While steeply orbiting Saturn twice during these past two weeks, Cassini's activities were just about as numerous as during any other two-week period. The science and engineering teams squeezed in some time off during the holidays, but they also managed to accomplish all the work that Cassini requires in real time, including the largest B-branch Reaction Control System maneuver to date, while continuing to develop command sequences that will control Cassini over the months to come.
During this period, including two more periapses near the F-ring, Cassini's instruments continue to exploit the unique features of the highly inclined ring-grazing orbits to almost constantly make observations of Saturn and its environment, its rings, and some of its moons. Among them, some were worth special mention as follows.
Wednesday, Dec. 14 (DOY 349)
Cassini's Cosmic Dust Analyzer (CDA) observed dust for 13 hours today while crossing south-to-north through Saturn's equatorial plane in the vicinity of the planet-like moon Titan.
Thursday, Dec. 15 (DOY 350)
Late in the day, Cassini passed within 35,000 kilometers of Titan. During this non-targeted flyby (so named because it required no propulsive maneuvers), the Imaging Science Subsystem (ISS), the Composite Infrared Spectrometer (CIRS) and the Visible and Infrared Mapping Spectrometer (VIMS) were all involved in making observations of the huge moon over the course of more than 30 hours.
Friday, Dec. 16 (DOY 351)
A publication by Cornell University on December 8 about the age of Saturn's C ring was featured today: http://www.news.cornell.edu/stories/2016/12/scientists-sweep-stodgy-stature-saturns-c-ring.
Sunday, Dec. 18 (DOY 353)
Cassini observed two stellar ring occultations today. One was led by CIRS, and the other by UVIS. CIRS then spent nearly four hours making direct observations of the rings, which included some of the instrument's highest-resolution thermal ring scans in the entire mission.
Today's ring-plane crossing occurred at about the same radial distance from the F ring as did last week's. Of course this repeated a week later on December 26.
After today's crossing, Cassini made its closest-ever approach to Saturn's little moon Pandora, flying within just 20,000 km of its surface. For about two hours, all the optical instruments examined the object, which orbits Saturn just outside the F ring.
Both before and after the unique Pandora flyby, ISS spent 1.4 hours obtaining some of the Mission's highest-resolution observations of the rings, with the other instruments participating.
Monday, Dec. 19 (DOY 354)
CIRS looked for structures in the A ring's "halo" region, where there are unusual photometric properties, believed to result from strong resonances with at least one moon. CIRS was just barely able to resolve the region with its mid-infrared sensors during the four-hour observation.
Next, the Radio Science team carried out a ring occultation experiment while the spacecraft spent nearly ten hours passing behind the rings as seen from Earth.
Tuesday, Dec. 20 (DOY 355)
Today VIMS took its turn observing a stellar ring occultation. This one lasted 4.2 hours; CIRS participated.
Wednesday, Dec. 21 (DOY 356)
Images from Sunday's observations of Pandora are featured here: https://saturn.jpl.nasa.gov/resources/7576.
A blast from the past appeared on the Cassini website today, recalling the spacecraft's delivery of the Huygens Titan atmospheric Probe: https://saturn.jpl.nasa.gov/news/2980/christmas-2004-cassini-delivers-a-very-special-gift.
Titan's clouds are the subject of another article today: https://saturn.jpl.nasa.gov/resources/7577.
Friday, Dec. 23 (DOY 358)
Acting on commands sent near real time, Cassini turned and fired its small rocket thrusters for 239 seconds, carrying out Orbit Trim Maneuver (OTM)-468. This was the near-apoapsis maneuver for Cassini's final targeted Titan encounter, T-126, which will take place in April. The OTM imparted a change in velocity of 0.22 meter per second to the spacecraft.
On a historical note, the flight team commanded Cassini to switch to the redundant B-branch of the Reaction Control System (RCS) in 2009 after the A-branch began to show signs of possible failure. The RCS is used for attitude control and small maneuvers like OTM-468. OTM-468 was the largest B-branch maneuver to date and all indications are that the B-branch is still functioning perfectly.
Images from Saturn's northern region were the subject of another feature on the Cassini website: https://saturn.jpl.nasa.gov/resources/7573.
Tuesday, Dec. 27 (DOY 362)
The DSN communicated with and tracked Cassini 17 times over the past two weeks, using stations in Spain, California, and Australia.
Cassini is executing its set of F-ring-grazing orbits of Saturn, with a period of 7.2 days in a plane inclined 63.7 degrees from the planet's equatorial plane. These 20 orbits are nearly identical, with Cassini's nearest point at about 150,000 km and furthest point at about 1.28 million km from Saturn. Speeds relative to Saturn at those points (periapsis and apoapsis), are close to 76,150 km per hour and 9,000 km/h respectively.
The most recent spacecraft tracking and telemetry data were obtained on December 28, using one of the 34-meter diameter DSN stations 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/anomalies
These illustrations depict Cassini's path up to mid-day December 20 http://go.nasa.gov/2ef7mL6 and Dec 27 http://go.nasa.gov/2epayr1. Comparing them, it's clear that the shape of Cassini's orbit remains constant, completing one orbit of Saturn every 7.2 days.
The countdown clock in Mission Control shows 261 days until the end of the mission.
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: https://saturn.jpl.nasa.gov/mission/saturn-tour/where-is-cassini-now.