Cassini: The Bigger Picture
My perspective as a former JPLer
This is the text version of Randy Cassingham’s “Uncommon Sense” podcast with his thoughts on Cassini’s End of Mission, recorded on 15 September 2017 after the spacecraft’s “loss of signal” was confirmed. If you prefer to listen to this essay, you can download the MP3 or stream it from the episode’s Show Page. The podcast serves Randy’s life mission to promote more thinking in the world.
I traveled to southern California for Cassini’s End of Mission. The spacecraft has been orbiting Saturn since 2004. During my literal tenure at NASA’s Jet Propulsion Laboratory, I didn’t work on Cassini, but I still felt a special affinity for her, and I’d like to give you a different perspective.
Now, I’m not going to give you an exhaustive list of the Cassini mission’s accomplishments, as amazing as those are: if you want that, just about every science-related publication or TV program or web site can pile a ton of data and gorgeous photos on you. I’m not going to try to compete on their turf even if I did start my career as a science writer. Instead, I’m going to tell you some behind the scenes stuff in hopes that you’ll see a much bigger picture.
When Cassini was being built at JPL, every few weeks I’d use some of my lunch hour to walk down to the gallery overlooking the clean room to watch the techs working on integrating all the scientific instruments into Cassini before launch. It took years, since it was a big and very complex spacecraft: 6.8 meters high, 4 meters wide — or 22 by 13 feet. It held 12 science instruments. Not just the obvious cameras, but a magnetometer to measure magnetic fields; imaging radar; multiple kinds of spectrometers to learn the composition of things like plasma, Saturn’s rings, and ions; a cosmic dust analyzer; and more.
The resulting package — the spacecraft — was huge! I took a lot of photos during my visits, which are in a box …somewhere. But the official photographers could be in the clean room once they dressed up in those bunny suits, so I’ll use an official so you can see just how big the thing was. All together, with fuel, it weighed 5,600 kg, or 12,300 lb. — six tons. So it’s pretty much bigger and heavier than a fully loaded commercial delivery van.
And that loaded van was launched on October 15, 1997, about 15 months after I left JPL to work full time on my own writing and publishing. Saturn, depending on where it is in its orbit, is about 1.2 to 1.7 billion kilometers away from Earth, or about 746 million to a little over a billion miles. The neat thing is, to get there, they didn’t point the rocket at Saturn. Instead, they pointed it at Venus, pretty much the exact wrong direction. Why? Because Cassini was so big and heavy, we didn’t have a big enough rocket to get it to Saturn. By looping it around Venus, they could get a gravity slingshot effect — a boost to get it up to a higher speed so it could get all the way there.
But it’s actually a lot more complicated than that: after its launch on October 15, and its Venus flyby on April 26 the next year, Cassini looped around and got a second gravity assist from Venus on June 24 of the year after that. Then two months later, it zoomed by Earth and got a third boost. It skimmed by us at only 720 miles above the Earth’s surface, diving deep into our planet’s gravity well to get up to speed. But think about this: 720 miles is well inside the orbit of communications satellites! So after flying for almost two years, and more than a billion miles, it was still only 720 miles from Earth. But now it was going pretty darned fast, and could get to Saturn.
But first, one more step: after flying by Earth it went by the gas giant Jupiter on December 30, 2000, for yet another boost, and then, finally, Cassini arrived at Saturn three and a half years later, on July 1, 2004. In all, more than six and a half years after launch. It flew what’s called a VVEJGA trajectory — Venus-Venus-Earth-Jupiter Gravity Assist. By using these gravity boosts it didn’t just get there, but by not using its fuel to get speed, it could reserve that fuel so it could move around Saturn and its moons to collect the most science data possible once it got there. And by the way yes: a planet’s gravity can also be used to slow a spacecraft down when it arrives, so it doesn’t just go shooting by.
And here’s what I find totally mind-boggling about all of this: you can’t just point a spacecraft at any old planet and figure it’s going to get flung out in the direction you want it to go. You have to first figure out where the planets are going to be when the spacecraft gets there, and aim the spacecraft exactly correctly so that the planet’s gravity will pull it in, thereby speeding it up into the right direction, rather than pull it in and pound it into its surface and making a pretty crater. And you have to know precisely not just what direction it will go, but at what speed when it’s flung out the other side. Just pause for a moment to consider the thinking involved behind that. Not just the concept that it can be done in the first place, but how to do it, and do it right. And remember that all the planets aren’t just sitting there: they’re all in orbit around the sun — they’re moving targets! And we have to figure out how to swing by, get flung out behind that target rather than crashing into it, and zoom into space toward where we know the next target will be …often years later!
And the whole time, as the spacecraft is speeding out toward its destination, in this case Saturn, what’s happening? It’s slowing down because it’s being pulled on by the gigantic gravity of the sun, which is more massive than all the planets combined. Which, of course, also has to be taken into account — in advance, because that changes when the spacecraft will get to the next destination for the next gravity boost.
And here you thought high school algebra was hard. Well, at least I did: and that’s why I’m a writer and not a scientist!
Check out this neat chart that shows how the spacecraft sped up going toward Venus (and the nearby sun), and slowed down after it went by, sped up as it got another gravity assist, and went slower, and slower, and slower as it was pulled on by the sun during its long journey until WHOOSH! it was captured by Saturn’s gravity — its “orbital insertion” where it went into orbit around Saturn, and then how it was pushed and pulled as it moved around there. It’s years of fascinating data condensed into one little chart that shows the whole path to Saturn in a fascinating way:
Did I say “mind-boggling”? But this is what thinking gets us. And as we saw not just with Cassini but with many other missions, they do get there, usually within seconds of what the plans called for. And those plans were all put into place in advance by brilliant men and women, white, black, Asian, Latino, and other, Americans and citizens of many other nations, who work together, and together understand how all of this works. And why? Not just to get there: the trip isn’t the thing. The spacecraft doesn’t just need to get there, it and its scientific instruments have to survive the journey. Its instruments baked by intense heat when it’s near the sun, and frozen by the cold of space a billion miles away from the sun. Think about that: can you put your computer into a standby mode for seven years, and then say “Wake Up! Time to do intense work for 13 years?!” Yeah, mine wouldn’t either.
Cassini’s mission at Saturn started July 1, 2004, and now here we are at the end, on September 15, 2017. Its computers still worked. Its radio transmitters still worked. Its cameras still worked. And all those systems and more have been working together to get photos and data back to Earth twenty years after launch — and remember, all of those parts had to be purchased years before that to be tested, assembled into components, and integrated into the spacecraft. With few exceptions, its parts have been working for 30 years without any possibility that we can send a technician to fix anything that might fail.
In other words, the whole mission is running on 1980s technology. And for the most part, working perfectly.
But wait: there’s more.
Cassini was not just the first probe to go into orbit around Saturn to get a long and detailed look at that planet and its moons: it also carried Huygens — a lander that was dropped onto Titan, Saturn’s largest moon. It was named for the Dutch astronomer Christiaan Huygens, who discovered the moon in 1655. Titan is so large, it’s bigger than the planet Mercury — and it has an atmosphere. And not just an atmosphere, but one so thick that spacecraft can’t see through it with anything but radar. And oceans or lakes filled with liquid hydrocarbons, such as ethane and methane.
But hey: they can land a probe there for a closeup look around, and Cassini could relay its pictures and measurements of what it found. It could, and it did. And the project planners had to figure out how to do that, too. It was the first time we landed something on a moon other than our own. They thought it would only survive for a maximum of 30 minutes after landing, so it was designed to get and relay most of its data on the way down to the surface. But it actually survived on the surface for about an hour and a half.
And, again, you can go to just about any science site, or JPL itself, to get all sorts of information on what Cassini and Huygens found. But I want to share one picture Cassini took since it’s absolutely astounding, as well as help put things into perspective. At one point, the controllers at JPL realized they had a good opportunity when Cassini was behind Saturn’s rings: Earth would be visible through the gap in the rings, known as the Cassini Divison. That gap, and then the spacecraft, was named after the 15th Century Italian astronomer Giovanni Cassini, who first saw the gap in the rings with his crude telescope.
The controllers had the spacecraft take that picture to show Earth, way in the distance, in the rings’ gap. When they got it downloaded, they noticed a tiny speck off to the side. What the heck IS that? Then they realized that the photo also shows our own moon — they didn’t think Cassini would be able to see that. But it did, and that’s how good its camera was.
And let me give you one little tidbit to show you just how clever the mission planners were, even on the fly. You’ve probably heard why Cassini was deliberately flown into Saturn’s atmosphere this morning to burn up. Since mission launch, they’ve realized that two of Saturn’s moons, Enceladus and Titan, might have the conditions to host life — or at least have what they call “prebiotic” environments. In order to avoid the unlikely possibility of Cassini someday colliding with one of these moons, NASA chose to safely dispose of the spacecraft in the atmosphere of Saturn. This will ensure that Cassini cannot contaminate any habitat or potential life on those moons.
Yes, Cassini was built in a clean room, and it’s been in space for years, but they can’t be absolutely sure the spacecraft is completely, entirely, sterile. They don’t want there to be any possibility that we could accidentally introduce Earthly microbes into those moons by crashing the spacecraft there, even 10,000 years from now. So by running the spacecraft into Saturn’s atmosphere, they can be sure it will be fully incinerated.
In the leadup to the End of Mission, I sat in on a briefing by Trina Ray, one of the mission science planners who has worked most of her 28-year career on the Cassini mission and, she says, “Still loving it!” She noted that when Cassini arrived at Saturn, it had about 2,000 lbs of fuel. As it was doing its last orbits, it had 20 — plus or minus 10! The idea was to use the very last of its fuel keeping the spacecraft stable — its radio antenna pointed at Earth — so it can relay as much information as possible in its final minutes before the spacecraft tumbled out of control in Saturn’s atmosphere, ending the signal lock when Cassini can no longer point its antenna toward home.
As I was writing this all up for the podcast, something occurred to me: they destroyed Cassini so it couldn’t possibly hit Titan or Enceladus, perhaps contaminating them, but they landed Huygens on Titan. Wouldn’t that contaminate Titan? I wasn’t sure about the details on that, so I called Trina to ask. I caught her on her way home to try to get some sleep after Cassini’s plunge. She said it’s a good question …and they anticipated that. (Well of course they did!) Here’s the answer: if Cassini had crashed into Titan, it was so massive it wouldn’t have burned up completely, and that’s why they figured microbes inside could survive, and maybe contaminate the surface. Probes meant to land on a body are sterilized in one of two ways: either by alcohol, as in any medical environment, or with heat, as also used in medical environments, like baking surgical tools in an autoclave. So indeed Huygens was liberally bathed in alcohol, but they also baked it — on the way down to Titan! It had a heat shield that kept it from getting burned up in the atmosphere during descent, but that didn’t keep it from getting pretty hot: they calculated that it would get plenty hot enough to kill any microbes through and through. Again, thinking not only anticipated problems, but came up with solutions.
So again, Cassini’s End of Mission isn’t just about disposing of the spacecraft, it still performed science right to the end. Once they decided to destroy Cassini, Trina said in the briefing, they put out word to all the science teams: what do you want to do in the last orbits — orbits that will go inside the rings, which is to say between the rings and Saturn itself? They wouldn’t fly the spacecraft inside the rings before, because it was too dangerous. Crashing the spacecraft? Well, what’s dangerous now? They went for broke, and came up with new science objectives for those last orbits. The scientists figured out what they could realistically get, and how many orbits it would to take to get that data. For instance, the magnetometer team wanted four orbits, Radio Science team wanted six orbits — and the antenna had to be pointed at Earth for those. And so on. The total to get it all: 36 orbits. The available number of orbits as the fuel was running out: 22.
So then they had to sit down and figure out what things could be done at the same time: could spectroscopy data collection co-exist with, say, radio astronomy experiments? They were able to plan it all out so everyone could get the data they needed in the time they had.
Here’s one example that Trina told us about that, again, for me, was mind-boggling. One thing we wanted to learn more about all along about Saturn was its amazing and beautiful rings. To help scientists understand just how they work, how they stay so perfectly formed. To get that understanding, they need to know their mass — essentially, how heavy they are. They tried all sorts of things to get that measurement, especially of the thickest ring structure, known as the B ring. They tried to see through it with a camera, with the spacecraft on one side of the B ring and the bright sun on the other side; they thought they could figure out the mass by how much sunlight was filtered out. They know how bright the sun is, so if they could see, say, 5 percent of the sunlight coming through, they’d be able to do the math and figure out the ring’s mass. The problem: the B ring filtered out 100 percent of the sunlight, so they couldn’t do the math.
