It’s time to say goodbye to an old friend, NASA’s Cassini spacecraft, which has been orbiting within the Saturnian system since 2004. But why does a seemingly healthy spacecraft and mission need to come to an end? Today we look back at the mission, some of the amazing discoveries, and why its finale was necessary.
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David Joseph Wesley’s Goodbye Cassini video with Fraser
NASA Cassini Mission
Cassini Science Overview
Dr. Carolyn Porco
Miles Traer’s Twitter cartoon Cassini
NASA Office of Planetary Protection
NASA Live: JPL – Previewing Cassini’s Grand Finale
NASA at Saturn: Cassini’s Grand Finale
NASA Mission Control Live: Cassini’s Finale at Saturn
Transcription services provided by: GMR Transcription
Fraser: Fraser, here. Once again, it’s time to thank Casper Mattress for their generous support of Astronomy Cast. I’ve been using my Casper Mattress now for several years, and I can tell you it’s great both in the wintertime and in the summertime, and it’s really holding up after all this time. I still look forward to going to sleep at night in my Casper Mattress. Half the fun of getting a Casper Mattress is that they ship it in this small box and the thing unfolds. I’m sure they’re using some kind of pocket dimension. They’re really breaking the laws of physics to actually get this thing to you. They do free shipping in the U.S. and Canada.
You can try it out for 100 days, and then return it free if it’s not for you. And Casper is gonna give you a discount if you wanna get a new mattress. So, go to Casper.com/astro and use the promo code: astro and get $50.00 towards any mattress purchase. Terms and conditions apply. So, once again, just go to Casper.com/astro, use the promo code: astro and get $50.00 off any mattress purchase. Terms and conditions apply. Thanks, Casper.
Astronomy Cast, Episode 457: Cassini in Memoriam. Welcome to Astronomy Cast, a weekly facts-based journey through the Cosmos, where we help you understand not only what we know, but how we know what we know. My name is Fraser Cain. I’m the publisher of Universe Today. With me is Dr. Pamela Gay, the Director of Technology and Citizen Science at the Astronomical Society of the Pacific and the Director of CosmoQuest. Hey, Pamela, how you doing?
Pamela: I’m doing well. How are you doing, Fraser?
Fraser: Doing great. Even though this is gonna be a super sad episode. There won’t be a dry eye in the place. Let’s talk about the Patreon for Astronomy Cast now that we’re here for an all-new season.
Pamela: So, as we go into our 11th year, for the past 10 years, Fraser and I have pretty much done this out of the goodness of our hearts, by which we mean we weren’t bringing in enough money to pay anyone other than Susie and Chad, and before Chad, Preston. And before Susie, well, there were a lot of people. Anyways, we’ve always been a really small crew and we’ve done this on a shoestring, and we’d like to upgrade the whole shebang, and more to the point, I’d like our quality to be a little bit closer to the quality of Fraser’s Weekly Guide to Space.
We know what quality looks like, we know we can do this, but in order to do it, we need to pay ourselves. So, can you guys help? Can you please, please go to Patreon.com/AstronomyCast and give a little. If even 10 percent of you give $10.00, we’re done begging. We’re set. I know that’s not gonna happen, but can at least 10 percent of you at least give $1.00? Maybe? Maybe?
Fraser: I mean, I think we’ve demonstrated that we’ve been here for a while, and we’ll continue to do this job and it would be wonderful with your ongoing support. So, just go to Patreon.com/AstronomyCast to support us. Even $1.00 a month is huge and makes a big difference in us being able to pay the team, the server, the other fundraising things that we do, as well as, theoretically, us. So, alright. Onto the show. It’s time to say goodbye to an old friend. NASA’s Cassini spacecraft, which has been orbiting within the Centurion System since 2004.
But why does a seemingly healthy spacecraft in mission need to come to an end? Today we look back at the mission, some of the amazing discoveries, and why its finale was necessary. Alright, Pamela, so when we’re recording this right now, we are actually a couple of days away from the finale. We’re actually recording the show on the 12th. When’s the finale?
Pamela: It’s Friday. It’s September 15th, just a month shy of poor Cassini’s launchaversary.
Pamela: It launched on October 17th, 1997, so Cassini almost lived long enough to drink, but not quite.
Fraser: But that’s still amazing, right? 1997, that was – well, in Canada it would be fine, but I started doing Universe Today in 1999. So, when I started reporting, I was already reporting that Cassini had already left, had already been launched, and I think, if I remember, the first thing that we were talking about was the fly-by that it was gonna do of Earth. So, it actually made a couple of fly-bys of some other planets before making its way out to Saturn.
Pamela: Right. So, in 1998, it did a fly-by of Venus, and then again in 1999 it did a second fly-by of Venus. This is where we use gravity to increase velocity of the spacecraft without having to use as much fuel, which is good because it needed all that fuel, now that it’s out at Saturn, than it did in 1999, an Earth-moon fly-by. It went through the asteroids just a little bit shy of 2001. At the end of 2000, it went and explored Jupiter, did some camera testing in 2002, and then 2004 was when the gorgeous thing started to happen.
Fraser: And again, now I remember reporting on this stuff item by item, and we got a chance, not only to see these better and better images of Saturn as Cassini was getting closer and closer, but I remember the first moon, and I think it was Hyperion was this first – it looked like a sponge that it went past and began starting to do these observations of the moons and Saturn.
Pamela: And for me, this was actually the beginning of my podcasting days because it was back in February of 2005 that we launched Slacker Astronomy. And Cassini and all of the things that it went through was the early days of us talking about the voices in our heads and all they had to say about science. So, it’s weird that we grew up as children with Voyager, but then as science communicators, we really grew up with Cassini.
Fraser: You hit the nail right on the head. Cassini has been our constant companion throughout both of our science communication careers. For the length of time that both of us have been doing this, Cassini has been this mainstay. And I’ve mentioned this previous times of the podcast where I would get emails from Dr. Carolyn Porco when I had made some mistake, or failed to provide the credit, or just needed a little more information, and she was right there to clean things up, fix it, make sure I understood the right way to describe the images and the signs that we were seeing.
Pamela: And I was lucky that I didn’t experience any of that because I think it would’ve completely mortified me because she’s one of the senior women scientists that the rest of us are like, “Wow. She went through a lot. Let’s live up to that if we can. But on my side, this was our chance to figure out how to tell the story. It was our chance to have a story that included so much diverse science, so much hardware, software, men, women who’d been working on this their entire lives, the spacecraft that had an amazing legacy. So, this was one of those rare instances where you have the narrative, the heritage, the science, and all the pretty, shiny pictures, all bound up in one little mission.
Fraser: And I think as well, one of the other parts that was really groundbreaking with Cassini was that it had this second mini-mission connected to it, which was the Huygens Probe developed by the European Space Agency. So, you had this collaboration between NASA and the Europeans to create this dual mission with this amazing little buddy.
Pamela: Yeah, and this really pointed the way to Rosetta and Philae where you have the main spacecraft and then the little parasitic guy that flies along like a lamprey that jumps off when things get interesting. And Huygens totally worked, and totally brought us science that – I think some people had anticipated that Titan would be amazing. We knew that it was weird from the Voyager missions that suddenly discovered, “Oh, this little moon. We can’t see through its atmosphere. This is sad and disappointing. Let’s send something back to go through the atmosphere.” And little Huygens did that.
And on this world, that’s at the triple point of methane, where you have methane, can snow, can flow, can beat as the gashes form. On this world, you have a geology that’s shaped by methane with river deltas and lakes and all this amazing, icy geology that because of its lower gravity, it is like Miniature Earth, but stinkier.
Fraser: Right. And the fact that what was down there beneath these clouds was essentially completely unknown. They could only see from Earth with the powerful telescopes that there was some kind of hydrocarbon in the atmosphere and it was incredible intriguing when it sent the Voyagers. Voyager 1 made a close fly-by, but couldn’t see through this hydrocarbon, so one of Cassini’s main purposes was being able to both land on the surface thanks to Huygens, but also be able to peer through this thick cloud and be able to see the surface. And what it found, I think, nobody expected, and as you said, one of the most amazing discoveries in the whole solar system.
Pamela: And this is essentially a world that doesn’t appear to be in complete chemical equilibrium using chemistry as we know it, so we either have to redefine geology, redefine chemistry, or find life, pick one at least when it could be all three, and as we look, we see these lakes that are even more than just straight out methane. There’s a variety of different hydrocarbons. They’re perfectly mirror smooth and with the variety of instruments that Cassini took with it – this is a hefty spacecraft. It is able to see in multiple wavelengths. It’s able to take samples. It has radar combining all of its capabilities, it was able to show us that other worlds have great lakes and other worlds have rain, and weather, and storms, that are not just dust but are also wet. It’s not just water.
Fraser: Who knew that the best way to explore Titan would be with a submarine? That that’s what we need in the future is some kind of submarine, a boat, a sailboat to better explore the oceans and seas on Titan.
Pamela: And how lucky did we get with Huygens that when it landed, it didn’t land in one of these lakes, that instead it basically landed somewhere that kinda went crunch.
Fraser: It could’ve. It was able to float.
Pamela: It was, but I think the fact that it landed somewhere where the soil just sort went crunch, like stepping out onto ice covered snow is somehow even more satisfying.
Fraser: Yeah, absolutely. But that was just one of the moons. It saw Iapetus with its amazing ridge. It saw Rhea and Dione, but I think the most interesting one is what it found at Enceladus.
Pamela: See, I’m still gonna go with Pan the ravioli.
Fraser: Sure, the UFO? I think it looks like a little UFO, but I understand that it also looks like a ravioli. Maybe raviolis look like UFOs. But Enceladus, for pure scientific value, again, one of the most amazing discoveries in the entire solar system.
Pamela: And this is, again, a world that we expected to be kinda boring. It was shiny, we knew that. We figured it was icy because shiny usually leads to icy. And when we first got pictures of it, sure, it’s fairly cratered. It’s not hugely cratered, but it’s definitely up there when compared to Europa, but when you start looking, you start noticing this is a little moon that has a geyser issue. Specifically, it has these dark cracks, these dark stripes, and we, on a fairly regular basis, see geysers of material coming out of these stripes that Cassini was actually able to fly through and sample, and these are ice geysers. How awesome is that?
Fraser: Yeah, absolutely. And now further on observations have shown that coming out of these geysers are hydrogen that could be the ingredients, the food for life deep down below the ice on Enceladus. Perhaps the same situation exists on Europa, whole different planet, but still.
Pamela: Europa’s a moon, not a planet. Moon or world, pick one.
Fraser: I meant around Jupiter, so Europa is orbiting Jupiter, not Saturn, but both of these places are now premium places to search for life and will be the – what Cassini showed us was that these are some of the most interesting places in the entire solar system and that this is where we need to bring far more energy, and technology, and exploration to in the future.
Pamela: And it wasn’t just the moons of Saturn, it’s also the rings and the dynamics of them that we’re still just beginning to understand as Cassini in its final days and its final dives gets up close and personal with the wave structure of the rings.
Fraser: Because it was there at Saturn for so long, it got a chance to go through a huge chunk of Saturn’s seasons, so it got to see the planet move through these seasons and see how the changes in temperature affected the storm patterns on Saturn. It got to see that enormous polar vortex, this hexagon shaped storm that is sort of around Saturn’s pole – north pole for sure, and see that with just amazing clarity.
Pamela: And what’s amazing, as you said, we got to see it go from solstice with the sun at its extreme point on one pole through equinox heading back through the other set of phases, and so we’ve now got to see this seasonal cycle both in terms of how storms will stripe around the entire Saturnian world, these big white startings of hurricane and then evolving, essentially into a band of storms. And we’ve seen in parallel storms on Titan showing us that you can get weather in multiple places, and this is all new and exciting for us to get to figure out.
Fraser: And you mentioned the rings. I think one of the most amazing part of the rings is that they got to the bottom of how several of the rings are formed and also actually could see some of the fine, fine details in the rings, like these wonderful little moons that move through and put gravity waves into the rings. How they’re these moons that can create these crazy wall structures in the rings. Things again, that nobody ever expected to see.
Pamela: And as we’re looking at this, there was a fabulous cartoon that came out today – again, today being September 12th, episode going out on, I believe, the 18th – Miles Traer – and we’ll work on getting this up in our show notes – put out a cartoon on his Twitter GeoMiles account where poor little Cassini is orbiting and orbiting and orbiting and is like, “Whoa, hexagon! Cool! I’ll collect more data.” And everyone’s like, “That makes no sense,” and Cassini’s like, “Oh, I’m sorry. I’ll try harder,” and then trying to measure the Saturn day, still can’t figure it out, that still makes no sense.
And Cassini is getting more and more frustrated as Cassini fails in his little robotic heart to be able to provide Earth with data that helps explain all of these mysteries of these mirror-smooth lakes, of all these ice plumes on Enceladus, of the age of Saturn’s rings. We still don’t know. There’s still, like every other week, it seems like a press release on Saturn’s ring age.
Fraser: They’re young. They’re old. They’re young. They’re old.
Pamela: And the magnetosphere, all of this, it’s one of these cases of we sent the spacecraft. We were going to get all the answers, and what we really got was we tripled the number of questions, and that’s really what science is about. It’s about having questions, trying to answer them, and constantly evolving. The parameter space we don’t know. Well, hopefully making little, tiny inroads, a little, tiny step at a time as we work to understand more, and more, and more.
So, yes. We saw the waves from the gravitational dynamic of the moons creating these gorgeous structures in the rings. We have seen weird pile-ups of organics on one side and crystal white ice on the other side of moons. We’ve seen so many different things that we’ve begun to start to have models to start to understand. But mostly, we just have questions.
Fraser: Right. Which is, as you said, that is the job of the scientific missions. They answer the previous set, or try to answer the previous set, and then uncover a whole new set of questions, and Cassini would be no different from that. Earlier this year, was when the mission – it was extended several times, but really earlier this year was when the mission started to begin its finale, and there was gonna be 25 close passes through the rings leading to what’s gonna be happening on Friday. So, let’s talk about why Cassini has to die. Why does Cassini have to die?
Pamela: Well, at the end of the day, when we launched Cassini back when I was still working on my Master’s Degree, and didn’t have any time for this stuff called planets, back in 1997 and in the years and years before that when this mission was getting planned, argued for, constructed, all of that, the idea that there could be life out in the Saturn system wasn’t an idea we had. And so, it isn’t a sterile spacecraft. In fact, it was deemed a Category 2 moon for Titan. That means you can just go there unsterilized your space craft, hop on down, explore, we’re all good. But our ideas of what is and isn’t habitable have evolved in the past 20 years.
And as our understand has evolved, we’ve realized, well, expletive. If we accidentally crash Cassini into Enceladus, into Titan, into these worlds that just might be able to support extreme forms of life, we could kill everything before we learn about it by sending our own germs and bugs that just might be lethal. So, as the mission begins to run out of fuel to do maneuvers, we need to use some of what’s left of that fuel to essentially sterilize the Saturn system as best we can by killing our own germs in the atmosphere of Saturn.
Fraser: Yeah, and this is not the first time this has happened. This was done was Galileo as well, again, to reduce the risk that it was going to crash into Europa. It will be the future for Juno.
Pamela: This is what we do to the spacecraft we love. We kill them.
Fraser: We kill them. Right. And so, you’re gonna get this – I mean, if you just left it, before you could control it, before you could do this controlled finale, the spacecraft would just be going around and around within the system and the chances of it eventually crashing into one of those moons is just too high, too much of a risk to take, and so they have to crash it.
Pamela: And in general, this has been a fairly risky mission if you think about it. We have a ring world, which means it’s ringed with dust and debris that can hit and destroy your spaceship. It is a world with a very complex gravitational field. We just used the mass of Titan to – the language they’re using, which I really don’t like, is to kiss Cassini –
Fraser: Shove it into Saturn.
Pamela: Yeah, the idea that Titan’s gravity was the kiss of death to Titan, a lot of the imagery associated with the reporting right now between saying that Cassini is committing suicide, that it bad imagery. Don’t use that, people.
Fraser: It’s being murdered.
Pamela: Yes. This is willful murder by mission control. Cassini is not –
Fraser: It’s being sacrificed? That’s the way to say it.
Pamela: Yes. It is dying that other life forms that may or may not exist can live.
Fraser: Yeah. So, over the next couple of days, how are things going to play out? I think I got an email from Dr. Porco just a day ago that they’ve just finished the 22nd. You said they did this final fly-by of Titan to get that final slingshot maneuver to be on its death trajectory. How are things gonna play out now?
Pamela: So, at this point, they’re sending back data. This is something I know a bunch of us are really looking forward to because that was one of the closest – I think it was actually the closest that we’ve gotten to Titan. And at about 5:00 a.m. Pacific, 8:00 a.m. Eastern, on Friday is when we should get the final signal.
Now, the final signal doesn’t mean the spacecraft is dead. It means the spacecraft has stopped transmitting. The spacecraft will begin to enter the atmosphere at probably about 3:45 a.m. Pacific, and it takes time for the signal to get to us, so there’s this disconnect between – the light of if we were able to watch it die won’t have reached us yet when we get the final signal, but it will have already entered the atmosphere, but the light won’t have gotten – it’s relativity.
Fraser: Right, right, right. You have to wait for the time for the spacecraft signals to reach us thanks to the slow speed of light.
Pamela: And so, here we are, the northern summer solstice has begun. It saw the beginning of that new spring on the world and now it’s gotten its up-close and personal views of the rings. It has performed science like a champ sending us back, gigabyte upon gigabyte of data. And now, the final orbit is under way. And what’s kind of amazing is it began this final grand finale of 22 orbits a long, long time ago because it’s not a fast process. It’s a big world. These orbits have been dragging it out past the outer rings, and so it takes time and we’ve all been sadly watching these last 22 orbits go by.
Fraser: Now, a couple of questions that I’m sure people are gonna have is how long will it take? Like, how deep will it get into Saturn before it’s destroyed? Will it reach the surface?
Pamela: Well, no. No. It will be crushed like a Styrofoam cup going into the ocean. We don’t know exactly when it’s gonna die. There’s just certain things that we can’t know.
Fraser: It won’t get far.
Pamela: It won’t get far, no.
Fraser: Do you remember when Galileo was crashed into Jupiter? People were freaked out that it was gonna turn Jupiter into a second sun.
Fraser: I guarantee they’re thinking about that this time. So, what would it take to turn Saturn into a second sun?
Pamela: Several Jupiters. By that, I mean, like, 20 of them.
Pamela: Well, if you want it to only burn briefly with tritium, you don’t need quite as many, but yeah, it’s not gonna happen. We’re good. We’re good. So, here’s what we’re looking at that will already have transpired by the time we hear this. Those of you listening to this as a podcast, well, it was Tuesday night, the night that after we’re done recording this, that the downlink of that final Titan data began.
Then on Thursday, there was the final images to be taken late afternoon here in the United States, and the spacecraft turned its antennas towards us and spent that evening sending us back a playback of the data recorder and for 14 ½ hours, it just continuously said, “Here’s my data. Go understand my world.” It ended its communications with science. The signal got passed from station to station where it was actually the Australian station that got to say goodbye.
Fraser: At this point, you will hopefully have seen the final picture, which has not been sent yet, but it’s gonna be that closest possible photo of the surface of Saturn, or the cloud tops of Saturn. Although, I mean, the close pictures that have already come back are just amazing. How much closer it had ever been. But we will get that final picture as it’s roaring in to wrap up the mission.
Pamela: It’s really those ring crossing pictures that get me. Just seeing all the fine structure in the rings. And like I said, they still, on a regular basis, are like, “They’re new. They’re old. They’re new.” We still have so much more to learn.
Fraser: You’re literally tearing up.
Pamela: I’m trying not to cry.
Fraser: Alright, well I think we may have another conversation next week, so I hope we get that final picture. I hope that final picture is amazing. I know the science was worth it. Thank you, Pamela, and we’ll talk to you next week.
Pamela: Sounds great, Fraser. Talk to you later.
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Duration: 30 minutes