NASA's Cassini spacecraft ends its 20-year run around Saturn

This story first appeared on FiveThirtyEight and is reprinted with permission.

Just six months after the Cassini spacecraft arrived at Saturn, its cameras caught something spectacular. It was Jan. 16, 2005, and Cassini was zipping past Enceladus, a bright, tiny moon just 313 miles in diameter. Cassini saw a fuzzy peak, like a tuft of hair standing up. It was a vapor plume, and it meant the little moon was spurting something into space.

As Cassini flew around Enceladus, scientists turned the spacecraft's suite of instruments and sensors to the apparent geyser and discovered that it was water ice. "To actually see this plume of water vapor and water ice particles coming out of the south pole of a moon that is only 300 miles across was absolutely astonishing," said Linda Spilker, Cassini project scientist at NASA's Jet Propulsion Laboratory.

As Cassini prepares to end its 20-year run with a death dive into Saturn on Friday, the spacecraft's two decades of images have made the Saturn system one of the most recognizable, most familiar and yet most breathtakingly mysterious places in the solar system. Pictures like those of the icy geyser will be Cassini's lasting legacy.

Images from Cassini led to the discovery of a system of icy geysers on Enceladus, one of Saturn's 62 known moons. (NASA)

The pictures were worth another three visits. Engineers made this movie from another flyby that November.

Later, Cassini flew directly through the plume and found evidence of a salty global ocean full of hydrothermal activity, building blocks of possible life. This discovery cemented Enceladus as one of the best places within our solar system to look for life outside of Earth - sorry, Mars. It was one of the most sensational findings in recent space history, and certainly one of Cassini's highlights. And it happened not because of hard-to-pronounce space technology, but because of a snapshot.

Pretty space photos may captivate the public, but they're not the backbone of modern astronomy. Given the hard limits on weight and size that space travel imposes, many scientists would rather prioritize instruments like mass spectrometers, which are chemistry workhorses, or radiometers and magnetometers, which measure radiation and magnetic fields. With those tools, astronomers can tease apart fingerprints of individual chemicals and construct detailed simulations of how planets and moons work. Cameras are often an afterthought, literally.

Take the Juno probe, which arrived at Jupiter last summer. It wasn't originally meant to carry a camera at all. Its early designs didn't have room for one, given all the radiation shielding required for Juno's science instruments to survive the stupefyingly horrendous environment of the solar system's biggest planet. Ultimately, the mission's lead scientist, Scott Bolton, decided he couldn't send a billion-dollar probe all the way to Jupiter and not equip it with the tool billions of Earthlings use to catalog their own planet, so the probe launched with a wide-angle JunoCam, "the people's camera." The images Juno produces, just like those from Cassini, are data, too.

Jupiter's south pole, as seen by NASA's Juno spacecraft, is spangled with cyclones up to 600 miles in diameter. This image combines multiple views from the JunoCam instrument in daylight, enhanced color and 3-D, taken from an altitude of 32,000 miles above Jupiter's cloud tops. (NASA)

Through photos, Cassini has made clear that everything about Saturn is odd and surprising. It has a blue hexagonal hurricane at its north pole. Its biggest moon has rivers carved by liquid methane. It is swaddled in iridescent, moving rings. Images, not data sets, capture the public's imagination.

Pictures speak to how we think about the universe - in images and narratives. We are driven by emotion and mystery, and moved by beauty. We use stories to make sense of the world. Data provides raw material for constructing those stories, but ultimately, our minds want images too. And as Cassini and Juno have shown, sometimes pictures provide the most stunning data of all, upending everything we think we know.

"Enceladus has no business existing," said Curt Niebur, a Cassini scientist at NASA. Such a tiny moon, so far from the sun, is unlikely to be warm enough for liquid to persist; moreover, if it's spewing liquid, it should have spent its supply a long time ago. "And yet there it is, practically screaming at us, 'Look at me, I completely invalidate all of your assumptions about the solar system,'" Niebur said.

