The Cassini orbiter is ready to die. Nearly 20 years after it departed Earth, and 13 years after it arrived at Saturn, the workhorse spacecraft is nearly spent. Its fate was orbitally sealed on Monday, when it flew by Titan, Saturn's largest moon, for one last gravity assist. Mission leads dubbed this Cassini's "goodbye kiss."
It was also a kiss of death, as Titan swung Cassini towards its final destination. On Friday at around 6:32 AM ET, the spacecraft will plunge into the ringed gas giant. It will radio back data for as long as possible, capturing a taste of Saturn's atmosphere and the closest visuals of the planet humankind has ever witnessed, before it meteorically vaporizes in the skies, a dramatic end to its tenure as the most productive interplanetary orbiter in decades.
"There isn't anything like Cassini," Jonathan Lunine, a planetary scientist who has worked on Cassini-Huygens from its inception, told me at his Cornell University office in Ithaca, New York. "Now, it's going to be in my mental rearview mirror. It's going to be a strange feeling."
Saturn has beckoned to countless generations of skywatchers, including the orbiter's namesake, the Italian astronomer Giovanni Domenico Cassini (1625-1712). Radially, the planet is around nine times larger than our own, and is extremely voluminous, containing enough space to fit over 700 Earths inside it.
It's no wonder that this exotic system has captivated so many people across history, but its far-flung location has been a major roadblock in studying it. Saturn's distance from our planet fluctuates between 1.2 billion kilometers (746 million miles) and 1.7 billion kilometers (one billion miles) at its farthest, which makes Cassini's epic seven-year trek to the system that much more impressive.
As a joint project between NASA, the European Space Agency (ESA), and the Italian Space Agency (ASI), the Cassini-Huygens mission also has a special legacy as one of the most successful collaborations in space exploration history.
Harnessing the talents of thousands of experts across the world over a timespan of 35 years—from its beginnings in 1982, when the mission was formally conceived, through its launch in 1997, to Friday's self-destruction—Cassini is a multinational and multigenerational labor of love. People have understandably grown attached to this spacefarer, and that personal connection shines through when they speak about it.
"It's really a sad feeling," Nicolas Altobelli, who began working on the mission as a graduate student in 2000, and is now the ESA project scientist for Cassini-Huygens, told me over Skype from Madrid, Spain.
"Cassini has been with me for all my professional life so far," he said. "It's very weird to think that Cassini will not be there somewhere in the sky."
Cassini has undeniably earned its retirement. During its trailblazing adventures, the spacecraft discovered several Saturnian moons, witnessed a rare storm rage in the planet's northern hemisphere, directly sampled the watery plumes of the potentially habitable world Enceladus, collected grains of interstellar dust, and delivered the Huygens lander to Titan's otherworldly surface, sticking the first planetary landing ever achieved in the outer solar system.
In addition to racking up these stunning milestones, the orbiter has snapped around 400,000 pictures of the Saturn system, including breathtaking views of Earth from its distant vantagepoint. It has relayed back terabytes of data about its adopted home, revolutionized our understanding of Saturn, and raised a fresh slate of intriguing mysteries that will have to be resolved by its successors.
Titan has been one of Cassini's priorities during this last act. This strange moon, which is slightly larger than Mercury, shares a particularly special relationship with the orbiter. Hidden under a veil of atmospheric haze, Titan has been the source of intense curiosity since its discovery by the Dutch polymath Christiaan Huygens in 1655. Voyager 1 imaged Titan up close in 1980, and determined the composition, temperature, and density of its atmosphere, which sparked theories that liquid hydrocarbons might be under the moon's cloudy skies. ESA and NASA were inspired to partner Cassini with a lander to expose the tantalizing landscape on the surface.
The result was the ESA-led Huygens lander, which hitched a ride with Cassini to Saturn. After separating from the orbiter in December 2004, Huygens entered Titan's thick atmosphere and successfully parachuted to the ground on January 15, 2005.
Lunine, an expert on Titan, worked on Huygens, and served as the co-investigator of the probe's Gas Chromatograph Mass Spectrometer instrument, which catalogued the composition of the moon's atmosphere. When Huygens' descent snapshots began to arrive back at Earth, he was with the imaging team at ESA's Space Operations Center in Darmstadt, Germany. The portrait Huygens painted of its final destination was "spectacular," Lunine said, somehow both strikingly alien and eerily familiar.
