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Cassini Significant Events 8/12/15 – 8/18/15

Cassini is currently orbiting Saturn with a period of 21.8 days in a plane inclined 0.5 degree from the planet's equatorial plane. The most recent spacecraft tracking and telemetry data were obtained on Aug. 18 using a 70-meter diameter Deep Space Network 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 .

Cassini's main event this week was the mission-final close flyby of Saturn's moon Dione, which included another realtime measurement of its gravitational field. In addition to the imaging and other observations Cassini made during the close encounter, the Radio Science team used the Deep Space Network's exquisitely precise capability to measure the Doppler shift of the spacecraft's continuous radio-frequency signal. Once all the known "baseline" motions of Earth and Saturn and spacecraft are subtracted out, the remaining Doppler shift not only accurately indicates Dione's mass, but also provides insight about how its mass is internally distributed.

Cassini acquired 814 images this week, among them views of Dione of course, plus coverage of Saturn's moons Hyperion, Tethys, Titan and Bestla. Saturn's G ring was also imaged, plus the E ring and the moon Enceladus whose plumes continue to generate that ring. All of these provide excellent reasons for exploring Cassini's raw-image web page ( http://saturn.jpl.nasa.gov/photos/raw ).

Wednesday, Aug. 12 (DOY 224)

The Imaging Science Subsystem (ISS) trained its wide-angle camera on Saturn for a storm-watch observation. This repeated seven more times throughout the week, each two-minute observation squeezed in while Cassini's telescopes were already pointing near the planet; the Visible and Infrared Mapping Spectrometer (VIMS) participated in three of them.

When today's storm watch was completed, the Ultraviolet Imaging Spectrograph (UVIS) made a mosaic scan of Saturn’s magnetosphere, wrapping up the series of observations that started last week. Today's lasted nearly 10 hours, for a total of 80 hours mapping the distribution of oxygen near the planet.

Next, ISS, VIMS, and the Composite Infrared Spectrometer (CIRS) performed a 90-minute observation in the Titan monitoring campaign; the hazy moon was 1.5 million kilometers away from the spacecraft. Finally, ISS carried out a 60-minute observation in the satellite orbit campaign, looking for objects orbiting near Saturn. This pair of observations repeated on Friday, and again on Saturday, at similar distances from their respective targets.

Thursday, Aug. 13 (DOY 225)

A news release was released today in anticipation of Cassini's flyby of Dione on Aug. 17: http://saturn.jpl.nasa.gov/news/newsreleases/newsrelease20150813/ .

Friday, Aug. 14 (DOY 226)

Today while the spacecraft was closing in on Dione, the flight team had an opportunity to make a slight adjustment in its velocity by firing the small rocket thrusters. However, navigation solutions based on the most recent tracking data from the Deep Space Network showed that this would not be necessary, since Cassini was basically right on target. Orbit Trim Maneuver (OTM) 418 was therefore cancelled.

UVIS spent six hours imaging Saturn's thermosphere -- the thin, hot layer above the visible limb. Data from this observation will help determine the density there, which will be useful in planning Cassini's 2017 Proximal Orbit phase.

Saturday, Aug. 15 (DOY 227)

VIMS spent three hours today doing a special calibration, which, along with data from activities on Aug. 18, served as a preliminary test of a numeric thermal model for the instrument that has been in development for the past eighteen months. Following this activity, ISS spent 65 minutes tracking the transit of Saturn's moon Tethys as it passed across the face of the more distant moon Hyperion. When this was completed, VIMS turned the spacecraft so its telescopes could observe Saturn's E ring and G ring for 9.5 hours while they were sunlit at a phase angle of 80 degrees; CIRS rode along.

Sunday, Aug. 16 (DOY 228)

VIMS studied Saturn's atmosphere by tracking the distant red star 56 Leo for 80 minutes as it was occulted by the planet. Following this, CIRS observed Saturn’s atmosphere in the far-infrared part of the spectrum for nearly 11 hours, collecting data to determine temperatures in the upper troposphere and tropopause.

Monday, Aug. 17 (DOY 229)

The Radio and Plasma Wave Science instrument (RPWS) and the other Magnetospheric and Plasma Science (MAPS) instruments participated in today's Dione encounter. Activities are further described on the flyby page, complete with video clip:
http://saturn.jpl.nasa.gov/news/newsreleases/newsrelease20150813/ .

Amid observations of Dione during the close encounter, Cassini happened to come within 42,000 kilometers of Tethys. CIRS took advantage of this, and turned to examine Tethys and the thermal anomaly on its surface. ISS, UVIS and VIMS participated in the one-hour observation. The thermal anomaly was described here in 2012: http://saturn.jpl.nasa.gov/news/cassinifeatures/feature20121126 .

Tuesday, Aug. 18 (DOY 230)

Cassini coasted through periapsis in its Saturn Orbit #220 early today. It came within 223,000 kilometers of Saturn's hazy visible atmosphere, going 56,032 kilometers per hour relative to the planet.

ISS led a 30-minute observation of Saturn's small, enigmatic moon Enceladus, while it was sunlit at a phase angle of 90 degrees. CIRS, UVIS and VIMS participated as riders. UVIS then continued watching Enceladus for another 1.5 hours, gathering measurements of the satellite’s ultraviolet albedo at varying longitudes. CIRS and VIMS rode along with UVIS. Closing out this suite of Enceladus watching, CIRS took control for an hour to favorably orient one of its fields of view, with UVIS and VIMS riding along.

Next, ISS and VIMS tracked Saturn's six-kilometer diameter irregular moon Bestla. Named after a frost giantess from Norse mythology, it occupies a retrograde, highly inclined, and highly eccentric orbit that takes it more than 20 million kilometers from the planet. After one hour, ISS had the spacecraft turn and begin a two-hour observation of Enceladus's plumes, with CIRS, UVIS and VIMS riding along. Finally, ISS and VIMS turned back to Bestla and tracked it for another 5.75 hours. Data from the observations should help determine Bestla's shape and pole-axis orientation.

Closing out a busy day, the spacecraft turned once more to Tethys so CIRS could search for emissivity variations. UVIS and VIMS rode along with the three-hour observation.

During the past week, the DSN communicated with and tracked Cassini on 12 occasions, using deep space stations in Spain and California. A total of 19 individual command files were uplinked, and about 1,500 megabytes of telemetry data were downlinked and captured at rates as high as 124,426 bits per second.

This illustration shows Cassini's position in Saturn orbit on Aug. 18: http://saturn.jpl.nasa.gov/news/cassinifeatures/feature20121126 .

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:
http://saturn.jpl.nasa.gov/mission/presentposition/ .