Planetary Radio • Apr 26, 2023

Juice mission liftoff: A new era in icy moon exploration begins

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On This Episode

Olivier witasse portrait

Olivier Witasse

Project Scientist for ESA’s Juice mission

Jack kiraly portrait 2023

Jack Kiraly

Director of Government Relations for The Planetary Society

Casey dreier tps mars

Casey Dreier

Chief of Space Policy for The Planetary Society

Bruce betts portrait hq library

Bruce Betts

Chief Scientist / LightSail Program Manager for The Planetary Society

Sarah al ahmed headshot

Sarah Al-Ahmed

Planetary Radio Host and Producer for The Planetary Society

Join us as we celebrate the successful launch of the European Space Agency's Juice mission! Our special guest, project scientist Olivier Witasse, shares the thrilling details about the launch and the next steps for this groundbreaking mission to explore Jupiter’s icy moons. We'll also recap the inspiring events of last week's Planetary Society Digital Day of Action, where space enthusiasts and advocates united to make an impact on the future of space exploration. Don't miss our resident astronomer, Bruce Betts, as he guides us through the upcoming night sky in What's Up.

Juice at Jupiter
Juice at Jupiter An artists’ impression of ESA’s JUICE spacecraft at Jupiter.Image: ESA / NASA / DLR
Ganymede from Juno 2021
Ganymede from Juno 2021 An image of Ganymede obtained by the JunoCam imager during Juno’s June 7, 2021, flyby of Jupiter's icy moon.Image: NASA/JPL-Caltech/SwRI/MSSS
Europa NASA's Voyager 2 spacecraft captured the images used to make this high-resolution mosaic of Europa in 1979. The mosaic has been rotated 90 degrees clockwise from its original orientation.Image: NASA/Ted Stryk/Edited by The Planetary Society
Global Callisto in Color
Global Callisto in Color The picture, taken in May 2001, is the only complete global color image of Callisto obtained by NASA's Galileo spacecraft. Callisto's surface is uniformly cratered but is not uniform in color or brightness. Scientists believe the brighter areas are mainly ice and the darker areas are highly eroded, ice-poor material.Image: NASA/JPL/DLR

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Sarah Al-Ahmed: Juice launches to Jupiter, this week on Planetary Radio. I'm Sarah Al-Ahmed of The Planetary Society with more of the human adventure across our solar system and beyond. If you like the suspense of rocket launches, last week was a blast. We're celebrating the successful launch of the European Space Agency's Juice mission, with project scientist Olivier Witasse. He'll tell us about the launch and the next steps for the mission as it makes its way to Jupiter and its icy moons. We'll also take a look back at last week's Planetary Society Digital Day of Action where space lovers and advocates joined forces to make an impact on the future of space exploration. And as always, we'll turn to our resident astronomer, Bruce Betts, for What's Up and a guide to this week's night sky. We've got to start our space news this week with the Starship. No way of sugarcoating this one, SpaceX's Starship exploded during its first orbital test launch. What can we say? Space is hard. The company's super heavy booster and Starship spacecraft blasted off on April 20th from the Starbase launch facility in Boca Chica, Texas. It self-destructed several minutes after taking off, in what SpaceX described as a rapid unscheduled disassembly. The Starship is the most powerful rocket ever tested. It might take a couple of attempts to get it right, and even though the rocket exploded, the test still represents a step forward in the company's ambition to send humans to the moon and Mars aboard Starship. And here's a fun one. China has selected an asteroid for a deflection test. At the recent Biennial Planetary Defense Conference, a representative of the mission announced that the near Earth object 2019VL5 will be the target of a dual spacecraft asteroid deflection and observation test, which is planned to launch in 2025. Much like NASA's DART mission, that purposefully crashed into an asteroid last year, the new Chinese spacecraft will smash into a roughly 30 meter, or 100 foot asteroid, to attempt to alter its velocity. Another spacecraft will observe the impact and evaluate its effect on the asteroid. The more planetary defense missions we have, the better. We'd also like to send a huge congratulations to Planetary Society Board member Britney Schmidt. As a planetary scientist, Britney does research in Antarctica for conditions that are analogous to frozen deserts on worlds like Mars. Time Magazine named her one of the 100 most influential people of 2023 because of her research on an enormous glacier that's melting due to climate change. Congratulations Brittany, and thanks for fighting the good fight for planet Earth. You can learn more about these and other stories in the April 21st edition of The Downlink, our weekly newsletter. You can read it or subscribe to have it sent to your inbox for free every Friday at On Tuesday, April 18th, The Planetary Society held its Digital Day of Action. Here's Casey Dreier, our Chief of Space Policy, and Jack Kiraly, our Director of Government Relations, with the details. Thanks for joining me, Casey and Jack.

Casey Dreier: Great to be here, Sarah.

Jack Kiraly: Great to be here.

Sarah Al-Ahmed: This past week on April 18th, we held our Digital Day of Action here at The Planetary Society. I had a great time doing it, but Casey, you've been the main organizer of this event since its inception, really. What is our Day of Action and why are these events so important to what we do as an organization?

Casey Dreier: Our Day of Action started as the in-person opportunity to go and advocate directly to members of Congress and their staff in Washington DC, and we had really gotten into a nice groove with those. We were getting hundreds of people showing up, really positive experiences in Washington DC, and then of course COVID hit. So we couldn't do those in person. And the last few years we'd done what I called a Virtual Day of Action, where we had organized the same level of meetings, person to person via computer, doing the same type of activity. This year, 2023, we're back. We have an in-person day of action, but it's in September. But we still wanted to retain an opportunity for people to participate in an advocacy opportunity, and a Day of Action from wherever they live. We heard a lot of feedback from our members that either financially or physically, or any other number of reasons, that it's very difficult to get to Washington DC, which I totally understand. It's across the country for me. And we wanted to retain an option for this. And so we called this our first Digital Day of Action. It's a slightly lower lift commitment. We're not organizing one-to-one meetings, but we're asking people to all act together. So we had a number of very specific letters to Congress that people could sign. Jack and I gave a prep rally, which is his term, which I really love. And we talked about why advocacy works, what advocacy we're trying to do. We had special guests from the Veritas mission talking about the potential that they can do at Venus if they get this mission back on track. And of course, special words from our president, Bethany Ehlmann, a professional planetary scientist and PI-ed her own planetary mission. It was a really great opportunity and really the first of this, what I see now as a one two day of action punch every year. Starting next year, we reverse these. In the fall of next year, we'll do our next digital day of action where anyone can participate, and we go back to spring in-person day of actions next year. So it's just we're expanding the opportunities for people to participate.

