Outer space is everywhere: Not only are we physically surrounded by it, but we’re inundated with images of it, both real and fictional. NASA’s long-lived Cassini mission is ending this week, just after its even longer-lived Voyager mission marked its 40th anniversary. SpaceX is about to launch the most powerful operational rocket in the world. Star Trek is returning to TV, The Martian author Andy Weir is returning to bookshelves, and Destiny 2 and a new Metroid release are bringing gamers back to the stars. Please join us at The Ringer as we celebrate and explore the cultural resonance and science of space all week long.
Early Friday morning, NASA’s Cassini spacecraft will make a precisely planned plunge into the atmosphere of Saturn at 76,000 mph, ending one of the most awe-inspiring engineering feats in human history.
In the 20 years (minus a month) since its launch, Cassini — the longest-operating non-Earth orbiter — has covered almost 5 billion miles, orbiting Saturn nearly 300 times as it’s crisscrossed the most scenic section of our solar system. With its complex path plotted out from afar, the spacecraft has mapped Saturn’s ring structure, imaged massive storms on the gas giant’s surface, and discovered six moons. It’s also identified features of two previously discovered moons, Titan and Enceladus, that established those bodies as among the most likely locations of extraterrestrial life. Until Cassini first flew by, we didn’t know that icy Enceladus emits jets of water from a subsurface ocean. And before Cassini gave the European Space Agency’s Huygens lander a lift to remote Titan, we didn’t suspect that the solar system’s second-largest moon has standing liquid on its surface.
Up until its final moments, Cassini will continue to capture information that humanity has no other way to obtain — and then, courteously, disintegrate in Saturn’s clouds so as to avoid running out of fuel and potentially crashing on and contaminating the moons that might harbor non-Earth organisms. Over the course of Cassini’s twice-extended travels, the craft has recorded almost half a million images, ranging from exquisite close-ups of Saturn and Jupiter to perspective-altering portraits of a distant, comparatively tiny Earth. Even aside from their scientific value, those images have educated, comforted, moved, and inspired countless civilians. Looking at the latest serene vistas beamed back from Saturn has never failed to improve my mood.
So much bad news everywhere, but I’m going thru Saturn pictures right now and I have some glimmer of hope for the future. pic.twitter.com/xf1IexSOFN— Phil Plait (@BadAstronomer) September 10, 2017
There’s no shortage of lists chronicling Cassini’s accomplishments and most memorable photos, but almost all were assembled by outside observers. To memorialize the mission, we wanted to go straight to the source, soliciting contributions from members of the Cassini science team, some of whom have spent decades devoted to their tasks. I asked each respondent to nominate one Cassini image that they considered the most personally or scientifically significant, and to write a brief blurb about why they made that particular pick. Their selections and explanations are presented below, organized by subject. I can think of no better tribute to the hundreds of scientists from 17 countries who’ve helped make Cassini a success than to let 28 of them, who shared their favorites via email, take curtain calls for the roles that they’ve played in bringing this mind-blowing beauty to us.
Amy Simon, Composite Infrared Spectrometer (CIRS) co-investigator: This image is from Cassini’s flyby of Jupiter in late 2000. Even though Cassini was only partway on its journey to Saturn, this was a spectacular preview of the wonderful science yet to come. The flyby was a gravity assist to boost Cassini on its way, but we were able to use the opportunity to test spacecraft operations and to check out the capabilities of each of the instruments. This led to several Jupiter system science discoveries, as well as allowing direct comparisons to what we’d find at Saturn with those same instruments!
Tony Del Genio, Imaging Science Subsystem (ISS) team member: This image shows Saturn’s night side as viewed by Cassini. The planet’s atmosphere and rings are backlit — that is, illuminated from behind by the sun. To me, Saturn is the most beautiful planet in our solar system, and this image shows it at its best. Just to the left and above the most prominent set of rings, and inside two of the faint outer rings, we can see Earth, the barely perceptible “pale blue dot” that Carl Sagan first mused about. This image is meaningful to me on several different levels. First, it was no accident that this image was acquired. It was planned far in advance, with knowledge of what would be in the camera’s view at this time, and as such it is a testament to the predictive power of science, something that is sadly underappreciated in our society today. Second, it is a view that no human has ever seen — from the outside looking in toward the inner solar system where we reside. Third, during the Cassini mission’s lifetime, we have gone from not knowing of the existence of any planets outside our solar system to having discovered thousands of them. The search for habitable, and perhaps inhabited, planets is now underway. This image reminds us that from far away, our own fascinating, beautiful, sometimes frustrating planet is nothing more than an insignificant point in the sky. It makes me even more excited to think about what surprises other seemingly insignificant “dots” out there may hold for us if we have the will to search for them, and it emphasizes the commitment we must all make to preserve the beauty of the planet that we have for future generations. What Sagan said decades ago is even more true today.
