Aside from one short, routine look at the weather on Saturn's giant moon Titan, all of Cassini's multi-spectral "eyes" spent this week making their specialized observations of the 120,000-kilometer-wide planet and its 280,000-kilometer-wide main ring system, from a varied range of viewing angles.
Wednesday, Sept. 7 (DOY 251)
Having passed apoapsis the previous day, Cassini is slowly building speed on its way back to the periapsis passage coming up on Monday. From its high vantage point today, the Visible and Infrared Mapping Spectrometer (VIMS) stared at Saturn's northern auroral region for eight hours, while the other telescopic instruments rode along. These are the Composite Infrared Spectrometer (CIRS), the Imaging Science Subsystem (ISS), and the Ultraviolet Imaging Spectrograph (UVIS).
Next, CIRS spent eleven hours mapping the planet at mid-infrared wavelengths; this period covered one Saturn rotation. ISS and VIMS rode along. These were the final science activities controlled by the S95 command sequence.
Cassini's synthetic-aperture-radar instrument provides the highest available resolution views of Titan's surface beneath its thick, hazy atmosphere. A news feature released today includes extraordinary images, and a video, showing vast fields of sand dunes on that alien world: /news/12933/titans-dunes-and-other-features-emerge-in-new-images .
Thursday, Sept. 8 (DOY 252)
The S96 command sequence, which was uplinked to the spacecraft the previous week, began exercising its 10-week-long control over Cassini's activities today. The first science observation from the new sequence was an 11-hour CIRS observation. The instrument's 19-inch-aperture telescope stared at one spot, collecting data on the composition of Saturn's atmosphere while the planet rotated through the instrument's field of view. UVIS and VIMS rode along. Next, UVIS took the lead for about 12.5 hours to image Saturn's northern hemisphere in the extreme- and far-ultraviolet parts of the spectrum, with CIRS and ISS riding along. This observation was repeated on Sunday, though for half as long and with VIMS joining the other riders.
Friday, Sept. 9 (DOY 253)
VIMS led an observation of Saturn's sunlit northern polar region, where an enigmatic hexagonal cloud feature surrounds a persistent polar hurricane. The observation lasted one Saturn rotation, with CIRS, ISS and UVIS riding along.
Saturday, Sept. 10 (DOY 254)
ISS performed a 90-minute Titan monitoring observation, with CIRS and VIMS as riders. VIMS then spent nearly 11 hours mapping Saturn's northern hemisphere, with CIRS and ISS observing as riders. This illustration shows the remarkable viewing geometry from Cassini over the past few days.
Sunday, Sept. 11 (DOY 255)
"Opposition effect" is the name given to the apparent brightening of a reflective target when the light source is directly behind the observer. An example of the resulting zero-phase point on Saturn's rings is evident here: /resources/13238 .
Today, VIMS led a three-hour zero-phase observation of the rings. As the spacecraft moved along its orbit, the zero-phase point moved across the rings, from the innermost D ring out to the F ring. VIMS, along with CIRS and UVIS, measured the strength and angular width of that bright spot, to learn about the ring particles' typical grain sizes, surface roughness and packing density. Of note, this scan occurred at the highest solar-elevation angle ever seen in the mission.
At the end of the day, ISS made a mosaic imaging Saturn’s disk at low phase angle. CIRS, UVIS, and VIMS rode along with this 2.75-hour observation.
Eighty days from today, Cassini will begin its series of F-ring orbits, in which periapsis will occur just outside the bright, ever-changing narrow strand.
Monday, Sept. 12 (DOY 256)
Cassini plunged southward through Saturn's ring plane today, during a tracking and communications session with the Deep Space Network (DSN). VIMS then controlled spacecraft pointing for eight hours to make mosaics of Saturn's southern polar region. CIRS rode along, as the two infrared-capable instruments took in the gas giant's dark late-autumn hemisphere. The observation was repeated on Tuesday. When today's was done, UVIS took the lead for six hours to observe the planet's south-polar auroral zone. VIMS, UVIS, and ISS rode along.
Cassini sped through periapsis in its orbit #242 of Saturn during the UVIS-led observation. Since Cassini has had no recent close encounters or Orbit Trim Maneuver rocket firings, its speed and distance from the planet were about the same as in the previous two periapsis passages.
This week's featured image is an unusual view from the dark side of Saturn: /resources/17500 .
Several Cassini scientists participated in the "Biosignatures of Extant Life on Ocean Worlds" conference at the Goddard Space Flight Center in Greenbelt, Maryland, which began today. The goal of this three-day workshop was to evolve understanding on the detectability of extant life on ocean worlds, such as Jupiter's moon Europa and Saturn's geyser-moon Enceladus.
Tuesday, Sept. 13 (DOY 257)
CIRS started the day by making a map of Saturn’s south polar region, measuring temperatures within the planet's southern vortex for eight hours, with VIMS riding along.
For the day's last science observation, UVIS spent 1.75 hours scanning Saturn's thin illuminated crescent. ISS, VIMS, and CIRS rode along. The viewing geometry today was very different from that at the beginning of the week.
The Deep Space Network communicated with and tracked Cassini four times this week, using stations in Australia. A total of 30 individual commands were uplinked, and about 1,200 megabytes of telemetry data were downlinked and captured at rates as high as 124,426 bits per second.
Cassini is orbiting Saturn with a period of 12 days in a plane inclined 53.7 degrees from the planet's equatorial plane. The most recent spacecraft tracking and telemetry data were obtained on Sept. 14, using the 70-meter diameter DSN station in Australia.
Milestones spanning the whole orbital tour are listed here: https://saturn.jpl.nasa.gov/mission/saturn-tour/tour-dates .
Information on the present position and speed of the Cassini spacecraft may be found on the "Present Position" page at: