Planetary Radio • Aug 25, 2021

Europa Clipper sails toward launch

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Laureano alberto cangahuala

L. Alberto (Al) Cangahuala

Europa Clipper mission system manager

Bruce betts portrait hq library

Bruce Betts

Chief Scientist / LightSail Program Manager for The Planetary Society

Kaplan mat headshot 0114a print

Mat Kaplan

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

Jupiter’s moon Europa hides a vast water ocean under a protective layer of ice. The Europa Clipper mission will send a powerful orbiter to investigate. Mission system manager L. Alberto (Al) Cangahuala tells us about the great strides made toward a planned 2024 launch and the challenges ahead. Bruce Betts faces one of the greatest challenges for any parent: getting a new college freshman installed in a distant university. Our chief scientist takes a break from the preparation to share the night sky and a new space trivia contest.

Europa Clipper Poster
Europa Clipper Poster Journey to an ocean world. Artwork for NASA's Europa Clipper mission.Image: NASA/ Jet Propulsion Laboratory-Caltech
Prepping Europa Clipper's propulsion tanks
Prepping Europa Clipper's propulsion tanks Contamination control engineers in a clean room at NASA's Goddard Space Flight Center in Greenbelt, Maryland, evaluate a propellant tank before it is installed in NASA's Europa Clipper spacecraft. The tank is one of two that will be used to hold the spacecraft's propellant. It will be inserted into the cylinder seen at left in the background, one of two cylinders that make up the propulsion module.Image: NASA/GSFC Denny Henry
Building a nadir deck for Europa Clipper
Building a nadir deck for Europa Clipper Engineers in a clean room at NASA's Jet Propulsion Laboratory in Southern California build the nadir deck for NASA's Europa Clipper spacecraft. The deck will stabilize the spacecraft's sensors and help the mission team ensure its instruments are oriented correctly.Image: NASA/JPL-Caltech
Europa Clipper Science Instruments
Europa Clipper Science Instruments Europa Clipper is a huge spacecraft packed with science instruments to scan Europa from above and directly sample the moon's tenuous atmosphere.Image: NASA / JHUAPL

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Trivia Contest

This Week’s Question:

Name every type of spacecraft that has carried humans into Earth orbit or beyond.

This Week’s Prize:

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

To submit your answer:

Complete the contest entry form at or write to us at [email protected] no later than Wednesday, September 1 at 8am Pacific Time. Be sure to include your name and mailing address.

Last week's question:

How many orbits of Saturn did the Cassini spacecraft complete?


The winner will be revealed next week.

Question from the Aug. 11, 2021 space trivia contest:

What is the tallest mountain on Venus?


The tallest mountain peak on Venus is Skadi Mons in the Maxwell Montes range.


Mat Kaplan: Preparing Europa Clipper for its voyage to Jupiter's ocean moon. 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. Designing, reviewing, redesigning, building, integrating, and testing it every step along the way, creating a spacecraft is not work for the timid or anyone lacking patience.

Mat Kaplan: Then there's living in a state of healthy paranoia. That's what mission system manager, Al Cangahuala, calls it. That's to say nothing of doing all this during one of the least healthy times in a hundred years, and yet Europa Clipper is making steady progress towards the beginning of its mission to that world with a hidden water ocean, the one that scientists and science fiction writers like the late Arthur C. Clarke have long suspected might be a likely place to find life. We'll get a great update from Al in a few minutes.

Mat Kaplan: I hope you're hearing this before Saturday, August 28th. That's when Sailing the Light, The Planetary Society's documentary about our LightSail project will premiere on YouTube. We'll watch the film together and then I'll welcome society leaders, Bill Nye, Bruce Betts, and Jennifer Vaughn for a conversation that will include your questions. It starts at 1:00 PM Eastern 1700 UTC on Saturday, the 28th. There's a direct link on our homepage,

Mat Kaplan: Don't worry if you've missed the live celebration. We'll make it available on demand. We're also coming up on the 2021 Humans to Mars Summit from Explore Mars. I've learned that I'll once again moderate the closing session with a bunch of Mars all stars answering a simple question, why Mars? You can register for this free gathering at We've also got the link on this week's episode page at H2M will run September 13 through 15. Wait till you see the amazing lineup of speakers and panelists, including NASA administrator, Bill Nelson.

Mat Kaplan: Is it an impact crater, a volcano? No, it's the first hole drilled by Perseverance in Jezero Crater on Mars. But the image that tops the August 20 edition of the downlinks sure looks bigger. We're waiting for the rover to make another attempt to collect a sample for eventual return to Earth. It's almost certain that the Japanese space agencies, Martian Moons eXploration mission will bring back a sample from Phobos years earlier. A new study says ancient asteroids could have blasted life from the surface of the red planet to its moon.

Mat Kaplan: You've probably heard that development of NASA's human landing system is once again on hold. Blue Origin has filed a suit against NASA because of the agency's award of only one contract to competitor SpaceX. So you can forget about boots back on the moon by 2024. Then again, that was never going to happen so soon. These stories and more great stuff, go to

Mat Kaplan: Laureano Alberto Cangahuala is simply Al to almost everyone. Al has one of those minds that guides spacecraft across the solar system. And then through the complicated trajectories that enable them to gather wonderful science, he managed the NASA Jet Propulsion Lab's mission, design, and navigation section. Prior to becoming part of the Europa Clipper team, he was a NASA astronaut selection finalist in 2000 and currently serves on the astrodynamics committee of the International Astronautical Federation.

Mat Kaplan: He didn't stop with BSMS and PhD degrees in aeronautics, astronautics, but also earned a BS in Earth, atmospheric and planetary science from MIT. If you stay through the end of our conversation, recorded just hours before we published this week's show, you'll understand why he loves making presentations to high school and college audiences about our exploration of the solar system. You'll also hear me mention Al's colleague and past Planetary Radio guest, Bob Pappalardo. Bob is project scientist for Europa Clipper at JPL. Al, thank you so much for joining us on Planetary Radio, and welcome.

Al Cangahuala: Thank you. It's a pleasure to be here.

Mat Kaplan: We have talked with Europa Clipper project scientist, Bob Pappalardo, and other members of the science team on the mission, but not that often with other colleagues like you who are helping to design all of this. So it's a pleasure to do this, especially as we see, in a fairly recent press release from JPL, that the spacecraft is coming together pretty well. Do you feel good about the progress?

Al Cangahuala: Oh, we're very excited. This is a very exciting time. To think that we are doing our work and seeing the hardware come together, and our plans for bringing this hardware to remote places of the solar system, to do these unique investigations has really been a shot in the arm.

Mat Kaplan: I'm going to get more deeply into the status of the spacecraft and the mission overall. But just a couple of words about what do you do first. I noted that you've been at JPL for nearly 30 years in a variety of jobs. I want to tell you that I very much enjoyed... I didn't get to watch all of it, but I think I watched two of the five portions of a course that you teach, deep space navigation, which I guess is a specialty of yours.

Al Cangahuala: Yeah. That was my technical background coming to JPL, navigation, determining the trajectories of the spacecraft headed towards and at remote destinations in the solar system, being able to know where to point the ground antennas at Earth, towards the spacecraft to send commands and receive data. Yeah, it's been very exciting because this type of work gives you the opportunity to learn about the science of many different missions, those around the Earth, between the Earth and the moon, and beyond.

Mat Kaplan: We have noted that you have worked on a lot of missions in your tenure there at JPL. I also saw that your PhD is in space geodesy. What is that?

Al Cangahuala: That's correct. Geodesy is a branch or a multidisciplinary aspect of science having to do with reconciling all these different attributes of the Earth, its shape, how the shape changes, its rotation dynamics, its measuring physical attributes of the Earth and how they change, and how they're influenced by the presence of the moon, the sun, and other bodies. In order to navigate spacecrafts, you have to know how the Earth is navigating, so to speak, around the sun.

Al Cangahuala: And so, in the research I did earlier, we were developing systems to monitor deformations of the Earth's crust using signals between Earth stations and Earth orbiting satellites. Knowing the spacecraft's orbit was important, but not as important as knowing the changes of the locations of the stations due to geological activity. Now we've turned it around. We know pretty much where the Earth stations are. Now we want to know where the spacecraft is. So it's been interesting to reverse the problem as my career has continued.

Mat Kaplan: That is a fascinating progression. Your title on the mission is mission system manager. What does that work include?

Al Cangahuala: The mission system is the ensemble of the things that are needed to properly operate the spacecraft and its instruments at its remote destination. One big aspect of this mission is the trajectory. We don't just fly to Europa and land. We're going to fly by Europa many times. Each flyby is tailored to achieve some of the science as part of the total science goals of the mission. Each flyby gives us an opportunity to come close to Europa, but each flyby changes the trajectory of the spacecraft significantly.

