Remembering Cassini - Getting to Saturn
February 27, 2024
Dear Friends,
Twenty-years ago this month, the Cassini spacecraft began its official approach to the Saturnian system. For those of us who had been assigned by NASA 14 years earlier to undertake the long voyage across the solar system to Saturn, it was a time of heightened vigilance, nervous jitters, and great exhilaration at what lay ahead.
Once Cassini leapt off Launch Pad 40 at Cape Canaveral on October 15, 1997, cleared the launch tower, climbed skyward and veered eastward, it began a circuitous, 7-year, 2.2-billion-mile journey around and across the Solar System to reach its destination, 10 astronomical units away from the Sun. Those seven years were neither idle nor easy. A great many tasks needed to be completed to ready us for our exploration of the Saturn system, and the less pressing ones were postponed until the spacecraft and its scientific payload were safely on their way. With that assured, all of us on the Project began to settle into a new rhythm as we adjusted to a life in flight and a whole new set of activities.
I had been tasked in late 1990 with leading the 14-person Cassini Imaging Team, a group of scientists whom a gauntlet of scientific review panels and senior NASA officials had selected to conduct a scientific exploration of Saturn and its environs with what would be the mission’s prime imaging instrument. The Imaging Science Subsystem (ISS) consisted of both a high-resolution narrow-angle and a low-resolution wide-angle telescope. It was tradition within the space program to refer to these as cameras. Together with assorted electronics and the software to operate them, the Cassini cameras composed the most sophisticated, highest resolution, 2-dimensional imaging system ever carried into the outer solar system. The whole package had been specifically designed to image the bodies in orbit around Saturn, and in its sensitivity, optical resolving power, stray-light rejection, the variety of data collection and compression modes, the range of the electromagnetic spectrum that it could see, from the ultraviolet into the near-infrared, and its many spectral filters that would slice that spectrum into narrow bands, and more, it represented a significant advance over its predecessor carried on the Voyager spacecraft. We on the imaging team, and indeed the rest of the world, had every good reason to expect that we'd all be mind-blown once we got to Saturn. And the record now shows … we were not disappointed!
Cassini was the first planetary mission in which mission operations would be distributed among the science teams and not centralized at the Jet Propulsion Laboratory as had been done in years past. This was a genius move, as it was (certainly in my case) less costly and put the job of developing sequences of observational commands, destined for uplink to the spacecraft from the Deep Space Network around the globe, in the hands of those who cared about the outcome the most ... the scientists. Accordingly, all the science team leaders like myself were responsible for establishing the uplink and downlink ground operations for their instruments at their home institutions. In my case, this entailed designing, assembling, and staffing the laboratory in which ground operations for the ISS would take place. I called this organization CICLOPS, an acronym that took me an entire month to conjure. It stands for Cassini Imaging Central Laboratory for Operations. (Wildly brilliant, don’t you think!?) With a staff of 12 personnel at our peak, we wrote the computer code that enabled my science team members to choose camera parameters and thereby build their own imaging sequences, and developed software, procedures, and protocols for image handling, pointing reconstruction, annotating, cataloguing, scientific analysis, and archiving once the images made their way back to Earth and to our lab. Unique to CICLOPS was the responsibility of processing and captioning for public consumption select images of either scientific interest or outstanding beauty, and we built the tools to do that, too.
Of course, there was also the monstrous job in which all the science teams had to engage … working with the mission designers to determine what the nominal 4-year tour of the Saturn system would look like, and, once the tour was set, planning in great detail every single minute of it, something that amazes me even more in hindsight. Imagine planning every minute of the next 4 years of your life and committing every detail of it to paper. That would be onerous enough, but on Cassini it involved participating for many years in negotiations that took place via grinding weekly telecons, sometimes multiple telecons a week, attended by the mission's scientists and sometimes engineers too, in the service of those two objectives. It was an extremely wearying, contentious, and political part of the mission and not my favorite, though not nearly as political as the Project would later become as we got closer to Saturn. I was asked once, during the Q&A of a public lecture I had given, what this many-years-long activity was like. I answered, 'It works like the American Congress'. The audience laughed. They got it immediately.
Another necessary but more pleasant post-launch activity for Cassini's engineers and science teams was the in-flight instrument checkouts during which the spacecraft subsystems and all the scientific instruments were tested. In our case, images were taken at infrequent intervals with both cameras, through various spectral filters and camera states, of stars that had been well-studied from the Earth and whose brightnesses were stable and well-known — eg, Spica, Fomalhaut, the Pleiades — for calibration and for assessing instrument performance. Calibration is the process by which we determine how much signal, and the uncertainty on that signal, our camera system — optics, electronics, and software — would yield for a particular input intensity of light. Knowing precisely this translation from one form of information to another is required if we are to make scientific use of an image and, say, determine the physical or chemical processes that might be producing the amount of light recorded in the image. Bob West, an atmospheric scientist on my team who later became my Deputy Team Leader, was the uncontested king of calibration, taking great care to plan and acquire the images we needed during the entire mission. With the exception of one scare, when it seemed that our high-resolution camera, critical for our science as well as the optical navigation of the spacecraft, had badly lost focus — thankfully, it had not and the problem was reasonably easy to fix — our cameras performed faithfully throughout the rest of the mission.
Our first two years of flight were spent close to home, marking time in the inner solar system, encircling the Sun, patiently awaiting an infrequent planetary alignment between Jupiter and Saturn that would power the final leg of our journey. These two giant outer planets find themselves about every 20 years in a configuration that permits the flight of a Saturn-bound spacecraft to sail by Jupiter sufficiently close to steal a modicum of Jovian orbital energy and thereby significantly shorten its voyage. This maneuver, well known to all space aficionados as a 'gravity assist', was used to exquisite advantage in a very rare, every-176-years alignment of Jupiter, Saturn, Uranus and Neptune in Voyager's Grand Tour of the outer planets from 1979 to 1989. Similarly, Cassini's launch date and solar system trajectory had been specifically designed to allow passage through the Jupiter-Saturn “planet gate” that opened near the turn of the millennium.
During that two year wait, we paid visit to Venus twice (in April 1998 and again in June 1999) and the Earth once (August 1999). All three flybys provided some measure of gravity assistance, but also presented opportunities for the Project’s scientists and engineering flight teams to test, and collect data on, the spacecraft and science instruments during rapidly changing flyby conditions. As temperatures at Venus were too hot for our cameras to work well, and the spacecraft orientation during Earth flyby geometry did not favor imaging of Earth, imaging with our cameras was limited to the Moon, which served exceptionally well as a calibration test.
Caption: Cassini’s Earth-Moon Flyby
Our moon is not the most visually exciting object in the solar system but nonetheless, I was delighted with the results. Here was our first real 2-dimensional planetary body imaged with our cameras and the images looked great!
Caption: Cassini Images the Moon
With the Earth flyby successfully completed, we set our sails for the high seas. First up? Passage through the asteroid belt.
Now, the asteroid belt at first blush might sound like a bad dream to a passing spacecraft, but it's not the danger it seems. One of the main objectives of the Pioneer mission and its two craft, launched in 1972 and 1973 to Jupiter and Saturn, was to verify safe passage through the belt for the following outbound spacecraft that would truly investigate, in detail, all the outer giant planetary systems … the Voyagers launched in 1977. Pioneers 10 and 11 sped through the asteroid belt unscathed as did Voyagers 1 and 2. We have since learned that despite 100,000's of known asteroids, and a total number likely in the millions, the belt is mostly empty space. You'd have to be very lucky to enjoy a close, unplanned encounter with an asteroid.
It took us about 7 months to pass through the belt. We entered sometime during fall 1999 and exited around May 2000, without a hitch. But how remarkable it was that, with no intention whatsoever, on January 23, 2000, about mid-way through, we came within 1 million miles of Asteroid 2685 Masursky. That’s not a close encounter but it
was, in fact, an amazing and poignant coincidence for those of us on my team who had also been Voyager imaging team members. Harold Masursky was a renowned geologist, one of those early planetary explorers chosen to participate in the historic Mercury and Apollo programs, the Viking mission to Mars, and the Voyager mission. It was on Voyager that I came to know Hal. He was an expert punster, a joy to be around, and has become, like the other early planetary explorers who populated the Voyager imaging team, an integral part of my cherished memories of the time I spent on that iconic mission so long ago. I was privileged to serve with him and very happy that we were able to capture an image of his asteroid.
A single pixel in an image taken with our narrow-angle camera at a range of 1 million miles was ~6 miles across. In our highest resolution image, the body was only just resolved. Still, we were able to eke out some scientific tidbits. Asteroid 2685 Masursky was likely ~19 - 25 miles wide and seemed not to be of the asteroid category to which it had been assigned … not too shabby for a serendipitous rendezvous. [I would love to know if anyone has since confirmed or refuted our findings, or has done any further analysis. If you know, please pass the information on to me in the comments below.]
By June 2000, the asteroid belt was behind us and glorious Jupiter, our next port of call, was up ahead. Arrival at Jupiter is always significant because it means you've officially entered the outer solar system. So, our encounter with Jupiter was not only a great boost to our spirits, but critical to our preparations for Saturn. Closest approach would be a distant 6 million miles from the planet's cloud decks. We wouldn't be buzzing the planet the way Voyager 1 and 2 did, with their much closer flybys of 174,000 and 401,000 miles, respectively. That meant we wouldn't be taking high resolution images of anything on the planet or its single narrow ring. But, because of Cassini’s leisurely flyby, the large data storage capacity and high data-downlink rate of the spacecraft, and the superior imaging characteristics of our cameras, what we could do that even the Galileo orbiter at Jupiter, with its impaired antenna, could not was acquire over the several months preceding closest approach very high-quality, data-voluminous movies of Jupiter's changing atmosphere that were better than those returned by the two Voyagers during their Jupiter flybys in 1979. We could even capture time-varying processes within the jovian satellite and ring systems, if there were any to be seen. In other words, our distant flyby of Jupiter would be Voyager class and, in some regards, even better.
Such a major planetary flyby requires a commensurate level of planning. Trouble was, we were woefully unprepared! The science teams had neither the funding nor time to hire software developers and sequencing personnel to assist in developing Jupiter observational plans and instrument commands. The Project personnel at JPL hadn't yet figured out how the remote ground systems supporting each science team would efficiently interface with JPL. And I don't think anyone was even thinking about information traveling in the opposite direction: what do we do when all that data hits the ground? Sufficient funding for such developments wouldn’t appear for another 2 years.
Consequently, Jupiter flyby became a high-stress, chaotic, making-it-up-as-we-go-along affair. With no assistance, I ended up doing all the sequence designing for the atmospheric and ring sequences myself, participating in telecons to negotiate how observing time and downlinked data volume would be allocated across the science teams, and using low-brow spreadsheets and text files (would you believe?) to send image designs and camera commands to JPL over email.
Despite the makeshift accommodations and frenetic pace, in the end Jupiter encounter turned out to be a blessing ... a detailed, full dress rehearsal that all of us on Cassini sorely needed before getting to Saturn. For me, the sleeves-rolled-up, in-the-trenches perspective made it crystal clear just exactly what capabilities, software, and experience CICLOPS would need to conduct operations at Saturn. It also gave me the insights and know-how to hire and train appropriately skilled support personnel and to efficiently lead both science team and staff members in preparing for the biggest task of all ... a 4-year expedition through the Saturnian system.
We did some fabulous science at Jupiter. Were it not for the fact that Jupiter was not our main gig and our findings there were understandably eclipsed by what we found at Saturn 3.5 years later, our encounter with Jupiter would have been far more celebrated than it was. Data collection began on October 1, 2000 and ended on March 22, 2001. We acquired about 26,000 images, well exceeding the clutch of the Voyager spacecraft at Jupiter of ~18,000 images each. The glorious, planet-wide movies we captured, rendered by team member Andrew Ingersoll and his associates, are still the best made thus far of Jupiter's kaleidoscopic atmospheric motions. Here's one of our most remarkable … smoothly interpolated to remove noticeable jumps between frames and, note well, rendered from an unrealistic reference frame … in 2D:
… and in 3D:
Caption: Interpolated Movies of Jupiter … in 2D and in 3D
Movies and still atmospheric images revealed features of Jupiter's atmosphere that had not been seen before. Among them was the presence of coherent zonal jets in Jupiter's polar region up to very high latitude; the presence of small vortices, both in the mottled polar region and at low latitude, that get carried around by the regions' zonal winds, and the relatively long-lives of both; the obvious lack in Jupiter's polar
Caption: Jupiter’s Polar Winds
jets of the large-amplitude excursions in latitude that are seen in the polar jets of both the Earth and Saturn; the detection of vigorous convective storms that are restricted to Jupiter's dark belts and not its white zones, as had been traditionally believed; and more.
We even managed to make a movie, planned by team member Alfred McEwen, of Jupiter's volcanic moon, Io, passing through the shadow of Jupiter … a movie that
made very clear that faint, diffuse emissions arising from Io are due to atmospheric aurorae caused, as they are on Earth, by the collisions of charged particles with gases in the atmosphere, and that the gradually shifting positions of the glows are due to the changing orientation of Jupiter's magnetic field. These results were visual confirmation that the aurorae are caused by the electrical currents flowing between Io and Jupiter along the planet’s field lines. We even processed the movie in color, where the emissions we colored red (and would look red to your eye if you were there) likely arise from the atomic oxygen in the tenuous Io atmosphere, and the emissions colored blue, which are visually deeper down in the atmosphere and thus closer to the surface, likely
correspond to near-ultraviolet emissions (which we can’t actually see with our eyes) from molecular sulfur dioxide, which itself would be nearer the surface than oxygen because it is heavier.