Sarah Al-Ahmed: And Jack, you've participated in our in-person Day of Action many times before as a volunteer. This is your first time doing this as an employee of The Planetary Society, and your first digital version of this, what was your experience like during the event this year?

Jack Kiraly: It's a lot of firsts all around. So for many years I did the in-person Day of Action. Now I had the benefit of downtown DC was only about four or five Metro stops away. Still a bit of a harrowing adventure if you've ever taken the DC Metro, but it was a fantastic exercise in showing just how great our community is. And we did it in our member community, quick plug for the member community if you're not already in it. And it was a great opportunity for people from all over the world to come together to celebrate space exploration, and to advance our goals as an organization. The Day of Action historically has been a very US-centric event, and this one incorporated some opportunities for our international partners, and our members all around the globe to take part in advocating for Veritas and Mars Sample Return and all the other missions that we have that are priorities for us.

Casey Dreier: We always try to find opportunities for our international partners to participate, and we've taken specific actions either in support of Hera, and European Space Agency, or Canadian space policy. So this is an extension. We're really trying to work this in so everybody has a chance to participate together, which is the societal aspect of The Planetary Society.

Sarah Al-Ahmed: And that brings up a great question, which was one that you had to address during the prep rally. We have members all around the world who want to get into space advocacy, but our Day of Action focuses primarily on NASA, advocating for their budget and their missions. Why is that such an effective way for us to be using our advocacy time to advance space science and exploration?

Casey Dreier: Well, there's a couple practical reasons. The majority of our members are in the United States. NASA is the largest space program, and through its international partnerships, it really sets the agenda for a lot of other nations. NASA also has, through the particulars of the US Democratic system, a lot of very direct opportunities for citizens to weigh in, whether through their representatives, or to NASA itself. The system is designed for very direct public input. It's a bit more complicated and a few steps removed in a lot of other parliamentary systems, or the US, or the European Space Agency, which is itself comprised of various space agencies representing their various national interests. So it's very direct in that sense. But one thing I always like to say to our members, and we've done this in the past, if you see opportunities for rallying members of your country or members of your... If you're an ESA, and you see an opportunity within ESA for public input, let us know. And this is the real value of, Jack and I are just two people focusing on the US space policy primarily. But when we see opportunities other places, and we trust our members will bring those to us, we can act on those, and we have in the past.

Sarah Al-Ahmed: And I feel like it's a lot easier for our members to communicate that to us now because now we have our member community app. I loved seeing everyone in that community getting engaged in our Day of Action. It was no longer just spaced out across social media. It was all in one place together with all of us celebrating. How many people actually joined in during our member community event?

Casey Dreier: I think we had 130 active people watching.

Sarah Al-Ahmed: So cool.

Casey Dreier: And that's pretty good for advocacy. And I'd say one thing when Jack and I are very happy, we hit our goal during the Day of Action of more than 1000 individual actions supporting Veritas, which is I think very, very good for a Venus mission that is still in the early development stages. That is a relatively modest mission in terms of size and scope. It's just it punches above its weight. We're just very excited to see so much traction happening with this. And this is on top of the letter to Congress that we have, others like the American Geophysical Union signing on, other top universities around the country, and we're really getting the word out. And this is something that we've actively been really putting a lot of time in, and I should say Jack in particular has been putting a lot of time in, wearing out his shoes in Capitol Hill.

Sarah Al-Ahmed: How's that been, Jack? I know you've been going literally office by office, trying to talk to people.

Jack Kiraly: Yeah, and I'll be doing the same thing this coming Friday. So I guess past Friday when this episode airs. I'm going to be on Capitol Hill again with another member of the science team from Veritas. It's been a fantastic experience. We've gotten a lot of warm reception from folks on Capitol Hill, because they understand the importance of following through on the decadal priorities, as part of the Planetary Science Decadal Survey. Veritas is one of the many missions that are expressly endorsed in that document. And then outside of that, this is the first US-led mission to Venus since Magellan, which decommissioned in 1994. So there is a national prestige element to this, as well as the scientific benefit that we're going to get. So the welcome that we've received has been warm, and we are just pressing the pavement, making sure that the message is heard. There's a lot of competing interests in Washington right now. If you turn on the news, you can see some of them. So we're just making sure that planetary science, and it being really a very aspirational part of what the federal government does, and something that basically every member of Congress can agree to, that this is an opportunity for cohesion on Capitol Hill to support a very important mission that's going to be bringing back very important science.

Sarah Al-Ahmed: I think one of my favorite things about the Day of Action is that we're giving people an easy place to start when they want to learn how to advocate for what matters to them. No one wants to argue against us exploring space. That's a good feel-good opportunity. But I've heard many times from people that participate in this event that this was their beginning point, this was their happy way to get into advocacy, and it taught them how to do more in the future. Have you guys heard that same sentiment?

Jack Kiraly: Yeah, we have.

Casey Dreier: Absolutely. And that's by design, and it's always really encouraging to hear that. My goal here at The Society has always been to lower the activation energy of turning space fans into space advocates. And the whole system can be very intimidating. And particularly if you're walking through the Halls of Congress, which is this ornate 19th century building, and you're wearing a suit, and you're saying, "Who am I to be doing this, or telling anybody anything?" But the point is you are you, this is, the system is designed to listen to you. And when we schedule your meetings, we give a ton of training online and in person, lots of background. You are walking in very prepared. And we've heard that from a lot of people who just come in, they say they felt super confident, they're going in with other people, so they have a team supporting them. And then at the end of the day, a lot of people, they just feel better about the democratic system that we have, which is just a great bonus, really. That we feel more confident about our whole representative democracy. And it's a really wonderful way to engage in that system. And we do try very hard to say, "You are doing this for the first time. Congratulations, welcome, come on in. The water is warm."

Jack Kiraly: And I will say in the initial feedback that we've gotten from the folks that participated in the Digital Day of Action, two thirds of them, this was their first Day of Action event. So obviously this is digital, and people could join from anywhere, and people did from all over the globe. And we are really looking forward to on September 18th, 17th is the training day, 18th is the day on the Hill. We really look forward to welcoming any number of new space advocates to our ranks.

Casey Dreier: Can we plug that Sarah? Can we plug the registration for the Day of Action, which is live now? You can register for it There, I did the plug.

