Cassini is currently orbiting Saturn with a period of 18.9-days in a plane inclined 0.3 degree from the planet's equatorial plane. The most recent spacecraft tracking and telemetry data were obtained on June 3 using the 70-meter diameter Deep Space Network 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/news/significantevents/anomalies .
Investigations into Saturn's atmosphere took center stage for the first part of this week, prior to Cassini's final encounter with Hyperion. Changes in Saturn’s atmosphere in the aftermath of the great storm of 2010-2011 were of central interest. Cassini's Radar instrument can operate in a “ microwave radiometry" mode, in addition to its more familiar synthetic-aperture imaging mode. The latter provides the highest-resolution views of Titan's surface. The instrument's altimetry mode and scatterometry mode are also mainly employed during Titan encounters. In the radiometry mode used this week, Cassini’s Radar measured the abundances of certain chemical species in Saturn’s atmosphere -- such as water and ammonia -- by picking up emissions that these molecules naturally radiate in the radio-frequency part of the spectrum. In this way Cassini can look deep into the planet's dynamic weather systems.
Wednesday, May 27 (DOY 147)
After finishing up a global map of Saturn, started the previous day by the Visible and Infrared Mapping Spectrometer (VIMS), Cassini turned to point its body-fixed dish antenna to Earth for about nine hours of two-way digital communications and radiometric tracking. The Deep Space Network (DSN) used its massive 70-meter diameter dish in Australia for this session.
After the scheduled "end of track" with the DSN, the Visible and Infrared Mapping Spectrometer (VIMS) and Radar began to take turns rotating the spacecraft for the next 37 hours, while Saturn itself rotated more than 3.5 times beneath the spacecraft; VIMS and the other Optical Remote Sensing instruments have fields of view that are at right angles to the high-gain dish antenna that radar uses, which is the same one used for communication. VIMS was looking at ammonia clouds and dynamics deep in Saturn's atmosphere; radar made its own global map of the microwave emissions coming from ammonia and other constituents. This suite of observations -- the third and final of this type for the remainder of the Cassini mission -- was deemed important enough that dual playbacks were programmed using different DSN sessions. This minimized any chance of data loss, for example a thunderstorm on Earth drowning out telemetry from Cassini. This represented the longest-ever "dual playback" of the entire mission.
Today the one-way light time (and communications delay) reached its minimum for the year; it was 74 minutes 35 seconds. This was four days past opposition, when Earth is closest to Saturn, but Cassini had been closing more of the distance to Earth while moving in its orbit of Saturn since then.
Thursday, May 28 (DOY 148)
While radar was carrying out the observation it started on the previous day, Cassini coasted through periapsis in its Saturn orbit #216. It came within 189,000 kilometers of the planet's visible limb, and had reached a speed of 59,945 kilometers per hour relative to Saturn.
Friday, May 29 (DOY 149)
VIMS finished off the Saturn deep-atmosphere observations early in the day. For the next science activity, Cassini turned towards Saturn's largest moon Titan for 90 minutes so the Imaging Science Subsystem (ISS), the Composite Infrared Spectrometer (CIRS), and VIMS could look for any recent changes in its atmosphere. Last, the Ultraviolet Imaging Spectrograph (UVIS) turned to observe Saturn’s aurorae for nearly seven hours. CIRS and VIMS rode along with the observation.
NASA's Astronomy Picture of the Day featured images of Saturn taken from Earth’s surface on the night of opposition http://apod.nasa.gov/apod/ap150529.html .
Saturday, May 30 (DOY 150)
VIMS controlled spacecraft pointing for nearly eight hours to watch the planet’s northern auroral region. CIRS, ISS and UVIS took data as riders.
Sunday, May 31 (DOY 151)
ISS turned to Saturn's small moon Enceladus and tracked it for 3.5 hours, observing its icy south-polar plumes while they were backlit by the Sun. Next ISS turned and tracked the 270-kilometer wide moon Hyperion for nearly 10 hours. This was a non-targeted encounter, meaning that Cassini did not have to plan any propulsive orbit adjustments for the flyby. This was Cassini's last chance to further characterize the strange, low-density object's chaotic rotation and surface features. Accordingly, before the observation began, the flight team sent realtime commands to update the spacecraft's knowledge of pointing vectors from Cassini to Hyperion, a distance of some 34,000 kilometers. As a result, Hyperion's spongy-looking face appeared squarely centered in the spacecraft's telescopes.
After ISS finished taking images, and before turning away from Hyperion, CIRS spent five hours collecting thermal-infrared spectra, to study its emissivity.
Cassini scientists attended the joint Juno/Cassini Jupiter-Saturn Magnetosphere meeting at the Georgia Institute of Technology (GIT) today. Members from both teams gave overviews of planned science activities at Jupiter and Saturn. During the coming week, the Magnetospheres of the Outer Planets meeting also convened at GIT; the first two days were heavily scheduled with Cassini-related presentations.
Monday, June 1 (DOY 152)
CIRS observed Saturn’s atmosphere in the mid-infrared part of the spectrum for 23 hours to better determine temperatures in the upper troposphere and tropopause.
Saturn's icy moons Dione and Rhea were both captured in one frame, and were featured today as both a still image and an animated GIF:
Tuesday, June 2 (DOY 153)
VIMS, CIRS and UVIS observed Saturn's E ring and G ring for 13.5 hours while they were sunlit at a phase angle of 130 degrees, for 13.5 hours today. These dusty rings lie outside of the main ring system (the A, B, and C rings). Both before and after this observation, ISS turned to Saturn for a two-minute storm-watch study; VIMS participated in the first of the pair.
Images that were successfully downlinked from Cassini's final encounter with Hyperion were the subject of a news release today:
During the past week, the DSN communicated with and tracked Cassini on nine occasions, using stations in Spain, California, and Australia. A total of 7,177 individual commands were uplinked, and about 2,000 megabytes of telemetry data were downlinked and captured at rates as high as 142,201 bits per second.
This illustration shows Cassini's position on June 2:
Milestones spanning the whole orbital tour are listed here: http://saturn.jpl.nasa.gov/mission/saturntourdates .
Information on the present position and speed of the Cassini spacecraft may be found on the "Present Position" page at: