Planetary Radio • May 25, 2022

Cassini’s Dramatic End: A Planetary Radio Reprise

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On This Episode

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James Green

Senior Advisor to NASA and host of Gravity Assist

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John Grunsfeld

Board of Directors of The Planetary Society; President and CEO of Endless Frontiers Associates

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Ralph Lorenz

Planetary Scientist for Johns Hopkins University Applied Physics Lab

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Bob Pappalardo

Europa Clipper Mission Project Scientist for Jet Propulsion Laboratory

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Linda Spilker

Voyager Mission Project Scientist at NASA's Jet Propulsion Laboratory

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Ellen Stofan

Planetary Geologist and Director of the National Air and Space Museum

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Bruce Betts

Chief Scientist / LightSail Program Manager for The Planetary Society

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Mat Kaplan

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

With Mat Kaplan in London for Planetary Radio Live, we bring back one of the most moving events in the history of our show. The Cassini orbiter plunged into Saturn in the early hours of September 15, 2017. Hundreds gathered to mark the end of this remarkable voyage, including former NASA chief scientists Jim Green, John Grunsfeld and Ellen Stofan. Mat talked with them and many others on that memorable morning. Then we’ll check in with Bruce Betts for a brand new What’s Up.

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Cassini grand finale promotional Video: NASA / JPL-Caltech

Beckman Mall
Beckman Mall Looking across the Caltech Beckman Mall in the pre-dawn hours before Cassini’s end of mission.Image: Mat Kaplan
Media reps gather at JPL prior to Cassini end of mission media briefing
Media reps gather at JPL prior to Cassini end of mission media briefing Image: Mat Kaplan
Jim Green and Mat Kaplan
Jim Green and Mat Kaplan NASA Planetary Sciences Division Director Jim Green and Mat Kaplan.Image: Pauline Acalin
Cassini End of Mission Media Briefing
Cassini End of Mission Media Briefing Earl Maize, Julie Webster and Linda Spilker at Cassini End of Mission Media BriefingImage: Mat Kaplan

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Name all the United States planetary spacecraft (those that went beyond Earth orbit) that launched in the 1980s.

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

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What Messier catalog object could have been named after a Natalie Portman movie?


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Question from the May 11, 2022 space trivia contest:

Why is there a depiction of a snake on the Perseverance rover?


No contest answer this week! Join us next time for the announcement of two winners.


Mat Kaplan: Looking back to the end of Cassini on a special edition of Planetary Radio.

Mat Kaplan: Welcome. I'm Mat Kaplan of The Planetary Society with more of the human adventure across our solar system and beyond. I'm in the United Kingdom as this week's show is published, hopefully walking through the sunny English countryside. It has been a very busy couple of weeks on the road, but with the Humans to Mars summit last week, and our Moons Symphony Planetary Radio live show in London a couple of days ago, that's why we will reprise our coverage of one of the greatest days in the history of planetary science and exploration.

Mat Kaplan: You'll hear my reporting from Caltech and the Jet Propulsion Lab on the morning of September 15th, 2017. That's when the magnificent Cassini Mission at Saturn came to an end. We'll follow it with a brand new What's up segment, and Bruce Betts. Bruce will deliver a new space trivia contest, but we won't announce a winner this week. We'll make up for this by naming two winners next time. I don't have headlines from our newsletter, The Downlink, but you can still find the latest edition at I won't be surprised if it features that terrific news from the worldwide, Event Horizon Telescope team, a couple of weeks ago. If you haven't seen their image of the black hole at the center of our galaxy, well, you've probably been hiding under an asteroid. I hope to welcome someone from the EHT team soon. What you're about to hear was originally aired in our September 20, 2017 episode. That was just five days after Cassini plunged into the great ringed world that it had orbited for 13 years.

Mat Kaplan: It was a difficult choice. Should I be at JPL for the last moments of the Cassini spacecraft as it plunges into Saturn's atmosphere? Or should I go to nearby Caltech? Where hundreds of Cassini team members, their families and friends were gathering. I decided to head for the Caltech campus and I'm glad I did. By the time I arrived at 4:00 AM, the party on the huge grass covered Beckman mall had been underway for hours. There was less than an hour to go before the light speed delayed evidence of Cassini's loss would reach Earth. Small clusters of people stood in front of huge video monitors, carrying the live feed from JPL, here and there more solitary men and women sat alone looking up at the screens. It wasn't long before I started running into people I know.

Mat Kaplan: Big surprise, two guys that I was attempting to watch an eclipse with in Carbondale, well, Mike, you left for better skies, but you were with us the night before for Planetary Radio Live, Mike Kentrianakis. And Tom Economou, who is on the Cassini team, right? What was your role with Cassini, Tom?

Thanasis Economou: I am a CO-I.

Mat Kaplan: A co-investigator, mm-hmm (affirmative).

Thanasis Economou: Co-investigator and a Cassini Dust Analyzer. And we have an instrument that detects the size, measure the size and the density of the dust and the rings. And on the rings of satellites. So we have a lot of data throughout all these years sent back.

Mat Kaplan: Is the dust analyzer still in use? Will it be as Cassini makes its plunge into the atmosphere?

Thanasis Economou: We have data until the end of the spacecraft.

Mat Kaplan: Mike, I thought you were just an eclipse chaser, not a spacecraft chaser.

Mike Kentrianakis: Oh, a spacecraft, that's a good one. Okay. Yes, this is a very exciting event and I'm an amateur astronomer. So all my life, I've just love studying the planets and the stars. And knowing Cassini for the 20 years that it's, nearly 20 years that it's been out in space and that it's coming to an end, having an invite here by Tom Economou, a good friend and fellow eclipse chaser around the world with me, I was very excited to be able to join here in Caltech and see this.

Mat Kaplan: So no regrets about getting up this early.

Mike Kentrianakis: No, this is actually very interesting that we're all here at three o'clock in the morning, pretending like the sun is not out, like it were an eclipse or something, you know, going too early in the morning.

