Cassini is currently orbiting Saturn with a period of 12.7 days in a plane inclined 1.3 degrees from the planet's equatorial plane. The most recent spacecraft tracking and telemetry data were obtained on Dec. 9 using the 70-meter diameter Deep Space Network (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/news/significantevents/anomalies .
Time-tagged commands, stored on board Cassini as the S92 sequence, orchestrated the spacecraft's activities this week; S means Saturn tour; 92 is the integer count. Meanwhile on Earth, Sequence Implementation Process team members in the US, in Germany and in the UK, continued their work on creating additional 10-week command sequences. S93 is planned to begin executing on February 7, and S94 starts on April 18. The final sequence is S101, and it is slated to begin executing on July 10, 2017.
Wednesday, Dec. 2 (DOY 336)
Today was Cassini's sixth day facing its high-gain antenna constantly toward Earth, with the secondary axis selected to enhance the Magnetospheric and Plasma Science (MAPS) instruments' collection of data about Saturn's magnetotail. The Radio Science Superior Conjunction Experiment continued taking advantage of the opportunity to probe the solar corona with Cassini's continuous radio signals; this will end on Dec. 14. Saturn is now officially a "morning object," rising just before the Sun. It's too close to the Sun, though, for any telescope viewing until it moves much farther west in our sky during the new year.
During a routine DSN tracking and communications session today, the flight team uplinked commands to fine-tune some vector quantities stored in Cassini's Attitude and Articulation Control subsystem. The refined vectors will ensure proper pointing of Cassini's telescopes during some important non-targeted encounters coming up on Sunday.
Thursday, Dec. 3 (DOY 337)
As seen from Earth, the angle between Cassini and the Sun increased beyond three degrees today, so the Imaging Science Subsystem (ISS) went back to work. First, it made a 30-minute observation in the satellite orbit campaign, looking for small objects near the planet. Given the increased angle from the radio-noisy Sun, communications will be robust enough to return the usual amounts of imaging and other science data from Cassini via telemetry.
Next, ISS spent an hour observing the transit of Saturn's small moon Mimas as the 396-kilometer diameter object passed in front of Enceladus, the 500-km wide active icy moon. When this was over, the Composite Infrared Spectrometer (CIRS) joined ISS and VIMS for a 90-minute monitoring of hazy Titan; Saturn's largest moon was at a distance of 1.4 million km from Cassini's telescopes. This observation was repeated on the following day from a similar distance. CIRS then called the shots for 11 hours to observe Saturn’s atmosphere, in an effort to determine temperatures in its upper troposphere and tropopause. Finally, it was Enceladus's turn to pass in front of Mimas, and ISS observed this transit for one hour.
Continuing to move inbound towards periapsis, Cassini crossed the orbit of Saturn's spongy-appearing, low-density moon Hyperion, which orbits the planet at a distance of 1.5 million km. Next, Cassini crossed Titan's orbit, at 1.2 million km out. The Cosmic Dust Analyzer (CDA) took full advantage of this crossing of paths and performed a 20-hour, high-priority scan where streams of dust from each of those moons may be found. During this period dedicated to CDA, ISS and the Visible and Infrared Mapping Spectrometer (VIMS) also made some observations of their own.
NASA's Astronomy Picture of the Day featured Cassini's view of Saturn's small but active moon Enceladus in front of the rings, everything backlit by the Sun. Looking closely, one can detect the plume emanating from Enceladus's south polar region: http://apod.nasa.gov/apod/ap151203.html .
Friday, Dec. 4 (DOY 338)
Cassini's instrumentation was designed with Titan in mind. While the human eye cannot see through the planet-like moon's hazy atmosphere, VIMS routinely obtains images of its surface. Moreover with VIMS, every pixel tells a story, providing detailed infrared spectral data that can yield information about the surface composition. The latest VIMS mosaic of Titan's previously veiled surface is featured at: http://saturn.jpl.nasa.gov/photos/imagedetails/index.cfm?imageId=5278 .
Saturday, Dec. 5 (DOY 339)
The Magnetospheric Imaging Instrument (MIMI) had the spacecraft roll for nearly nine hours to study Saturn’s inner magnetosphere. After this, the Ultraviolet Imaging Spectrograph (UVIS) spent four hours observing Saturn’s aurora; VIMS rode along.