"The images were unworldly; they were like Earth in terms of certain features. Then, in other ways, they didn't look at all like Earth," he recalled. "We saw gullies in the side of a hill that looked like they had definitely been carved by [liquid methane] rainfall."
The images revealed a world that is weirder than anyone imagined, sculpted by seasonal weather, possible cryovolcanism, and hydrocarbon seas, which are the only stable surface liquids known in the solar system, apart from Earth's bodies of water. Since Huygens' historic landing, and with Cassini's help, scientists have marveled over Titan's "magic" disappearing islands, counterintuitive dune patterns, and electrically charged sand. With temperatures of around minus 179.2 °C (minus 354.56 °F), the moon is far too chilly to support liquid water at its surface, but it may have a subsurface water ocean under its icy crust.
These unique features, in tandem with Titan's rich organic chemistry, have sparked debates over its potential habitability. No life has been detected there, but then again, Cassini-Huygens was not specifically equipped to assess habitability of any of the other worlds it explored.
Given the adventures that Titan and Cassini have shared, it's fitting that the moon was responsible for tugging the orbiter into its Grand Finale victory lap back in April, before pushing it toward its grave this week.
Titan is not the only Saturnian moon that has divulged its secrets to Cassini. Enceladus, a walnut-shaped oddball measuring only 500 kilometers (310 miles) in diameter, was also thrown into sharp focus. Several close Cassini flybys revealed that the moon was spewing the contents of a subsurface liquid ocean from icy surface cracks, literally spilling its guts out to the solar system.
"Of equal import to what we've learned about Titan is what we've learned about Enceladus," Lunine said. "Before Cassini arrived at Saturn, it was clear Enceladus was peculiar, but we didn't know there was a plume."
Enceladus was a major research target for Cassini from the start, but the surprise discovery of the plume in 2005 left mission leads brainstorming ways to directly sample these jets of moon juice. Cassini got to pull off this maneuver on October 28, 2015, diving through the moon's sprinkler at an altitude of only 50 kilometers (30 miles) above the surface, and collecting a spray of alien ocean particles.
"What we have seen fantastically demonstrated by Cassini is that those icy moons are able to be geologically active," Altobelli told me. For instance, an April 2017 study co-authored by Lunine and published in Science detected molecular hydrogen and silicates in the plume, which means that the moon is likely to be hydrothermally active. On Earth, hydrothermal vents on the ocean floor support thriving communities of life, so it's plausible that Enceladus has its own biology too.
"It's a very Earthlike environment that could, for all we know, support life," Lunine said.
The prospect of alien inhabitants in the Saturn system is the reason why Cassini must die, as mission leads do not want to risk the spacecraft colliding with worlds that could be contaminated by Earth microbes. So for now, Enceladus' habitability remains an open question to be resolved by future spacecraft, such as the proposed mission Enceladus Life Finder (ELF).
Beyond these exciting revelations about Titan and Enceladus, Cassini provided unprecedented visuals of the known worlds Phoebe, Iapetus, Rhea, Hyperion, and Dione, and discovered six tiny moons called Methone, Pallene, Polydeuces, Daphnis, Anthe, and Aegaeon.
It also used its Cosmic Dust Analyzer (CDA) tool to trap submillimeter dust grains from the interstellar medium. These small particles, primarily composed of calcium, carbon, iron, magnesium, silicon, and sulfur, are direct samples of this extrasolar void, sifted from the abundance of icy particles produced by the Saturn system. As interstellar interlopers, they can be mined for clues about the broader environment from which the solar system was born some 4.5 billion years ago.
"It is the end of a human adventure"
"It's exactly like a deep space survey with a telescope, except instead of photons we are getting dust particles," Altobelli, who is a member of the CDA team, told me. "You get the end product of millions of years of processing in the interstellar medium."
The scope of Cassini's mission ranged from capturing particles smaller than red blood cells to mapping out planetary-scale atmospheric dynamics to testing the general theory of relativity that governs the known universe. Cassini took wide shots and close-ups across several wavelengths and from wildly different angles. It constrained, and in some cases solved, long-debated theories about Saturn and the solar system. It has intimately studied the planet's rings, and during its final months, it captured the view between this icy belt and the gas giant for the first time.
Perhaps most importantly, Cassini made a powerful case to further explore Saturn and its dazzling satellites, and address this system's copious unsolved mysteries. Unfortunately, due to budget constraints and changing cultures at NASA and ESA, there's no project of this scale on the horizon to follow up on Cassini's efforts.
"This whole legacy of understanding Saturn to a much greater depth than before actually leads us to a dilemma," Lunine said. "If we want to address all the exciting things that Cassini has discovered, we need a super-orbiter system." Think: Life-sniffing probes to fly through Enceladus' plume, boats and aerial vehicles for Titan, and other accoutrements.
But the next generation of Saturn missions is more likely to be deployed piecemeal to study specific objectives, each one with a budget of under $1 billion (Cassini-Huygens cost about $3.26 billion). Several of these concepts focus on exploration of Enceladus' plumes and Titan's lakes, including Lunine's projects ELF and the aquatic vehicle Titan Mare Explorer (TiME). Other scientists have proposed Titan drones and Saturn atmospheric entry probes.
Many of these proposed missions have been selected for development by NASA, but as of now, they still remain in administrative limbo.
NASA and ESA have sketched out broad details of a possible Cassini-Huygens successor called the Titan Saturn System Mission (TSSM) that would bundle a Titan aerial balloon and the TiME boat concept in with a new Saturn orbiter. But that project is now on hiatus, ousted by a preference for visiting Jupiter and its own icy moons, including Europa. For now, NASA's public manifest of planned Saturn missions remains sadly empty.
The uncertain future for Saturnian exploration underscores the singular adventure of Cassini-Huygens. Though this trusty orbiter will cease to exist Friday, the years it spent exploring the Saturn system are etched in its dispatches.
"This is really something people should know," Altobelli said, "that these missions are selected for delivering data that can be preserved for future generations. Most of the Cassini data has already been archived. This provides the possibility for anyone in the world to access the data and analyze them." While Cassini will no longer be relaying back information every day, its scientific legacy is far from over.
Moreover, there is a prismatic range of ways to consume, interpret, and represent Cassini's motherlode of data that transcend science. Take the work of Matt Russo, a postdoctoral fellow at the Canadian Institute for Theoretical Astrophysics, who specializes in converting the resonances of astronomical systems into musical compositions. Inspired by the Grand Finale, he produced scores based on Cassini's trajectory and its observations of Saturn's rings and moons.
Video: SYSTEM Sounds
"Saturn is just jam-packed with resonances," Russo told me over the phone. "Within the ring system, you have every resonance you could want with any of the moons. It's really the most musical object in the solar system."
Such interdisciplinary revelations mined from Cassini will continue, and that is some consolation to the international network of people who are about to lose the emissary they raised through a rocky bureaucratic infancy, blasted into deep space, and guided through an eventful expedition that ended in its atmospheric cremation.
"This, in some ways, looks almost like a one-of-a-kind thing," Lunine said, "because Cassini has had a much more intimate collaboration than other missions. Every instrument on both the probe and the orbiter was an international collaboration between the US and a European country."
"I hate to think it was a fluke," he continued. "But it was a great example of how, by working together with Europe, the US could do something greater."
The sense of gratitude is echoed by team members from around the world.
"It is the end of a human adventure," Altobelli said. "This type of project, where you have so many people trying to get the best science from the mission—and you have to negotiate—little by little, you get a family spirit developing."
On the eve of its death, Cassini has brought people together for one final daredevil maneuver. NASA will livestream the scene at the Jet Propulsion Laboratory in Pasadena, where team members have convened for the orbiter's fatal dive.
By the time the last radio signal is received on Earth, at approximately 7:55 AM ET Friday, the orbiter will have joined its astronomer namesake as part of history, and merged with the beautiful planet it has come to know so well.
We're not likely to hear any robotic chattering from the Saturn system for a long time, and the outer solar system will seem lonely. But Cassini-Huygens, even as a mission of the past, will be a beacon of what is possible when Earthlings work together to explore new frontiers. Goodbye and godspeed to a beloved interplanetary trooper.
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