Sen—Cassini has orbited Saturn for almost eleven years now. But not all orbits are the same. Using flybys of Saturn's largest moon to adjust its course, Cassini has tweaked the size, shape, and most importantly, the orientation of its orbit. For the last couple of years, Cassini's orbit has had a high inclination, taking it far above and below Saturn's equatorial plane, giving it top-down and bottom-up views of the gassy giant and its spectacular ring system.
Image credit: NASA/JPL/SSI/Gordan Ugarkovic
Cassini shifted into a new mission phase on March 16, when a flyby of Titan brought its orbital inclination down to less than one degree for the first time since May 2012. Orbiting nearly within the ring plane, Cassini's view collapses the disk of the rings into hardly more than a line across the screen. Watch the Cassini raw images page for views of Saturn like this one, taken in 2011. In the three years since Cassini last had this point of view, the north pole has been more and more illuminated by the Sun; Cassini will see the shadow of the rings falling farther south on Saturn's disk, and the winter polar atmosphere will have shifted to a methane-blue color.
Image credits: NASA / JPL / SSI / Gordan Ugarkovic
In the image above, Saturn's second largest moon, Rhea, occults the rings. I love equatorial Cassini orbits because they allow the spacecraft to have frequent encounters with the icy moons, each of them like another Voyager flyby. Cassini will orbit near Saturn's equator for the rest of 2015, and plans high-resolution imaging of larger moons Mimas, Enceladus, Tethys, Dione, Rhea, Hyperion, and Titan, as well as smaller moons Telesto, Epimetheus, Prometheus, and Atlas. One such flyby, of Rhea, happened on February 10; the next close flyby will be of Tethys, on April 11. Although Cassini has flown past these moons before, there are still gaps in our global maps, particularly in their north poles, which have never been as well-lit as they are now.
But even when Cassini does not fly very close to moons, equatorial orbits offer opportunities for breathtaking views. For instance, yesterday Cassini planned to catch Titan, Rhea, and Mimas within a single camera field of view. At the time that I write this blog entry, those images have not yet been transmitted to Earth, but I will be checking the Cassini raw images website frequently in order to see them. Three moons is a lot for a single photo, but it doesn't come close to Cassini's record, which (I think) is five moons with one shot:
Cassini caught five moons at the edge of Saturn's ring system in this natural color photo from July 29, 2011. From left to right the moons are Janus, Pandora, Enceladus, Mimas, and Rhea. Image credit: NASA / JPL / SSI / color composite by Emily Lakdawalla
Even more fun, these mutual event observations are almost always captured as movies, where multiple individual frames separated by seconds or minutes follow the mutual motions of the moons. You can stack these into flip-books and animate them, watching the moons dance with each other. The flickering is caused by Cassini cycling through different color-filters; if you wanted to spend a lot of effort, you could assemble this animation in color, seeing orange Titan pass behind the yellow-ochre rings and ringmoons and icy white Dione.
On September 17, 2011 at about 1200 UTC, Cassini pointed at the mid-sized moon Dione and watched as it transited Titan while the rings and two small moons passed by. Pan is a tiny object orbiting within the A ring's Encke gap, while Pandora is the outer shepherd satellite of the F ring. Image credit: NASA / JPL / SSI / animation by Emily Lakdawalla
Of course, Cassini doesn't do mutual event observations just for fun. The important purpose behind them is to perform highly detailed astrometric measurements; seeing two moons at once in the same frame tightly constrains the positions and velocities of the moons in their orbits. Since all of the moons' motions depend upon the gravitational influences of all the other moons and the rings, their individual motions aren't simple Keplerian ones, but have wiggles and modulations over time, setting up waves and instabilities in the rings that encourage the coalescence and disruption of little concentrations of masses. It's like a baby solar system in miniature, and over its 11 years of operation Cassini has been able to watch its dynamics play out in exquisite detail.
But all good things must come to an end. 2015 will be the last year that Cassini enjoys equatorial orbits, mutual events, and frequent close flybys of Saturn's moons (other than Titan, which it must always return to for orbit-tweaking gravity assists).
In January 2016, a Titan flyby will launch Cassini up and out of the ring plane again, for its last inclined phase. Then, at the very end of 2016, another Titan flyby will shrink Cassini's orbit so that it actually passes between the innermost D ring and Saturn's cloud tops on every elliptical path around the planet. That will signal the beginning of the "proximal orbit" phase of the mission, in which gravitational effects will drag Cassini's periapsis closer and closer to the planet. On September 15, 2017, its periapsis will have shrunk to 98% of Saturn's radius—that is, to a point within Saturn's atmosphere—and the mission will come to a natural end.