The Cassini Project Science Group met this week at the Jet Propulsion Laboratory in Pasadena, the 43rd in a series of three-per-year where various members of the project, from NASA Headquarters to the spacecraft operations team to mission planners and scientists. This meeting coincided with the tenth anniversary of Cassini’s launch on October 15, 1997. We marked the occasion with a cake and some group pictures and then immediately got to the business of how Cassini will die.
Cassini is now in the final year of its nominal four-year mission, though a detailed plan for a two-year extension has been proposed and is awaiting final approval from NASA. But the time to figure out what will happen even after that two year extended mission (the “XM”) is now, given the time it takes to plan the complicated trajectory of the spacecraft through the Saturn system. Dubbed the “XXM”, this secondary extended mission would nominally conclude with the end of the mission.
The spacecraft is remarkably healthy and has sufficient resources to continue operating for many years, so there is a long list of possible options for the XXM. The scientific drivers for the XXM argue for a significantly longer XXM than the two-year XM so that features that change slowly can be studied. A Saturn season is 7 years long, so at the end of the XM Cassini will have observed about one season. To study Titan’s seasons (which march in lock step with Saturn’s), seasonal changes in Saturn’s atmosphere, and seasonal changes on Saturn’s rings (expected to affect production of spokes as well as the temperature of the particles), there are strong scientific reasons to have the XXM go for an additional Saturnian season. Studies of the vapor-spewing moon Enceladus for a long time period would tell us about variability in the activity of this intriguing moon. We still have a lot to learn about how the activity of that moon works. Studies of the magnetosphere will benefit from seeing changes over the course of the 11-year solar cycle. There is a lot of work to do before a decision is made on the Cassini XXM, but the potential is there for dramatic new discoveries by observing the planet and its moons and rings for more than a decade to see how things evolve on that timescale.
One more example of variability in the system centers around Saturn’s F ring. This unusual ring shows clumps that come and go, and it is strongly perturbed by nearby moons Pandora and Prometheus. The F ring seen by Cassini is quite different in many ways than the ring seen by Voyager 25 years ago. The number of strands is different, and the frequency and morphology of clumps within the main F ring core is quite different between the Voyager era and the Cassini era. With a lengthy Cassini XXM, we could actually observe these kinds of changes occurring. One possible reason for these changes is the changing orientation of the eccentric (non-circular) F ring with respect to its nearby shepherd moon Prometheus, also on an eccentric orbit. The period for the change in the alignment of pericenters of the orbits of the F ring and Prometheus (the points where they are closest to Saturn) is 17 years. The F ring is fundamentally intriguing because it is at the boundary between rings and moons, where tidal forces barely succeed in thwarting accretion of moons. Without the gravitational stirring of Pandora and Prometheus, perhaps long-lived moonlets would form in the F ring. A long-term observing campaign of the F ring in a lengthy Cassini XXM would illuminate the processes of satellite accretion and gravitational stirring by the moons and how that affects accretion.
As for the eventual “end of life” of Cassini (and yes, that’s what it’s really called), there are many possibilities, including a Galileo-like crash into Saturn.