Sarah Al-Ahmed: Yeah, you can absolutely sign up for our Day of Action later on in September at Or if you're sad that you missed our Day of Action, our Digital Day of Action this last week, there is still time. All of our assets for advocating for these missions are still available on our website. So you can go check out all of our advocacy actions at

Casey Dreier: That's it. Always an opportunity to be an advocate for space.

Jack Kiraly: And this year in particular, marathon, not a sprint. We need to keep up that pressure, and there's a number of exciting missions we really need to get their funding for this year, and really looking forward to securing that with your help, and the help of the members of The Planetary Society.

Sarah Al-Ahmed: Well, thanks for joining me, Casey and Jack. I'm so grateful that we do events like this, and I can see it working. Every moment we all band together to try to advocate for what we care about, it always makes the world a better place. So thanks for being that happy starting place for so many people to get into space advocacy.

Casey Dreier: Thanks, Sarah. And I have to say it works for us too. It just, maybe speak for Jack here, but just seeing all the people engage and take action, and give that feedback, very heartwarming, very inspirational for us. It really energizes us, too, to get out there and keep doing this every day. So I just want to give thanks to everybody who participated, and everyone who, listening to this, will be so inspired, will participate in the future, when they take actions online or join us in September. I'll just thank you in advance, because it's very inspiring to see you all do this.

Jack Kiraly: I'm going to ditto that. It was very heartwarming to see so many people coming together from across the entire planet to advocate and support the space program. So thank you, thank you, thank you, and I look forward to seeing as many of you as possible in September.

Sarah Al-Ahmed: The European Space Agency's Jupiter Icy Moons Explorer Mission, or Juice, launched from French Guiana on April 14th. This begins the mission's eight year journey to Jupiter, where it will study the moons Europa, Callisto, and Ganymede. En route to the Jovian system the spacecraft will perform the first ever Earth-moon dual gravity assist flybys in August 2024, followed by a Venus flyby, and then two more Earth flybys before reaching Jupiter in 2031. Jupiter's three largest icy moons, Europa, Ganymede, and Callisto, may host liquid water oceans beneath their icy crusts. This has prompted decades of speculation about these moons, how they formed and evolved over time, how they interact with Jupiter, and about the potential for life on these worlds. ESA's Juice mission aims to investigate their habitability, and expand our understanding of potentially life harboring locations in the universe. Dr. Olivier Witasse is a planetary scientist and a project scientist for the Juice mission. He joined the European Space Agency in 2003, and has worked on a number of missions, including the Huygens probe to Titan, Venus Express, Chandrayaan-1, Mars Express, and the ExoMars Trace Gas Orbiter. That's an impressive resume. In 2015, he joined the Juice Mission team and turned his sights to Jupiter. Hi, Olivier.

Olivier Witasse: Hi. Hello Sarah. How are you doing?

Sarah Al-Ahmed: I'm doing really well, and it's been a really exciting week. I want to congratulate you and everyone on your team for the successful launch of the Juice mission.

Olivier Witasse: Yeah, thank you very much. It was a big week last week, and that's the start of the new phase, and the start of the journey to Jupiter.

Sarah Al-Ahmed: How are you and the rest of the team feeling? Did you celebrate afterwards?

Olivier Witasse: Yes, of course, of course. We had a few celebrations. I was in our operation center in Germany, so people were split between our operation center in Germany, and of course the main site in Kourou, in French Guiana. So on both sides there were a lot of celebration, and in the operation center now people are working very hard for the first phase of the mission. So I can relax a little bit from my side, but there are people who are still working very hard on this project.

Sarah Al-Ahmed: I know that you guys had about a one second launch window, in order to achieve the correct orbit. What's the status of the mission? Is everything on track right now?

Olivier Witasse: As you have seen, so we launched with one day delay. There was some bad weather and lightning activity at the time of the launch on the 13th of April, so we postponed to the 14th. It launched on time. I mean we had one second to launch, so we did it, and then the launch sequence went absolutely perfectly. All the parameters were nominal, I mean everything was on schedule. So the separation, the injection to space, I mean it couldn't have been better than that.

Sarah Al-Ahmed: It's good to hear.

Olivier Witasse: The James Webb Telescope, the injection was perfect. There was a lot of discussion about that. So for Juice, it's the same. So we don't need any correction maneuver in the next week, so that's good. Then after the separation, there was the acquisition of the first signal from Juice. Here, there was a little bit of delay, so people got a bit nervous, but then it went well, and the deployment of the solar panels happened a little bit earlier than expected. So all in all, it went very, very well.

Sarah Al-Ahmed: That's perfect, because I know just like the James Webb Space Telescope, making sure that you do the perfect launch and that you don't have to use that fuel means that you can do a whole lot more when you get to your target, which is perfect, because you're going to need that fuel to get this mission into orbit around Ganymede at some point.

Olivier Witasse: Now we need a lot of fuel, in fact, more than half of the spacecraft mass is fuel because we have a lot of maneuvers. We have a few maneuvers during the cruise phase. Then we have a big maneuver when we are at Jupiter. Another major up size maneuver when we arrive at Ganymede, to enter an orbit around Ganymede. And in between, we have 34 flybys of the moons of Jupiter, where every time we do a flyby, we need a little bit of fuel to correct if needed. So we need a lot of propellant, so it's good that the launch was perfect. Then we can keep a little bit of margins for the future milestones.

Sarah Al-Ahmed: And for people who are just now learning about this Juice mission, what is this mission going to do and why is it so important?

Olivier Witasse: Everything is in the title. Juice means Jupiter Icy Moons Explorer. So that means we'll explore Jupiter and the icy moons, which are Europa, Callisto, and Ganymede. And one of the big question for our mission is to understand whether there are habitable places within the icy moons of Jupiter, around a planet like Jupiter. The main question is to explore then the liquid water. Because when we talk about habitability, the first thing to check is the presence of liquid water. And there is liquid water inside the icy moon. It's strange to think about it, but underneath the surface of Europa, Ganymede, and maybe Callisto, there is more liquid water than on Earth, and Juice will explore that. And because we want to understand habitable places around Jupiter, we will explore Jupiter as well. So the atmosphere, the magnetosphere, and also the full system. So the other moons. Io, very interesting moon, the dust and how everything is connected to each other. For example, how the moons are connected to Jupiter via the gravitational force. As a result there is tides on the surfaces of the moons. So we'll explore that. And they're also linked to Jupiter via the magnetic field line. So there is a lot of magnetic things to discuss, to explore. So it's very a rich mission, and very broad, and yeah, we are very excited by it.

