The curtain has opened on the Cassini mission’s final act. In the early hours of December 4, NASA’s long-lived Saturn explorer swung low over the planet’s F ring—the first of many ring-grazing orbits that will take place over the next four months.
It’s a major change of pace for the school bus-size spacecraft, which spent the past 12 years exploring the Saturn system. During its prolific mission, the craft mostly kept its distance from the ringed planet and instead swooped by some of the 62 enigmatic moons that spangle its skies.
Now, though, that changes. With its fuel running out, the spacecraft is thundering toward its final days in orbit around Saturn, and over the next nine months, Cassini will be focusing on the planet and its rings.
At the end of November, Cassini swung high above the Saturnian system and set itself on a course that will see it graze the edge of Saturn’s F ring 20 times before the end of April.
During the first of those ring-grazing orbits, which climaxed this weekend as Cassini neared the rings, the spacecraft will be beaming radio signals back to Earth, staring at the poles of the planet, and studying the small moons Tethys and Enceladus.
The outermost of Saturn’s easily visible rings, the F ring is also one of the most active. Spirals, kinks, and knots regularly appear in the 310-mile-wide disk of icy particles, which is bounded on the inside by a small moon, Prometheus, and shepherded on the outside by another small moon, Pandora.
“This ring is ever-changing, with bright streamers and tenuous filaments sometimes flowing out from it,” says Cassini project scientist Linda Spilker of NASA’s Jet Propulsion Laboratory. “At other times, the moon Prometheus dips in and out of the F ring, creating channels in the ring material.”
Cassini’s ring-grazing orbits should offer scientists a decent view of the outer edge of the massive A ring, which is just inside the shifting, comparatively thin F band. It’s here that small moonlets hiding within the ring betray their presence by leaving propeller-shaped wakes in the icy particles.
More than mere curiosities, these tiny moonlets could behave a lot like nascent, still-forming planets. Scientists are hoping to use them as proxies to better understand how the planets grew in our solar system and how worlds form around faraway stars.
Scientists also hope that as Cassini flies high over Saturn, the spacecraft will get a good look at that strange hexagon adorning Saturn’s north pole. Though the six-sided jet stream has been reproduced in the lab, it is perplexing because of its longevity and the fact that nothing else like it exists in our vicinity.
“We still don’t understand how the hexagon is maintaining its non-circular shape,” Spilker says. “It is unique in the solar system.”
After the ring-grazing orbits end in April, things start to get really exciting.
As planners imagined the end of Cassini, they decided to ask the probe to go where no spacecraft has gone before: not just near Saturn’s rings, but between the planet and its giant bangles.
During what mission scientists have named the “Grand Finale,” Cassini will loop around the ringed world 22 times, threading that thin space between Saturn and its rings.
The maneuver is not without risk. Colliding with one of Saturn’s icy ring particles could be disastrous. But managers aren’t too worried. On that first plunge through the void, they’re going to protect Cassini by having the craft use its high-gain antenna as a shield.
Still, “there is always a chance that a ring particle might hit Cassini in a key area, and it will stop functioning,” Spilker says.
Assuming all goes to plan, these orbits will offer scientists the chance to study Saturn’s magnetic field and its gassy, high-pressure interior like never before, and maybe finally begin to understand how the innards of giant planets are put together.
Perhaps more excitingly, Cassini could also answer one of the most nagging questions about the Saturnian system: Just how long has Saturn been wearing its planetary hula hoops?
Currently, scientists disagree about whether the rings are as old as the planet itself or are the younger remnants of a stray moon or unfortunate comet that wandered too close to Saturn and got shredded by its gravitational power.
As Cassini swoops and dives during the final part of its mission, it will be able to measure the mass of the rings and collect a key clue that could help unlock that answer.
In September 2017, Cassini will take its final bow as it plunges into Saturn itself, frantically sending data back to Earth until it burns up in the planet’s atmosphere.
“Cassini is like a good friend, and I will feel sad to say goodbye on that final day,” Spilker says.
But, she notes, “after Cassini is vaporized in Saturn’s atmosphere, her atoms will now be part of Saturn and remain there forever. I hope that we can return to the Saturn system some time in the near future to continue unraveling the many mysteries that Cassini has revealed.”