Nearly 20 years ago, NASA’s Cassini spacecraft left Earth and began a seven-year journey across the solar system. Its destination? Ringed Saturn and its many, many moons. After more than a decade exploring the system, Cassini will plunge to its doom in the planet’s atmosphere on September 15, 2017. Here, we take a look at Cassini’s voyage and its greatest achievements.
THIS IS SATURN
The second largest planet in our solar system is known for its dramatic ring system and dozens of moons. Shown here are the major moons beyond the rings. The planet itself is big enough for 764 Earths to fit inside.
CASSINI ENTERS SATURN ORBIT
Cassini slipped into orbit in 2004 and quickly began an acrobatic exploration of the planet and its myriad moons. Some of those worlds, revealed by the data and imagery Cassini has beamed home, are now thought to be among the best places to look for extraterrestrial life-forms.
ENCELADUS, AN ERUPTING OCEAN WORLD
Enceladus may be a tiny moon, but it has a pretty big reputation among scientists looking for life beyond Earth.
Spotted by Cassini in 2005, dozens of saltwater geysers erupt from fissures in the moon’s south pole. Some of the erupting material falls back to the surface like a dusting of snow, but the rest escapes into space, where it’s responsible for crafting Saturn’s thin, far-flung E ring.
The jets are fueled by a global sea beneath Enceladus’s icy crust. When Cassini flew through and sampled the plumes, it discovered that the alien water contains all the ingredients we believe are necessary for life to exist. Chemicals in the geysers also offer hints that hydrothermal vents are active on the ocean floor. What’s more, the hidden ocean has potentially existed for billions of years—more than enough time for life to evolve.
TITAN, A HAZY MIRROR-EARTH
Wrapped in a thick, orange atmosphere, Titan is a world that appears deceptively Earthlike: Its surface is sculpted by flowing rivers, rainstorms sprinkle its dune fields with seasonal showers, and winds make waves in its alien seas.
But the chemistry on Saturn’s largest moon is decidedly unearthly. Instead of water, its lakes are dominated by ethane and methane, which exist as gases on our planet but are flowing liquids at Titan’s frigid temperatures.
As on Earth, this moon’s puffy atmosphere is mostly made of nitrogen, but it also contains all sorts of strange organic compounds, including one that might be able to self-assemble into truly alien membranes.
Shortly after arriving in the Saturn system, Cassini dropped the Huygens lander onto Titan—the first time humans had set a spacecraft on the surface of an alien moon. The lander parachuted through the atmosphere and beamed back images and data from the icy surface for several hours before forever falling silent.
MANY, MANY MOONS
Saturn’s skies are home to more than 60 moons. Some of them are round like planets, while others resemble a variety of vegetables. The biggest moon, Titan, is larger than the planet Mercury.
Rhea, Saturn’s second largest satellite, was once suspected of having its own ring, but closer looks by Cassini didn’t find any evidence of that. Instead, a March 2010 flyby suggested the moon has an extremely thin, oxygen-dominated atmosphere.
Another moon, Hyperion, looks like an alien honeycomb—perhaps the work of meteorites sinking into its spongy surface. Dione is riven with curious chasms, Mimas is scarred by an enormous crater, Tethys is stained with blood-red streaks, and tiny Pan and Atlas are shaped like UFOs.
The diversity of worlds orbiting Saturn is both incredible and perplexing, and scientists aren’t quite sure how these moons all managed to form around the same planet.
In late 2010, an enormous storm erupted in Saturn’s northern hemisphere. Over two months, the storm spawned a tail that grew to be more than 118,000 miles long—the tail eventually wrapped itself around the entire planet and collided with the storm’s great white eye.
Cassini watched as the storm churned and dissipated, leaving almost no traces in the planet’s yellowish atmosphere. The spacecraft determined that at its most intense, the storm was dredging water ice from the planet’s depths and depositing it atop the clouds, along with large crystals of condensed ammonia. Cassini also attempted to record the storm’s lightning, but it was flashing so furiously—more than 10 times a second—that the spacecraft couldn’t keep up.
This was the longest and most intense storm seen on Saturn. Scientists now suspect that such tempests are seasonal, emerging during the planet’s spring and summer.