So they tried the same thing, except sending a radio signal between Earth and the spacecraft through the B ring. It’s the same idea: what percentage of the radio signal got through? The answer: zero. No way to do the math. But get this: by flying the spacecraft on the inside the rings, which remember was too dangerous to do before, they’ll be able to measure what the gravity pull is from the planet, and the gravity pull from the rings, and then they can do the math that lets them figure out the mass of the rings! That’s how sensitive Cassini’s instruments are: even being that close to the gigantic planet and its strong gravity, they’ll still be able to also see how much those thin rings are pulling on the spacecraft from the other direction, and get a decent measurement of the rings. That’s … just … astounding!
As Trina Ray put it in our conference, “There’s nothing better for a scientist to say than ‘I don’t know’!” It just means they have more work to do: more to learn. And they came up with another way — probably a foolproof way — to get the mass of the B ring at the last moment.
And that’s why I love the contrast between my old job and my new job. I got to work with people who had the imagination and creativity to think of doing this stuff, and the ability to think about how to get the answers they want so they can understand how this universe is put together. We have the towering intellect of thinking scientists that’s like the gigantic mass of Saturn on one side, which really puts into perspective the other side, like a guy who wraps duct tape around his head after deciding that would be an effective disguise to help him get away with robbing a Kentucky liquor store.
This is True is thought-provoking entertainment, but what does that really mean? Yes, we’re entertained by the stories of people not thinking, but they also give us context about the human condition so that we can marvel on the fact that a species that includes the duct tape robber also includes the scientists that are figuring out the universe by putting thinking to good use.
And: in addition to the scientists thinking about putting technology to work, there are the engineers who figure out the technology that the scientists use. The exploration that JPL is doing wasn’t possible even just 50 years ago, when we were going to the moon using computers whose memory board was a matrix of ferrite beads wrapped — by hand — with copper wire. They need the geeks too, to come up with the new technology to do the work.
And: before we could do this kind of exploration for real, before we had that technology available, there were flights of imagination by another kind of hero of mine: writers.
Just like Jupiter happened to be in a good position to help fling Cassini toward Saturn a little quicker, I happened to be in town for a special meeting last night: the meeting of the oldest continuously operating science fiction club in the world, LASFS: the Los Angeles Science Fantasy Society, founded in 1934. What made this meeting special? I wanted to hear some the discussion about one of their recently deceased members, science fiction author Jerry Pournelle.
When the technology isn’t ready — or we’re not ready to spend the money to use the technology we do have — humans can still explore ideas using their own creativity. Before we went to the moon, we went there in stories, imagining what we might find. By exploring creative scenarios in stories and novels, we thought of ways to overcome obstacles. What if, for instance, the moon’s surface is several yards deep with loose dust, making a safe landing impossible? That was what some thought could happen, and there were contingencies put into place in case that did happen in 1969.
David Gerrold, a science fiction writer, also a member of LASFS and also someone I met via the JPL Writers Club and have kept in touch with, he put it this way: “This is the primary function of science fiction — to be the Research and Development Division of the Human Species. This literature is the laboratory in which we consider the universe and our place in it. It is the place where we ask, ‘Who are we and what is our purpose here? What does it mean to be a human being?’” Brilliant!
There are a lot of great minds in speculative fiction, and Dr. Pournelle was one of them — and part of the reason he was this week’s Honorary Unsubscribe. But that “Dr.” title isn’t at all honorary, and it’s one of the reasons I say he had a great mind: Pournelle earned two doctorates, in psychology and political science. And a master’s, in experimental statistics and systems engineering; he also studied mathematics. After all that, he worked in military strategy and aerospace before turning to fiction full time — all those interests informed his writing, making it more realistic and believable. Yeah: R&D!
And if you talk to the scientists who are making it all happen in real life? Most will tell you that their imaginations were initially sparked by science fiction, from the golden age novelists to Star Trek and, more recently, shows like Babylon 5 and Firefly. Fiction sparks the imagination of the scientists, and their scientific discoveries spark the imaginations of the people creating the fiction. What a wonderful cycle, ever building and refining and moving all of us forward under the power … of pure thought. Think about that: they sure did.
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Randy Cassingham is the author of This is True, one of the first online subscription features (weekly starting in 1994). He often writes about the space exploration accomplishments of humanity in his blog.
