Cassini made its first Saturn ring plane crossing in the "grazing" category this week. It passed southbound just outside the main rings early Sunday morning local time, going some 76,000 kilometers per hour with respect to the planet. Thanks to JPL's world-class navigational capabilities, last Tuesday's flyby of Titan provided the "gravity assist" that sapped precisely enough of Cassini’s orbital energy to fling the spacecraft over near the F ring, and reduced the robot's orbital period to 7.2 days. Cassini will do 20 of these F-ring-grazing orbits, gaining unprecedented views and measurements of the planet Saturn and its rings, moons, and magnetosphere.
Wednesday, Nov. 30 (DOY 335)
Cassini coasted through apoapsis today, marking the start of its Saturn orbit #251. Meanwhile, the Composite Infrared Spectrometer (CIRS) continued to lead examinations of Saturn's planet-like moon Titan for 19 hours, while receding from the previous day's close encounter T-125. The Visible and Infrared Mapping Spectrometer (VIMS) rode along the whole time, and the Ultraviolet Imaging Spectrograph (UVIS) joined in for about 15 hours. Their observations help cover seasonal changes in Titan's stratosphere as its southern hemisphere approaches the winter solstice, which comes every 29.5 Earth-years. They also help monitor the evolution of Titan's liquid-hydrocarbon reservoirs.
Thursday, Dec. 1 (DOY 336)
Today, CIRS observed Saturn's northern hemisphere for 8.3 hours to measure atmospheric composition. VIMS and UVIS rode along.
Friday, Dec. 2 (DOY 337)
CIRS and VIMS continued watching Saturn over the course of one complete planetary rotation, mapping it at mid-infrared wavelengths to detail troposphere and tropopause temperatures. UVIS took the lead next, to map Saturn's sunlit north in the extreme- and far-ultraviolet parts of the spectrum. CIRS, ISS, and VIMS rode along.
Saturday, Dec. 3 (DOY 338)
With Cassini now diving in close over Saturn's high northern latitudes towards an historic ring plane crossing, VIMS held the reins for nine hours to collect spectral images, one after another, of the area famous for its giant hexagon. CIRS and the Imaging Science Subsystem (ISS) rode along. The Radio and Plasma Wave Science (RPWS) instrument adjusted its settings to be able to best monitor conditions around the upcoming ring-plane crossing.
Sunday, Dec. 4 (DOY 339)
While Cassini continued speeding towards the outer edge of Saturn's main rings, UVIS and VIMS stared at Saturn's north polar auroral region for 3.7 hours. Next, UVIS turned to track the bright blue star Beta Crucis for one hour, and watched as it entered occultation behind Saturn. The observation provided detailed vertical profiles of several hydrocarbon species and aerosols in Saturn’s thermosphere and high stratosphere. Previous UVIS stellar occultations showed Saturn's atmosphere expanding until about 2010; it has since contracted to some extent.
Having received realtime commands to perform Orbit Trim Maneuver (OTM)-467, Cassini turned and fired its main rocket engine for six seconds. The resulting one-meter-per second change in velocity took out the trajectory error that was induced during Tuesday's close flyby of massive Titan. That was the flyby whose "gravity assist" sapped precisely enough of Cassini’s orbital energy to fling the robot in close to the F ring, and reduced its period to 7.2 days.
The ring plane crossing came next. Early in the morning local time, the Cassini Mission Control console was buzzing with activity. The 70-meter diameter Deep Space Network (DSN) station in Spain provided radiometric tracking data for the Navigation team. Live telemetry from the spacecraft confirmed its health on the way through this territory. This page provides a clear illustration of Cassini's newest path: .
Gliding through periapsis, CIRS turned to watch the small but active moon Enceladus, and scanned its effusive south polar region for 4.2 hours. The other instruments rode along. This observation was the first in a campaign to determine whether Enceladus's heat varies with its orbital location (as its plumes are known to do). In the middle of the observation period, Cassini turned to Tethys to search for more red-stripe regions on the icy moon's surface.
Monday, Dec. 5 (DOY 340)
The spacecraft had whipped rapidly around to the far side of Saturn's rings by early today, and trained its high-gain antenna on Earth for an 8.3-hour Radio Science ring occultation experiment. The DSN participated using stations in Spain and Australia, and the European Space Agency used a station in Australia. Experiments like today's provide information about the physical properties of the ring system.
On the heels of the Radio Science experiment, ISS reacquired and tracked the orbits of individual known propeller-like features ( ), looking toward the unlit side of the rings. CIRS rode along. These objects' orbits are known to change on many time-scales. Measuring their positions at as many times as possible is essential to understanding the causes of this behavior. Propellers in Saturn’s rings are the only objects ever to have had their orbits tracked while embedded in a disk, a configuration analogous to planetesimals in early solar systems.
Next, CIRS examined the innermost portion of Saturn's C ring for four hours, obtaining spectra at far-infrared wavelengths. The data will help determine ring-particle composition.
An image featured today shows one of Cassini's views of Saturn's spongey-appearing moon Hyperion: .
Tuesday, Dec. 6 (DOY 341)
At 10.3 hours in length, today's ISS Enceladus observation was one of the longest in the plume monitoring campaign. Short-term plume brightness variations are excellent for testing theories of how the plume is produced. All the other optical instruments rode along.
This page shows some of the latest close-up images of Saturn's northern regions from Cassini's unprecedented new orbital vantage point: .
The Deep Space Network communicated with and tracked Cassini 15 times this week, using stations in Spain and Australia; an ESA station also participated one time from Australia. The Radio Science team is conducting a study of the near-Sun environment while the spacecraft is close to superior conjunction. A total of 165 individual commands were uplinked, and about 2,625 megabytes of telemetry data were downlinked and captured at rates as high as 110,601 bits per second.
Wrap up:
Cassini is orbiting Saturn with a period of 7.2 days in a plane inclined 63.7 degrees from the planet's equatorial plane. The near and distant points to Saturn in Cassini's orbit will be occurring at about the same altitudes for a total of 20 orbits: about 150,000 km and 1.28 million km from the planet, respectively. Speeds relative to Saturn at those points (periapsis and apoapsis), will be close to 76,150 km per hour and 9,000 km per hour respectively.
The most recent spacecraft tracking and telemetry data were obtained on Dec. 7, using the 70-meter diameter DSN 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 .


Cassini position on Dec. 6, 2016 An illustration of Cassini's path up to mid-day Dec. 6, 2016. This chart offers a comparative view of the change to ring-grazing periapsis altitude. At that time, the countdown clock in Mission Control was showing 282 days until the end of the mission.