Seasons Changing on Saturn

Todd J. Barber, Cassini lead propulsion engineer

February may be a short month, but it was long on Cassini science observations at Saturn, even during the first week! For starters, just as Punxsy Phil was emerging from his hollow stump in western Pennsylvania on Groundhog Day, Cassini was observing light and shadows as well on the intriguing moon Rhea during a non-targeted flyby. I’m sure the entire flight team is excited for our close Rhea encounter next year, particularly since this icy satellite of Saturn humbled us with the perplexing discovery of a potential ring system, the first ever seen around a moon. Until next year, though, these more distant encounters will have to suffice, whetting our voracious and bottomless scientific appetites. Also, unlike Punxsy Phil’s checkered history in prognosticating six more weeks of winter, Cassini can confidently state that seasons are changing on Saturn. The north polar region will soon see the sun for the first time in nearly fifteen years, and another harbinger of seasonal change — ring spokes — should start becoming more prevalent as well. Spokes in Saturn’s rings were one of the most captivating discoveries from Voyager, so we are only too happy to continue observing these beautiful, transient features with Cassini.

Engineering enjoyed a stroke of good fortune early in the month as well. As we prepared for a thrilling Titan-50 flyby on Feb. 7, our approach maneuver, OTM-181, was canceled. Often times skipping approach maneuvers incurs a small but justifiable propellant cost, but this time all the stars aligned in our favor. We actually saved about 1.2 meters per second (2.7 mph) in delta-V by canceling this maneuver, with no deleterious impacts to T50 science! Turning to the trusty old rocket equation, which I’ve joked I must use at least once per day as a propulsion engineer to get paid, I calculate this saved us about 1.0 kg (2.2 pounds) of propellant. That’s a good day in the world of engineering! OTM-182, a small clean-up maneuver (2.1 seconds on the main engine) following T50 went off without a hitch as well.

Speaking of the T50 flyby, this low altitude buzz of the solar system’s second-largest moon went swimmingly. We use Titan to shape future orbits with its large gravity field, yet another example of the wonderfully powerful technique of gravity assist. Of course, the low altitudes required for orbit modification thrill Cassini scientists as well, particularly when “sniffing” the atmosphere. To that end, our Ion and Neutral Mass Spectrometer (INMS) instrument was prime for this flyby, but a “ride along” was provided for radar as well. INMS was investigating a magnetospheric boundary region in Titan’s mid-Southern hemisphere, while radar executed both inbound and outbound altimetry along with imaging data for the mountains southwest of Tsegihi. Not to be outdone, our infrared and ultraviolet instruments also investigated atmospheric temperatures, minor hydrocarbon species, new gases, nitriles, cloud mapping, nitrogen and monatomic hydrogen emissions, scattering of haze aerosols — you name it! To me this sounds like a year of AP Chemistry squeezed into one brief flyby, but no one “crams” for a scientific test like Cassini, pulling all-nighters with ease, all without an ounce of caffeine. Even when our robotic emissary requires the same of her flight team on Earth, we are only too happy to oblige (with copious quantities of coffee, of course).