Cassini spent this week plunging in orbit towards Saturn's ring plane, passing rapidly through it, and then climbing back "up" and out all the way to apoapsis. This loop gave the spacecraft plenty of opportunities to study all the rings from every angle, using a number of highly effective techniques including the usual telescopic remote-sensing, but also direct sampling of dust, plus three stellar occultations and a solar occultation.
Wednesday, Oct. 12 (DOY 286)
Cassini started the day with some spectacular views of Saturn and the rings, looking "down" towards their sunlit side, from about 850,000 kilometers "above." Taking advantage of the situation, the Composite Infrared Spectrometer (CIRS) controlled the spacecraft's pointing for five hours, and made observations intended to constrain theoretical models on the composition and structure of the most opaque region of Saturn's B ring.
Next, the Visible and Infrared Mapping Spectrometer (VIMS) trained its field of view on the red star Gamma Crucis, to watch it being occulted by Saturn's main rings. CIRS, and the Imaging Science Subsystem (ISS), rode along. The egress occultation started behind the inner C ring, and proceeded to cover the rest of the main rings out through the narrow F ring. The viewing geometry illustrated here catches the red star in the lower right as it goes behind the Cassini Division, having emerged from behind the B ring and on its way out to the A ring.
When the stellar occultation was over, ISS spent one hour recovering sight of some of the intriguing features in the A ring that have been dubbed "propellers" due to the shape of the visible wakes they create. These bodies are large enough to clear out ring particles in their immediate vicinity, yet too small to be moons in their own right, as they cannot clear a complete gap in the rings. CIRS rode along, as did the Ultraviolet Imaging Spectrograph (UVIS).
Thursday, Oct. 13 (DOY 287)
VIMS led another stellar occultation today. While Cassini was speeding through lower and lower latitudes, the bright red the star Alpha Scorpii, more commonly known as Antares, made an appearance, and its light probed the rings for about five hours. This time the star ingressed from outside the F ring, inward to behind the innermost D ring, and then egressed across the main rings again. CIRS and UVIS rode along.
Next, VIMS turned back towards the rings, leading CIRS and UVIS in making a radial mosaic across the sunlit side of the rings for three hours. The scientific goal was to learn how the spectral contrast between narrow features in the rings differs between their sunlit and dark sides. At this point, Cassini had descended almost all the way to the ring plane. Being at a distance of around 350,000 km from the planet, the spacecraft had begun sweeping through Saturn's vast, diffuse E ring, which is made up of particles ejected by the small moon Enceladus's icy plume. The Cosmic Dust Analyzer (CDA) spent three hours measuring the characteristics of those E-ring particles that it happened to encounter.
At periapsis passage today, the spacecraft had come within 280,000 km of Saturn's visible edge, going 48,467 km per hour relative to the planet. At this point, VIMS had started a three-hour observation of the dark side of Saturn's rings to complement the previous day's observation of their sunlit side. CIRS and UVIS rode along.
CIRS took the reins next, leading UVIS in an observation of the dark side of the rings, which will be contrasted with an observation Cassini's view of the sunlit side from Oct. 10. By seeing the rings from opposite sides, but at otherwise similar geometries, scientists will measure the contrast in thermal emission between their sunlit and dark sides, and infer how thermal energy is distributed across the ring plane.
With the CIRS-led activity done, ISS revisited some "propellers" in Saturn's A ring for 2.3 hours. This time the view was from well below the rings' dark side, thanks to Cassini's swift motion past periapsis. CIRS and UVIS rode along.
In one week from now, on Oct. 27, Saturn will form a straight line above Venus and Antares in Earth's western sky, just after sunset: /news/12950/saturn-fades-into-the-sunset .
Friday, Oct. 14 (DOY 288)
ISS trained its cameras on Saturn's largest moon Titan for a 90-minute observation in the Titan meteorological campaign to monitor its atmospheric activity. VIMS rode along. This observation was repeated on Monday.
ISS turned to Saturn's rings again for six hours, with CIRS riding along. The first four hours were joined by UVIS, and dedicated to the outer edge of the broad A ring, the narrow F ring, and the region between them. Interactions there between small moons, large objects embedded within the rings, and the rings themselves, make for interesting and insightful scientific study. This region is near the Roche zone, where tidal stress can break an object to pieces.