Thanasis Economou: We could not resist seeing the last moment of our beloved spacecraft for so many years.

Mat Kaplan: Thank you, gentlemen. And, thanks again for being part of that great night at Southern Illinois University for Planetary Radio Live.

Mike Kentrianakis: Thank you, Mat. That was a wonderful show that you had there.

Thanasis Economou: Yeah. We have a chance to do it again in a few more years.

Mat Kaplan: I'll see you in 2024.

Thanasis Economou: We're looking forward to that. Yes.

Mat Kaplan: All right, guys. We'll get ready for the big moment here when we lose the signal.

Mat Kaplan: I move closer to one of the big screens as the end of the mission approached. With others, I watched two displays of the radio bands Cassini was using to communicate with the Deep Space Network, all in real time, as it sped toward its doom. A spike in the middle of each display, represented the signal coming from the spacecraft. When that spike disappeared, we would know that Cassini was no longer able to fight the buffeting of Saturn's atmosphere as it strained to keep its big radio dish pointed at our pale blue dot. A few seconds more and the mighty probe that had spent two decades in space would be torn apart and vaporized. Here's what I heard in the last moments as I stood with a small clutch of nervously joking team members.

Speaker 4: We're just starting to see the thrusters fire more and more.

Mike Kentrianakis: Stay on target, stay on target.

Speaker 5: Radio signal still holding, 30 seconds.

Speaker 6: It's going to be really embarrassing if the signal stays on like a minute too late.

Mat Kaplan: That'd be great, which brings me over here. [inaudible 00:06:16]. Still there.

Speaker 7: There's a spike in the middle.

Mat Kaplan: Hmm?

Speaker 7: There's a spike in the middle.

Speaker 6: So, that's the signal strength of the radio signal. So the fact that it's narrow is all the power in one little band, which is why it's almost one little band. Yeah. So when that peak will just drop, basically, when we lose [inaudible 00:06:45].

Speaker 7: Okay.

Speaker 6: Oh, oh. There's still something there. Oh, now it's gone. So, I wonder if it,

Mat Kaplan: And there we go.

Speaker 6: Oh, it's popped back.

Mat Kaplan: Tumbling.

Speaker 6: Yeah. It's probably tumbling. Yeah.

Mat Kaplan: Yeah.

Speaker 6: That will be interesting to see a plot of afterwards.

Speaker 4: Go ahead, [inaudible 00:07:11] director.

Speaker 6: Oh, well. That's that. End of a 27 year adventure.

Speaker 5: The signal from the spacecraft just gone and within the next 45 seconds so will be the spacecraft. I hope you're all as deeply proud of this amazing accomplishment, congratulations to you all. This has been an incredible mission, an incredible spacecraft, and you're all an incredible team. I'm going to call this the end of mission, project manager, [inaudible 00:07:48].

Ralph Lorenz: I'm Ralph Lorenz, I worked on the Huygens Probe in ESA, in 1990, in the Netherlands. I then did a PhD at the University of Kent, building some of Huygens instrumentation. I worked on the radar instrument. So my entire career, 27 years, has been spent on Cassini. Four institutions, three different countries, it's been an amazing adventure.

Mat Kaplan: A trooper, right to the end. It didn't last a bit longer than was predicted?

Ralph Lorenz: Well, it looked like the telemetry dropped out and then maybe just popped back for a moment. Maybe the spacecraft was tumbling, but yeah, it was sending data right to the end. It's going to be very exciting to see what it learned about the composition of Saturn's atmosphere.

Mat Kaplan: Take us back to the Huygens Probe, the history, making work that it did.

Ralph Lorenz: Well, the Huygens Probe gave us that in situ look, it was there, the ground truth, a lot of what we've learned from Cassini and its remote sensing has really been sort of validated and laid out by what Huygens measured there and then. It was able to see Titan much closer. And it's fun, just looking back at the... I was there in Germany at the Huygens Encounter and we were watching a very similar plot of radio signal strength. And there was that little spike of radio energy, just the frequency of supposed to appear told us that after this long journey that Huygens had come out of its heat shield and the parachute was out and it was transmitting. We knew the mission hadn't been lost without trace like Beagle 2 had a year before. So it's kind of poignant just seeing the same kind of plot of radio data as Cassini met its final end.

Mat Kaplan: And if you don't mind becoming a little philosophical for a moment or two about the collaboration, the partnership that was represented by the work that the people at JPL, NASA, did with the people at ESA, the Italian Space Agency to make this mission the success it has been for the last 20 years.

Ralph Lorenz: Well, that's one of the great triumphs of Cassini Huygens, is that it has been this international collaboration that has drawn on the best qualities of all the participants. I think there was a time in the early nineties where the Cassini Mission was under threat, budget-wise. And I think it was only because of the international dimension that the mission was really preserved against that threat. You know, I, myself am a product of this international collaboration. I'm British, I worked in the Netherlands and the UK and in the US, and it's been really amazing to be part of this sort of big international family.

Mat Kaplan: So, now what? As someone said, this mission is really far from over.

Ralph Lorenz: Well, the data are going to keep us busy for decades to come, I'm sure. It's been very, very productive, but it's also laid the groundwork for future exploration. We've seen how amazing a place Titan is, with its seas and sand dunes, an active hydrologic cycle. We've seen the plumes of Enceladus. So, those are two destinations in particular that we want to go back to. Some of us have been involved in mission proposals, things to fly through the plumes of Enceladus and study them more closely with more advanced instrumentation than Cassini has. Or, to go back to Titan with perhaps a boat. We actually have a proposal in for a Quadcopter Lander, using Titans low gravity and thick atmosphere to land softly and then take off and land somewhere else. So there's a lot of possibilities for the future. It takes a long time for these things to happen as Cassini has shown us. But you know, there's a lot of possibilities in the future.