Sarah Al-Ahmed: And this is the first dedicated Jupiter mission by the European Space Agency, right? What sparked the creation of this mission?

Olivier Witasse: Yeah, so that's the first time we go to Jupiter. So that's a very challenging mission. It's one of our biggest mission in ESA, at least in the solar system exploration, as a follow-up of the Rosetta mission, for example, this kind of big mission. So a lot of challenges. First we have to have a mission which last 10 to 15 years. So there is a lifetime. We go to Jupiter, which is a very hard radiation environment, very difficult to resist for the electronics, the spacecraft, the instrument. We go there where it's cold around Jupiter, while doing the cruise to Jupiter, we go via Venus. Venus, it's a warm environment. So we have to design a spacecraft which can work in the cold environment, in the warm environment. Plus we'll do a lot of very sensitive measurement of electric and magnetic field. That's one of the measurement, very useful to detect the liquid water underneath the surfaces. And then we need to have a very clean spacecraft from the electromagnetic point of view, we don't want to measure what is coming out of the spacecraft. So the design was extremely challenging for this. And because we are far from the sun, little power, little, little solar illumination. That means we had to embark huge solar panels. So if you see how the spacecraft look like, you see the big solar panel with a cross-shaped, 85 square meter of solar panels. So huge. So we need to find the right solar panels which can work in the cold environment, with low illumination conditions, in the radiation environment. So all in all it's a very challenging mission.

Sarah Al-Ahmed: Yeah. With NASA's Juno mission, they put it on an orbit that took it very far away from Jupiter and then back in, and the readings of the radiation coming off this thing are unreal. So how is the spacecraft grappling with that level of radiation?

Olivier Witasse: That's one of the big issue of any mission to Jupiter. So that means you have to check what you can do. That depends on your science objective. For example, the general science objectives are to study Jupiter in detail, the interior, the atmosphere, the gravity field, the magnetic field. So they have a polar orbit, and they go very, very close to Jupiter, and then they go very, very far, not to be in the radiation belt all the time. So they designed the trajectory depending on their science goal. For Juice, we are mainly interested in the icy moon, so we need to orbit in the equatorial plane of Jupiter. So a different orbit than Juno, because it's a polar orbi. Juice is an equatorial orbit because we want to fly by the moon. So we need to stay in this equatorial plane where there is a lot of radiation environment. So we need to stay at reasonable distance from Jupiter. So that's why we don't go to Io, because Io is very close. We go only twice to Europa, because also Europa is very interesting, but relatively close. And we focus on Ganymede, because it's at a reasonable distance from Jupiter. Ganymede is a very unique moon, very, very interesting, so we made Ganymede as our prime target. So the first thing to cope with at the radiation environment is to make sure that your trajectory is fine with that. So not too close to Jupiter, for example. And then in the design of the spacecraft, we protect the very sensitive electronics of the instrument, and the spacecraft inside the satellite in the core, in the middle of the satellite, in what we call vault. So we have two big cavities in which we put all the sensitive element of Juice, so such they are protected from the radiation environment. And we have more than one hundreds of shielding, a bit everywhere in the spacecraft, to shield the particular sensitive area. And on the solar panels, which are outside the spacecraft, we put a layer of cover glass, to protect, again, from the radiation environment. So you see we have thought a lot about the design, and that was the main requirement is to protect the spacecraft as much as possible from the radiation environment.

Sarah Al-Ahmed: And you need to keep all of those scientific instruments able to do their job, and there's a lot of them on the spacecraft. What are the most important instruments on board Juice to help it do its mission?

Olivier Witasse: Well we have 10 instruments, and they have been selected at the same time of course, and each of them to achieve specific objectives of the mission. And they have been selected such that they can work also together to address all the big questions of Juice. So there is no one instrument particularly more important than the others. They're all very important because they have selected all together to fit the science objective of the Juice mission. We have three big packages. So we have the remote sensing instruments, so the eyes of Juice, so all the cameras, the spectrometers. So we have four of them, because we want to take images and to study the geology of the moons and the atmosphere of Jupiter. And then we have spectrometers covering radio, near infrared, visible, and UV, for the atmospheres and for the surface. So that's how Juice will see everything around the spacecraft. Then we have a package which is called geophysics. That's a very interesting one, because that's the first time we fly this kind of instrument to the moons of Jupiter. Here we have a radar to study the first 10 kilometers of the crust, so to penetrate the ice. So we'll see the first 10 kilometers, how is the structure? So that's the first time we will do that. And we have also a laser altimeter, which is very, very interesting, because we'll measure the topography of the moon. So we send laser shots, and then from that we can see the topography. So if you have small hills, craters, and so on. But what is very interesting is we will come back to the same point many, many times, and we will see the tides of the moon. So how the height of the moon change with time. And that is a way to study the interior, and the liquid water. We have a radio science experiment to study the gravity field, and then we have what we can call the north of the spacecraft, is institute measurement of particles, electric field, magnetic field, radio wave, to study in situ what is around the spacecraft, in terms of particles and in particular for the magnetosphere. And that's [inaudible 00:26:31], so to detect shifts of exertions. Each of them are very useful, and they will be all providing a small piece of the puzzle.

Sarah Al-Ahmed: I'm really excited to see what it can tell us about whether or not there actually are in fact subsurface oceans on these bodies. And the way that particularly Ganymede interacts with the magnetic field of Jupiter is so wacky, that I cannot wait to learn more about this. And as I was learning more about these instruments, I learned, too, that there's an experiment which is called the Planetary Radio Interferometry and Doppler Experiment, or PRIDE, and as the host of Planetary Radio, that made me very happy.

Olivier Witasse: Yeah, this one I did mention because it's not an instrument, it's an investigation. So there is no hardware on the spacecraft. They will use the communication system of the spacecraft. So how we transmit the data to the Earth, they will listen to big radio telescopes everywhere around the Earth. And then they will do a special measurement to detect the speed of the spacecraft, and more importantly the position of the spacecraft in the plane of the sky. And then we'll get very interesting information, for example, on the positioning of the moons in the Jupiter system, so their orbits, and that's an interesting topic for Juice as well.

Sarah Al-Ahmed: Yeah, it'd be very useful to be able to verify the positions and movements of these spacecraft from afar from Earth. I've been so excited about this for so long. Jupiter has what, 92 moons I think at this point? But these ones have the potential for subsurface oceans, and therefore the potential for being habitable. Juice isn't going to be directly detecting life, but what can it teach us about the potential for habitability on these moons?

Olivier Witasse: Well first is to... It's to really confirm that there are liquid water. We are pretty sure there is liquid water at Europa, relatively sure that there is liquid water on Ganymede. Callisto, there is a question mark. So Callisto is also quite interesting. And the first thing when you want to discuss habitability is to really understand the properties of liquid water. So because we don't know where it is, so at which depth, or we have some ideas, of course, some indication. But for example, the subsurface ocean at Ganymede could be at 100 kilometers underneath the surface. But it can be 110, 100, 150. So we need to know. It's important. We don't know exactly the depth of those oceans. So is it 100 kilometers, 50, 80? We need to know. Because we want to know the amount of water that you have there. And also the composition. We know they are salty because we detect them with the magnetometer. So we know they are salty. That's an interesting piece of physics and detection, by the way. But we don't know how much salt do they have there, and the composition is quite important to characterize liquid water, is it an interesting water for life, et cetera. We'll also be studying the radiation environment because it's good to know the radiation environment. I mean, on Earth we are happy to have the magnetic field, then we have less radiation coming from the sun. So what is the case at Europa when there is no internal magnetic field, and Europa is close to Jupiter? What is the case at Ganymede, which is a bit further away, I mean much further away, with an internal magnetic field? That's the only moon to generate its own magnetic field. So very, very special. So what is the role of this magnetic field? Does that help to protect, not at all? How this interaction between the magnetic field of Ganymede and the magnetic field of Jupiter, and what is the case at Callisto? The moon, which is much further away, with the Galilean moons. So in principle it's better for the radiation environment at Callisto, but at the same time the moon is far from Jupiter, so the tidal activity is very weak. The moon has probably not evolved since its formation. When you look at the surface of Callisto, it's plenty of craters. So that means the moon, the surface is very young, probably there is not much geological activity. So is there a liquid water underneath the surface, or not? That's an interesting question, and the finding, either yes or no, will be interesting. And then you can compare those three moons. So Europa, which is active with the possible geysers, very interesting ocean, but close to Jupiter. Then you have on the other hand, you have Callisto, which is dead. We don't know if there is an ocean. In principle it's not very interesting for habitability and life, but who knows? And then you have Ganymede in between, so a big question mark. So that will be very interesting, to know more about those three moons and then to compare them, and to understand better the question of habitability, and whether around Jupiter there is interesting place to study life. And then to study life, we need another mission.

Sarah Al-Ahmed: Yeah, well thankfully we have NASA's Europa Clipper mission coming up for Europa. But you make a great point, which is that Europa doesn't have a global magnetic field, and that might be necessary, I mean depending on the thickness of the ice. That's really fascinating. I'm really excited too, about the potential that we might be getting pictures of geysers on these worlds. We do have evidence that that might be happening on Europa, but who knows about Ganymede? Those images might be mind blowing.

Olivier Witasse: Exactly. Exactly, exactly. We will see when we are even at Europa, the situation is not very clear for the geysers. So we'll see a little bit with Juice, mainly with Europa Clipper. We are very happy that there will be two spacecraft in the Jupiter system at the same time. By the way, we already started to collaborate with the two science teams, so that's great. Then yes, maybe there will be some activity that we detect at Ganymede. We will see. Yeah, no, it's very, very exciting.

Sarah Al-Ahmed: We'll be right back with the rest of my interview with Olivier Witasse, after a short message.

Casey Dreier: Hi, it's Casey Dreier. I'm the Chief of Space Policy here at The Planetary Society, and I just want to take a moment to thank you for being part of the world's largest and most influential nonprofit space organization. Together we are on a remarkable journey, to make a better future for humankind through space science and exploration. Your support in the United States enables our team, my team, to work to make sure every US Representative and Senator in our Congress understands why space exploration is a critical part of US national policy, from workforce technology, to science, to even international relations. Each year, we urge Congress to maintain robust funding for NASA's ongoing missions, guaranteeing that they stay on course and achieve their scientific goals. Now look, there are 81 new faces in Congress and several returning members that are joining space related committees for the first time. All of these could influence NASA's future endeavors, and already they have begun to discuss next year's NASA budget. The time to act is right now. So in addition to writing Congress yourself, which you can do if you live in the United States, there's something you can do to bolster our advocacy work. And anyone can do this, anywhere in the world, and that's to make a donation. Thanks to the generosity of a fellow Planetary Society member, your gift today will be matched up to $75,000. This ensures our advocacy efforts will continue to thrive. Now, to make your contribution, you can visit On behalf of everyone at The Planetary Society, thank you for your support and helping us champion the future exploration of space.

Sarah Al-Ahmed: Juice is arriving in 2031, right? Am I remembering correctly that Europa Clipper is arriving in 2030?

Olivier Witasse: Yeah, yeah, there are a few months before us if they launch next year. So that means there will be an overlap of maybe two, three, four years between the two missions. So that's great. Yeah, yeah, that's absolutely great.

Sarah Al-Ahmed: And that's so exciting. Something that I learned recently when I was learning more about recent results from Juno is that Ganymede's magnetic field is actually creating an interaction with Jupiter's magnetic field that is causing aurora, that trail behind this moon. Is there anything that we can do to study those auroras in this interaction with Juice?

Olivier Witasse: So in fact there are two types of aurora. So the one at Ganymede, due to the magnetic field of Ganymede, we have imaged that with the Hubble Space Telescope, and with other ground-based observations of observatories. Those auroras on Ganymede, you can use them to study the interaction between the magnetic field of Ganymede, and the magnetic field of Jupiter. But also you can use them to study the liquid water ocean underneath the Ganymede surface, and that's a very interesting piece of physics. It's because the oval on Ganymede, so we have the ovals like on Earth, this oval oscillates with time. So there is a small oscillation of this aurora rate, and the level of these oscillations depend if there is a liquid water ocean underneath the surface. Because the ocean includes salt, and then this ocean generate electric currents, and those currents disturb the magnetic field around Ganymede. It's quite complex. We call it an induced magnetic field. And then this induced magnetic field competes with the magnetic field of Jupiter. That's the physics of the magnetic field. And that means the auroras that they are generated because of the magnetic field of Ganymede, they could see the presence of this liquid water ocean. So to study this light, you can get something on the entire of Ganymede, so it's quite interesting. But then because Ganymede is connected to Jupiter with the magnetic field line, so there is an invisible connection between Jupiter and the moon, and the charge particle, they can travel via this magnetic field line. Because the charged particle, they like the magnetic field, they follow the magnetic field line, and then those particles can impact the atmosphere of Jupiter around the poles of Jupiter, because the connection to this field line occur at the pole. So when you look at the poles and the auroras of Jupiter, you not only see the normal auroras at Jupiter, but you see also the auroras due to the moons, we call that the footprint of the moon. So you see that for Io, for Europa, for Ganymede, maybe Callisto, here the situation is less clear. So you see the situation is quite complex, but at the end it will be quite interesting to study.

Sarah Al-Ahmed: You said there's a spectrometer on board. What can that teach us about the potential for habitability here? Are there certain elements the spacecraft might be looking for?

Olivier Witasse: Yeah, so we have a near infrared visible spectrometer, which it's a very useful instrument to measure the composition of what you observe. Because you take images, and then at the same time, with the spectrum in the visible infrared, you get the composition. So if you look at the surface of the moon, you can get information on the composition. For example, different ices, different molecules, organics. And there are many molecules we have not identified. For example, on the surface of Europa, this orange pattern on the surface, it's not very clear what it is, and it can be organic molecules. So if we detect molecules which are interesting for as a biomarker, or that interesting molecule for as a building block for life, that will be important to identify to detect. So we'll do that at Europa, Ganymede, and Callisto, and then we'll see what we get. With the spectrometer, you can also study the thin atmosphere of the moons. We call that exosphere, it's a very small atmosphere, composed of water vapor, oxygen, a bit of CO2. And then if you study the atmosphere, it's also interesting because what is in the atmosphere is partly due to what is on the surface, because there is some interaction between the atmosphere and the surface. So yes, the spectrometers will be used for this.

Sarah Al-Ahmed: How thick are the atmospheres on these moons? They must be fairly tenuous.

Olivier Witasse: Yeah, yeah, it's very tenuous. I mean it's only 10 to the minus six atmosphere, so it's almost nothing. I mean they go very high in altitude, but the density is pretty small. At Europa it might be a bit denser because of the possible plumes, or geysers. And with our two flybys of Europa, we have a mass spectrometer that it will be detect the molecule that are present in the atmosphere. So also we'll have here a direct detection of molecules. So we have the remote spectrometer that we look at the light from which you can get the composition, but there will also in situ spectrometer, with which you really detect what molecules do you have in the atmosphere?

Sarah Al-Ahmed: And with the gravitational readings on these worlds, I know we'll be able to know more about the liquid water ocean underneath. But will we be able to get enough information to make, say, diagrams of the different layers within these moons, how thick the ice is, or if they have cores, for example?

Olivier Witasse: Yeah, so here particularly, we will do that at Ganymede, not so much at Europa, because we are only with two flybys. It will be a bit difficult. But partly at Callisto, because Callisto we have 21 flybys, so it's a lot. But yeah, Ganymede, because at the end of the mission we'll be orbiting Ganymede for nine months, one year. With our gravity experiment, from an experiment called radio science, we will get the gravity feel coefficient of the moon, and from that you can get the entire structure of the moon. So we'll do a real tomography, what do we know about the entire? Because we think that there is a first icy shell, so the ice, then there is liquid water, then there is probably another layer of ice. And then the mantle may be made of rocks and then the iron core, because if there is a magnetic field, there must be an iron core. But that's more or less all what we know. But with Juice, with the radio experiment, together with the laser altimeter and also partly a little bit with the camera, we have a lot of investigation that can compliment to each other. It's quite nice. We will get, in principle, the entire structure of Ganymede. And that, to understand the habitability of a body, like a moon, a planet, it's very important to know the entire structure.

Sarah Al-Ahmed: I love the idea, too, that this altimeter will allow us to probably make a global map of Ganymede, and actually see all the different peaks and layers, the topography of this world, in a way that we've never been able to see before. That's going to be really exciting.

Olivier Witasse: The laser altimeter, it's a great tool. I mean when we got the first data at Mars from the NASA MOLA laser altimeter, everybody was completely crazy about that. So we'll do the same at Ganymede. The images can be used also to image the surface in stereo, so we can get also a 3D view, thanks to the images. I mean when you look at that at Mars, it's also quite fantastic, so we'll do the same at Ganymede. The radar will get also quite interesting information on the subsurface, but also on the surface. So yes, if we put all the data together, it will be very, very exciting.

Sarah Al-Ahmed: I can just imagine a future where people are in virtual reality, in stereo, staring down at this world. I always love those experiences. They're so fun.

Olivier Witasse: Yeah, indeed, indeed. I hope we will make it.

Sarah Al-Ahmed: Is there anything that you think you would be surprised to learn from this mission once it reaches these moons?

Olivier Witasse: In fact, this is what we like also in planetary exploration. So we built the mission, we designed a mission to address many, many questions. So we want to do that, we want to learn that. But in addition, there is always surprises. What is also quite interesting is which surprises will come, so just need to wait.

Sarah Al-Ahmed: Yeah, we have no idea. Is there anything that you personally are really interested to learn more about?

Olivier Witasse: So in term of planetary science, it's just the dream, because you have everything. Plus what is quite interesting for me is also the study of oceans underneath the surface. And I think that will trigger a lot of new studies in comparison with our own ocean. And at the moment, planetary science and oceanography are completely decoupled. I mean it's completely two different communities. I mean we have the scientists on Earth, that study the ocean, and there are many interesting stuff, the dynamics, the soil, the current, the link to the global warming, the climate, the clouds, et cetera. But then we'll be studying the oceans out there at Ganymede, Europa, and maybe Callisto. And that will trigger a lot of studies, because how can we have an ocean between two layers of ice, and that Europa between one layer of ice and a layer of rocks, what's happening inside? Do we have hydrothermal activity at Europa? The dynamics, the pressure, the temperature? And then I'm pretty sure we'll come back to the people doing oceanography on Earth to understand more. And they will also ask, "What do we learn here?" So that, I think it's a very interesting aspect of the mission. So I will see more and more connection between geophysics in the Earth, and geophysics in the outer planets. So I think that's quite interesting, and I look forward to it already.

Sarah Al-Ahmed: Yeah. And I think we've seen this happen with past missions, particularly to Mars, where we had to call in geologists and be like, "What is going on with this rock on Mars?" And it's created a whole new field of science where people are doing the geology of other worlds. So who knows how many people might inadvertently find their way into exo-oceanography because of this mission? That's really cool.

