Photo of Saturn at its equinox, when the sun is exactly overhead at the planet's equator.

From 20 degrees above the ring plane, Cassini’s wide angle camera shot 75 exposures in succession for this mosaic showing Saturn, its rings and a few of its moons a day and a half after exact Saturn equinox, when the sun’s disk was exactly overhead at the planet’s equator. Image credit: NASA/JPL/Space Science Institute

Before Cassini, we had only brief glimpses of the discoveries awaiting us at Saturn. Pioneer 11 and Voyagers 1 and 2 conducted flybys decades ago, taking pictures, measurements and observations as they zoomed past. These missions shed new light on Saturn’s complicated ring system, discovered new moons and made the first measurements of Saturn’s magnetosphere. But these quick encounters didn’t allow time for more extensive scientific research. 

Cassini changed all that. It began the first in-depth, up-close study of Saturn and its system of rings and moons in 2004. It became the first spacecraft to orbit Saturn, beginning a mission that yielded troves of new insights over more than a decade. The Saturnian system proved to be rich ground for exploration and discoveries, and Cassini's science findings changed the course of future planetary exploration.

"We're looking at a string of remarkable discoveries -- about Saturn's magnificent rings, its amazing moons, its dynamic magnetosphere and Titan's surface and atmosphere," said Linda Spilker, Cassini's project scientist. "Some of the mission highlights so far include discovering that Titan has Earth-like processes and that the small moon Enceladus has a hot-spot at its southern pole, plus jets on the surface that spew out ice crystals, and liquid water beneath its surface."

Cassini let us see beneath Titan's thick orange haze. This synthetic aperture radar image taken by Cassini has abundant evidence for flowing liquids, from sinuous, wide river channels to shorter, more chaotic drainage patterns. Image credit: NASA/JPL-Caltech/ASI

Studying Saturn’s Many Moons

Some of the most surprising scientific findings have come from encounters with Saturn’s fascinating, dynamic moons. 

Cassini's observations of Saturn's largest moon, Titan, have given scientists a glimpse of what Earth might have been like before life evolved. They now believe Titan possesses many parallels to Earth, including lakes, rivers, channels, dunes, rain, clouds, mountains and possibly volcanoes.

The data from the Cassini spacecraft and the European Space Agency's Huygens probe, which plunged through Titan's dense, smoggy atmosphere to land on its surface in 2005, have generated hundreds of scientific articles and been the subject of special issues of the world’s most important scientific journals.

Enceladus, too, proved to be a rich source of discovery. The spray of icy particles from the surface jets forms a towering plume three times taller than the width of Enceladus itself. Cassini confirmed that the plume feeds particles into Saturn's most expansive ring, the E ring. The spacecraft has come as close as 15 miles (25 kilometers) from the moon's icy surface during its investigation, revealing the presence of a global subsurface ocean that might have conditions suitable for life.

Saturn's moon Enceladus backlit by the sun show the fountain-like sources of the fine spray of material that towers over the south polar region. Image credit: NASA/JPL/Space Science Institute

Extending the Mission

The Cassini mission has seen two mission extensions, allowing for more flybys, investigations and measurements, over a longer span of time.

When its initial four-year tour of the Saturn system was complete in 2008, the Cassini-Huygens saga had brought a new dimension of understanding to the complex and diverse Saturn system. 

The two-year Cassini Equinox Mission brought continued excitement. During that first extended mission, the spacecraft made 60 additional orbits of Saturn, 26 flybys of Titan, seven of Enceladus, and one each of Dione, Rhea and Helene. The Equinox mission allowed for observations of Saturn's rings as the sun lit them edge-on, revealing a host of never-before-seen insights into the rings' structure.

Since 2010, the spacecraft has conducted a second, seven-year-long, extended mission called the Cassini Solstice Mission. This final mission will conclude with a phase known as The Grand Finale -- 22 deep dives between Saturn's cloud tops and innermost ring before it plunges into the giant planet's atmosphere.

Why “Equinox” and “Solstice”?

Saturn’s axis is tilted with respect to its orbit around the sun, similar to Earth. And just like on Earth, Saturn has seasons as a result of this tilt. But on Saturn, one season lasts seven Earth years.

Cassini initially arrived just after Saturn's northern winter solstice, and the Solstice Mission continues until a few months past the northern summer solstice in May 2017. A complete seasonal period on Saturn has never been studied at this level of detail. The Solstice mission schedule allows for an additional 155 orbits around the planet, 54 flybys of Titan and 11 flybys of the icy moon Enceladus.