Mat Kaplan: Few people seemed interested in leaving the mall when the spacecraft went silent. For many of them, this event marked the end of years, even decades of research, engineering support and all the other tasks it had taken to make Cassini such a success. Moving away from the big monitors, I saw the leader of the Europa Clipper Mission, now preparing to build that spacecraft that will fly low over Jupiter's moon, Europa. Tasting the geysers that may carry signs of life in the ocean that hides deep below that world's thick ice. But Bob Pappalardo's focus had been Cassini before he moved to this new mission.

Bob Pappalardo: I wouldn't miss this for the world. I was the project scientist for the Cassini Equinox Mission for two years. It was during that time that we proposed to NASA, a finale to the Cassini Mission that consisted of orbiting Saturn and flying between the planet and the rings. And NASA loved it and said, yeah, let's do it. To see it come to fruition has been just incredible. The Cassini Grand Finale has been scientifically so rich. The data, the analysis is just starting to trickle out, but there are hints of just incredible information there.

Mat Kaplan: And we don't even yet know the wonders that have been returned by Cassini, that data that has just arrived on Earth.

Bob Pappalardo: That's right. There's data that was broadcast back in real time this evening. And you might notice some of the individual scientists gathering together around computers here at the event, because they're seeing what that data looks like for the first time together.

Mat Kaplan: Linda Spilker was your deputy, right? Who moved up then to, succeeded you?

Bob Pappalardo: That's correct. Linda has been with the mission since the beginning. I came in for a brief time and then went off to work full time on Europa, getting that mission going.

Mat Kaplan: And so you represent not only this mission's legacy, but the future because of the Europa Clipper Mission, how has the Cassini Mission and what it has told us about the Saturnian System informed your work to take us to Europa and sniff the plumes there?

Bob Pappalardo: Cassini's informed us both scientifically and in terms of figuring out how to best do the Europa Mission. Scientifically, we see the incredible information coming back from the moons of Saturn, from Enceladus. We're learning so much about how icy satellites work and that's giving us a new perspective on Europa and the moons of Jupiter. So we just get a little hint from Saturn of the rich science that will certainly come from Europa. But also remember, we used to be planning an orbiter around Europa, but what we learned from Cassini's exploration of Titan is that making many, many Flybys can tell us about the world globally, can give us an incredibly rich amount of information as Cassini did with Titan. And of course Cassini's limited in how low it can go because of the atmosphere of Titan. Something like a thousand kilometers above the surface is as low as it can go. But at Europa, like Cassini can do it in Enceladus, we can skim the deck as low as 25 kilometers from the surface, which is what we'll do on many of the Europa Flybys.

Mat Kaplan: And like many people, I can't wait. And as mission builds on mission, would you like to see something like the Europa Clipper Mission at Enceladus?

Bob Pappalardo: There have been several concepts for how to explore Enceladus in the future. Some have talked about a combined Enceladus-Titan mission, some have talked about, could we do some sort of sample return from Enceladus, some have talked about collecting samples and analyzing them in situ at Enceladus. So we'll see what the right next mission architecture is. But yeah, I sure want to get back there in my lifetime.

Mat Kaplan: Current status, just a quick update on where you are with the Clipper Mission?

Bob Pappalardo: We are in Phase B, Mission Development. We are just entering what they call PDR season, Preliminary Design Review. So for each of the instruments and for each of the subsystems of the spacecraft, there'll be a review. We've started those and they'll continue way into the middle, or actually till the end, nearly the end of 2018, when we have a preliminary design review, then for the entire mission. Things are going really well. We're racing along. I'm very pleased at how well the science looks and mission looks for Europa.

Mat Kaplan: Thank you, Bob. Best of luck with that mission that so many of us are looking forward to, and congratulations on the milestone achieved here this morning.

Bob Pappalardo: Thank you. It's a bittersweet occasion.

Mat Kaplan: Time was short, if I was to reach JPL a few minutes before a 6:30 AM media briefing, but I could not resist saying hello to one more person standing under the still dark skies at Caltech. You never know who you meet at Caltech at five o'clock in the morning. Ellen Stofan, you are part of the team?

Ellen Stofan: Yeah, I've been with the Cassini Radar since about 2001 or 2002, which for me, seems like a significant portion of my life, but I've been on it for so much less than so many of the team.

Mat Kaplan: I wasn't sure I was making the right decision, whether to go to JPL where my colleague Emily is, or to come here. I'm really glad I came here.

Ellen Stofan: All the instrument teams are here and we're all like family, because we've been together for so long, meetings, ups, downs, working on papers together. And so it's really special to me. It's like a huge family reunion here.

Mat Kaplan: A team member, but you also, maybe not anymore, but you can bring sort of the NASA Headquarters view of this as well, it's a triumph.

Ellen Stofan: Yeah. As the now former chief scientist, when you look across what NASA accomplishes from earth science, studying the sun, studying the universe, obviously for me, the study of the solar system is amazing and Cassini has been really the crown jewel for NASA. For one thing, just looking this morning, as they've gone through all the old images of Saturn that we've taken over the last 14 years, it's aesthetically beautiful. I mean, to me, there's something magical about the Saturn system that it really is the most beautiful place in the solar system.

Mat Kaplan: And we need to go back.

Ellen Stofan: We do need to go back. So about five years ago, I proposed to send a boat to one of the seas at the north pole of one of Saturn's moons.

Mat Kaplan: This just came up with Ralph Lorenz.

Ellen Stofan: Yeah. I'm sure it did. We really would like to go back. Titan is an amazing world. It's got these seas of liquid hydrocarbons. It's a fascinating place, sort of a push on what are the limits to life in the solar system. And then of course, there's Enceladus spitting out its oceans, waiting for us to come sample them and figure out if there could be life.

Mat Kaplan: All we have to do is go.

Ellen Stofan: All we have to do is go back. We know what to measure. We know where to go. We know how to do it. We just need to go.

Mat Kaplan: You told me just before, when I asked you to do this, you had just stopped crying.

Ellen Stofan: Oh yeah. But now I'm going to start again, it's a huge part of your life.