Olivier Witasse: Yes. And then there is another link, which is also very interesting for Juice, is the link with our solar system and other solar systems, because we'll be studying Jupiter as a small solar system. So as you said, we have 90 moons, maybe there are more. So many, many, many moons orbiting Jupiter, a big planet. The dust rings also. If you learn more about Jupiter, we could learn more about our own solar system, how it was formed, how it evolved. It's very important. We've learned a few years ago that the image of the solar system that we see now, it's not the same as the one when it was formed. The planets, in particular the giant planets, they migrate. So now we see them where they are, but in the past they were closer to the sun, and then they went back further away. So it's quite interesting to study the evolution of the solar system. So studying Jupiter as a small solar system can help also understanding that. And the same for the extrasolar systems that we discover very regularly. And with James Webb, there will be a lot of new data, because Jupiter, you have everything. You have big moons, water, a big planet. What we know about it with Juice, we'll apply that to other systems. So there is also the link between planetary science and exoplanetary science, so it's very rich.

Sarah Al-Ahmed: Yeah, my focus when I was going in undergrad was exoplanet detection. So the fact that we're now in a phase of research where we're going to be getting enough data on these moons, so then we can compare it to the spectra of exoplanets light years away, the fact that we can even look at the atmospheres, or analyze the surface composition of these things with JWST, is still blowing my mind. I'm still not able to grapple with that power.

Olivier Witasse: Yeah, no, indeed, indeed, indeed. And maybe we will find the water world. I mean if we can call water world, those moons around the giant planet, not only around Jupiter, but also Saturn, Uranus, and Neptune, maybe there are many more water worlds everywhere.

Sarah Al-Ahmed: Yeah. But it might be possible that worlds like Earth where there's water on the surface that isn't obscured by ice might be far more rare. If we're looking at our solar system as an example, there are many, many ocean worlds, but only one Earth. Understanding the surface composition of these moons a little better could, not tell us that these worlds far away have subsurface oceans, but could at least help us indicate you're going to have some weird secrets hidden underneath that ice. Well, this is all very exciting and I cannot wait for these results to come out. How is this mission going to be sharing all the science data with the rest of the world?

Olivier Witasse: Here, this is a standout procedure. So when we take the data by with Juice, of course we download all the images, data, et cetera to Earth. Then we give that to the scientist who have built the instrument. They have a few months to understand how the instrument works, so if they need to do correction, or we call that calibration, that can do some data processing to make sure that the data can be correctly interpreted. Because we don't want to say mistakes with the data set. And then we will put that into the public archive, so accessible to all the scientific community, but to everybody. So after a few months after we receive the data on Earth, the data will be publicly available so everybody can check what we have done at Jupiter.

Sarah Al-Ahmed: Yeah, between that and Europa Clipper collaborating together, we're going to have such a better understanding of how these moons operate. And I want to know the answer, I want to know for sure whether or not there are subsurface oceans on these moons, because so frequently will you talk about these subsurface oceans, but it's still very speculative. We aren't 100% sure, and I want that assured so we can say that there might be.

Olivier Witasse: So in 10 years from now, we'll know, I think.

Sarah Al-Ahmed: Yeah, that's really exciting. We've been thinking about this for ages, I mean even since Voyager flew by Jupiter. So I'm so happy that we have this mission to finally help us definitively answer these questions. Well thanks so much for sharing this mission with us, Olivier, and good luck to you and all of your team in these next phases of the mission. It's going to take a while to get there, but we're all going to have to be patient, and I'm sure that's the worst for your team.

Olivier Witasse: I think we have been working on the last 15 years, so we can wait a little bit more.

Sarah Al-Ahmed: Thanks, Olivier.

Olivier Witasse: Thank you, Sarah.

Sarah Al-Ahmed: This is a moment in space history that deserves to be marked. We're embarking on a new era of icy moon discovery with the launch of Juice, and the upcoming Europa Clipper mission. Europa, Ganymede, and Callisto have captured the imaginations of scientists and science fiction writers for ages. And there's no wonder why. These worlds represent untapped reservoirs of knowledge that could reshape our understanding of habitability in the cosmos. It's hard to say how rare worlds like Earth truly are with their liquid water oceans and abundant life, but icy moons are everywhere. These are the beginnings of a deeper understanding of our place in space. And who knows how many icy worlds have creatures thriving in their oceans under a vault of ice? Only time and science will tell. Now let's check in with Bruce Betts, the Chief Scientist of The Planetary Society, for What's Up. Hey, Bruce.

Bruce Betts: Hi Sarah.

Sarah Al-Ahmed: Now what a week. I mean, Day of Action was cool. Starship exploded.

Bruce Betts: Yeah, I don't know how to follow that, other than telling people what's up in the night sky. Oh, don't forget, a hybrid solar eclipse and the Lyrid meteor shower all were a party.

Sarah Al-Ahmed: It's true.

Bruce Betts: But we've got more. We've got Venus, our buddy is just hanging out. Always is. I just keep saying it because it keeps being there over in the west, after sunset. Mars moving over, moving gradually towards Venus, up higher in the sky, looking reddish, a lot dimmer. And then in the predawn, Saturn's getting higher and higher in the east. The night of the fifth through the sixth, an above average meteor shower, the Eta Aquarids will be partying. Here's the good news, bad news. If you're in the Northern Hemisphere, hey, you get to see most things, but this one's going to be tougher. Southern Hemisphere, this is in a better position for you to see it. Unfortunately, there's going to be a nearly full moon, and so it'll be tougher to see. But that's the evening of the fifth/sixth, a few days before or after. Pretty good meteor shower, especially Southern Hemisphere, so go out and check that out. And yeah, it's pretty chill other than that.

Sarah Al-Ahmed: Yeah, the only time I'm ever sad that a full moon is out is during a meteor shower.

Bruce Betts: We move on to this week in space history. The first image of Earth from the surface of the moon, by a human built robotic spacecraft, was taken by Surveyor 3 on April 30th, 1967. And it's not very spectacular, compared to now, but it's pretty substantial when it's the first time you do that. It was a few months earlier they got the first images from orbit as well. All right, we move on to random space fact.

Sarah Al-Ahmed: What do you got this week, Bruce?

Bruce Betts: I've got something that requires a brain, sorry, which is the speed of sound, commonly called mach one. The speed of sound at the Mars surface is a little over 70% of mach one at the surface of the Earth. So on the surface of the Earth it's 340 meters per second, or about 781 miles per hour. 340 meters per second goes down to 244 meters per second and 546 miles per hour. So you've got, because it's dependent on things like what the atmosphere is made of, and temperature, and of course both those numbers vary depending on your altitude, and depending on the temperature. But ballpark bottom line is you hit supersonic at a lower speed, which is something when people think about, which people occasionally do, and I was involved in at some point, Mars airplanes, it's a whole different world because you go supersonic at a slower speed, which requires different design.