The Enceladean plume pictures are just one example of how Cassini's images have brought not only Saturn but the entire Kronian system into breathtaking, Art Deco relief. The twin Voyager probes were the first to resolve the fuzzy dots of Jupiter and Saturn into swirling giant planets, but Cassini turned the planets and their moons into worlds - almost-homes, places we could imagine going and being, and maybe encountering other beings, too.

Cassini also brought into focus Titan, a behemoth orb with lakes and rivers of methane and ethane and wind-carved dunes made of hydrocarbon-coated ice pebbles. When Cassini deployed a lander, the Huygens probe, onto Titan's surface in 2005, it unveiled a reddish terrain, a hazy sky and what appeared to be a dry lakebed. "I still give myself goosebumps thinking about seeing that first image - I'll never forget it," said Earl Maize, a Cassini project scientist at the Jet Propulsion Laboratory. Through Huygens, Cassini demonstrated that Titan is the only other world in the solar system, to our knowledge, where liquid flows today.

Cassini was designed as an explorer mission, which allowed its controllers to nimbly adjust its orbit to look at interesting features on its moons, in its rings and on the planet itself. Scientists affectionately call Cassini's haphazard-looking orbital path the "ball of yarn."

It's fitting that Cassini's final days will be spent in what scientists are calling "the last picture show." First, it took one last look at Titan, whose gravity sent Cassini barreling inexorably toward a collision with Saturn. Then it will zoom in on Enceladus and watch as the tiny moon sets behind Saturn. Then it will look at "Peggy," a moonlet in Saturn's rings that might be breaking away and becoming a moon in its own right. It will view the inexplicable hexagonal hurricane raging at Saturn's north pole. And, finally, Cassini will shoot photos of a "propeller," a strange feature in Saturn's ring system that might also be a moon being born.

This movie is pieced together from 128 images of Saturn's north pole. Within the hexagon are several cloud structures, including a massive hurricane centered on the pole. The hurricane's eye is about 50 times larger than the average hurricane eye on Earth. This image includes false color to enhance detail; to human eyes, the hexagon and north pole would appear in hues of gold and blue. (NASA)

The pictures will beam back to Earth on Sept. 14, and then Cassini will turn away from our planet and toward its fateful encounter with Saturn. "It's sort of like taking a last look around your apartment or house just before you move out," Spilker said. "You walk around the downstairs. As you walk upstairs, you run your fingers across the bannister. You look at your old room, and memories across the years come flooding back."

Cassini's end also marks the beginning of a transition in solar system exploration. After Cassini, Juno is next on interplanetary death row. In 2021, it, too, is slated to dive into the cloud tops of its host planet, to protect Europa and other possible life-harboring moons of Jupiter. And after that, Earth won't have a single emissary to the outer planets.

A few new missions are in the planning stages. NASA already announced plans for a satellite called the Europa Clipper, which will repeatedly sail past the Jovian moon to study whether Europa is a livable world. The Jupiter Icy Moons Explorer (JUICE) is a European mission with similar goals. But neither will launch until sometime in the 2020s. Possible missions to Titan and Enceladus are still competing for funding.

The Voyagers launched 40 years ago, and their travels set the course for solar system exploration through two generations. Cassini launched 20 years ago next month, and it has done the same. As it falls apart and becomes one with Saturn, mission controllers will shut down their command terminals and turn to each other for hugs and high-fives. The scientists and engineers will take some time to celebrate and to evaluate all they've done. And then they will get to work building the next set of robot eyes.

A few years from now, peering through cameras not yet constructed, the next generation of explorers will again prepare to see something new in the cold darkness of the outer solar system. Enceladus might be in the sights again. And there we might learn, finally, that we are not alone.

Scientists have used images such as these to map the positions of features on Enceladus, in part to measure changes in the moon's rotation. Grooves and ridges, at far left and center, suggest tectonic activity. Many of its craters have been deformed or relaxed over time. (NASA)