Mat Kaplan: Thank you, Ellen.

Ellen Stofan: Thanks.

Mat Kaplan: When we return, you'll hear highlights from the JPL Media Briefing that followed the end of the Cassini Mission, including a few words from project scientist, Linda Spilker. And we'll visit with the longtime head of NASA's Planetary Science Division, Jim Green and retired astronaut and NASA Associate Administrator. John Grunsfeld. This is Planetary Radio.

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Mat Kaplan: Welcome back to Planetary Radio. I'm Mat Kaplan, back with more coverage of the end of the Cassini Mission, on Friday, September 15th. I arrived at JPL and found a choice parking spot with my press pass hanging around my neck. I entered Von Karman Auditorium, where, as a reporter from my college radio station, I had witnessed the landing of Viking 1 on Mars, that was 41 years ago. The old auditorium has probably hosted more planetary science mission, press conferences than any other structure on Earth. This morning, the room was once again packed with TV cameras, reporters tapping on laptops, bright lights, and more than a few people in purple Cassini shirts.

Mat Kaplan: We'll have a link to the entire briefing on this week show page, but here are the opening statements from Cassini Program Manager, Earl Maize, Cassini Spacecraft Operations Manager, Julie Webster, and our old friend, the Cassini Project Scientist, Linda Spilker. By the way, these three leaders of the mission were our guests for the live celebration of Cassini, we held at Caltech on Monday night, September 18th. You'll hear portions of that event on next week's show. Here is Earl Maize.

Earl Maize: There are times in this world when things just line up, when everything is just about perfect, a child's laugh, a desert sunset, and this morning, it just couldn't have been better. And if you think about that moment, what we've been waiting for, for this entire seven years, everything clicked out just right. And then we can step back and say the same thing about the Cassini Mission, a superb machine in an amazing place, doing everything we could possibly do to reveal the mysteries and secrets of our solar system.

Earl Maize: This morning, a lone explorer, a machine made by humankind finished its mission 900 million miles away. The nearest observer wouldn't even know until 84 minutes later, that Cassini was gone. To the very end, the spacecraft did everything we asked. That ground systems report was superb and we believe we got every last second of data. It's already back in Arizona and I think the analysts are already working on it. So we have indeed accomplished exactly what we set out to do, complete this mission with a Saturn probe.

Earl Maize: Maybe just a little bit about the legacy of this mission. We've built the blocks and both scientifically and engineering-wise for the next set of missions. Europa will capitalize upon our engineering expertise and techniques and the instruments that we have developed for Cassini, 30 years later almost, will be that much better and more sophisticated and tuned for the environment that we're in. The scientific and engineering collaboration, I think will be a hallmark for future missions. Cassini presented a unique set of challenges to the science and engineers, of course, there's that ever lasting tension between science goals and engineering conservatism, but also this put the scientists in contention with each other and with the engineers. And the mix of this, as an experiment in sociology was an astonishing success. And I believe that future missions are also going to learn how to cooperate and how to get the very best of their systems. Well, with Cassini leading the example.

Earl Maize: We've been able to repurpose the spacecraft in all sorts of unique ways. And as you just saw a little while ago, we turned it into an atmospheric probe. So every piece of it's been used for a benefit of exploration. I've got to thank the many thousands of people. We had three space agencies, 17 member nations contributing to the launch of the hardware, the mission. Hundreds of contractors, thousands of individuals in science and engineering. And we have to reach back, all the way back to the early eighties for the folks that did those thankless cost exercises over and over again, all the way up to now, the people that sent the very final commands. Thank you. And the gratitude I believe of the world should be bestowed upon you for the accomplishments of this mission.

Earl Maize: We also need to thank our many millions of fans, the heartwarming buzz that we've gotten from social media, from educational region throughout the world, the media, the more traditional media as well has just been great. Telling the Cassini story, inspiring the next set of explorers is just absolutely as important to us as the scientific results we've found. So thank you very much for that. The Cassini Mission ended this morning, high over the clouds of Saturn. The spacecraft is gone, thanks and farewell, faithful Explorer, but the legacy of Cassini has just begun. The effect that Cassini has and will have on the future of planetary exploration will go on for decades. Thank you and long live Cassini.

Julie Webster: Okay, thank you. I almost have no words. I was supposed to give the chronology, I've been on this mission since it was built. And one of the people, one of the privileged few that actually sat inside this spacecraft before it was put together. My last image was inside of the parts and the wiring as we went in. We've had 13 years at Saturn, but 20 years of an incredible spacecraft that was designed by people. And I can't emphasize this enough, that had 30 years of experience when they designed it, they took all the lessons learned from the Voyagers and the Galileos and the Magellans and Mars Observer, and built a perfect spacecraft, right to the last end. The whole electronic system of the spacecraft ran at room temperature. That's an amazing accomplishment.

Julie Webster: And that speaks to all the individual engineers that built the spacecraft to last. The Mission Planning Team and the Navigation Team that designed their trajectory to get the best bang for the buck, with the scientists. And I remember the mission planners going back and forth with the scientists, there was like seven different trajectories chosen at this point. And I think the goal was to make all scientists equally unhappy. And the goal for our team, my team, and the Navigation Team was to make it last. And I think you saw this morning that we did. We got, actually almost 30 seconds longer than we predicted. It didn't seem like it to me because it was in the flash of an eye. All night long, the minutes seemed like a long time. And then all of a sudden it was over. Cassini as a spacecraft could have gone on a long time, but it accomplished its mission at Saturn.

Julie Webster: We did everything that the scientist asked us to do, and we're really over. During that time, we traveled 4.9 billion miles. We did 292 and a half, 293 orbits, all unique orbits around Saturn, shaped by the Navigation Team and by the Spacecraft Team, by the nav saying, point here and go change your speed this way, the spacecraft performing it flawlessly. We did 360 burns. We planned 472 maneuvers. We executed 360, a little more than half of those on the main engine. The last 21 weeks since April, I was a lot more nervous in April when we dove through the first time I could barely speak, I could barely breathe when we were waiting for that signal to say that we got through inside the rings. And this last time, I have no words because it did exactly what it said it was supposed to do

Linda Spilker: Even better.

