Daphnis and Saturn's rings

Saturn’s wavemaker moon Daphnis flexes its gravitational influence over Saturn’s rings in this new view of the tiny moon. › Full caption

On Monday, for the seventh time in the Ring Grazing orbit phase, Cassini crossed southward at high speed through the ring plane, just outside Saturn's ever-changing F ring. In addition to the in-situ activities of the fields and particles instruments, the geometry of this crossing also offered a “best-ever” view of the small moon Daphnis, which orbits inside the Keeper Gap. A feature and spectacular image are can be viewed at /resources/17589/ .

Wednesday, Jan. 11 (DOY 011)

We often refer to the spacecraft command sequences that control most of the Cassini’s activities. These onboard sequences are further divided into a series of segments. The segments, which can be as short as a few hours or a long as several weeks are carefully stitched together to make optimal use of the orbital position, observing times, and resources. The segments are often named for the predominant geometry or observing priority along with the orbit number.

Two Imaging Science Subsystem (ISS)-led observations closed out the Rings_255_256 segment today. The first was another installment in the observing campaign to monitor Titan's atmosphere. The geometry of this observation was particularly favorable for ISS, the Composite Infrared Spectrometer (CIRS), and the Visible and Infrared Mapping Spectrometer (VIMS) to look for changes in the hazes that surround Saturn's largest moon. ISS then pointed its cameras back in the direction of Saturn's faint rings for nearly eight hours, taking advantage of the high-phase geometry which preferentially highlights the fainter components of the ring system. The segment wrapped up with a final downlink over the Deep Space Network’s (DSN’s) Goldstone complex.

Towards the end of the day, the spacecraft started Saturn_257_258, a long segment spanning about 1.5 orbits (starting before apoapsis of Rev 257 and ending after apoapsis of Rev 258). The segment began with a 1.5 hour ISS haze observation of Titan’s atmosphere as part of the Titan Monitoring Campaign with CIRS and VIMS riders.

Jan. 14 marks the 12th anniversary of the Huygens probe landing on Titan; a feature released today takes a look back at that momentous day: /news/12987/huygens-ground-truth-from-an-alien-moon .

Thursday, Jan. 12 (DOY 012)

The Ultraviolet Imaging Spectrograph (UVIS) performed the first of a series of Extreme and Far Ultraviolet observations mapping the planet to study the distribution of high altitude atmospheric hazes and organic compounds in Saturn’s far northern hemisphere. The observations went on for over 17 hours; CIRS, ISS, and VIMS rode along.

Near the end of the day, the spacecraft coasted through apoapsis, beginning orbit #257.

Friday, Jan. 13 (DOY 013)

After Cassini completed the first downlink of the segment and passed apoapsis, control was passed to ISS for another haze observation of Titan’s atmosphere for the Titan Monitoring Campaign with CIRS and VIMS riding along. Next, UVIS and ISS performed instrument calibrations using an occultation of the star Alpha Virginis, popularly known as Spica located in the constellation Virgo. The observation period ended with a nine-hour CIRS observation studying the composition of the northern Saturn atmosphere; UVIS and VIMS rode along.

A feature on how the European Space Agency’s deep-space radio dishes on two continents are helping return Cassini data safely to Earth can be found at /news/12989/catching-cassinis-call .

Saturday, Jan. 14 (DOY 014)

A third installment of the Titan Monitoring Campaign had ISS studying Titan’s atmosphere with a VIMS rider. VIMS then took the lead in a six-hour mosaic mapping of Saturn’s northern hemisphere along with CIRS as a rider. UVIS then followed with a collaborative observation with VIMS; first staring at the northern aurora for six hours, then making repeated slews across the illuminated northern polar aurora for six hours (CIRS, ISS, and VIMS rode along). UVIS then handed off to VIMS again for a Saturn North Pole movie for a full rotation of the planet (11 hours) with CIRS riding along.

Monday, Jan. 16 (DOY 016)

After the Solid State Recorder (SSR) was cleared off during an important 70-meter antenna, downlink pass, the densely packed periapsis observation period began with a VIMS North Pole mosaic map for four hours (CIRS, ISS, and UVIS rode along). CIRS then completed a six hour Saturn limb mid-IR sounding to obtain stratospheric thermal structure (UVIS and ISS were riders).

Some observations are so important to the mission that they are laid out in the advance, well before other observations are incorporated into the timeline. These are called pre-integrated events or PIEs and are of the highest priority.

One such PIE was the 2.5 hour ISS collaborative observation with CIRS of the ring-moon Daphnis. It ranked as the highest priority observation for this orbit as Cassini passed through periapsis close to the edge of Saturn’s F ring. A UVIS rider collected ultraviolet albedo measurements, while the VIMS rider studied Daphnis’s composition. This unique and relatively close flyby—the best in the entire mission--offered nearly four times better resolution of the moon than was previously obtained. This will allow scientists to explore its body/equatorial ridge morphology, and greatly improve the moon’s density estimation with better determination of the moon’s volume. Such information improves the origin story of Daphnis, the rings, and other small moons. A spectacular image of Daphnis is available at /resources/17589/ .

During the ring plane crossing CDA, RPWS, and INMS studied the F-Ring particles, dust, and atmosphere.

Exiting periapsis, VIMS collected high resolution images of Saturn’s southern hemisphere with mosaics at varying southern latitudes; CIRS, ISS and UVIS participated as riders.

Only four hours after periapsis, CIRS began the segment’s second PIE. This observation, also of high priority for the orbit, performed a four-hour high-resolution Saturn illuminated limb integration with mid-infrared sounding to obtain stratospheric thermal structure. UVIS and VIMS rode along.

This unique configuration, and proximity to Saturn (between 4 and 6 Saturn radii) provided a vertical profile of temperature and hydrocarbon abundances throughout Saturn's stratosphere. The latitude selected—five degrees south -- exhibits a dynamical effect known as the quasi-quadrennial oscillation. Layers of hot and cold air move vertically near Saturn's equator on a time scale of several Earth years. Limb measurements from CIRS are the only way to study this phenomenon on Saturn. This effect is also operative in the Earth's stratosphere with a 28-month period.

VIMS finished out the day by creating Saturn’s South Pole region mosaic maps for 2.5 hours, while CIRS and UVIS rode along.

As Cassini crossed the ring plane just outside the F ring, the Ion and Neutral Mass Spectrometer (INMS) pointed for 2.2 hours in the direction from which particles loosely associated with the ring were expected to arrive. Before the observation was done, Cassini sped through periapsis in its orbit #256 of Saturn, just 20 minutes after a ring plane crossing.

Back on Earth, Cassini’s fans were treated to a view of Saturn and its rings that is impossible to see from our home planet: /resources/17585 .

Tuesday, Jan. 17 (DOY 017)

VIMS continued a block of various observations with a 1.5-hour southern hemisphere mosaic centered at 30 degrees south latitude. Then VIMS switched its focus to mapping the Saturn equatorial region. Next, VIMS and a collaborative UVIS rider used their instruments’ solar ports to observe a Solar Ring occultation as the Sun crossed through the rings and behind Saturn. UVIS then took advantage of the dark south polar region during the solar occultation to perform several slews over the southern auroral oval between 55 degrees south latitude and the southern pole.

The final PIE of the segment was the Radio Science Subsystem’s (RSS) Saturn egress-only atmospheric occultation. The RSS Ka, X, and S-band signals peered through Saturn’s atmosphere to profile its thermal structure and possible seasonal variation at the mid-northern latitudes. RSS then continued to follow the Earth as it was occulted by Saturn’s rings in order to detect detailed ring structure and composition.

Wrap up:

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. The 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 Jan. 18, 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 http://saturn.jpl.nasa.gov/anomalies .

The countdown clock in Mission Control shows 241 days until the end of the mission.

This illustration depicts Cassini's path up to mid-day Jan. 17, 2017. This illustration depicts Cassini's path up to mid-day Jan. 17, 2017.

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