Stop the presses. Cassini's close approach to Saturn's little moon Pan resulted in some images of a very unusual moon within the rings. Ring scientists predicted Pan's existence and location before it was discovered, and it was found in 1990 by examining images that Voyager 2 had taken nearly a decade earlier. Up until now, Cassini has been seeing Pan as a small bright speck within Saturn's 325-kilometer-wide Encke Gap in the A ring, where the little moon spends its orbital life. This week, thanks to Cassini's ongoing series of ring-grazing orbits, Pan joins the A-list of fascinating objects at Saturn. No matter what earthly thing it may resemble, a walnut, a hat, or a flying saucer, the scarred little ball of ice is definitely wearing a skirt. Obviously, dust particles must have accreted there from within the Encke gap, forming the tall snow-drift around its middle. Saturn's gravitation likely limits the height of those dunes on the little moon.
Wednesday, March 8 (DOY 067)
Having passed periapsis in its Orbit #264 the prior day, Cassini was seeing an eerily dark planet with dark rings, as illustrated here. The Visible and Infrared Mapping Spectrometer (VIMS) spent 6.5 hours observing Saturn's un-illuminated south-polar region. Two more of the Optical Remote-Sensing (ORS) instruments, the Composite Infrared Spectrometer (CIRS) and the Ultraviolet Imaging Spectrograph (UVIS) made observations, riding along since their telescopes all point in the same direction. CIRS finished up its ride-along observation after 2.5 hours, while the others continued examining more of Saturn's dark atmosphere, including latitudes where storms are frequent.
The Sun passed behind Saturn's rings next, and then behind the planet itself. The Imaging Science Subsystem led observations for 3.4 hours during this opportunity with high-phase-angle lighting of Saturn's limb, where sunlight was refracted through its high-altitude haze; VIMS and UVIS participated at times. This was interrupted about mid-way by a 2.6-hour UVIS observation, staring at Saturn's south aurora, with ISS and VIMS riding.
The day ended with a communications session with the Deep Space Network (DSN), during which Cassini downlinked telemetry containing engineering data, and science data including those close-up observations of Pan.
Thursday, March 9 (DOY 068)
CIRS spent eight hours keeping its field of view trained on one spot on Saturn, staring and acquiring spectral data with high signal-to-noise ratio, to study the atmosphere's composition. This was a collaborative activity with VIMS, which took an image once every hour.
Next, ISS took the reins for two hours to observe Saturn's small but active moon Enceladus, as part of a plume monitoring campaign. This campaign watches brightness variations in the ejecta plume on short timescales, which is excellent for testing theories of how the plume is produced.
Two days ago, Cassini snapped its close-up images of Pan. One day ago, the DSN captured the data from the spacecraft. Today the images of strange Pan are available for all to see, and to speculate about: /news/13005/cassini-reveals-strange-shape-of-saturns-moon-pan.
Friday, March 10 (DOY 069)
Cassini had two observations to make today. The first was a 90-minute Titan monitoring campaign activity, in which ISS, CIRS, and VIMS watched Saturn's planet-like moon from 1.9 million km away. ISS and CIRS then turned the spacecraft and observed Saturn's faint outer rings for 11 hours at high phase angles, as part of ongoing efforts to monitor dusty rings for temporal and seasonal variations.
An image of Saturn's small icy moon Mimas, taken last November, was featured today: /resources/17612.
Saturday, March 11 (DOY 070)
At zero-phase illumination, when the Sun is directly behind the observer, unique effects can appear. Today Cassini was in such a position with respect to Saturn's big two-tone moon Iapetus. ISS, UVIS and VIMS jointly observed Iapetus for 14.5 hours to help characterize different areas on the surface, spanning the dark terrain and the bright terrain. During the observation, VIMS explored spectral contrasts in both the bright water-ice terrains, and the dark regions in which absorption bands of both aromatic and aliphatic organic materials are seen. The data will allow scientists to compare the spectral contrast with data at larger phase angles from previous flybys.