Julie Webster: Even better, even better. As it always did, as it always did. Cassini will have questions for the scientists that will keep them up at night. Well, I no longer have a spacecraft that will keep me up at night. And I think after a few days, I think I'm going to really miss that. And don't ask me tomorrow, if I'm ready to build another one, but you can ask me next month. 20 years of test labs and flight hardware and support hardware and a team of 150 people to disperse and break down, but right to the end, it did everything.

Julie Webster: A lot of the team is in here right now and I want to thank again, the Navigation, the Spacecraft Team, the Real Time Operations that also worked the thankless hours, that were the ones that called me in the middle of the night. It went perfect. And I really thought I was going to be more sad about the spacecraft, but I'm not. The spacecraft did everything we asked it to do, everything, right to the very end, that's all you can do for anybody. That's all you want for any human let alone a robot.

Speaker 14: Did any of you have tears today? [inaudible 00:31:41].

Julie Webster: Earl started it. He did. He did have tears.

Earl Maize: I think we messed it up a little bit. I must admit. It was a very emotional moment.

Julie Webster: It's perfect. I played the Moody Blues, My Wildest Dreams, coming in and out of the lab the last few days. So I'd blast it in the car going home and I'd blast it coming back in. This has truly been beyond my wildest dreams.

Linda Spilker: Well, for me, this has been an incredible journey with Cassini that spanned 30 years. I was with the mission from when it was just an idea, after the Voyager Flybys. And now to see it through to the end is truly amazing. And to share that with my family, my personal family and my Cassini family, what a wonderful experience. When I look back over the Cassini Mission, I see a mission that was running a 13 year marathon of scientific discovery. And this last orbit was just the last lap. And so we stood in celebration of successfully completing the race. And I know I stood there with a mixture of applause and tears, because it felt so much like losing a friend, a spacecraft I'd gotten to know so well. And yet in looking ahead, both an end and a beginning, there's so much left, so much incredible science left to figure out and understand, decades worth. Science that will span a generation.

Linda Spilker: When I think about Cassini going in, I know that there's a piece of me there in heart and soul, because I know we signed our signatures on a list of sheets. Those sheets were scanned in and put on a CD, and that CD is onboard Cassini. So a little piece of me went into Saturn's atmosphere along with Cassini. So what an incredible ride. And just lasting for so long. I want to step back just a little bit. If we could go to the first slide, please, this is an image put together by our Visual and Infrared Mapping Spectrometer Team. They did a spectacular job turning around this data set that just came down last night. And this is a view in the infrared, at five microns you can see the heat energy coming out of Saturn. And this is the place where Cassini took its final plunge.

Linda Spilker: And if we go to the next graphic, see the little [aerolits 00:34:18] there, that's where we think Cassini went into the atmosphere of Saturn. So, what an incredible ride and to get that, that was the very last set of VIMS images that came back from Cassini. And so here it is, turned around very quickly for you to see. If we go on to the next set, we had our last Downlink of images, and I'll just look at this and you can share what we saw for our final set of images and data. Saturn, and Enceladus is setting behind Saturn, how beautiful.

Linda Spilker: Part of the mosaic of Saturn and the rings in color. Our last look at this incredible system. Titan, you can see the lakes and seas at the north pole and the haze and the limb, and even better view of the lakes. There's daftness creating its wake along the edge of the Keeler gap and the beautiful structure in the rings. Another view, looking out across the rings, the bright B Ring, the dark Cassini Division snuggled next to it. Views that we're going to miss for a long time to come. That little tiny propeller, that little object just above the dark gap, a large set of ring particles together, trying to force open a gap. And here's Cassini's final image.

Linda Spilker: So what an incredible, incredible, wonderful set of data. And as we went into the atmosphere, we had eight of our science instruments on, including the Ion and Neutral Mass Spectrometer. We had the magnetometer, we were collecting gravity data there to answer questions about Saturn itself, but in particular, trying to understand, as we probed deeper into the atmosphere, the hydrogen to helium ratio. You can't measure helium unless you're directly measuring, you can infer it, you can model it, but to be there and directly measure and sample, that was absolutely amazing. And so that team is hard at work right now, looking at their data and trying to assess what they saw in those very final moments. And I'm sure they'll be very happy that Julie was able to get the spacecraft to survive those extra seconds as we plunged on in. And then of course, the longer term analysis, as I said, that will go on for years.

Linda Spilker: And I just want to thank everyone as well. In particular, The International Science Team, a lot of them are down at Caltech. We had too many to try and fit all at JPL. And so they're down and they've been celebrating and I've heard having a great time from the reports I've heard. And also to thank the public, as Earl said, who have come along with us. And when I think about Cassini, I think Cassini's final gift to humanity was the fact that we knew the day, the hour, the minute, and now the second of the plunge. And so we could gather together with the scientists, the engineers, with the public, with our own families, you can think of us as a giant worldwide Cassini family, and share this final moment of the plunge and have that memory to add to our Cassini scrapbooks. And if I had one thing I could say to Cassini, I'd say goodbye Cassini, thanks for the ringside seat at Saturn. And as Thomas said, we'll be back. Thank you.

Mat Kaplan: Earl Maize, Julie Webster and Linda Spilker of the Cassini Mission. A little bonus before we go on. My colleague, Emily Lakdawalla was in the throng of reporters that morning at JPL. She was the only one who got to ask two questions.

Emily Lakdawalla: So, Emily Lakdawalla with a follow up, what does it mean that you had this signal for 30 seconds longer? Is that just the usual overperformance of the DSN and locking on the spacecraft? Or does it mean that the spacecraft was able to fight the atmosphere for longer?