Sandrine Guerlet, CIRS co-investigator: This picture is from the beginning of the mission, when it was the end of summer in Saturn’s southern hemisphere and winter in the northern hemisphere. From Cassini’s unique point of view, one can see the projected shadows of the different rings on the planet, and the striking change in color between the two hemispheres, yellowish in summer and blue in winter. From the Earth, we see Saturn as a yellow disk and cannot observe or study the winter hemisphere, partly due to the 26.7° tilt of Saturn’s rotation axis, and also because it is hidden behind the ring’s shadows. This picture (among others) revealed the changing color of Saturn’s atmosphere, which we think is related to the quantity of aerosols changing the way the light from the sun is absorbed and reflected in Saturn’s atmosphere (a more hazy atmosphere during summer and a clearer atmosphere during winter). This is just one example of the special insight Cassini brought us by orbiting Saturn, allowing us to witness the change of season (a year on Saturn lasts 30 Earth years!) in both hemispheres in multiple ways — not only colors, but also atmospheric temperatures and composition.
Gordon Bjoraker, CIRS co-investigator: At any given time, Saturn has several small storms. They have low contrast due to haze particles in Saturn’s upper atmosphere. However, roughly every 30 years a Great White Spot erupts on Saturn that is so bright that amateur astronomers can easily observe it with small telescopes. This bright white cloud is confined to a narrow latitude band, but over several months it expands in longitude until it encircles the planet. Amateur astronomers detected a new storm in December 2010. A few weeks later, Cassini began observing the storm and continued to follow it for the next year. The Visible and Infrared Mapping Spectrometer (VIMS) on Cassini detected a mixture of water ice and ammonia ice in the storm clouds. This provides evidence of violent updrafts that carried water ice from the deep water clouds up around 200 kilometers into the upper atmosphere where they were observed. The storm also perturbed Saturn’s normally quiet stratosphere, creating two hot spots, or “beacons,” that merged and cooled over the next three years. Cassini’s Composite Infrared Spectrometer detected enhanced temperatures and larger amounts of hydrocarbons and water vapor in the beacons. We don’t know what triggers Saturn’s storms every 20 to 30 years, although Cassini scientists have proposed a few ideas. Even more puzzling is what caused Saturn’s stratosphere to heat up by 80 degrees C (or 140 degrees F). Some type of wave activity is suspected, but we don’t know for sure.
Liming Li, participating scientist for ISS, VIMS, and CIRS: Each planet or satellite absorbs solar energy from solar radiation (mainly concentrated in the visible wavelengths) and at the same time emits outgoing energy by thermal radiation (mainly concentrated in the infrared radiation). The balance between the absorbed solar energy and the emitted thermal energy is called the radiant energy budget, and is one of the most important factors for an astronomical body. This is an image of Saturn recorded by the Cassini VIMS. I selected it because the Cassini observations help us better understand the radiant energy budgets of Saturn and its largest satellite, Titan.
Glenn S. Orton, CIRS co-investigator and ground-based support astronomer: The first look at [Saturn’s] monster south polar “hurricane” appealed to me because it’s something we sort of predicted from the 2004 February observation that we published in Science a year later, but also because it was the first time we saw a real “eye wall” any place outside the Earth. And it’s hot! [Saturn also has a monster north polar hurricane and hexagon, which is also hot.]
Donald E. Shemansky, Ultraviolet Imaging Spectrograph (UVIS) co-investigator: [This is] an image extracted from Cassini UVIS system scans in 2005, in H2 emission from Saturn. The H2 emission structure in the upper atmosphere of Saturn is significant. When the Voyager spacecraft were launched in 1977, the hydrogen upper thermosphere temperatures of Jupiter and Saturn were predicted to be in the 350 to 250 Kelvin range, because the heat source was expected to be primarily solar flux deposition. Both planets were found to have greatly expanded atmospheres with temperatures of 1200 and 900 Kelvin. The cause of the high-altitude heating is an electrodynamic interaction with the magnetosphere, coupled with solar resonances in the H2 internal structure.
Joshua Colwell, UVIS co-investigator: This picture is somewhat infamous within the Cassini community because it is a false-color image made from uncalibrated ultraviolet data. I created this image from UVIS data the day after Cassini arrived at Saturn. As the only color image coming out of the project in those early days, and being visually pleasing, it got a lot of publicity and was even a Time magazine picture of the year in 2004. The images are always credited to the institution, but this is a highly processed image that took me a fair amount of time to construct from our raw data, and it was a PR hit, so it is a sentimental favorite. I think it’s visually appealing, and I made it!