How cool is that?!
And I'm sure those who were watching us at the time remember our last Jupiter image release, about three years after the images were on the ground, after an enormous amount of work on the part of one of the team's associates Ashwin Vasavada. I called it “The Greatest Jupiter Portrait” because at the time it was and, to me, still is.
Caption: Cassini’s Greatest Jupiter Portrait
Do read the caption and go to the link there to bring up the full-sized image.
Once passed Jupiter, we returned to our in-flight calibration observations, stealing longing glimpses of Saturn from time to time … one from a distance of 177 million miles, or about 2 astronomical units, on October 21, 2002, and another from 69.2
Caption: Cassini first sights Saturn
million miles, on November 9, 2003, when it had obviously grown in size and greater detail could be seen, including 5 of its satellites:
And the day did finally come, 20 years ago this month, when we arrived at Saturn's doorstep. On February 6, 2004, our official approach to Saturn began. The pace of activities within the Project picked up and data collection on Saturn and everything around it started in earnest … rapid and steady. It felt like we had stepped onto a conveyor belt and strapped ourselves in, knowing it wouldn’t stop for the next four years. (Little did we know then that our years at Saturn would extend to 13!)
At this time 20 years ago, my team members and I were not yet seasoned Saturn explorers — no one was — and I, for one, was anxious about the magnitude of the task before us. We did not yet have our imaging ground system fully up and running and we were still using spreadsheets! But the images that we were seeing on our computer screens were breathtaking. Habituated as I was to impressions of Saturn, its rings, and moons gleaned from Voyager images of 23 years earlier, I was stunned at the clarity of Cassini's images in comparison. It was like opening your eyes after LASIK surgery and suddenly there before you is the world as it really is.
Our first image sequences taken on February 6 were atmospheric movies of the type we had designed at Jupiter for the very same purpose: to take advantage of the large distance from Saturn, which would never be so large again once we were in orbit, and make long duration, planet-wide, multi-wavelength movies of the planet's atmosphere to study its meteorology and sound its atmosphere. We eventually made movies of the rings, too … to search for the famous spokes, discovered on approach to Saturn by Voyager, and any new moons within or near the rings. We also executed special imaging searches for new satellites throughout the inner satellite system, from the rings out to the orbit of the moon Iapetus, and planned routine images of the known moons, big and small, to improve their orbits.
Of course, everyone on the mission was terrifically interested in seeing the hitherto unseen surface of Saturn’s largest, hazy moon Titan, the largest single expanse of unexplored terrain remaining in our solar system before Cassini got there, and it was from the beginning a top scientific objective for us imaging folks and indeed the entire Cassini mission. Routine imaging of Titan would begin when our resolution became comparable to that seen from the Earth.
And June would deliver a planned, very close flyby of the retrograde moon, Phoebe ... a body 125 miles wide, believed to have been captured into Saturn orbit billions of years ago, as Saturn and its inner moons were forming. We didn’t want to miss the chance to see a Saturnian interloper up close!
Though we anticipated being in orbit for at least four years, and taking 100s of thousands of images while we were there, we also knew that if we didn't make it into Saturn orbit, if something went wrong ... say, a valve in the main engine, intended to slow Cassini's acceleration so it could be inserted into orbit, didn't work, or the spacecraft was fatally damaged by a ring particle while it flew through the ring plane during the insertion maneuver ... and the mission devolved to a simple flyby, the information we collected during those 5 months on approach would be all we’d have for 14 years of effort. It became both scientifically and emotionally important that we did the approach sequence right.
On February 27, 20 years ago today, we released our first image of Saturn as an
Caption: Approach to Saturn Begins
announcement to the world that after toiling 7 years to get to the launch pad, and another 7 years journeying to Saturn, the show had finally begun. On that day, I advised visitors to the CICLOPS website, “Prepare to be amazed!”
Over the next year or so, I hope to remind you just how amazing it all turned out to be, and hope that you follow along with me here and encourage others to do the same, as we look back at those privileged years of wide-eyed wonder traveling Saturn.
- Carolyn Porco
The best space mission of all time, for now! Pluto is my second best though it is just too effing short. Cassini is still how a space mission should be, effective, comprehensive, exciting, personal, kind, etc etc. Used to visit ciclops.org just to check and recheck, I still am remembering that website after all these years!
I have 9,493 of them on Twitter. Once the book is out I will plan lots of reposts to promote it.