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Cassini is currently orbiting Saturn with a 28-day period in a plane inclined 0.3 degree from the planet's equatorial plane. The most recent spacecraft tracking and telemetry data were obtained on April 16 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.
Before Voyager 1 and Voyager 2 had seen the natural satellites of all the outer planets, it was thought they'd be relatively boring, dead worlds, much like our own Moon. This week, with its orbit back in the plane of Saturn's rings and moons, Cassini enjoyed
Voyager-distance-class, non-targeted encounters with Saturn's moons Tethys and Dione; these are not (yet) known to be geologically active like some moons (e.g., the geyser-moon Enceladus, and planet-like Titan, which Cassini also observed this week), but they're
far from boring. Building upon Voyager's quick reconnaissance, as well as Cassini's own previous encounters, the spacecraft obtained images and other data this week about these worlds from new perspectives.
To clarify: "targeted" encounters are those for which Cassini's Navigation team plans and executes propulsive events -- Orbit Trim Maneuvers. For example, a targeted Titan encounter occurs during most of the spacecraft's orbits of Saturn. Targeted encounters
with other icy satellites are less common; during Cassini's remaining mission, there are two targeted encounters with Dione planned, and then three with Enceladus. This week's "non-targeted" visits required no special expenditures of propellant.
Wednesday, April 8 (DOY 098)
The Composite Infrared Spectrometer (CIRS) finished up a 23-hour long observation of Saturn’s atmosphere that was started on the previous day, in an effort to determine temperatures in the upper troposphere and tropopause. While CIRS controlled spacecraft
pointing, the Visible and Infrared Mapping Spectrometer (VIMS) also took data as a ride-along instrument. As soon as this finished, the Imaging Science Subsystem (ISS) spent two minutes making a storm-watch observation on Saturn. Next, ISS, CIRS and VIMS had
Cassini turn to perform an observation in the Titan monitoring campaign; the giant, hazy moon was 1.7 million kilometers from Cassini's telescopes.
Thursday, April 9 (DOY 099)
ISS carried out a one-hour observation in the satellite orbit campaign, sighting objects near the planet. Then, prior to a nine-hour communications and tracking session via the Deep Space Network, the spacecraft remained earth-pointed for eight hours for
the Magnetospheric and Plasma Science (MAPS) instruments to collect data. After another quick ISS storm watch, the Ultraviolet Imaging Spectrograph (UVIS) began a 12-hour observation of Saturn’s aurora. The other telescopic optical remote sensing (ORS) instruments,
ISS, CIRS, and VIMS, rode along.
Friday, April 10 (DOY 100)
As soon as ISS finished another two-minute Saturn storm watch, VIMS spent 1.25 hours in the driver's seat to track the red star 56 Leonis, which is bright in the infrared though dim by visual standards, as it sank into Saturn’s atmosphere. Spectral data
obtained from stellar occultations like this one are valuable sources of information about the structure, density, and chemical properties of Saturn's atmosphere. Once this was complete, ISS turned to begin another observation of Saturn's outer irregular moon
Paaliaq, with VIMS riding along. This moon was described in last week's account, at which time ISS observed it for 38 hours. Today's eight-hour observation adds to the "light-curve" data, helping characterize its surface brightness and its shape as it rotates.
Saturday, April 11 (DOY 101)
Cassini's ORS instruments spent more than 16 hours making best use of today's non-targeted encounter with Saturn's 1,060-kilometer wide, icy moon Tethys. Objectives included investigating its anomalous thermal signature, which Cassini saw in 2012 (see
http://go.nasa.gov/1JL0rDY). While the Tethys observations were in progress, Cassini sped through periapsis, the bottom of its 3.2 million-kilometer high "roller coaster" orbital ellipse. In free-fall, the 2,248-kilogram spacecraft had sped to 51,423 kilometers
per hour relative to Saturn, and came as close as 277,000 kilometers from the planet's hazy limb.
