So during Cassini’s cruise, Phoebe traveled around Saturn more than four times. 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. 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). The red streaks are narrow, curved lines on the moon's surface, only a few miles wide but several hundred miles long. Unusual arc-shaped, reddish streaks cut across the surface of Saturn's ice-rich moon Tethys in this enhanced-color mosaic. The 2004 Phoebe flyby alone was a spectacular feat if for no other reason than the distances, time and motion involved. But while some of Cassini’s discoveries were happy accidents, the flybys themselves were intentional and carefully planned. “They are very, very mysterious.”Ĭassini’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. “The streaks look like they’re painted on,” Buratti said. 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. Red features are rare on Saturn’s moons, and scientists found no relationship between the streaks and the moon’s surface topography. 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. Scientists can come up with reasonable explanations for some of the surprising features of Saturn’s moons, but others remain unsolved. At least two of Saturn's other small moons have a similar ridge or "skirt" around their middles. This animation of Cassini images shows Saturn's moon Pan during a March 2017 flyby. “And we wouldn’t have known that without these flybys.” “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 because these moons are so small, the ridges are their dominant surface feature. Other moons in the Saturn system have ridges, too, built not of their own ring particles but of Saturn’s. 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. Scientists remain uncertain how the ridge formed. 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. 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. Scientists aren't sure what led to the ridge's formation. As a result, scientists learned of exotic features they would have otherwise missed.Ĭassini scientists found that Saturn's two-tone moon Iapetus has a ridge along its equator. 2008) - providing jaw-dropping views of surface features, and up-close inspections of the moons’ temperatures, compositions and other characteristics. Saturn's moons came to life only through Cassini’s many close flybys.Ĭassini 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. 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. 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. 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. 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. That changed in June 2004.Īs Cassini entered the realm of Saturn, the spacecraft passed within 1,300 miles (2,100 kilometers) of Phoebe on June 11. 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.
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