Cassini is orbiting Saturn with a period of 16 days in a plane inclined 49.2 degrees from the planet's equatorial plane. The most recent spacecraft tracking and telemetry data were obtained on Aug. 10, using one of the 34-meter diameter Deep Space Network 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.
Occultation experiments are valuable for obtaining high quality information about rings, atmospheres, and other phenomena of interest. During an occultation, some object visible in the background becomes hidden behind a target: a bright, distant star goes behind a ring or an atmosphere, or the Sun sets behind a planet or a moon. Sometimes it’s the spacecraft sending its radio signals through atmospheres or rings. No less than five times this week, Cassini took the opportunity to observe occultations using different background stars, as well as our own central star, the Sun.
Wednesday, Aug. 3 (DOY 216)
Three of Cassini's telescopic instruments observed Saturn's largest satellite Titan for 90 minutes today. The Imaging Science Subsystem (ISS), the Composite Infrared Spectrometer (CIRS), and the Visible and Infrared Mapping Spectrometer (VIMS) watched the huge moon's atmospheric activity from a distance of 2.3 million kilometers. Next, ISS and VIMS made a 60-minute observation as part of the satellite orbit campaign, looking for small bodies near Saturn. These two observations were repeated on the following day, and a third Titan monitoring occurred on Friday. Finally today, ISS and CIRS observed the orbital arc of Saturn's G ring for 9.4 hours in the vicinity of the small moon Aegaeon.
Seeing Saturn in the infrared part of the spectrum reveals astounding cloud patterns that are normally hidden beneath the planet's high-altitude haze: https://saturn.jpl.nasa.gov/resources/7395/?category=amateur_images .
Thursday, Aug. 4 (DOY 217)
CIRS watched the sunlit rings for nine hours, obtaining spectra in the thermal infrared to study ring structure and particle composition. ISS and VIMS participated; this observation repeated on the following day, though with the Ultraviolet Imaging Spectrograph riding along, rather than ISS.
Friday, Aug. 5 (DOY 218)
For 38 minutes, ISS tracked features in Saturn's rings that have been dubbed "propellers" (http://go.nasa.gov/17oqTWF). The observation was repeated on Saturday for 90 minutes. Following today's, the bright red star Alpha Scorpii, also known as Antares, appeared to pass behind the rings from Cassini's vantage point. VIMS observed this stellar occultation, recording the brightness variations as ring material along the line of sight occulted the star.
After the occultation, an observation led by UVIS focused on Saturn's moon Tethys, in which the instrument measured the reflected brightness at ultraviolet wavelengths down to an illumination phase-angle of just 1.6 degrees. Low-phase observations such as this one help scientists unravel the causes of the "opposition surge" in brightness seen on many airless solar-system bodies.
Saturday, Aug. 6 (DOY 219)
With the Sun and Earth at its back, Cassini's view of Saturn today was similar to the view from the vicinity of the home planet, but much closer of course. For five hours, ISS took images of Saturn's rings through its complement of spectral filters, building up a mosaic of the rings at multiple wavelengths. These images will be compared to a series of multi-filter images of Saturn's rings created by the Hubble Space Telescope, which orbits close to Earth. CIRS, UVIS, and VIMS rode along, making their own observations.
When the image series was complete, CIRS spent three hours obtaining multiple radial scans of Saturn's main rings at low-phase and low-elevation angles, as part of a campaign to map out the observed thermal emission from the rings at a variety of angles. UVIS rode along.
Late in the day, CDA took control of spacecraft pointing for 4.4 hours to collect particles orbiting near Saturn's ring plane while the spacecraft sped southward through that plane. The point of passage was 688,000 km from Saturn, about mid-way between the orbits of Titan and Rhea.
Sunday, Aug. 7 (DOY 220)
For 7.5 hours, ISS imaged the outer edge of Saturn's A ring, the thin strand of the F ring, and the region between the two. The objective was to study the complex dynamics near the Roche zone, that arc where tidal forces begin to overwhelm and tear apart any object held together by gravity alone. UVIS and VIMS rode along.
Cassini coasted through periapsis in its orbit #239 of Saturn, coming within 517,000 km of the cloud tops at 40 degrees south latitude. The spacecraft had reached a speed of 36,757 km per hour relative to the planet.
Next came another stellar occultation, this one lasting 6.3 hours. The red star R Cassiopeiae crossed inward from the F ring to the inner B ring, and then back out past the F ring. CIRS and ISS rode along.
Cassini's image of an old friend was selected as NASA's Astronomy Picture of the Day: http://apod.nasa.gov/apod/ap160807.html .
Monday, Aug. 8 (DOY 221)
Today, VIMS observed for 2.25 hours while Saturn's rings occulted another star; CIRS and ISS rode along again. This time the red star was Rho Persei, and it cut across the full radial extent of Saturn's main rings, slicing through the planet's shadow on the rings.
Following this, ISS monitored some of the fainter regions in Saturn's rings for 15.7 hours, taking advantage of the high-phase sunlight, which highlights faint ring features. CIRS, UVIS, and VIMS rode along.
VIMS and UVIS are equipped with off-axis ports designed to allow observations of the Sun while keeping the optics and sensors safe from intense sunlight. These instruments observed the Sun being occulted by Saturn's A ring, starting late in the day. Once the bright Sun was safely behind Saturn's disk, VIMS turned to observe the faint, innermost D ring for 3.5 hours, taking advantage of the high-phase geometry to observe small particles in this tenuous ring. CIRS and ISS rode along. Cassini's revealing view is illustrated here: http://go.nasa.gov/2aN66zh .
As seen in today's featured image, Saturn's shadow, cast onto the rings, reaches just past the Cassini Division these days, as the Sun ascends towards its 2017 solstice height: https://saturn.jpl.nasa.gov/resources/7399 .
Tuesday, Aug. 9 (DOY 222)
As the Sun reappeared from behind Saturn's limb, VIMS and UVIS once again used their solar ports and recorded the egress occultation, literally sunrise. Our central star emerged and crossed the entirety of Saturn's main ring system from Cassini's perspective. CIRS then observed the unlit side of the inner A ring. The spectra obtained during this six-hour observation will be averaged to constrain the composition and structure of this region of the A ring.
Titan loomed ever closer to Cassini, coming less than the distance from Earth to our Moon, as the day ended. Cassini's Radio Science Subsystem (RSS) team began preparing for the Cassini Mission's final gravity-field measurement of Titan during the following day's T-122 flyby. Using a Deep Space Network station in Spain, RSS began capturing the the first several hours of that upcoming gravity measurement.
On Monday, Tuesday, and Wednesday this week, while Cassini's optical instruments were already pointing at or near Saturn, ISS carried out two-minute Saturn storm-watch observations.
The DSN communicated with and tracked Cassini on 12 occasions this week, using stations in Spain, California and Australia. A total of 16 individual commands were uplinked, and about 2,050 megabytes of telemetry data were downlinked and captured at rates as high as 142,201 bits per second.
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 .