Al Cangahuala: One interesting piece of trivia is that we bring a lot of propellant with us to accomplish this mission. But the change in velocity of the spacecraft, that can be accomplished with that propellant is about 5% of the total trajectory change that you need. The rest of it, we get from flying by Europa, and in this case, the Earth and Mars as well.

Al Cangahuala: So to us, Europe is not just the science destination. It's also a propellant depot as well. One big aspect of the mission system work is the design of the trajectories and navigating to that trajectory. Then there's the mission operations. There is the determination of how we're going to operate the spacecraft, at what cadence are we going to be uplinking commands, how will we respond to anomalies, and so forth. Then there's the ground data system, which is needed to support all these activities.

Al Cangahuala: One other aspect of which I should also add is the mission planning. In addition to having the trajectory, it's like you're having the blank slate set up there. Now you need to put in the activities that you normally would expect to perform, to accomplish the science. That includes direct observations of Europa, as well as other calibration activities to support the operation of the instruments in the spacecraft subsystems.

Mat Kaplan: I have several other questions to ask you about how this spacecraft will get its job done once it is in orbit around Jupiter. That's something else we might come back to. Really, even though it's a Europa mission, I know it's going to be orbiting Jupiter, much like Juno is as we speak. But I'll come back to that. You've also touched on a topic which personally the audience knows I like to bring up frequently, because it doesn't get the attention that it deserves. That is the field of systems engineering, which seems to be so critical to the success of a mission like this, but of so many types of projects.

Al Cangahuala: Yes. I think to accomplish these missions, you need engineering in depth and breadth. I think system engineering is a very important component of that. A change in one domain area could lead to changes in another domain area that aren't easily anticipated or modeled. So having a good cadre of system engineers is really essential to making this all happen. In addition to looking at those changes where one thing happens here, another thing happens here, we also want to measure risk and robustness.

Al Cangahuala: Measuring that across different domain areas can be a challenge to quantify what it means to be safe in terms of energy and power for the spacecraft, versus a thermal considerations, versus navigation considerations. These are different areas of risk that have to be weighed against each other properly in order to make good engineering decisions with design and how to operate the spacecraft. We have a great system engineering team to help us do that.

Mat Kaplan: So many different factors to weigh. As you've used, changing one thing even slightly over here may have unexpected consequences for our system over there. Man, my hats off to all of you who have to deal with this kind of stuff, although it does seem that JPL and others have gotten pretty good at this in the last few decades.

Al Cangahuala: Yeah. I think the cumulative experience of the partner institutions brought to bear on this is a great asset, and experience includes times when things have not gone well and what we have learned from them. Lessons learned can come in many forms like formal reports or just anecdotes, just talking with team members. It's important, I think, to cultivate a healthy level of paranoia about what could go wrong and to try to quantify it and see what can be done to mitigate what we perceive to be genuine risks. So risk management is very important. Managing the uncertainty is a very important part of the job.

Mat Kaplan: Not the first time we've heard reference to that healthy sense of paranoia. Sounds like a contradiction in terms, but I think we get it. Let's go back to talking about the status of the spacecraft. There are so many components of this spacecraft coming from so many different facilities, sometimes leaving one for another, and then going back again. I mean, that has to be a challenge in itself. How does the project keep track of everything that has to come together?

Al Cangahuala: Well, managing schedule and schedule margin is important part of the job. We do our best to kind of allocate times and milestones to accomplish the shuttling of the components needed to create the subsystems and modules resulting ultimately in the full assembly of the spacecraft, along with the instruments. From my first days on this project, the creation of the schedule and the management of the margin against that schedule has been a very important part of our work and something we track day in, day out, and month to month. I think that's gone really well, and we've had a great team of people with which to do that.

Mat Kaplan: Among those components of the spacecraft, of those nine instruments, which is quite a compliment of instruments that the spacecraft is going to carry... There are some great photos of those available as well, but we'll try and make available or at least get access to from that article about the current status of the mission. How are those coming together, because I know those are coming from all over as well.

Al Cangahuala: Yes. It's an interesting suite of instruments. Some of them are builds that are based on designs that are already flying in other parts of the solar system. So we get to leverage from the experiences building and operating those earlier versions of the instruments. So that's been a benefit. With regards to the instruments and the spacecraft subsystems, COVID has been a challenge. As part of tracking schedule and schedule margin, understanding where there may be pinch points because of COVID issues, whether it's at the institutions or suppliers, is something to project has really taken to heart and tried to measure and understand.

Al Cangahuala: I would also say that safety is a very important part of this too. In managing the schedule, safety has been a top priority. I'm really proud of our project management for making it clear that the health and safety of the team, including all the contributors, going out to the subcontractors and so forth, is the top priority. It's great to know that we have the backing of our leadership while we go through these really unprecedented times.

Mat Kaplan: Are you one of those who has had to do a lot of this work out of your home rather than at the lab where you could directly interact with people face to face?

Al Cangahuala: That's correct. Yes. Much of the mission system work at this point, the development, continuing refinement of the trajectories, the maturation of the mission operations processes, procedures, the development of the ground data system software, much of that work has essentially, or I'd just say, a vast majority of it has proceeded off lab in homes around Southern California, in other states. Yeah. We have made good progress under these, I'll say again, unprecedented circumstances.

Mat Kaplan: Let's hope unprecedented and also not repeated any time soon.

Al Cangahuala: Yeah.

Mat Kaplan: If I were to look down from that observation bay over at the high bay, that historic clean room at JPL, would I start to see the spacecraft coming together there, or is that still in the future?

Al Cangahuala: That's still in the future, but in other clean room areas at JPL, at APL, and Goddard, you would see activity these days.

Mat Kaplan: APL, of course, the Applied Physics Lab in Maryland, Johns Hopkins University, of course, which is a major contributor. And of course the Goddard Space Flight Center, another NASA center like JPL. Looking at artists' renderings of the spacecraft, I don't know if people... I know I didn't get an idea of just how big it is, just the propulsion module.

Al Cangahuala: Yeah. About 10 feet high. Yeah.

Mat Kaplan: Yeah. This isn't a CubeSat.

Al Cangahuala: Yeah.

Mat Kaplan: I guess that is one of the reasons why, looking at how you're going to begin its journey out there to the Jovian system has been so important. For a while there, of course, Congress had required that the launch take place on that big Space Launch System rocket that is still in development. But recently, we got the word that, no, a Europa Clipper is going to be up in the Falcon Heavy. I just wonder how much of a difference that makes for the mission, and making sure that you get out there to Europa as soon as possible.

Al Cangahuala: When we design these missions, when we reach, we call it preliminary design review or PDR, our challenge there is to explain the different possible ways we could do a job, the different options for making a possible decision. And for going to Jupiter, that's a pretty big trade. We had a design for the spacecraft and instruments, that was agnostic to a few different options to getting to Jupiter. We could go directly. We could use gravity assists in a few combinations. And I think that that was part of our message.

Al Cangahuala: Fast forward a few years later, when you get the critical design review, the onus now is to explain what you will do. That required having a decision on that. So we continue to show that we were ready to go with whatever the choice was going to be. Shortly after our review, we were starting to get that direction. And so, we're in a good place now that the announcement has been made, that we're going on a mega trajectory with this particular launch vehicle. So it's all come together and we're happy to see that crystallized now.

Mat Kaplan: That's great to hear and I hope that you guys can take some comfort in the fact that Falcon Heavy, they've done pretty well so far. We had our own LightSail 2 on one and it performed just fine for us and all those other payloads.

Al Cangahuala: That's right. Yeah.

Mat Kaplan: I talk with Scott Bolton, the principal investigator for the Juno mission, now and then, and he has talked about the contacts that are maintained between his team and yours. Your mission profile does seem, as I said, to have a lot in common with the Juno mission that is orbiting Jupiter. Have you had personal contact with folks on the Juno side, and have you seen some of the benefits of being able to talk to them? Not just about trajectory, but about things like surviving that terrible radiation out there near Jupiter.

Al Cangahuala: Yeah. Absolutely. We have many team members who have served on Juno in the past. Earlier in my career, in a role in line management, I supported Juno. So it was very exciting to watch them transition into their extended mission where they'll be performing flybys of the Galilean moons, including Europa. So we're very excited to hear about their science from these ongoing flybys. Yes.

Al Cangahuala: And then, during their prime mission, we did keep in touch on how the spacecraft was doing. Its orbit is inclined with respect to the Jupiter orbit plane. So it's a little different from the clipper orbit, but nonetheless, you're in the Jupiter environment and you're going to learn things. And so, we've been in communication about their findings. But I have to say, yeah, going back up the last couple months and looking a year ahead, these upcoming flybys are very much on our minds.

Mat Kaplan: Has to be reassuring to see that Juno has, if anything, survived that horrible radiation environment better than a lot of people expected.

Al Cangahuala: Yeah. The spacecraft has done great and we're very happy for them. Appreciate the opportunity to learn from their experiences.

