This late in the game, Cassini is still undertaking many new, scientifically important observations. The current F-ring-grazing orbits always bring the spacecraft close to Saturn, and once in a while Cassini happens to close in on one of the little moons that orbit near, or even within the rings. This week, Cassini made observations of the small moon Pan that were about eight times better than ever before. Pan orbits within the Encke Gap in the outer part of Saturn's broad A ring. Studying the little moon, especially near its equator, provides key clues to how such objects interact with rings. An image featured this week reveals even smaller embedded moonlets!
Perhaps even topping this, Cassini made a special set of observations of Saturn that should help pin down the ratio of helium to hydrogen that make up the planet's atmosphere. Previous attempts at this key measurement had failed for technical reasons, but this week's activity returned the sought-out data.
Wednesday, March 1 (DOY 060)
Cassini's Visible and Infrared Mapping Spectrometer (VIMS) controlled the spacecraft’s pointing to observe Saturn's rings for 5.4 hours while they were backlit by the Sun, which itself was blocked from Cassini's view by the giant planet. This viewing geometry meant that the smallest ring particles shone brightly in forward-scattered sunlight. The Imaging Science Subsystem (ISS) and the Composite Infrared Spectrometer (CIRS) rode along, making their own observations.
Next, VIMS had the spacecraft turn a little so that the instrument's solar port could watch the Sun directly for 1.5 hours, while it passed behind the rings. The Ultraviolet Imaging Spectrograph (UVIS) used its solar port to observe the solar ring occultation as well.
Thursday, March 2 (DOY 061)
Today, UVIS began a 24-hour study of Saturn's nighttime southern auroral region; CIRS and VIMS rode along.
In these reports, there have been mentions of "propeller" features in Saturn's rings. With Cassini's closeness to the rings during its most recent orbits of Saturn, these objects, which are of compelling scientific interest, are being observed as never before. The image featured today compares close-ups of propellers from both sides now, lit and unlit: /resources/17611 .
Friday, March 3 (DOY 062)
ISS, CIRS and VIMS turned to observe Saturn's largest satellite Titan for two hours, to monitor its thick atmosphere and weather. Then, while the spacecraft was rolling about its longitudinal axis for a Magnetometer calibration activity, UVIS conducted an interplanetary hydrogen survey.
Saturday, March 4 (DOY 063)
The spacecraft coasted through apoapsis early today, marking the start of Orbit #264. For four hours, the Cosmic Dust Analyzer (CDA) pointed in the ideal direction to measure the influx of meteors into the Saturn system.
These days, Cassini's apoapsis passages always occur out near the orbit of Titan, which is 1.2 million kilometers from Saturn. Today, Cassini began a 32-hour series of observations of Titan, while the big moon and the spacecraft came together for a non-targeted flyby. ISS, CIRS, and VIMS all participated in this opportunity.
The Cassini Mission (for one) would never have flown at all, were it not for important influence from a handful leading planetary scientists. Foremost among them was Dr. Toby Owen, who unfortunately passed away today. This page is a tribute to him: http://solarsystem.nasa.gov/people/owent&display=tribute .
Sunday, March 5 (DOY 064)
Today the spacecraft passed within 490,000 km of Titan during the non-targeted flyby, continuing its series of observations of the planet-like moon's surface and atmosphere over mid-northern latitudes. These observations are of particular scientific interest at this time while Titan's northern summer solstice is approaching. The geometry of this flyby made it possible for ISS to fill a gap that remained in surface mapping coverage.
Monday, March 6 (DOY 065)
Speeding in towards Saturn, UVIS took the reins today for 1.7 hours, in collaboration with ISS and VIMS. The target was Saturn's dayside northern auroral oval. Next, CIRS observed Saturn for two hours right at the point where a stellar occultation was going to occur one Saturn-day later (about 11 hours).
An image featured today clarifies an important puzzle about Saturn's small icy moon Enceladus, which Cassini has handily solved: /resources/17610 .
Tuesday, March 7 (DOY 066)
VIMS began the day observing Saturn’s north polar region for six hours with CIRS, ISS and UVIS riding along.
Next, VIMS tracked the red star Gamma Crucis for a little over four hours while the star passed behind Saturn's rings, and then went behind the planet. CIRS, UVIS, and ISS rode along during the ring occultation, which provided the highest quality profile of dense regions, such as the B ring. The observation should help improve models for understanding B-ring waves, and perhaps identify new features in the dense rings.
CIRS participated with VIMS in the high-quality Saturn occultation, building on its solo observation a Saturn-day earlier; these were intended to measure the ratio of helium to hydrogen making up planet's lower stratosphere.
More observations of Saturn's limb continued for nearly another 13 hours, with various Optical Remote-Sensing (ORS) instruments taking part at times. These observations were interrupted for 2.5 hours, though, when the little moon Pan came into view, closer to Cassini than ever before. All the ORS instruments examined Pan, which orbits Saturn within the Encke Gap in the outer A ring. This close view was important to capture, because the shape and surface morphology of ring-related satellites are important for understanding satellite-ring relations. (Appearances like Pan’s, by the way, are no surprise; observations are all planned out months and years in advance of sending commands.)
During the Pan observations, Cassini plunged through the ring plane, and then sped through periapsis in its Saturn orbit.
(Late breaking news: Check out the amazing images of Pan at https://saturn.jpl.nasa.gov/galleries/raw-images/ .)
The DSN communicated with and tracked Cassini on eight occasions this week, using stations in Australia, Spain, and California. A total of 7,595 individual commands were uplinked, and about 1,970 megabytes of telemetry data were downlinked and captured at rates as high as 110,601 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 1 using one of the 34-meter diameter DSN stations 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 192 days until the end of the mission.
Cassini's orbit looks the same again this week, but the positions of spacecraft and moons and the Sun are different, on this illust