was a fun time. Speaking of fun times, onto random space fact.

Mat Kaplan: All right, we'll take that for this week. We'll give you a buy.

Bruce Betts: Okay, thank you. So LightSail 2 will have been in space for almost three and a half years. LightSail 1, the test mission, which never attempted to solar sail and was designed to test all of our components and deployment, was launched to a much lower orbit. It was in space for about 25 days. And once they deployed the sail, they only were in space for another roughly seven days, because again, the high drag of having that sail out and the fact that they were in a much, much low, they were in a similar orbit to where we have been the last few days.

Mat Kaplan: Now, that's an interesting random space fact, that right there. Similar orbit at the moment.

Bruce Betts: Yeah, I mean, I exaggerate. They're in a similar pair G altitude. They're in a highly elliptical orbit and we're in a much less much closer to circular orbit.

Mat Kaplan: And it certainly did prepare us for the success of LightSail 2.

Bruce Betts: It did. We learned a lot. We learned a lot of things, which is a nice way of saying a lot of things didn't work, and we came up with ways to try to make sure those things would work or that it was capable of fixing itself if there was an issue on LightSail 2. And LightSail 1 was successful at its basic mission, which was to get all the way through deployment of a sail, and it did. We go onto the trivia question. I ask what former JPL director or directors have won the US National Medal of Science? How'd we do, Mat?

Mat Kaplan: This is interesting. There were a lot of people who only came up with one person. I don't know if anybody came up with two, but apparently there were three.

Bruce Betts: Dun, dun, dun

Mat Kaplan: As we learn in this poem from Gene Moonen in Washington. The first director so selected, Von Carmen was his name. His leadership in aeronautics would garner him acclaim. In the discipline engineering recognized by Gerald Ford, presented to William Pickering, the second director to win this award. And third, for physical sciences, Edward Stone selected here directing the Voyager mission out beyond the heliosphere.

Bruce Betts: That's very nice.

Mat Kaplan: Well done. We got this from Timothy Myers, listener in California. Dr. Von Carmen was the first recipient apparently of the National Medal of Science. That's according to Timothy. Here's our winner. He has been listening for a very long time. And I think, I don't know if my records are correct about this, because they're not perfect, but if I'm right, Ed Lupin in California, this is his first win. Congratulations, Edward. We're going to send you, Edward, a Planetary Society kick asteroid rubber asteroid. Congratulations once again.

Bruce Betts: Congratulations.

Mat Kaplan: We'll close with this. From Dave Fairchild in Kansas, he only salutes one of these recipients. This national Medal of Science, the president clearly can say, we like what you're doing in cool engineering, so thank you for showing the way. When William H Pickering won it, it recognized what he had done, to put an explorer in orbit, his JPL Stellar Home Run. Explorer 1. Yeah.

Bruce Betts: Explorer 1.

Mat Kaplan: First successful US satellite in orbit. Low Earth orbit, just like LightSail 2, getting lower all the time. Sorry. Sorry. Couldn't resist.

Bruce Betts: Hey, congratulations to Ed Stone on his retirement from 50 years as Voyager Project Scientist, and of course to your fabulous guest as the new project scientist.

Mat Kaplan: Thank you. I'll pass that along to Linda.

Bruce Betts: Thank you. So the question for you, in two weeks when we go over this answer, it will be our 20th anniversary Planetary Radio show. Did you know that, Mat?

Mat Kaplan: Is it something special?

Bruce Betts: Well, I don't know. I'm trying to make it special. So apparently, for marriages, you know how they've got all this silly paper, leather, whatever gifts you're supposed to give different anniversaries. Well, 20th anniversary traditional is china. No offense. But I would not give you china. The question for you out there is what would be an appropriate gift for a 20th anniversary of Planetary Radio? Go to We will judge it on however we judge it. Good luck, good hunting, and have fun.

Mat Kaplan: You have until Wednesday, November 23 at 8:00 AM in the morning to get us this answer. And tell us what you think the 20th anniversary of Planetary Radio should be celebrated with were you to give me a gift. I don't know, maybe the gifts you got.

Bruce Betts: Yeah, I mean, I'm not going to give you a gift anyway, so it's really all hypothetical. And sure you're not giving me a gift.

Mat Kaplan: I might have a gift for you on the 23rd. I actually have something in mind.

Bruce Betts: Oh geez.

Mat Kaplan: Now you'll have to stay in suspense.

Bruce Betts: Oh, I have to figure out something. I've got, here's an old can. Do you want that?

Mat Kaplan: Yeah. No, no, no. Give it some more thought. I think we're done.

Bruce Betts: All right. Everybody out there, look up in the night sky and think about fireballs, preferably in Dungeons and Dragons. Thank you and Goodnight.

Mat Kaplan: Or Fireball XL 5, which was my fave. He's Bruce Betts, the chief scientist of The Planetary Society. He joins us every week for What's Up. How could I forget to announce the new contest prize? It's a stunning Voyager t-shirt from our friends at You'll love it. Planetary Radio is produced by The Planetary Society in Pasadena, California. It's made possible by its members like me. Join our voyage by visiting Mark Hilverda and Ray Paoletta are our associate producers. Josh Doyle composed our theme, which is arranged and performed by Pieter Schlosser. Ad astra.