Earl Maize: I think it's a little bit of everything. The spacecraft, I think we did come in a little bit later than we thought, that just didn't delay the demise, it just delayed the start of it. But, well I thought the DSN has just been phenomenal. They've been tuned in as well as you could. And the spacecraft, all of our modeling is we don't have any real world experience with this. And so to be within 13 seconds of our predict is, for us, that's a homerun.

Mat Kaplan: Up on the stage in Von Karman Auditorium was a man who looked as happy as anyone in the room. He had a right to be pleased. Most of Cassini's time at Saturn has happened during his tenure as NASA's Head of Planetary Sciences. Jim Green, a great day for Planetary Science.

Jim Green: Absolutely, Mat, it's a beginning, in addition to what we think of as an ending with the loss of a spacecraft, but in reality, we have so many more things that we want to do. It really leaves us a legacy of fabulous data to continue to interrogate, find new and exciting things in it. And then also what we know already that has come out, allows us to plan for future missions.

Mat Kaplan: And when something like this happens, this kind of glorious success, I assume it makes it easier to go to Capitol hill and tell people how valuable this is.

Jim Green: Yeah, it does in the sense that we want to bring everyone along with all our missions, but we're still following what's in the Planetary Decadal. We still are marching to an important drum that the planetary community is all put together for us. I've been called a decadal zealot before, and I'm just happy with that because that really keeps us all together. What we're discovering now, and what we're discussing now really is going to largely feed into the next planetary decadal that will make this next decade, that begins in 2023 for planetary, that's when the start of the next decadal is, is going to be remarkably exciting. I can just guarantee it.

Mat Kaplan: You don't want to preempt that next decadal study, but what else do we need to do in the outer solar system?

Jim Green: Well, even in this decadal, there's two major planets that we really have only flown by and that's Uranus and Neptune. Now we call them ice giants because their composition is significantly different than Saturn and Jupiter, which we call gas giants. They also have an array of fabulous moons. Some of which we've gotten a glimpse of with the Voyagers and are already really excited about them. Like Triton, which is pretty spectacular. In fact, Triton is probably a Kuiper Belt Object, and I believe it's even bigger than Pluto.

Jim Green: So if it was orbiting the sun, it'd be another dwarf planet, I guess. But indeed we've already started making some studies of that. We just completed, with a science definition team, what are some of the things that we could do out at Uranus and Neptune? And we've come to a realization after tens of thousands of orbit trajectories and analysis that they do, that there's a window for which we could actually create two identical spacecraft and with the right gravity assists, launch them at the end of the next decade and go one to Uranus and one to Neptune and knock them both off. Pretty spectacular.

Mat Kaplan: Absolutely. We got a quick update from Bob Pappalardo a couple hours ago at Caltech about Europa Clipper, also moving along well from your end of that project?

Jim Green: Oh yeah. Europa Clipper is doing great. Yeah. I'm really, really proud of the team. You know, planetary is once again, I think pioneering another approach to larger missions. We want them. We want them to be highly capable. We want them to be strategically aligned.

Mat Kaplan: Are you talking about Flagship Missions?

Jim Green: I'm talking about Flagship missions. Another name for those are strategic. And typically they're the most expensive ones we do. For us to be able to really do the agenda that's in the decadal and look forward to a rather comprehensive program, well into the future, we're developing them under a cost constrained environment. We, from headquarters, are putting this constraint on our mission, on our teams and we're making trades all over the place to be able to keep our missions at a particular cost. And that's important for us because if we can contain our cost and we're getting better at this. So, this is remarkable, in the last 10 years that I've been at headquarters, it's remarkable about what we've been able to do in terms of maintaining costs, we then can do more missions. And that's, to me, one of the really critical aspects about what we're trying to do.

Mat Kaplan: And we talked about this on the last of the monthly version of our show that we do, The Space Policy Edition, about this report that came out recently about Flagship Missions. And it talked about this progress that is being made in estimating and controlling costs, which I guess is what you're talking about.

Jim Green: Yeah. I don't know how many people out there actually have had their kitchen done, but I can guarantee that you get your initial estimate and that's not really what you end up paying for. And yet how many kitchens have been done in the United States? What are we talking about? So, in space and planetary science, when we do one of a kind missions, going to remarkable locations, and we do the best job of estimating what it costs, it's really hard. It is just really hard to do, but we have to take everything into account. Now there is a lesson learned, to me, that came out of this mission right away that we just got to implement. And that is, we need bigger thrusters, because if we had a bigger thruster on Cassini, it would've last longer into the end of the atmosphere.

Mat Kaplan: Might have gone three minutes instead of a minute and a half.

Jim Green: Absolutely.

Mat Kaplan: There was a mention of New Frontiers proposals. Is there room in the outer solar system for those less expensive spacecraft, New Frontiers discovery?

Jim Green: Well, indeed, in the New Frontiers list is Saturn Probe getting into the atmosphere and much deeper. So when Cassini went through the rings, everyone says, okay, there's not much there, but the plasma wave instrument indicated that it really got hit by about six or seven particles. So when you think about the size of Cassini and the fact that the rings are pretty small height-wise and getting hit by six or seven particles, it's raining out there. It's not the latest hurricane you have, but that materials is fallen out of the rings. That's adding to the composition of Saturn. And then the composition of the rings as we can get it, is really coming from the top of this atmospheric measurement.

Jim Green: And here's the composition of the rings added to Saturn's atmosphere. So that's going to tell us perhaps where the origin of the rings comes from. It's that kind of science that's really exciting. So Saturn Probe, fabulous, wanted to do that. That was always on the list. But we've also added in Enceladus and Titan, and those are new, really new objects that Cassini's really proved in the Saturn environment, that we just absolutely have to go back to. Now, we may not be able to execute those under that cost cap, but scientists are going to give it a try. Well, we'll see. All the New Frontiers proposals are currently under evaluation.

Mat Kaplan: I can't remember. Have you been running Planetary Science during the entire time that Cassini has been at Saturn?

