Before coming to Science I was an editor at National Geographic, where I wrote a story on the first phase of Cassini’s historic encounter with Saturn. That journey ended last September, when the two-story-tall interplanetary probe dove into the yellowish haze of the giant planet’s upper atmosphere. Cassini was perhaps the most successful planetary mission ever conducted. Exploring Saturn, its gaudy rings and retinue of over 60 moons for 13 years, Cassini was like a voyage back in time, offering humanity clues to how the solar system took shape and gave rise to life. On the anniversary of Cassini’s finish, as Science publishes papers from the final phase of the mission, here are a few recollections of this remarkable exploration.
On 1 July 2004, I was at Cassini Mission Control at the Jet Propulsion Laboratory in Pasadena, California, when Cassini arrived at Saturn, ending a 7-year, 2.2-billion-mile journey. Traveling at 68,000 miles an hour, the spacecraft had to shed speed so that Saturn’s gravity could capture it. Firing engines, it dropped to within 13,000 miles of the planet’s butterscotch clouds after a daring passage through a wide gap in the outer rings. At that speed, hitting a single pebble could have ended the $3.4 billion mission.
The most intriguing of Saturn’s moons is the largest: Titan. In December 2004, 6 months after arriving at Saturn, Cassini launched the crown jewel of the mission, a saucer-shaped probe named Huygens that had ridden piggyback from Earth. Three weeks later, Huygens plunged into Titan’s smoggy atmosphere. Pummeled by air friction, the probe’s heat shield reached a temperature of thousands of degrees. Gradually, Huygens slowed and cooled. Parachutes opened, the heat shield dropped away, and Huygens drifted like a leaf on the winds of Titan, cameras and microphones recording the weather on this distant world. The first picture from the ground appeared—a garish orange landscape, strewn with rocks.
Other Huygens pictures from the descent showed hills carved by methane rain. Later observations from Cassini revealed seas and lakes of the chilly compound. A stunning picture emerged of a complex world, the only one in the solar system other than our own with stable liquid on the surface. Titan turned out to be a kind of frozen time capsule, with some conditions like those found on the very early Earth.
Saturn’s most prominent feature is the dazzling ring system. From edge to edge, the main rings span some 200,000 miles, nearly the distance from Earth to the moon. Yet the thickness of these bands of icy rubble averages only 150 feet. How the rings formed remains a mystery, although some scientists speculate that Saturn’s gravity tore apart an icy moon or a comet, strewing debris that provided the raw material. In Saturn’s rings today, tiny moons play the role of the planets. Each moon’s gravitational tug is minute. Yet the moons’ gravity helps maintain the rings by keeping the particles from straying from their orbits. A moon can also carve a gap between rings, and its gravity can send waves of density rippling through a nearby ring, like traffic speeding and slowing on a crowded freeway.
Bright as a beacon, ice-covered Enceladus reflects more light than any other body in the solar system. In 1980, Voyager images showed only a few large craters marring the moon’s surface, leading scientists to suspect that geologic processes were somehow erasing the scars. As the distant sun silhouetted Enceladus, Cassini made images of the tiny moon that showed geyser-like eruptions of water vapor and ice particles shooting far into space. Could this modest moon harbor life? Life as we know it requires liquid water, energy, and organic molecules. Life might be hiding just a few dozen feet below the ice in pockets of warm water, living off dissolved organic compounds and reproducing using some alien version of DNA—or an entirely different kind of genetic material. Future missions may sample the plumes of Enceladus or drift across the skies of Titan, seeking evidence of life on Saturn’s surprising worlds.
As Cassini’s epic mission concluded, the cameras took 80 images of the planet and ring system. Forty-two were montaged, creating this final somber mosaic on the cover of Science. “It is our goodbye to Saturn picture,” said Project Scientist Linda Spilker. Two days later, on 15 September 2017, Cassini’s engines fired one last time, slowing the spacecraft for a dramatic plunge into the Saturn’s atmosphere.
Bill Douthitt is the Photography Managing Editor at Science.