Luke Dones, ISS team member: Overall, Saturn’s rings are unimaginably flat compared to how wide they are. In some places, though, the rings are thicker because of disturbances from nearby or distant moons. The outer part of Saturn’s densest ring, the B ring, is one of those places. This image was taken on July 26, 2009, when the sun was almost edge-on to the rings. “Cliffs” in the rings more than a mile high cast shadows. Near those cliffs in the image, Saturn’s moon Mimas, which is well outside the rings, creates a gap, forming the inner part of the Cassini Division. I love this image because while studying Saturn’s rings can get kind of esoteric, anyone can see that something special is happening.
Carl Murray, ISS team member: This image was taken on April 15, 2013. The target at the center of the image was Prometheus, the inner F ring shepherd moon, and such images were part of a long-running observing campaign designed to improve the orbits of the small moons. However, the significance of this particular image was what it revealed at the edge of the A ring. It shows a bright, extended feature indicating the unexpected presence of an object, perhaps one that has just undergone a collision. By searching previous images we were able to find prediscovery detections of the object, and we have been targeting this region of the rings ever since, with lots more detections. Because we found the object on my mother-in-law’s birthday, April 19, we decided to nickname it “Peggy” in her honor. However, although we can see the gravitational effect of the object on nearby ring particles, Peggy herself has proved to be very elusive. This is not the most beautiful image, but it is certainly a special one to me.
Matthew Tiscareno, participating scientist: The “propellers” of Saturn’s rings — disk-embedded mini-moons that in some ways resemble baby planets — first appeared 12 years ago as smudges on my computer screen, and we have been relentlessly chasing them and learning about them ever since. To achieve the glorious detail we see here at the end of the Cassini mission is both satisfying and tantalizing. We have learned so much, and there is so much yet to learn.
Conor A. Nixon, CIRS Deputy Principal Investigator: This iconic image of Titan was taken in July 2009, and shows a “specular reflection,” or glint of the sun reflecting off one of its seas (Jingpo Lacus) just as astronauts see glints of the sun reflecting from the Earth’s oceans. This is marvelous to me for several reasons — firstly the sheer technological triumph that it took to pull off such a remarkable feat of photography, to have a spacecraft sailing past another world a billion miles away and to set up this amazing shot. But also because Titan is the only other place in the entire solar system (other than Earth) where you could see this miracle. Earth and Titan are twins, in a way — the only worlds where there are seas on the surface. Long-lost cousins indeed.
Jason Barnes, participating scientist: This August 11, 2014, Cassini Visible and Infrared Mapping Spectrometer view shows Titan’s north polar seas in a crescent illumination. The dark spots show Kraken Mare, Punga Mare, and Ligeia Mare, along with myriad smaller lakes. The bright yellow spot is a specular glint of the faraway sun, still outshining everything else on the moon.
Athena Coustenis, CIRS and Huygens Atmosphere Structure Instrument (HASI) and Descent Imager/Spectral Radiometer (DISR) co-investigator: This image from the Descent Imager/Spectral Radiometer (DISR) shows what Huygens “saw” with its near-infrared eyes after landing: a riverbed of methane and round ice pebbles which showed us that a fluid was running on Titan near the equator. The fluid is methane, and the reason the ground is rather dry is probably due to the fact that we were in a rather dry season in 2005. When I first saw this image, I was amazed and touched to discover with so much detail the landscape and the horizon of such a faraway object.
Laurence Soderblom, interdisciplinary scientist: This image was taken as the Cassini-Huygens Probe descended to Titan’s surface. This image set shows that the surface of Titan appears like many places on Earth! Rivers drain the highlands, and longitudinal bars align with the shorelines of the now-dry plain.
Jani Radebaugh, RADAR team associate: This region on Titan north of the equator and near the giant impact crater Menrva is significant for me because it is the first place we saw giant sand dunes in a Cassini Synthetic Aperture RADAR image. I remember looking at their wispy form, their regular spacing, and seeing them for their beauty, but having no idea what they were. Planetary landscape guru Vic Baker saw them and immediately said, “Well, those look like draa!” — an Arabic term for large, linear dunes found in the Arabian and Saharan deserts. It turns out that is exactly what they were, and we have since seen tens of thousands of dunes, made of organic sands from atmospheric processes, all across Titan. I never knew I could grow to know and love sand dunes, on Titan and on Earth, so much — and this was the start.
Charles Elachi and Stephen D. Wall, team leader and deputy team leader for the Cassini RADAR team: We have picked the [above] two RADAR images as two of our favorites. The first represents our first view of [Titan’s] dunes, although at the time we didn’t realize what they were and naively called them “cat scratches.” This image was taken early in the mission, and we were still getting used to what this amazing moon had in store for us. The second image is one of our first false-color images of the northern lakes. It represents a time when we had just begun to realize that Titan had a hydrologic cycle just like Earth, although a lot colder and with a different working fluid.