During the zero-phase observation, the spacecraft coasted through apoapsis, which marked the start of Orbit #265.
Finally today, ISS began a 13.3-hour observation of Saturn's narrow F ring, with CIRS and VIMS riding along. Observations such as this one provide short, medium, and long-term monitoring of features in the F ring, as well as regions in the outer A ring.
Sunday, March 12 (DOY 071)
Upon completion of the ring imaging, UVIS turned to track the bright blue star Eta Canis Majoris while the spacecraft's motion caused the star to pass behind Saturn’s rings. This stellar ring occultation, at about 27 degrees elevation off the ring plane, was useful for studying the three-dimensional structure of the rings, in particular the self-gravity wakes that dominate the A ring. These ephemeral agglomerates of ring particles are stretched out by Keplerian shear, and occultations at low elevation angles such as this, when combined with other occultations at different geometries, can reveal the vertical thickness of the clumps and how it varies across the ring system.
Monday, March 13 (DOY 072)
UVIS, CIRS, and VIMS performed an 8.5-hour series of stares and slews across Saturn's northern auroral region. They captured auroral dynamics across the full polar region, while the Magnetospheric and Plasma Science (MAPS) instruments measured the nearby plasma and mag-field conditions, some of which likely feed the auroras.
After an engineering activity and a DSN communications session, VIMS took control of spacecraft pointing to begin observing another stellar ring occultation. This time it was the red star Lambda Velorum, and the occultation lasted 4.4 hours. CIRS rode along.
NASA appropriately selected Cassini's newest image of Saturn's moon Pan to be the Astronomy Picture of the Day today: https://apod.nasa.gov/apod/ap170313.html.
Results of an observation of Enceladus by Cassini's Radar instrument are in. Used in its microwave radiometer mode, the instrument measured natural radiation from the surface and just below the surface of Enceladus's southern crevasses. The measurements implied constraints on the thickness of parts of the body's icy crust: /news/13007/cassini-sees-heat-below-the-icy-surface-of-enceladus.
Tuesday, March 14 (DOY 073)
CIRS observed the rings for three hours, as part of a campaign to measure the rings’ thermal emission at a wide variety of phase, emission, and solar elevation angles. The campaign has the potential to provide insight into the spin states, dynamics, and composition of ring particles.
ISS performed another one-hour observation as part of the satellite astrometry campaign to monitor the orbits of rocks near Saturn. This campaign has been running continuously since January 2004.
Next, ISS spent another 1.1 hours reacquiring and tracking the orbits of individual known "propellers" (see last week's report for more on these objects). CIRS rode along. Propeller orbits are known to change on many timescales, and measuring them often helps to understand the causes of this behavior. By the way, 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.
After the propeller activity, UVIS devoted 3.7 hours, with CIRS riding along, to making an observation of Saturn's illuminated north polar auroral region.
While many the world over celebrated "Pi day" (3.14) by eating pie today, Cassini had its own PIE, or Pre-Integrated Events. One such observation was ISS's 30-minute observation of Saturn's big icy moon Tethys, in hopes of learning more about the red streaks on its surface. Cassini's next serving of PIE was for the Ion and Neutral Mass Spectrometer (INMS). The spacecraft turned to favor this direct-sensing instrument while Cassini made its 15th dive through Saturn's ring plane, close outside the main rings, and periapsis passage. This observation was supported by the Cosmic Dust Analyzer (CDA), another direct-sensing instrument, that was pointed into "the wind" of oncoming dust.
Another image of Mimas was featured this week, as the result of Cassini's final encounter with the object early this year:/resources/17614/?category=images.
The DSN communicated with and tracked Cassini on eight occasions this week, using stations in Australia and California. A total of 18 individual commands were uplinked, and about 2,390 megabytes of telemetry data were downlinked and captured at rates as high as 124,426 bits per second.
Cassini is executing its set of F-ring-grazing orbits of Saturn, with a period of 7.2 days in a plane inclined 63.5 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 March 15 using the 70-meter diameter DSN station in California. 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 185 days until the end of the mission.