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During the summer of 2004, almost seven years after launching from Cape Canaveral Air Force Station, a spacecraft entered orbit around Saturn for the first time.
A NASA, European Space Agency (ESA) and Italian Space Agency partnership produced Cassini-Huygens, the uncrewed, fully robotic interplanetary spacecraft set to explore Saturn and its many moons.
And mission accomplished: In 2017, 20 years after its launch from Earth, the project concluded in a fiery blaze as Cassini disintegrated in Saturn’s atmosphere. During the mission, 4.9 billion miles were traveled, 453,048 images were taken, and 3,948 science papers were published about its findings, NASA says.
“Being able to spend that much time at one planet, studying various aspects of the planet, is pretty incredible,” says Matthew Shindell, the planetary science and exploration curator at the National Air and Space Museum.
The mission involved two separate vehicles: the Cassini orbiter and the Huygens probe, named after 17th-century astronomers Gian Domenico Cassini and Christiaan Huygens.
Cassini transported Huygens on their journey from Earth to Saturn. Cassini’s mission was to investigate Saturn and its system, while Huygens’ was to explore Titan, the planet’s largest moon. The Huygens probe separated from Cassini in late December 2004.
“It was considered a relatively simple arrangement because, you know, you just bolt the two things together, and then they pop apart and go do their own thing,” NASA program scientist Curt Niebur says. “It was, even within that simple arrangement, an exquisitely complex job to pull it off.”
Because Cassini was the larger spacecraft that hosted most of the mission’s scientific instruments and had a longer life span, the mission’s hyphenated name is often shortened to Cassini.
Cassini was “one of the largest, heaviest and most complex interplanetary spacecraft ever built,” according to the ESA.
Before Cassini, little was known about Saturn.
“We knew Saturn was big,” Niebur says. “We knew it had an atmosphere; that atmosphere was full of hydrogen and had a lot of other trace elements in it. We knew it had rings. We knew it had a lot of moons of various sizes. And we knew one of those moons—Titan—seemed to have an atmosphere. That’s it. That’s all we knew.”
But, after Cassini’s 294 completed orbits and 162 targeted flybys of Saturn’s moons across its voyage, a wealth of information was discovered.
Cassini found several moons orbiting Saturn, and as of 2023, the planet’s known number of moons in its orbit is almost 150.
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Titan
On January 14, 2005, the Huygens probe landed on Titan and revealed the its remarkable similarities to Earth.
“It was kind of called ‘the Earth in deep freeze,’” says Bonnie Buratti, a fellow and senior research scientist at NASA’s Jet Propulsion Laboratory.
The moon, with temperatures around minus 290 degrees Fahrenheit, has atmospheric pressure twice that of Earth. While its atmosphere is primarily nitrogen like Earth’s, instead of oxygen, it has methane.
“Titan is an organic chemistry powerhouse,” Niebur says. “It’s doing a lot of weird organic chemistry in its atmosphere. … That has a lot of implications not for habitability but for life, because complex organic chemistry was the step before life.”
NASA says it is “the only moon in the solar system with a dense atmosphere, and it’s the only world besides Earth that has standing bodies of liquid, including rivers, lakes and seas, on its surface.”
Titan also has a unique methane cycle analogous to Earth’s hydrologic cycle but based on methane rather than water.
At the National Air and Space Museum in Washington, D.C., Titan is part of the “Walking on Other Worlds” interactive experience.
“We give the public a sort of immersive experience of standing on the surface of Titan, looking out at these hydrocarbon lakes and this sort of dense, yellowish atmosphere,” Shindell says. “And that’s only possible for us to do because the Huygens probe landed and collected that data.”
Enceladus
Enceladus, another one of Saturn’s moons, also came to hold plenty of interest.
Niebur says that the Cassini team first thought, “‘Enceladus, that’s just a tiny ball of ice the size of Texas. We’ll fly by once, we’ll take some pictures, we’ll say we’ve been there, done that.’ And then we flew by, and it was, ‘Oh, this makes absolutely no sense whatsoever. Now we have to rewrite all of the textbooks about planetary science.’”
Enceladus was found to have active cryovolcanic plumes at its south pole, indicating the presence of a global subsurface saltwater ocean with hydrothermal vents that “may serve a source of nutrients just like the hydrothermal vents during the deep oceans on the Earth,” says Linda Spilker, a senior research and planetary scientist at the Jet Propulsion Laboratory.
On Earth, hydrothermal vents are considered potential environments for the origin of life. According to the American Museum of Natural History, “some of the thermophilic, or heat-loving, vent microbes are the most primitive organisms known on Earth.” Additionally, “complex organic molecules, the building blocks of life, are found at the vents” and “the deep ocean was one of the few places on the early Earth that was protected from frequent meteorite bombardments and lethal radiation.”
During its Enceladus survey, Cassini found 101 distinct geyser-like jets on the moon. The geysers, whose origins are believed to be in the below-surface ocean, spray water vapor and ice particles.
Spilker says that Cassini flew through the geysers on Enceladus seven times, directly sampled them and “found the basic ingredients for life,” such as carbon, hydrogen, oxygen, phosphorus, sulfur and nitrogen.
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The rings
Cassini was tasked with closely investigating the planet’s ring structures, and it uncovered significant swaths of data.
During its exploration of Saturn’s rings, Cassini found that they consist of materials ranging from “tiny dust grains all the way up to house- or mountain-sized objects, and of course, there are fewer of the larger-size particles,” Silker explains. Given the sizable radial extent of the rings, this finding was groundbreaking.
Cassini also captured the continuous chaos of the rings.
When people envision Saturn’s rings, they often see “this very stately, majestic, well-ordered classical orbit around Saturn, and it’s like listening to classical music; everything’s in its place and stays in its place,” Niebur says. “It’s not like that. It’s more like being at a motocross race where the track is muddy. And you see the motorcycles hit the hills and fly up, and they’re flinging mud everywhere. And there’s just stuff flying and colliding with each other. That’s what it's like in the rings. It’s crazy chaos in there.”
Before Cassini, scientists had long debated the age of Saturn’s rings. Silker says that “there were two camps, the old rings and the young rings, back and forth, back and forth, back and forth.”
Using the data from Cassini’s mission, various studies determined that Saturn’s rings are relatively young. A 2023 study published in Science Advances concluded that Saturn’s rings are no more than 400 million years old, far younger than Saturn’s 4.5-billion-year-old age.
Cassini was able to measure the mass of the rings. “And we had to do that in the final orbits by diving in between the rings and the planet,” Silker says.
Having available data on the mass of Saturn’s rings allowed scientists to estimate their age by seeing “how quickly incoming debris is added, and how that influences the way the rings change over time. Put those elements together, and one can get a better idea of how long they’ve been around and the time they’ve got left,” NASA’s Aaron McKinnon reports.
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What’s next for Saturn exploration?
As the 20th anniversary of Cassini’s entry into Saturn’s orbit passes, scientists worldwide are preparing for future missions to the ringed planet to build upon Cassini’s enduring discoveries.
NASA’s Dragonfly mission, which aims to further unravel Titan’s mysteries after its scheduled launch in 2028, will deploy a rotorcraft lander to hop across Titan’s surface, exploring everything from its subsurface ocean to its methane cycle.
“It’s essentially a drone with eight rotors,” Niebur says. “It’s about the size of a dining room table. And it is going to fly around Titan all by itself. … It’s a tremendously ambitious and exciting mission.”
NASA also hopes to investigate Enceladus further. Cassini’s discovery of evidence for a global liquid ocean on the moon makes an Enceladus lander a high priority.
In the meantime, people can continue to celebrate the achievements of Cassini-Huygens, which was, as the Royal Society hails it, “one of the most successful and groundbreaking space missions ever launched.”