Near the day's end, and right on the heels of the non-targeted Tethys encounter, Cassini enjoyed another fortuitous non-targeted visit. The 1,122-kilometer wide Dione has ice cliffs -- enormous tectonic fractures -- on its trailing hemisphere, which faced
Cassini during the encounter. An illustration seen here shows Cassini's location and observing geometry relative to Tethys and Dione; all move in a counter-clockwise direction in this view (the speed shown is relative to Saturn):
http://go.nasa.gov/1JKWYW0.
Sunday, April 12 (DOY 102)
The Dione observations continued today, including a search for any signs of extant activity such as plumes. Various ORS instruments mapped the curious trailing hemisphere. Upon parting company with Tethys and Dione, ISS turned to view Paaliaq again, this
time for 6.25 hours, with VIMS and UVIS riding along. (Recall that the Raw Images page always has Cassini's latest views:
http://saturn.jpl.nasa.gov/photos/raw/?start=1.)
The next science activity belonged to CIRS, which made a 12-hour observation of Saturn’s atmosphere in the far-infrared part of the spectrum, similar to Wednesday's 24-hour stare.
Monday, April 13 (DOY 103)
After giving ISS two minutes for another storm watch, UVIS, CIRS and VIMS began an auroral science campaign with a 14-hour observation of Saturn's poles. Late in the day, ISS, UVIS and VIMS began controlling spacecraft pointing for nearly 10 hours to watch
the plumes of the 504-kilometer wide Enceladus, 1.5 million kilometers away from Cassini, adding to measurements of their variability.
Cassini scientists gathered for the week at the European Geophysical Union general assembly in Vienna, Austria, where they gave presentations on all aspects of Saturn-system science.
Saturn seems to have a huge convective storm about once a year -- once a Saturn year, which lasts 29.45 Earth-years. So it was fortunate that Cassini was in position to observe the storm that began in December 2010. A news item released today describes
the findings of a NASA-funded investigation into the storms' underlying cause:
http://saturn.jpl.nasa.gov/news/newsreleases/newsrelease20150413 .
An image featured today shows Mimas lit mostly by Saturn's reflection of sunlight, along with a nice view of the rings. From Mimas inward are the bright, narrow F ring, the wide, smooth A ring, the dark, dense B ring and bright strands of the C ring:
http://saturn.jpl.nasa.gov/photos/imagedetails/index.cfm?imageId=5180 .
Tuesday, April 14 (DOY 104)
VIMS watched Saturn's rings for nine hours while Cassini gradually coasted through the ring plane at Saturn's equator, going towards the south. They were illuminated by the Sun at a phase angle of around 155 degrees. Finally, as if to celebrate the birthday
today of Christiaan Huygens's (1629-1695), who discovered Titan when he was 26, Cassini's ISS and VIMS carried out another 90-minute observation in the Titan monitoring campaign, this one from a distance of 1.7 million kilometers. ISS finished out the week
with another two-minute storm watch on Saturn.
Scientists have directly traced the paths that material follows as it erupts from Enceladus's south-polar jets and plumes, contributing to Saturn's E ring. It seems to flow along sinuous, ever-changing tendrils. The news release may be seen here:
http://saturn.jpl.nasa.gov/news/newsreleases/newsrelease20150414 .
During the past week, the Deep Space Network communicated with and tracked Cassini on five occasions, using stations in Australia and Spain. A total of 15 individual commands were uplinked, and about 1,072 megabytes of telemetry data were downlinked and
captured at rates as high as 142,201 bits per second.
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This illustration shows Cassini's position on April 14: http://go.nasa.gov/1OxJJL2 .
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Milestones spanning the whole orbital tour are listed here:
http://saturn.jpl.nasa.gov/mission/saturntourdates.
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For a glossary of technical terms relating to these events, click the "full story" link on this page:
http://saturn.jpl.nasa.gov/news/significantevents/ .
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Information on the present position and speed of the Cassini spacecraft may be found on the "Present Position" page at:
http://saturn.jpl.nasa.gov/mission/presentposition/
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