Sunday, Dec. 6 (DOY 340)
A first-time event occurred today for Cassini, after more than 17 years in flight: a conjunction between two of Saturn's irregular moons occurred within the space of a single ISS frame. The moons, Siarnaq und Ijiraq, both named after creatures in Inuit mythology, are distant members of Saturn's menagerie whose orbits reach as far as 22.7 million and 14.6 million km from Saturn respectively, requiring 896 days and 451 days to complete their orbits. ISS's observation lasted 3.6 hours.
CIRS took control of spacecraft pointing next, targeting the sunlit side of Saturn's mid-C ring for seven hours, obtaining spectral clues to its composition. This is the innermost of the three rings that are clearly visible from Earth in a moderate telescope.
Taking over pointing control from CIRS, ISS spent 2.6 hours observing several of the objects in the main rings dubbed "propellers" (see http://go.nasa.gov/17oqTWF). These bodies may hold clues to the origin and evolution of Saturn's vast ring system.
Next came a rapid series of non-targeted encounters, flybys which have required no dedicated propulsive maneuvers to achieve; ISS led the 2.6 hours of observations, with UVIS, CIRS and VIMS riding along. First was Epimetheus, 130 km in length, which orbits Saturn just outside the F ring; Next came Atlas, about half that size, which skirts immediately outside the A ring. Finally Prometheus, which is roughly the size of Epimetheus, was ready for its close-up; this unusual moon is known for an orbit that brings it into contact with particles in Saturn's F ring, one of the factors keeping that ring in constant flux. The F ring is the thin dusty strand just outside the A ring. This illustration shows the landscape: http://go.nasa.gov/1QSIeLa .
While these non-targeted encounters were taking place, Cassini sped through periapsis in its Saturn orbit #227. It came within 92,305 km above the planet's visible edge, and had reached 77,345 km per hour relative to Saturn. For comparison, the moon Epimetheus, seen in the illustration, travels in its orbit about 57,000 km per hour relative to the planet.
Monday, Dec. 7 (DOY 341)
CIRS stared at the unlit side of the mid-C ring for four hours, complementing the previous day's study of the lit side. On completion, ISS turned to observe Titan for 90 minutes, part of the Titan monitoring campaign, with VIMS riding along. The target was 1.8 million km away. Next, UVIS studied Saturn's atmosphere by watching the bright blue stars in Orion's belt known as Epsilon Orionis and Zeta Orionis for 2.5 hours, while they were occulted by the upper atmosphere. Ending a very productive day, ISS and UVIS watched Enceladus for 7.7 hours to monitor the variability of its icy plume, while it was backlit by the Sun.
Today, Earth's moon presented a beautiful, thin crescent in the morning sky. A similar view, though of a smaller, icy, moon of Saturn, was Cassini's featured image today: http://go.nasa.gov/1QSIeLa .
Tuesday, Dec. 8 (DOY 342)
VIMS and CIRS observed Saturn's faint E ring and G ring for 16 hours. Finally, as Cassini was again crossing the orbits of Titan and Hyperion while outbound from periapsis, CDA observed the dust in its path for 16 hours.
An image taken this week of Prometheus, gravitational thief of material from the F ring visible behind it, was featured today: http://saturn.jpl.nasa.gov/photos/imagedetails/index.cfm?imageId=5280 .
On nine occasions this week, while Cassini was pointing its optical instruments towards or nearly towards Saturn, ISS turned and made two-minute Saturn storm-watch observations. VIMS rode along with four of them. Also during the week, the Deep Space Network communicated with and tracked Cassini on six occasions, using stations in Spain, Australia, and California. A total of 64 individual commands were uplinked, and about 1,815 megabytes of telemetry data were downlinked and captured at rates as high as 110,601 bits per second.
This illustration shows Cassini's position early on Dec. 8: http://go.nasa.gov/1QSH56c . The format shows Cassini's path over most of its current orbit up to today; looking down from the north, all depicted objects (except the background stars of course) revolve counter-clockwise, including Saturn along its orange-colored orbit of the Sun.
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: