Cassini is currently orbiting Saturn with a period of 12.7 days in a plane inclined 1.3 degrees from the planet's equatorial plane. The most recent spacecraft tracking and telemetry data were obtained on Dec. 23 using the 70-meter diameter Deep Space Network station in Australia. The spacecraft continues to be in an excellent state of health with all of its subsystems operating normally except for the instrument issues described at http://saturn.jpl.nasa.gov/news/significantevents/anomalies.
During a week when many flight team members were enjoying some well-earned time off for the holidays, Cassini's scientific observations pressed on, taking advantage of some extraordinary opportunities in Saturn orbit.
Wednesday, Dec. 16 (DOY 350)
Cassini's Imaging Science Subsystem (ISS) led a two-hour observation in the Titan monitoring campaign from a distance of 600,000 kilometers. The Composite Infrared Spectrometer (CIRS) participated. Next came a one-hour satellite-orbit campaign observation in which ISS looked for small objects near Saturn. CIRS then stared at the planet for 11 hours, obtaining spectral data with a high signal-to-noise ratio, studying the atmosphere's composition. The Visible and Infrared Mapping Spectrometer (VIMS) rode along, making its own observations.
Thursday, Dec. 17 (DOY 351)
Today was devoted to viewing Titan. ISS started off, with CIRS and VIMS participating, for an observation lasting 13 hours. Next came a special opportunity, when the red star named RX Leporis passed behind Titan's atmosphere (by virtue of the spacecraft's and Titan’s relative motion). The pulsating red giant is interesting in its own right, but VIMS used this ingress occultation to learn more about the huge moon's atmosphere. This observation lasted 85 minutes. The viewing geometry is illustrated here; RX Leporis can be seen to the lower right of Titan, just before entering occultation: https://space.jpl.nasa.gov/cgi-bin/wspace?tbody=606&vbody=-82&month=12&day=17&year=2015&hour=18&minute=16&fovmul=1&rfov=3&bfov=30&showsc=1&showac=1 .
Friday, Dec. 18 (DOY 352)
CIRS observed Saturn's substantial icy moon Rhea for five hours today to study its surface composition, while the Ultraviolet Imaging Spectrograph (UVIS) rode along observing as well. UVIS then turned to watch Saturn’s aurora for just over five hours with VIMS riding along.
Saturday, Dec. 19 (DOY 353)
CIRS led ISS and UVIS in another study of Rhea's surface; this one lasted four hours. Next, moving inbound towards the planet, Cassini began passing through Saturn's diffuse E ring, which is made up of material being ejected from the small moon Enceladus. Throughout the ring passage, the spacecraft turned and rocked back and forth to permit the Cosmic Dust Analyzer (CDA) to make radial scans of the E-ring ice grains with very high spatial resolution, providing great sensitivity to compositional variations.
After nearly four hours, CDA's direct-sampling observation was done, and the spacecraft turned to point ISS's telescopes to the tiny moon Aegaeon. This object orbits Saturn within a bright arc of debris, amid the G ring, which is the next dusty ring inward towards Saturn from the E ring. For 90 minutes, ISS led observations with all the other optical remote-sensing (ORS) instruments participating: UVIS, VIMS, and CIRS. This encounter was the closest Cassini will ever come to Aegaeon, at 2,500 km. While it was a non-targeted encounter, meaning no propulsive maneuvers were needed to achieve it, commands were sent up near real time to update the pointing vectors stored in Cassini's Attitude and Articulation Control System (AACS). This step ensured that the little object would be properly positioned in the instruments' fields of view. The moon's dimensions are roughly 750 by 250 by 250 meters, and this observation, which covered a range of sunlit phase-angles, should resolve its shape and surface albedo for the first time. It should also reveal any other small bodies that might be in the vicinity.
Upon completion of the Aegaeon observation, and while the spacecraft was passing through periapsis, CIRS turned to examine Saturn's large icy moon Dione for 75 minutes, with VIMS and UVIS participating as riders.