Jim Green: Not quite. Not quite, almost. Yeah. So this is, I'm going into my 12th year now. Cassini always had a special place in my heart because when I went to Goddard Space Flight Center, I was working really hard to get on a Cassini team, proposal team. I didn't end up on a proposal that made it. Others beat us out. So, if I'd have won, I'd have been a member of the team and I wouldn't be doing what I'm doing either.

Mat Kaplan: Well some people would probably say, they're glad you didn't win because they want you doing what you're doing.

Jim Green: Some people. There might be other opinions, but yes, I would hope so.

Mat Kaplan: Any place else you would've rather been today?

Jim Green: Oh, no. Wild horses couldn't drag me away. You kidding? I mean, I would love also to have been down to the mall, at Caltech, because that's where a lot of our scientists were. I can't be in two places at once, but both I'm sure had... The excitement here was electric and I'm sure that's what was going on down in Caltech.

Mat Kaplan: It sure was. It was terrific. I'll end where we started. Great day for Planetary Science. Really a great day for science.

Jim Green: Absolutely. And more to come.

Mat Kaplan: Thanks, Jim.

Jim Green: My pleasure.

Mat Kaplan: Jim green of NASA. A few feet from Jim was one of his former NASA colleagues. We last talked with John Grunsfeld when he was still NASA's Associate Administrator heading its science mission directorate. Like Ellen Stofan, John had also served as the agency's chief scientist and he rode the space shuttle into low Earth orbit on five missions.

Mat Kaplan: You were in charge of the science mission director at NASA during a good piece of this mission that we just witnessed the end of, I can certainly understand why you're here, but how does it feel?

John Grunsfeld: Actually it's really gratifying. The Cassini Mission was, we can say that now, so incredible from start to finish. And it's a great example of what I think NASA science does best, which is to come up with something extraordinarily challenging, aspirational, where we are going somewhere that we don't know the answers, we are going into an environment where we don't know what we'll encounter. And Cassini of course had strong science rationale, strong science requirements. But here we are now, looking back at really decades of incredible science and incredible discoveries. And so when I start getting a little bit sad that the Cassini spacecraft is no longer, that it's now part of Saturn, I only have to think about the incredible plumes on Enceladus or the lakes on Titan or the incredible images that we've gotten of Saturn. And I start getting goosebumps as I am right now, that this has been an extraordinary ride and we'll continue because of the science.

John Grunsfeld: For so many of us, Saturn has a special place in our hearts because as wonderful as the rest of the solar system is, as wonderful as our universe is, our Hubble's Universe, when you're a kid and you're looking in a backyard telescope, Saturn is really the object that jumps out at you as wow. We live in an incredible universe. For so many of us, it has inspired us to go on to do science, to do astronomy, to do astrophysics, to become astronauts. And also Saturn is unique in that, of the gas giants, Jupiter and Saturn, Jupiter's radiation environment is so hazardous. I don't think people will ever go there. Ever is a strong word, but Saturn is a place we could actually go someday and explore as human beings. And so Cassini has given us our first taste of that. Of course, all of the science, all of the engineer and Cassini is a human effort. And who knows, humans may someday go and sail over the rings and wonder at their incredible beauty.

Mat Kaplan: A little choked up about that. As somebody who's been out there, you'd like to see humans out there, that far.

John Grunsfeld: Well, once we develop rocket engines, fusion drives, where we can physically go out and explore the solar system, as we do low earth orbit, I think Saturn will be a desirable place to go. We have to go back to the why, and the why is really the wonder, are we alone in the universe? To me, that's the grand question. And in order to answer that question, we will probably need to go and visit Europa. That's why we started the Europa Clipper. We will need to go and visit Enceladus. We will need to build telescopes that can look at planets around other stars. And all of these missions are, by any definition, large strategic missions, Flagships, and will be expensive.

John Grunsfeld: But if that's what it takes to answer that question and many more, it's worth it. And let's not forget that going to the outer planets is certainly the best way to get the most detailed information, but we will, in a little over a year, launch The James Webb Space Telescope. And it is so capable to observe Saturn, Saturn's moons, Neptune, Uranus, even diminutive Pluto. And of course, Jupiters moons. That it's almost like having a mission going to those planets. And I think we are going to be startled at how useful that is for learning more about these and enigmatic worlds.

Mat Kaplan: This mission, obviously its legacy is shared by thousands of people. You are certainly part of that group. Is there any place else you would've wanted to be today?

John Grunsfeld: No. As your listeners know, I'm retired from NASA now. And so I came here on a personal trip. I really wanted to be here, to be part of this because we love Cassini, long live Cassini.

Mat Kaplan: Thank you, John. Glad you made it.

John Grunsfeld: Thank you.

Mat Kaplan: John Grunsfeld, astronaut and former NASA Associate Administrator. My thanks to all of the guests you've heard on this week's show and to NASA and JPL and my congratulations to the 5,000 men and women of the Cassini team for a job well done. Ellen Stofan, former NASA Chief Scientist, former Director of the National Air and Space Museum is now Under Secretary for Science and Research at the Smithsonian Institution. With oversight of its Science Research Centers, along with the National Museum of Natural History and the National Zoo.

Mat Kaplan: Jim green would also become NASA's Chief Scientist, a job he has only just retired from. His hope for a mission to our solar systems ice giants has been furthered by the recent recommendation of the National Academy's Planetary Science Decadal survey of a Uranus orbiter. Linda Spilker remains the Cassini Project Scientist. She has also returned to the Voyager Mission as Deputy Project Scientist. She and other Cassini leaders joined me on stage at Caltech one week after the mission ended. We've got a link to that September 27th, 2017 episode on this week's show page at You'll hear her again soon, as part of my Planetary Radio Live panel in London. Here's that brand new visit with Bruce.

Mat Kaplan: Time on the special edition of Planetary Radio for a special edition of What's Up with the Chief Scientist of The Planetary Society, there's Bruce Betts. Welcome back.