Mat Kaplan: Al Cangahuala will share much more with us in a minute here on Planetary Radio.

Bruce Betts: Hi again, everyone. It's Bruce. Many of you know that I'm the program manager for The Planetary Society's LightSail program. LightSail made history with its launch and deployment in 2019, and it's still sailing. It will soon be featured in the Smithsonian's new FUTURES exhibition. Your support made this happen. LightSail still has much to teach us. Will you help us sail on into our extended mission? Your gift will sustain daily operations and help us inform future solar sailing missions like NASA's NEA Scout.

Bruce Betts: When you give today, your contribution will be matched up to $25,000 by a generous society member. Plus, when you give a hundred dollars or more, we will send you the official LightSail 2 extended mission patch to wear with pride. Make your contribution to science and history at That's Thanks.

Mat Kaplan: We mentioned that Juno and the Europa Clipper as well are going to be orbiting Jupiter. Now, in the case of Clipper, why not just orbit Europa?

Al Cangahuala: Europa has been a destination of high interest for decades. Many different mission architectures have been proposed for studying Europa, including that of a dedicated orbiter. One of the challenges is dealing with the fact that Europa is orbiting in this high radiation environment. Due to the strong magnetic field of Jupiter and the high energy particles trapped in it, if you're orbiting Europa, you're in the midst of that high radiation environment.

Al Cangahuala: And so, your time is limited unless you brought more and more shielding. If we were going to expend mass on the spacecraft design, we would rather put that mass into instruments, scientific instruments. That would be preferable to more shielding. Teams that have studied these types of missions in the past have struggled with it, and one idea that came out was to just dip in, do your science during a period of like, let's say, a day or so around closest approach to Europa.

Al Cangahuala: Then leave the high radiation environment and downlink data at let's say a more relaxed cadence, and then repeat. After a few dozen flybys, you could start to accrue the coverage that you would have achieved through a dedicated orbiter anyways. So we feel like we're getting the best of both worlds. No pun intended.

Mat Kaplan: Took me a moment. How do you describe... I mean, is it a particularly complex orbital trajectory that the Clipper will follow or is it maybe simpler than the one that Juno is doing at that incline, to the ecliptic?

Al Cangahuala: At a high level, if you step back and you would look at the diagram of the Europa Clipper orbit around Jupiter, and its closest approach to Jupiter, that's about the time when you would be encountering Europa. That orbit is sized so that its encounters with Europa are in resonance with the Europa orbit. Europa orbits Jupiter once every three and a half, 3.5, five days. We would look at orbits that would have a four-to-one or six-to-one resonance with Europa's orbit.

Al Cangahuala: So we would meet Jupiter... I'm sorry, we would meet Europa once every 14.2 or 21 point... Let me get my math right. 14.2 or 21.3 days, roughly every two to three weeks. So at a high level, we're just talking about exploiting a residence between the orbits. The listeners could just ping you a picture every two or three weeks we meet Europa. Now, if zoom in on the orbits and these encounters, and you looked at the ground, track, the angle at which we approach Europa does set up a change in the orbit of Europa Clipper, which sets up the ground track of the next flyby.

Al Cangahuala: You could think of a fly by as being organized like six or seven at a time to achieve coverage, so that from north to south, each flyby is covering some new territory. Those sequences or sets of flybys are referred to as crank-over-the-top sequences or COTs. Designing a COT as a unit to achieve a certain level of coverage of a hemisphere is the currency that we're talking about. We will use a COT to cover the anti-Jovian hemisphere, then another COT to go over the same place but going south to north.

Al Cangahuala: And so, in adding COTs together, you can really achieve really strong global, regional coverage of the hemisphere of Europa. And so, we'll do that first for the anti-Jovian hemisphere, and then we'll rotate through a [inaudible 00:25:55] and repeat doing flybys of the sub Jovian hemisphere in a similar fashion. This may be a bit of a stretch, but I used to tell people that, to me, a flyby is like a transistor, in a hardware sense. If you think of it that way, then a COT sequence is like a integrated circuit.

Al Cangahuala: Every transistor is in there with a specific purpose. This is how I explain it to the hardware people, that the COT sequence is really architecting multiple flybys to achieve something greater than you could achieve with a single flyby.

Mat Kaplan: Let me continue with your metaphor for a moment and think that maybe the comparison to a transistor, you were saying that tiny adjustments to the trajectory can have a much larger effect, the way a tiny change in current to a transistor can control something much bigger.

Al Cangahuala: Something like that. Yeah, the integration is what gives you the strength.

Mat Kaplan: Yeah. Okay.

Al Cangahuala: Most listeners are familiar with the concept of the benefits of a single flyby. But orchestrating them together to rotate the orbit or make adjustments to the orbit to reach new parts of the European surface, it's a great application of astrodynamics.

Mat Kaplan: Let's say that Clipper, on one of its passes, finds some particularly interesting feature on the surface of this moon, a plume, let's say. We should be so lucky. You've got to know that Bob Pappalardo and the rest of the science team are going to be pounding on your door and saying, "How soon can we get back to that? How soon would you be able to get back to a feature like that?"

Al Cangahuala: That's a great question, and that's really at the heart of the mission operations design, is working with science to understand, what can we respond to quickly? What really deserves more time to respond to, if a particular instrument encounter something interesting, like they detect a new species on a particular flyby and want to re-optimize their sensors, the operation of their sensors, to be more receptive to particular species, we can do that. We can support that with relatively little effort. I mean, our baseline design really accommodates that.

Al Cangahuala: Some of our instruments have gimbals like our narrow angle camera. What we can do is also support updating the mosaic of images that are going to be generated from flyby to flyby. We have some ability to readjust that mosaic, to respond to findings from images that have been downlinked along the way. It won't be instantaneous, but we do intend to allow for re-optimizing those profiles. Moving the trajectory is probably one of the items that requires the longest lead time.

Al Cangahuala: We've worked with science to socialize that and show making a change here means losing something that you might have already preplanned and accounted for. So we certainly have studied astrodynamics mechanisms to help us cover new things, but I think studying it and having a well instrumented in the case of a plume, having a well instrumented dedicated plume flyby is something that will require more lead time to plan and work out with science. So, to me, responding to findings is part of our job. There's a spectrum of response time constants, and the plume scenario is probably one of the ones with the longest time constants.

Mat Kaplan: I like how you put that, socializing the science team. That's a nice way to put it. There was another word you used, which we should clarify. When you mentioned species, I suspect you were talking about chemical species, not necessarily a biological species or living thing. Wouldn't that be nice?

Al Cangahuala: Yeah, yeah. Yeah, we're referring to elements and compounds. Yeah.

Mat Kaplan: Very exciting stuff. Are we on track for a launch in not very long, 2024?

Al Cangahuala: Yes. We're doing very well. Again, it is under challenging circumstances, but we're just really proud of how everyone has been pulling together to get this time. Yes. We're on track for a launch in October of 2024 on a Mars-Earth gravity assist trajectory to Jupiter. We'll arrive in a Jupiter system in April of 2030.

Mat Kaplan: 2030. All right. I'm not going to think right now about how old I'll be by then. I cannot wait, and there are a lot of us out here who are saying, "Godspeed," to a Europa Clipper and you and the entire team. Before I let you go, I want to ask how you ended up in this line of work.

Al Cangahuala: I've always been interested in exploring the solar system. I got hooked by watching the Voyager launches as a kid. In the back of my mind, I thought, "Well, wherever they do that, I'd like to find out more about that." Then I had the benefit of going to a science summer school. It was a hosted by the state of Pennsylvania as a junior in high school. There I learned about how to really propagate trajectories using software. We looked at propagating the trajectory of Skylab. Once I learned how it really works to develop model and design trajectories and navigate to them, I was hooked. It was just something I wanted to do ever since then.

Mat Kaplan: Just one more thing. I found a great quote from you on NASA's Solar System Exploration Facebook page. We'll link to this as well. You talked about a message that you once received from your grandfather in Columbia. Do you remember this?

Al Cangahuala: Yes. Yes. It was really a letter to me when I was just a baby hand. I guess I had written a card to him for Father's Day, and he took the moment to write back and just say that he was just really supportive of me and really wanted to make sure that... I mean, to him, his daughter, my mom, came to the United States to teach Spanish in US high schools and didn't know if she was going to come back. Was just sending a letter in support to her and to me, hoping that I would listen to my parents and follow in their footsteps, or try to be a good citizen of wherever I was, to be helpful to wherever you were going to be, in this case, my adoptive country.

Al Cangahuala: And so, yeah, I've always treasured that letter. Everyone's grandparents are special, but I feel like to have such a vision as to how a child's life was going to evolve, having never really left the country, his native country, says a lot to me. I've never forgotten that. I hope to live up to his expectations and really help make this kind of work something that is really inspiring to the younger people in particular.