The flight team updated AACS's pointing vectors for Enceladus in the same commanding session that fine-tuned the Aegaeon vectors. Now Cassini turned to that 504-km wide geologically active icy sphere, and began leading a series of observations during the 4,999-km altitude E-22 targeted encounter, which would include all the ORS instruments observing Enceladus for almost ten hours, finishing early the following day. Included was an opportunity that VIMS took advantage of, watching for 18 minutes while the bright red star Arcturus sliced behind Enceladus's plume. UVIS and CIRS also observed this unique occultation, which was designed to help understand the plume's opacity as a function of wavelength, as well as its composition and dust-to-gas ratio at specific locations.
Today's encounter was Cassini's final targeted flyby of Enceladus, though there will be some additional opportunities for the spacecraft to observe it during distant non-targeted encounters. This enigmatic body, now known to harbor a global ocean of liquid salty water below its icy crust, will beckon Earthlings until another mission can be designed and flown to the Saturn system.
Sunday, Dec. 20 (DOY 354)
With the final, highly successful Enceladus targeted encounter in the bag, CIRS made use of an opportunity to lead an observation of Saturn's large icy moon Dione at high phase angle, basically backlit, to look for any signs of potential outgassing. All the ORS instruments participated.
Next, UVIS took the reins to observe another stellar occultation for 3.5 hours. This time, it was the star Alnilam, or Epsilon Orionis, passing behind Saturn's upper atmosphere. Occultations by ultraviolet-bright stars such as this one can reveal atomic and molecular hydrogen, and some light hydrocarbons, in Saturn's atmosphere. They are especially valuable experiments because they provide detailed vertical profiles of these constituents. They also measure temperatures in the region of atmosphere where the heating mechanism is still unexplained, and where much of the conversion of methane to other hydrocarbons occurs.
Finally, ISS spent 5.3 hours making a distant observation of Saturn's irregular moon Hati, which has a diameter of about 6 km, and occupies an inclined retrograde orbit. This was the last of several Hati observations to determine its rotation period, pole direction, and shape; Cassini will not be observing Hati again.
Monday, Dec. 21 (DOY 355)
ISS, CIRS and VIMS performed an observation of Saturn's faint E and G rings while they were sunlit at high phase angles for close to six hours.
Today's exciting news is a report on Cassini's completion of the E-22 targeted encounter with Enceladus: http://saturn.jpl.nasa.gov/news/newsreleases/newsrelease20151221 .
Ever since the fast-flyby Voyager encounters in 1980 and 1981, Enceladus has been known to have both old, cratered, and fresh, smooth surface features (the difference is now attributed to snowy geysers from the south pole). A Cassini image featured today clearly shows both terrains: /resources/16286 .
Tuesday, Dec. 22 (DOY 356)
VIMS, with CIRS, observed Saturn's diffuse E and G rings for six hours. Finally, the flight team had an opportunity to fire Cassini's rocket thrusters to clean up the spacecraft's trajectory following the E-22 encounter. The Navigation team's solutions using current radiometric tracking data showed that this would not be necessary. Orbit Trim Maneuver (OTM)-434 was therefore cancelled.
Another image of our favorite Saturnian moon with a plume was featured today, this one showing the location of Enceladus's north pole: /resources/16290 .
On six occasions this week while Cassini was turned to point its optical instruments towards or nearly towards Saturn, ISS made two-minute Saturn storm-watch observations. VIMS rode along with two of them. Also during the past two weeks, the Deep Space Network communicated with and tracked Cassini on seven occasions (eight were scheduled, but one was unable to support), using stations in Australia.
This illustration shows Cassini's position early on Dec. 22: http://go.nasa.gov/1QSHFAU .
The format shows Cassini's path over most of its current orbit up to today; looking down from the north, all depicted objects (except the background stars of course) revolve counter-clockwise, including Saturn along its orange-colored orbit of the Sun.