LOOKING TOWARD HOME
In July 2013, Cassini slipped behind Saturn and swiveled to stare at the giant planet, backlit by the sun. It snapped several pictures, and in the final image, a pale blue dot is tucked beneath the planet’s gleaming rings: Earth, as seen from nearly 900 million miles away.
NASA knew of Cassini’s plans and alerted Earthlings to the upcoming portrait, sharing information about the exact time the image would be taken and telling people where to find Saturn in the sky. That Cassini photo is now known as “The Day the Earth Smiled,” because people around the world turned to look at Saturn and smiled for Cassini’s faraway eyes.
SATURN, THE RINGED GIANT
Befitting its tantalizing array of moons, Saturn itself is worth ogling as it treks around the sun in its 29-year orbit.
When Cassini arrived, Saturn’s northern pole was bathed in winter and was a deep, Neptunian blue. Jet streams in the planet’s atmosphere had shaped the cloud tops into a noticeable hexagon wide enough to stuff two Earths inside. The seemingly artificial shape provoked wild speculation until scientists showed they could naturally produce the pattern in the lab.
Since then, winter has passed into spring and summer. That hexagon still adorns the northern pole, but its color has changed: Once turquoise, it’s now a hazy beige-yellow, which probably has something to do with summer sunlight striking the atmosphere and crafting molecules that shine in different hues.
Saturn is perhaps most famous for its rings, a vast system of dust, rock, and ice that ranges from 300 feet to a mere 30 feet thick.
Stretching 50,000 miles from the planet’s belly, the rings are classified into several large chunks, lettered A through G. Several smaller rings and ringlets are named after nearby moons. Thanks to Cassini, we know that some of the gaps within the rings are carved and shepherded by small moons; others are formed by gravitational interactions with larger moons, such as Mimas. Fine, small-scale structures within the rings also record the motions of the planet’s interior via small ripples and waves.
The Rings of Saturn
miles from equator
Despite being studied and photographed countless times, those beautiful bangles are still fundamentally mysterious. Scientists don’t know how most of them formed or how old they are, but they hope Cassini will help answer these questions during its last few months in orbit, as it measures the amount of material in the rings from a closer range than ever before.
Saturn’s rings may seem placid and elegant, but appearances can be deceiving. Pruned, sculpted, and cleared by gravitational interactions, the countless particles that make up the rings hid several secrets from earthly eyes until Cassini stopped by for a visit.
In 2006, scientists spied tiny structures within Saturn’s A ring that looked a lot like propellers. Eventually, they realized the strange shapes were the fingerprints of tiny moons embedded within the rings themselves. Images from Cassini have since revealed dozens of these propellers. The largest of them, estimated to measure several hundred feet across, have been named after aviators such as Amelia Earhart and Louis Blériot.
More recently, scientists caught a moonlet in the process of being born. Named Peggy after the discoverer’s mother-in-law, the tiny moon appeared to be emerging from the edge of the A ring in 2013. In Cassini observations from 2015 and 2016, Peggy seems to have broken a bit after a collision with another Saturnian moonlet.
After zooming through the Saturn system for more than 13 years, Cassini is coming to the end of its mission. We knew it had to happen—after all, the spacecraft has a finite fuel supply, and it’s not possible to refill the gas tank from afar.
But mission designers saved the most daring moves for last. In December 2016, Cassini began buzzing the planet’s rings, and since April, it has been threading the space between them and the planet. Ultimately, Cassini will plunge into Saturn’s atmosphere.
The mission team has dubbed this portion of the adventure the Grand Finale.
Why kill Cassini instead of letting it orbit aimlessly? The team is worried about an uncontrolled crash into one of Saturn’s moons, particularly Titan or Enceladus, which are considered capable of hosting alien life. Potentially contaminating those moons with the husk of a spent spacecraft isn’t exactly polite. Instead Cassini will end its journey among the planet’s clouds. As it does, the spacecraft will swivel to face Earth one last time, beaming data home until it ignites and breaks apart, streaking like a meteorite through Saturn’s skies.
Additional contributions by Jason Treat, Matthew W. Chwastyk, NG Staff. Data: NASA/JPL-Caltech; NASA Goddard Space Flight Center
The angle of the sun and the positions of background stars in the 3-D graphics are for visual effect and do not reflect real-life conditions.