Bruce Betts: Thank you, man. I'm feeling special.

Mat Kaplan: I am too. I'm feeling really special because as people hear this, I'm in London, probably walking through the country and hopefully will have just enjoyed this Planetary Radio Live event that we did on Monday, May 23rd, about the Moon Symphony. So yeah, I'm a happy guy.

Bruce Betts: All right. Have a scone for me.

Mat Kaplan: I will. Yes. With clotted cream, which sounds so awful, but really is delicious.

Bruce Betts: Okay. Should we talk about the sky? The one you'll see, because you're staying in the Northern hemisphere.

Mat Kaplan: I'm ready.

Bruce Betts: Actually, this first stuff should be visible Northern or Southern hemisphere. And that is the planets in the pre-dawn sky. Venus and the moon, crescent moon, very close together, low on the Eastern horizon. Venus looking super bright, near the moon on the 26th and 27th. So shortly after this comes out. Interestingly, in our line of planets, we've got bright Jupiter and dimmer, but still bright reddish Mars moving very close together on the 29th. On the 29th, in the pre-dawn, look in the east and there's the super bright Venus down below, and then bright Jupiter and reddish Mars will be closer than a moon diameter to each other, in the sky. If you miss that morning, they'll be near each other for a few days before and after. We've got Jupiter moving relative to each other, up in the sky and Mars moving down in the sky. And then Saturn hanging out safely, far to the upper, right, just avoiding the tussle.

Bruce Betts: In the evening sky, I feel like I've abused it because all the planets are in the morning sky. If you look up high overhead and this one is a Northern hemisphere thing, sorry. You will see Arcturus, which you can also find by going to the Big Dipper and following the arc of the Big Dipper. And Arcturus, you'll come to a very bright star, Arcturus. Now, if you're up at three in the morning, then look up there and you'll see the bright Vega. That's our, This Week in Space History. No, it's not. This is our, This Week in Space History. 2008, the Phoenix Lander successfully landed in the polar regions of Mars, hunting ice and carrying, by the way, The Planetary Society's Vision of Mars mini DVD. The first library on Mars with science fiction, science fact, greetings from Carl Sagan and others, to future explorers of the red planet, as well as a whole bunch of names of people who wanted to go to Mars.

Mat Kaplan: And that naughty spacecraft dropped Mars dirt all over our nice pristine DVD, but that's all right.

Bruce Betts: It did. And I got to say, it made me really happy. It's like, oh, it's so cool, there's Mars dirt on our DVD. Enough about me. Let us move on to Random Space Fact or Random Space Fact.

Mat Kaplan: Nice, in honor of my trip to the UK. Thank you.

Bruce Betts: You're welcome. So this is a story of a spacecraft called, originally called AsiaSat 3, something I would normally not talk about, a telecommunication satellite destined, ideally for geosynchronous or geostationary orbit around the Earth to do communication. Well, it turns out, Russian launch vehicle screw up, and so it ended up in a bad orbit. Long story short, it was bought by [Hughes 00:57:05] and they sent it on several elliptical orbits. They had enough fuel to send it, get this 6,000 kilometers away from the moon. It went by the moon and used a gravity assist there and then a follow on to get back into, although not the orbit exactly they wanted, they made a geosynchronous orbit that was salvage the mission. And strangely enough, this sort of makes it the first commercial lunar spacecraft.

Mat Kaplan: Oh my goodness. Yeah, I suppose so. Fascinating. Man, I hope that whoever laid out that trajectory got a nice raise.

Bruce Betts: I read they patented the trajectory, which I didn't, could not have imagined.

Mat Kaplan: So, we don't have a winner this week, but stick around. We'll have two to make up for it next week, which means we can go straight on to a new contest because we do have one of those.

Bruce Betts: Good news is we don't have any losers this week either.

Mat Kaplan: That's true. That's true. We always have a lot more losers than winners.

Bruce Betts: We just don't have a question that corresponded to giving an answer a win this week because someone decided to travel the world during this period, but we've got a new one for you. Name all the US planetary spacecraft, wouldn't it be fun if I just stopped there? But instead, name all the US planetary spacecraft by which I'm defining beyond Earth orbit, including the moon. All US planetary spacecraft launched in the 1980s. Name them all, go to contest, launched in 1980s.

Mat Kaplan: I do know that mentioning the 1980s, this is going to make this a little bit easier than it might have been otherwise because at least it's going to save some space. That's enough, pun not intended. You have until Wednesday, June 1st at 8:00 AM Pacific time, to get us the answer for this one and the prize we're going to have for you, if you're chosen by and get this right, is in my hand. It's Mary Roach. Remember her Bruce? She wrote that great really funny book, Packing for Mars. One of the funniest, maybe the funniest guest I've ever had on the show other than you of course.

Bruce Betts: Oh, good save.

Mat Kaplan: She has now written Packing for Mars for kids. It's this beautiful hardcover book from Norton Young Readers. It's nicely illustrated and it has Mary's, what should be her patented humor, her approach to things, it's really a terrific read. Whether you're a kid or not, I enjoyed it very much and that's it. That's the prize. I think we're done.

Bruce Betts: All right, everybody go out there looking up the night sky and think about Mat in the tower of London as a visitor, maybe. Thank you and goodnight.

Mat Kaplan: Checking out the jewels. Well, not this trip, did that last time, had one of those nice talkative [B feeders 01:00:16], show us around the place. Queen's got a nice place, I got to admit. She has several nice places. Went to Windsor too.

Bruce Betts: On personal invitation?

Mat Kaplan: Yeah. Yeah, of course, but not the Queens. He's Bruce Betts, the Chief Scientist of The Planetary Society who joins us every week here for What's Up? Planetary Radio is produced by The Planetary Society in Pasadena, California, and it's made possible by its members. Become part of our adventure at Mark Hilverda and Rae Paoletta are our Associate Producers. Josh Doyle composed our theme, which is arranged and performed by Pieter Schlosser. Ad astra.