Lunine: This is Ligeia Mare, the best-known and best-studied of the great seas of Titan, and one we nearly had an opportunity to explore by marine navigation via the Titan Mare Explorer.
Hofgartner: I am partial to the images of Titan’s “Magic Island” region. My favorite is the one we released showing all of the sea known as Ligeia Mare and multiple panels of the Magic Island region.
Rosaly Lopes, RADAR team member: This image of Doom Mons on Titan is my favorite because this is the most likely cryovolcano (ice volcano) we found. The peak is nearly a mile high and stands next to a pit which is over a mile deep. The artificial colors come from VIMS and show that the composition of the material is different from that of its surroundings. The image itself is a Digital Terrain Model (DTM) made from RADAR images. The name is from Mount Doom of J.R.R. Tolkien’s Lord of the Rings.
The Smaller Moons
Morgan Cable, Project Science and system engineering assistant: I love this image of Titan and Epimetheus because it shows the incredible diversity of moons in the Saturn system, as well as their interconnectedness with the rings. The moons range in size from huge to tiny, and in physical and chemical properties, too. Titan is the second-largest moon in the solar system, and has an atmosphere (even thicker than Earth’s) and a surface covered in liquid hydrocarbon lakes and dunes of organic molecules. Meanwhile Epimetheus is tiny, mostly water-ice, and shaped like a potato. Hard to imagine that they are both part of the same Saturn family! And the rings, as stunning as ever, remind us that the moons and rings of Saturn are involved in a complex relationship that we are only beginning to understand.
Molly Bittner, systems engineer, Cassini Spacecraft Operations: This photo of Enceladus’ plumes was taken on October 28, 2015 during one of the sequences that I was Systems Lead on. I remember planning for this close flyby, with an altitude of only 49 km (30 miles) above the surface of the icy moon. Not only does this picture capture a scientific marvel, but it also represents one of the best qualities of the Cassini Mission: an extremely talented engineering team that sets up the spacecraft to conduct inspiring science time after time. All of the engineering subsystems, the navigation team, and the science planning team came together to make this flyby a success. When I look at this image, I remember all of the hard work that went into it, and I feel grateful to be part of such an amazing team.
Michelle Thomsen, Cassini Plasma Spectrometer (CAPS) co-investigator: There are many other similar images, all of them my favorite for the same reason, but this is an exemplary one. The reason it is my favorite is that, as a magnetospheric physicist, I appreciate the fact that the water plumes of Enceladus are the key to the contents, structure, and dynamics of the entire Saturnian magnetosphere. The water from this little moon provides the dominant plasma source for the magnetosphere. Combined with Saturn’s rapid rotation and strong magnetic field, this plasma drives an interchange motion that transports it out towards the solar wind, forming the first stage of a fascinating magnetospheric circulation process. So the plumes in this photo are the genesis of Saturn’s magnetosphere as we know it.
Todd J. Barber, propulsion lead engineer: I love that you can work on a mission for nearly two decades and still get completely gobsmacked by Cassini’s revelations. As we wrapped up our F-ring orbits in the spring of 2017, we caught this view of Saturn’s tiny moon, Pan. We already knew it was irregularly shaped, but close-up images of the moon utterly blew our minds. The solid skirt of material around Pan’s equator, gathered as the moon cleared out a gap within Saturn’s rings, was positively bizarre. The only downside was an inexplicable craving for tortellini, ravioli, empanadas, or a fried egg!
Candy Hansen, UVIS co-investigator: Helene shares Dione’s orbit, situated at one of the Lagrange points. This is my favorite image because its geography is so unexpected. It isn’t battered and covered with craters, as conventional wisdom would dictate. Rather, it has these beautiful landslide flow features. To see terrain like this — so, so different from a typical icy moon — makes space exploration fun. We’re always being surprised.
Bonnie Buratti, CIRS team member: I always wondered what a moon that was so fluffy it could easily float on water would look like. Well, the first image here is Hyperion: a sponge. (Those holes are really craters.)
The second image is of Iapetus, which was always mysterious to me. Half of it was as dark as tar, and the other half was as bright as snow. I always wondered what it would look like up close. Cassini’s first global view of Iapetus shows an even weirder moon [than we expected], with a strange rim 12 miles high around its equator. Cassini scientists still aren’t sure what it is. Perhaps it is a collapsed ring.
Trina Ray, Cassini Science Planning & Sequencing deputy: This montage was from the first targeted Titan flyby of the mission (known as “T-a”) and just looking at it brings back memories of the many meetings that the Titan Orbiter Science Team (TOST) needed in order to arrive at a final, conflict-free, negotiated plan, and the pure joy of seeing the images go up on a screen in a room full of Titan scientists and engineers who had gathered in October of 2004 for a live stream of the data. Everyone being puzzled and overjoyed with each image that hit the screen — priceless!
These contributions have been lightly edited for clarity and style.