An image of Saturn's moon Phoebe taken by the Cassini spacecraft in 2004.

Cassini captured this image of Saturn's moon Phoebe in June 2004. The encounter with Phoebe foreshadowed how the mission would revolutionize our understanding of Saturn's moons through Cassini's score's of close flybys of the natural satellites. › Full image and caption

Saturn’s moon Phoebe can seem unremarkable from Earth — just another run-of-the-mill space rock slowly tumbling along in its orbit, so distant and small that it’s invisible to most ground-based telescopes. That changed in June 2004. 

As Cassini entered the realm of Saturn, the spacecraft passed within 1,300 miles (2,100 kilometers) of Phoebe on June 11. Cassini’s instruments studied Phoebe and sent stunning images back to Earth, transforming it from a remote and vague speck into a place in its own right — a new world more than 130 miles (210 kilometers) wide. 

It was Cassini’s first flyby of a Saturnian moon, and it brought Phoebe and its cratered glory into sharp focus for the first time. The spacecraft was still millions of miles from Saturn and weeks from entering orbit, but the Phoebe encounter kicked off a new era of discovery on Saturn's orbiting worlds. The flyby also showed that Cassini could serve as our proxy — a human presence, in a way — making the Saturn system more real to humans than ever before. Saturn's moons came to life only through Cassini’s many close flybys.

Cassini has since performed scores of flybys of Saturn’s moons and has passed thrillingly close to some of them — 16 miles (25 kilometers) at the closest (Enceladus, in Oct. 2008) — providing jaw-dropping views of surface features, and up-close inspections of the moons’ temperatures, compositions and other characteristics. As a result, scientists learned of exotic features they would have otherwise missed.

An image captured by Cassini of Saturn's moon Iapetus.
Cassini scientists found that Saturn's two-tone moon Iapetus has a ridge along its equator. Scientists aren't sure what led to the ridge's formation.

One of those mysteries scientists hoped to solve during the Cassini mission was why one side of Saturn’s moon Iapetus is dark as charcoal while the other side is nearly white as snow. But when Cassini got close to Iapetus, scientists stumbled upon an entirely different puzzle: Iapetus has a ridge around its middle that makes the moon look a bit like a walnut. 

Scientists remain uncertain how the ridge formed. One possibility is that Iapetus once had rings of its own but the ring system collapsed, eventually building up a ridge around the moon’s equator.

Other moons in the Saturn system have ridges, too, built not of their own ring particles but of Saturn’s. And because these moons are so small, the ridges are their dominant surface feature.

“At least three of Saturn’s little moons have those little skirts,” said Bonnie Buratti, a senior research scientist at NASA’s Jet Propulsion Laboratory. “And we wouldn’t have known that without these flybys.”

An animation of Saturn's moon Pan made from several Cassini images.
This animation of Cassini images shows Saturn's moon Pan during a March 2017 flyby. At least two of Saturn's other small moons have a similar ridge or "skirt" around their middles.

Scientists can come up with reasonable explanations for some of the surprising features of Saturn’s moons, but others remain unsolved. In September 2005, Cassini visited Saturn’s moon Tethys and detected arc-shaped reddish streaks a few hundred miles long and a few miles wide running over the moon’s surface. 

Red features are rare on Saturn’s moons, and scientists found no relationship between the streaks and the moon’s surface topography. If the red appeared only in the bottoms of craters, or along surface fractures, then scientists would have more to work with — but the streaks run indifferently across everything. “The streaks look like they’re painted on,” Buratti said. “They are very, very mysterious.”

Cassini’s close flybys provided other astounding revelations, such as liquid methane lakes on Saturn's largest moon Titan, and water jets spraying out of the smaller moon Enceladus that led to the discovery of a possibly habitable subsurface ocean there. But while some of Cassini’s discoveries were happy accidents, the flybys themselves were intentional and carefully planned. The 2004 Phoebe flyby alone was a spectacular feat if for no other reason than the distances, time and motion involved.

Tethys
Unusual arc-shaped, reddish streaks cut across the surface of Saturn's ice-rich moon Tethys in this enhanced-color mosaic. The red streaks are narrow, curved lines on the moon's surface, only a few miles wide but several hundred miles long. Scientists don't know what causes them.

Just to get to Saturn, Cassini traveled a little more than 2 billion miles (nearly 3.5 billion kilometers), making two full orbits of the sun en route, and gaining speed through planetary gravity boosts (see sidebar). Phoebe takes about 1.5 Earth years to complete one orbit of Saturn, but Cassini took six years and nine months to get from Earth to Saturn. So during Cassini’s cruise, Phoebe traveled around Saturn more than four times. If the spacecraft’s velocity relative to Saturn had been a little off, or if Phoebe’s future position had been miscalculated, the little moon could have been millions of miles away when Cassini crossed Phoebe’s orbit.

Thanks to careful calculations, Cassini was right on target and passed nearer to Phoebe than Hawaii is to California. That was only the beginning of the expert navigation. 

Cassini used up most of its propellant by the time it began orbiting Saturn. So to chase down Saturn’s moons for the rest of the mission, the spacecraft used flybys of one moon in particular. “We use the gravity of Titan to change the trajectory of the spacecraft,” said Brent Buffington, a trajectory designer for the Cassini mission. Each of the 127 close Titan flybys was a gravity assist used to speed or slow the spacecraft and alter its trajectory, or flight path, in a precise way. 

But the spacecraft didn’t always fly as close to Titan as originally planned. “Titan’s dense atmosphere extended farther from the surface than we thought,” Buffington said. As a result, the Cassini navigation team had to modify a couple dozen Titan flybys so that the spacecraft was a safer distance from the moon’s atmosphere. That, Buffington said, is one of the exciting things about planetary exploration. “We think we know what we’re going to encounter, but sometimes we’re wrong and must adapt our strategies accordingly.”

Before the spacecraft runs out of propellant and can no longer be controlled, the flight team is disposing of Cassini on Sept. 15 in Saturn’s atmosphere to protect the pristine environments of Enceladus and Titan, both of which might be habitable.

On that day, for the first time in 13 years, Earthlings will have no presence in the Saturnian system — no sensors or other instruments to listen in case the moons want to confide more secrets. 

“Cassini has brought Saturn and its family of moons into our neighborhood, making them as familiar as your own backyard,” said Linda Spilker, Cassini’s project scientist. “With Cassini’s final heartbeat, our close personal connection to the Saturn system will be gone, but so much more remains to be discovered. We must go back!”

Saturn is the farthest world that any spacecraft has ever orbited, and Cassini is one of the most massive spacecraft ever flung into the outer solar system. But the farther you want an object to go, the faster it needs to travel and the more powerful its rocket needs to be. No U.S. rocket at the time was powerful enough to get Cassini to Saturn, so mission designers came up with a plan. Cassini would take the scenic route, flying by Venus twice, then back to Earth, followed by Jupiter, to accelerate enough to reach Saturn. Read More