Sarah Al-Ahmed: I hadn't even considered what that would mean for airplanes someday. I thought about it in the context of what that means for Martian microphones.

Bruce Betts: I mean that also is why sound... It's related to the differences in sound, and frequency shifts, and things like that.

Sarah Al-Ahmed: Yeah, that's really cool.

Bruce Betts: All right. Let us move on to the trivia question because usually we ask you one, and in this case I ask what was painted on the front of Yuri Gagarin's flight helmet, and why? How'd we do, Sarah?

Sarah Al-Ahmed: When you posed this question, I knew part of the answer, but it got weirder and weirder the more I read about. Yeah. And every answer that people sent in gave even more pieces to this puzzle. So it was really cool. Yeah, I mean the too long don't read version of this is that the Soviet Union wanted Yuri to be clearly labeled as a member of their nation when he was doing this. But the realty here is that about a year before Yuri Gagarin went on this wild flight into space for the first time, there was a US U2 pilot named Gary Powers that the Soviet Union shot out of the sky while he was doing surveillance. So everyone was on high guard in the situation and they were worried that if Yuri went off target and landed back in the Soviet Union, or even in another nation, he might be seen as an invader, or a spy, or something. So an engineer, about half an hour before he went up on his historic flight.

Bruce Betts: Yeah, that's where it gets funny.

Sarah Al-Ahmed: Yeah. Half an hour before. This is crazy. This guy spray-painted CCCP, which is of course the Soviet Union's version of USSR on his helmet. Just in case he landed back in the USSR, they wouldn't think that he was invading them from space.

Bruce Betts: Yeah, that's why I asked. Because I thought the story was fascinating. I knew he had that on his helmet, I didn't realize it had been such a last minute addition, or why.

Sarah Al-Ahmed: Right? Half an hour before.

Bruce Betts: So who talked to us? Who won?

Sarah Al-Ahmed: Oh, yeah. It was hard to pick on this one because so many people had partial answers, that kind of thing. But in the end, our winner is Merinda Weaver from New Hampshire, USA and Merinda. You're going to be getting a cool prize. It's our Yuri's Night gift set, so lots of stickers, and patches, and cool little fake tattoos of Yuri's face. Although I don't think that any of these images actually have the CCCP written on the helmet in them. I think they internationalized his image.

Bruce Betts: Glossed over that.

Sarah Al-Ahmed: Yeah.

Bruce Betts: Yeah. No, you read his speech before he flew and it was not internationalized at the time.

Sarah Al-Ahmed: Oh, spicy. I'm going to have to do that because this story was really cool to learn about.

Bruce Betts: I mean, I doubt he wrote it.

Sarah Al-Ahmed: Yeah. And I loved, too, all the comments we got on this. Some people were talking about their experiences, going to see statues of Yuri Gagarin, that kind of thing. But Pavel Kumatia, who lives in Belarus, wrote in and said, "Since childhood as a resident of the former USSR, he'd seen countless images of Yuri Gagarin and other Soviet cosmonauts with the CCCP on the helmets." But at that point it was already standardized, and so this was actually the beginning of that practice, and now we all know why. That's really cool.

Bruce Betts: Look how much we learned.

Sarah Al-Ahmed: We learned today. We got some other really good comments too. Mark Dunning in Florida USA wrote in, and I like this one, I'm going to share this with our T-shirt team, said, "Howdy, T-shirt request, DART punching an asteroid in the face for the dinosaurs." I'd totally buy it.

Bruce Betts: That works.

Sarah Al-Ahmed: Yeah. We could add it to our kick asteroid line. I like that. And then this one too, I wanted to share because Yun Woo Chang from South Korea wrote in to say, "Hi there, I've been enjoying Planetary radio for over two years now and keep up the good work," and I just wanted to say thank you, because this is the first person to write me from South Korea for Planetary Radio, so that was awesome.

Bruce Betts: Cool. No, we love the international listenership.

Sarah Al-Ahmed: Oh, yeah. It makes me so happy just hearing from people all around the world. Being on this side of it, and just seeing how much participation and love there is from people all around this planet, it just, it makes me so happy.

Bruce Betts: Softly gooey.

Sarah Al-Ahmed: So gooey, but all right, Bruce, what's our next trivia question? You going to stump them this time?

Bruce Betts: No, but I am going to deliver something in a format that has never before been used with the trivia question. This is a put things in order question. I'm going to give you five things. You're going to put them in chronological order. The following five things are all still going or still working. Put them in chronological order from oldest to youngest. If it's a spacecraft, start with their launch date, and for others, the first public release. All right. Get your notepads out. Here are your five things, the Mars Curiosity Rover, Planetary Radio, Minecraft, Mars Odyssey, and iPhones. Go to, and give us the five in chronological order from oldest to youngest.

Sarah Al-Ahmed: You've got a week to answer this one. You have to turn in your answer by May 3rd at 8:00 AM Pacific time, and the winner will get a Planetary Society beanie. I love these kinds of questions, because I feel like particularly with historical events that we learn about in classes and school, whenever someone puts all these things in context, in their order... Especially when they're disconnected, a historical event plus a TV show or something, it always blows my mind because there's so many things that happened in such close proximity, that are so distant in my brain. So it's always awesome.

Bruce Betts: Distant in your brain. How far distant can things get in your brain?

Sarah Al-Ahmed: I mean, there's an empirical answer to this, but it wouldn't reflect how far apart they actually feel in my mind.

Bruce Betts: Oh, I see. It was a figurative thing. Okay. All right.

Sarah Al-Ahmed: Yeah. About two centimeters maybe.

Bruce Betts: All right. I think this needs to end. All right everybody, go out there and look up in the night sky, and think about a wet dog nose on on the back of your arm, because that's what I have right now. Thank you, and goodnight.

Sarah Al-Ahmed: We've reached the end of this week's episode of Planetary Radio, but we'll be back next week with Matt Kaplan's adventures at the Planetary Defense Conference. Planetary Radio is produced by The Planetary Society in Pasadena, California and is made possible by our curious members. You can join us as we continue to daydream about crabs in the oceans of Europa at Mark Hilverda and Rae Paoletta are our associate producers. Andrew Lucas is our audio editor. Josh Doyle composed our theme, which was arranged and performed by Pieter Schlosser. And until next week, ad astra.