Al Cangahuala: Because when we talk about years like 2030, you have to think that the people who are going to operate this mission, as we go around Jupiter and flyby Europa, those people are in college and high school right now. So we need to reach out and inspire them. I feel like the way my grandfather inspired me is a model that I would like to emulate.

Mat Kaplan: I hope some of those young people who are the future scientists and engineers that will be a revealing Europa to us with Europa Clipper, are listening to us right now. I think you have given plenty of reason for pride, Al, a job well done. Thank you so much for giving us this status report on Europa Clipper, and for sharing that great experience, that great letter that your grandfather sent you years ago. Best of continued success to you and the rest of the Europa Clipper team.

Al Cangahuala: Thank you very much. Great speaking with you.

Mat Kaplan: Time again for What's Up on Planetary Radio. Here he is, the chief scientist of The Planetary Society. Planetary scientist, astronomer, all around great guy, Bruce Betts. Welcome.

Bruce Betts: Oh, thanks man.

Mat Kaplan: Oh, you really do need a vacation.

Bruce Betts: Oh, maybe I'll take one.

Mat Kaplan: Remember, that's why we're recording early this time.

Bruce Betts: Right.

Mat Kaplan: He needed a reminder. Here's a good start for that vacation. Wired magazine now, their last page, they do this thing called Six-Word Sci-Fi, and they give you a prompt and then they let people write in, submit their six-word Sci-Fi stories based on that prompt. Here's the one from the new issue. The prompt is, write a story about a casual encounter with aliens. Here's the winner. It's not my favorite. It has three aliens sitting in a call center. They have headset mics and the whole bit. That's not the funny part, but it's okay. It's not bad act. Why not? It's a secondary funny part.

Bruce Betts: Is the funny part that we're talking about the casual encounter with aliens? I find that funny. Seems unlikely. Go ahead, Mat. I'm sorry.

Mat Kaplan: Here's the response that, one, from @phone96 on Twitter, "So about your planet's extended warranty," which is pretty good, coming from a call center guy. Then here's my favorite. "Remember casual encounters with aliens. This from a John Wagner, quite unexpectedly, cocktail recipes were exchanged."

Bruce Betts: That is unexpected.

Mat Kaplan: Definitely. Tell us about the Night Sky. I bet you were expecting it would be great.

Bruce Betts: I think it will be great. There are planets and they're bright and you can see the brightest planet, besides Earth, over on the Western horizon early in the evening, Venus low down, but looking super bright. Turn your view to the other horizon over in the east and you've got Jupiter and Saturn rising, Jupiter, very bright, Saturn looking yellowish to its upper right. Check them out. I need a vacation.

Mat Kaplan: You're counting down, aren't you?

Bruce Betts: It's not really a vacation. I mean, it's only kind of a vacation. Taking a son to college, and exploring hurricanes that shouldn't be there. On to This Week in Space History.

Mat Kaplan: By the way, tell him congratulations.

Bruce Betts: I shall. Voyager 2 was busy two different times during this week in space history. It was flying by Saturn in 1981 and flying by Neptune in 1989.

Mat Kaplan: Just as we were told by Linda Spilker on last week's show. A double anniversary. Also the anniversary of one of the launches. I can't remember which one, but that took place in 1977.

Bruce Betts: They're still going strong-ish.

Mat Kaplan: Yeah.

Bruce Betts: On to Random Space Fact. I think you're going to like this one, Mat. The orbital speed of Mercury, how fast it's going around the sun, compared to the orbital speed of Neptune is about the same as the speed of a fast race car like Formula 1 or any car compared to the speed of a fast running human. Race car to human, Mercury to Neptune.

Mat Kaplan: That's a great one. Thank you for that. You're right. I love it.

Bruce Betts: Thought of that as I was falling asleep last night. Then I have confirmed the numbers.

Mat Kaplan: Oh, good. I'm glad you did the fact check.

Bruce Betts: Yeah, I didn't just make it up. All right. On to the Trivia Contest. I asked you, what is the tallest mountain on Venus? How did we do, Matt?

Mat Kaplan: We got some great responses this week, including a couple of poems. I'm going to open with one of those. It's from Jean Lewin in Washington. "Within the Maxwell Montes massif stands the tallest of the New Zealand peaks, measured up from the planet's mean radius since there's no sea level there, so to speak. Discovered through use of radio waves, Arecibo provided the means, confirmed a bit later by Pioneer 12 with a 1.7 gig radar beam. And though Venus we know itself is quite hot, you may want to wax up your skis because the top of old Skadi Mons, it's only 716 degrees."