For the next two hours, now joined by VIMS instead of UVIS, ISS took images at high-phase lighting angles of the clumped arc that is thought to be the source of Saturn's G ring; VIMS rode along. The arc's tiny particles light up brightly in forward-scattered light when sunlit from behind.
VIMS led UVIS in the day's final science activity, an occultation of the Sun by Saturn's rings. For 4.5 hours, the instruments' solar ports watched the Sun appear to move behind the rings, going from outside the F ring, and in to the middle of the B ring. Solar-occultation observations like these are useful for studying the size and spatial distributions of the smallest particles in the rings.
The week-long Cassini Project Science Group (PSG) meeting #70 wrapped up in Monrovia, California today. As part of the PSG, a "Geologically Young Rings & Moons" workshop examined the dynamical and geological evidence for and against a recent origin of Saturn's icy moons and rings. The Cassini magnetometer team met today at Caltech, with a primary focus on preparing for analysis of data to be acquired during Cassini's upcoming proximal orbits.
Saturday, Oct. 15 (DOY 289)
Today marked the 19th anniversary of Cassini's launch from Earth; the spacecraft will not survive to see its 20th: /news/12946/remembering-cassinis-beautiful-launch-19-years-ago .
CIRS spent 2.5 hours making radial scans of Saturn's main rings at thermal-infrared wavelengths; UVIS rode along. The observation will allow scientists to understand how the thermal emission from the rings varies with the lighting phase angle, and with Cassini's elevation above the rings.
Next, VIMS and CIRS turned to track the bright, familiar red star Alpha Orionis, better known as Betelgeuse. The line of sight to the star cut across the A ring, turned around in the middle of the Cassini Division, then egressed again, allowing VIMS to sample the A ring at different longitudes, which is particularly useful for studying azimuthal asymmetries in the ring, such as wake structures. The viewing angle today was enormously different from just a few days ago; this illustration captures Betelgeuse in the Cassini Division.
Finally today, ISS took images of the fainter components of Saturn's rings for nearly six hours. The sunlight was still at high-phase illumination, making the finest particles light up the brightest; CIRS, UVIS, and VIMS rode along.
Sunday, Oct. 16 (DOY 290)
ISS observed Saturn's faintest rings at low elevation and high phase, with CIRS and UVIS participating. Finally, ISS began a 17-hour observation of Saturn's irregular moon, Tarvos. With an estimated diameter of 15 km, this is one of the larger irregular moons of Saturn. In its highly inclined, highly elliptical orbit, Tarvos's distance to the ringed planet varies between about 8.4 million and 28 million km. The body was named after a bull-god in Gaulish mythology.
Monday, Oct. 17 (DOY 291)
After ISS completed a two-minute Saturn storm-watch observation, CIRS began a 23-hour scan of Saturn’s atmosphere at mid-infrared wavelengths to measure upper-troposphere and tropopause temperatures.
Tuesday, Oct. 18 (DOY 292)
Cassini coasted through apoapsis today, marking the start of its orbit #246 of Saturn. It had reached 1.399 million km from Saturn, and slowed to 11,666 km per hour relative to the planet. From this high vantage, VIMS, CIRS, and UVIS observed the once-again sunlit side of Saturn's rings for seven hours, in order to obtain additional latitude and lighting-phase coverage.
The Deep Space Network communicated with and tracked Cassini eight times this week, using stations in Spain, Australia and California. A total of 6,1570 individual commands were uplinked, and about 1,294 megabytes of telemetry data were downlinked and captured at rates as high as 110,601 bits per second.
Cassini is orbiting Saturn with a period of 9.6 days in a plane inclined 57.9 degrees from the planet's equatorial plane. The most recent spacecraft tracking and telemetry data were obtained on Oct. 19, using one of the 34-meter diameter DSN stations in Spain. 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 .
Cassini's path up to mid-day Oct. 18 is illustrated here: http://go.nasa.gov/2cEsjkh . At that time, the countdown clock in Mission Control was showing 331 days until the end of the mission.
An illustration of Cassini's path up to mid-day Oct. 18.