Cassini's orbital path this week closely resembled last week's, in which the plunge from 1.3 million kilometers high resulted in another fast passage through Saturn's thin upper atmosphere. That passage was the 19th proximal ring-plane crossing out of the Mission's total of 22. But the scientific investigations on this orbit took a slightly different turn, addressing a varied set of objectives.
Wednesday, Aug. 16 (DOY 228)
Early today, Cassini was coasting "up" towards apoapsis, slowing as it reached the distance of Titan's orbit, around mid-day.
The Imaging Science Subsystem (ISS) controlled pointing for 90 minutes during this period to monitor the weather on Titan. Saturn’s largest moon was more than 2 million km away, on the other side of the planet, but its back-lit, high-phase illumination was favorable for seeing the hazes in its thick atmosphere. This illustration shows the viewing and lighting geometry of the target: https://go.nasa.gov/2w4L8WO. Next, the Ultraviolet Imaging Spectrograph (UVIS) tracked the blue star Beta Orionis, commonly known as Rigel, to record a tracking occultation as it passed behind Saturn's rings. Its apparent passage approximated the speed of the ring particles as they orbited Saturn. Since UVIS can sample at hundreds of times each second, such occultations allow scientists to characterize the azimuthal structure of the rings at extremely fine spatial scales.
Finally today, ISS rotated the spacecraft to train its telescopes on Saturn's small, irregular satellite Kiviuq, which was described last week. As it was slewing toward the target, ISS took the opportunity to dwell on the planet Neptune briefly, since it happened to be in its field of view. VIMS rode along, and the spacecraft's orientation during this five-hour observation was chosen to optimize data collection for the Cosmic Dust Analyzer (CDA).
Thursday, Aug. 17 (DOY 229)
During ISS's observations, Cassini's apoapsis passage marked the start of its Orbit #289 of Saturn. Next, the Composite Infrared Spectrometer (CIRS) observed the middle strands in Saturn's sunlit C ring. The observation gathered data on the composition and structure of this unique region, which seems to be enriched with an unidentified non-icy component relative to the rest of the C ring. VIMS and UVIS rode along. ISS then took over for a five-hour observation of the fainter components of Saturn's ring system at high-phase illumination, with CIRS and VIMS riding.
Friday, Aug. 18 (DOY 230)
Today the subject continued to be Saturn's rings. First VIMS spent five hours making a mosaic of the rings while CIRS rode along, then ISS targeted the outer, narrow, F ring for 17 hours, which was long enough to watch particles pass through a full revolution around the giant planet. CIRS and VIMS participated.
Saturday, Aug. 19 (DOY 231)
After the ISS F-ring observation wound up, it was time for an engineering activity, then a communications and tracking session with the Deep Space Network (DSN), one of nine this week.
This week's news feature is about the great diversity found among Saturn's moons: https://saturn.jpl.nasa.gov/news/3088/nine-ways-cassini-matters-no-5.
Sunday, Aug. 20 (DOY 232)
ISS led CIRS and VIMS in another 90-minute Titan weather-monitoring observation today, and then UVIS began nine hours of observing Saturn's bright northern auroral oval; CIRS rode along.
One hour before this week's high-speed periapsis passage, CIRS began taking advantage of its specially-planned "cold" period, in which the spacecraft's orientation was managed so that the ORS instruments' thermal radiators had lots of deep space to point toward, rather than the much warmer planet or its rings. CIRS's task was to look from inside, targeting outward across the rings, with ISS riding along to capture enough images during the 90-minute observation to create a movie.
The spacecraft switched to its small attitude-control rocket thrusters for today's 19th ring-plane passage between Saturn and the D ring. This was the second of the five final crossings where the spacecraft briefly passes through the upper reaches of Saturn’s atmosphere. Using thrusters, the attitude control system handily offset the torque imparted to the spacecraft by atmospheric drag.
Following ring-plane crossing and periapsis passage, CIRS held onto the reins, still in its cold orientation. This time it looked inward to Saturn and mapped its southern latitudes, while VIMS rode along. A main objective was to measure temperatures in the planet's southern vortex. Finally today, VIMS began a seven-hour stare at Saturn's night-time southern aurora, with CIRS and ISS riding along.
Monday, Aug. 21 (DOY 233)
Many members of the Cassini flight team had travelled to places on Earth where they would have a high-phase observation of our own Moon, watching even as it blocked out the Sun's entire disc. Some of those who remained behind at JPL, where only a partial solar eclipse was visible, put together this collection of eclipses in the Saturn system: https://saturn.jpl.nasa.gov/news/3101/spectacular-eclipses-in-the-saturn-system.
Back at the ring world, VIMS began leading another observation of Saturn's southern hemisphere, including the south polar region. CIRS, ISS, and UVIS rode along for the 13-hour mapping operation.
Today's featured image shows one of Saturn's moons in a new light: https://saturn.jpl.nasa.gov/resources/7729.
Wednesday, Aug. 22 (DOY 234)
CDA was given control of spacecraft pointing today for 15 hours to capture and directly observe particles while the spacecraft passed through Saturn's ring plane, going north again on the way out to the following day's apoapsis.
The DSN communicated with and tracked Cassini on nine occasions this week, using stations in California, Spain, and Australia. A total of 875 individual commands were uplinked, and about 1,445 megabytes of science and engineering telemetry data were downlinked and captured at rates as high as 124,426 bits per second.
Cassini is executing its set of 22 Grand Finale Proximal orbits, which have a period of 6.5 days, in a plane inclined 61.7 degrees from the planet's equatorial plane. Each orbit stretches out to an apoapsis altitude of about 1,272,000 km from Saturn, where the spacecraft's planet-relative speed is around 6,000 km/hr. At periapsis, the distance shrinks to about 2,500 km above Saturn's visible atmosphere (for reference, Saturn is about 120,660 km in diameter), and the speed is around 123,000 km/hr.
The most recent spacecraft tracking and telemetry data were obtained on Aug. 23 using the 70-meter diameter DSN stations at Goldstone, 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 https://saturn.jpl.nasa.gov/anomalies.
The countdown clock in Mission Control shows 23 days until the end of the Mission.
This page offers all the details of the Mission's ending: <https://saturn.jpl.nasa.gov/mission/grand-finale/overview/>
Milestones spanning the whole orbital tour are listed here:
Information on the present position and speed of the Cassini spacecraft may be found on the "Present Position" page at:
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