Bruce Betts: That sounds delightful and a relaxing vacation spot.

Mat Kaplan: Yeah, I'll say. That theme continued, as you'll hear in a moment. Here is our winner, a first-time winner. Congratulations to Milena Mandich, who is in Serbia. I think this may be our first winner from that nation. She said, "Maxwell Montes mountain range. That's the mountain, basically. The highest peak is Skadi Mons named after the Norse goddess of winter." She adds her birthday is in December. "So this is destiny to listen for the first time to your podcast and for this to be the question."

Mat Kaplan: Sure. Why not, Milena? If you say. We're happy to congratulate you and glad to welcome you to the show as well. What a way to start. You have won yourself a copy of Light in the Darkness: Black Holes, the Universe, and Us, by astrophysicist, Heino, Falcke, published by a HarperOne. A fascinating book. We'll have the publisher send that your way.

Bruce Betts: Excellent. Maxwell Montes also is one of my favorite answers to a trivia that I asked many, many, many moons ago, which is it's the only feature on Venus not named for a female character or actual person. That isn't quite true. Alpha and Beta Regio. But it's the only male name on Venus named for famous Maxwell equations. Dude who did a bunch of great science.

Mat Kaplan: We had several entrants who were delighted to discover, because you had given them that hint, that that was what you were looking for, the only feature on Venus named after a human male. We did get some other stuff who have some interesting extreme sports in mind. Daniel Sorkin in New York, he said that a Skadi Mons rises to about 11 kilometers, seven miles above the planet's mean radius. Not in the same league as Olympus Mons on Mars, but still would be quite a climb.

Mat Kaplan: Thorsten Zimmer in Germany, and this is legit. "It's also featured in the Climbers Guide to the Galaxy, a truly indispensable guide for every serious future mountain climber." It's at, but we'll put up the direct link on this week's show page.

Bruce Betts: Wow.

Mat Kaplan: It does. It really is a very interesting fun article. It's well done with a few peaks around the solar system that are yet to be conquered. Finally, Chris Mills in Virginia said that it seems to be covered in snow made of lead and bismuth sulfide, perfect for making heavy metal snow angels.

Bruce Betts: Yeah.

Mat Kaplan: We do close with something from our poet Laureate, Dave Fairchild. "Maxwell Montes is a mountain standing tall on Venus plane, reaching almost seven miles in the altitude you'd gain, marked by basin, Cleopatra, quite an impact from above. Max and Cleo, what a combo on the planet named for love."

Bruce Betts: Oh.

Mat Kaplan: Isn't that sweet?

Bruce Betts: That's adorable.

Mat Kaplan: What have you got for next time?

Bruce Betts: Name every type of spacecraft that has carried humans into Earth orbit. I don't mean every individual spacecraft, but every type. For example, space shuttle counts as one, one type. Go to to find out how to enter. This is Earth orbit or beyond.

Mat Kaplan: There are people out there slapping their foreheads right now, but then there are other people, I can see you, who are already digging deep in the Wikipedia. You have until-

Bruce Betts: Or into your brain. It's an interesting question. How many can you name without looking it up?

Mat Kaplan: Yeah.

Bruce Betts: And then look it up and give us the right answers.

Mat Kaplan: You have until September 1st, Wednesday, September 1st, at 8:00 AM, to get us to this answer. And what the heck, we will award your chance to pick up another robotic spacecraft poster from, run by our friend, Thomas Romer. A whole huge series of your favorite missions, robotic missions, and soon to be including the third series of these, which, you know what? I don't have it in front of me, but I know it's Pioneer Viking and something else.

Mat Kaplan: Anyway, they're beautiful. We will give you your choice of those posters if picks you out and you come up with all those different human carrying spacecraft. I believe we're done.

Bruce Betts: All right, everybody. Go out there and look up the night sky and think about lions.

Mat Kaplan: And tigers and bears. Oh my. Have a great time on that little trip delivering your son to his new university. We'll see you next week.

Bruce Betts: All right. Take care.

Mat Kaplan: That's Bruce Betts, chief scientist of The Planetary Society who joins us every week for What's Up. Planetary Radio is produced by The Planetary Society in Pasadena, California, and is made possible by its always exploring members. Become part of their mission of discovery at Mark Hilverda and Jason Davis are our associate producers. Josh Doyle composed our theme, which is ranged and performed by Pieter Schlosser. Ad astra.