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Every "proximal plunge" brings unique scientific results. Cassini has made 16 of these close-in passages between Saturn’s rings and atmosphere now, and six more remain before the mission's end. Some have been dedicated to Radio Science
experiments, some have been dedicated to the direct-sensing in-situ instruments, and some have been dedicated to remote-sensing at various wavelengths. On Tuesday Aug. 1, Cassini favored its infrared instruments. Steps were taken to ensure that its instruments'
radiators were not facing the relatively warm – and very close – planet Saturn, nor were they facing the nearby ring system, both of which reflect warm sunlight. As the spacecraft dove through closest approach on that day, its infrared optical instruments
had their best shot.
Whenever possible during the mission, the optical instruments' thermal radiators have been kept pointed towards cold deep space, so that their sensitive electronic detectors can dump overboard much of the heat they generate, passively
cooling them for best performance. (These radiators are mounted facing 90 degrees from their boresights.) However, during the proximal plunges in between Saturn's rings and atmosphere, which occur on the daytime side of the planet, this has not always been
possible. This time, though, Cassini was commanded to maintain orientations which kept its infrared remote-sensing instruments as cold as possible. This way, observations at infrared wavelengths experienced the highest available signal-to-noise ratio.
Wednesday, July 26 (DOY 207)
Having just plunged past periapsis the preceding day, Cassini turned and trained its Ultraviolet Imaging Spectrograph (UVIS) on a single location at Saturn's night-side southern auroral zone for 3.6 hours; the Composite Infrared Spectrometer
(CIRS) and the Imaging Science Subsystem (ISS) rode along observing the aurora.
Continuing to speed outbound, late in the day the spacecraft had travelled more than halfway out to apoapsis, and it came within 500,000 kilometers of Titan, Saturn's largest moon. The Optical Remote Sensing (ORS) instruments spent
the next 21 hours observing it. The Imaging Science Subsystem (ISS) and the Composite Infrared Spectrometer (CIRS) led the observations, with the Visible and Infrared Mapping Spectrometer (VIMS) riding along at times. Of prime interest was monitoring clouds
on the planet-like moon as summer sets in there.
The Cosmic Dust Analyzer (CDA) collaborated with ISS, to monitor the nano-dust stream activity at low inclinations near Titan’s orbit of Saturn.
Today's news feature recounts several of Cassini's discoveries: https://saturn.jpl.nasa.gov/news/3092/nine-ways-cassini-matters-no-9.
Thursday, July 27 (DOY 208)
The second part of the S101 sequence of commands went up to Cassini today. After 2 hours 35 minutes, telemetry confirmed that each of the 10,657 individual commands was flawlessly received and stored on board. While there will be
additional short, realtime commands transmitted to the spacecraft nearly every day, today's uplink represented the final time for a long list of timed, background-sequence commands. This part of S101 will control the rest of Cassini's main science activities
through the end of the mission, including the special "bent-pipe" activity on September 15, during the fateful encounter with Saturn's atmosphere. Up until the moment when aerodynamic forces cause the spacecraft to tumble out of control, data from selected
in-situ instruments will be "piped" directly to Earth, instead of being recorded on board.
Friday, July 28 (DOY 209)
Today, ISS observed Saturn's irregular moon Kiviuq for 3.4 hours. A very dark-surfaced object only about 14 kilometers in diameter, Kiviuq’s namesake is an eternal hero, the wandering shaman of Inuit mythology. The icy object follows
an inclined, eccentric orbit, going as far as 14.8 million km out from the planet.
Interrupting ISS's Kiviuq watch, UVIS turned to the distant star Kappa Orionis, as it entered slowly into occultation behind Saturn's atmosphere, on its dark southern hemisphere. After 70 minutes, ISS took the reins again to observe
Kiviuq for another 18 hours. UVIS and VIMS rode along for both Kiviuq observations.
The observations of Kiviuq covered about 80 percent of the little object's rotation, and were intended to improve knowledge of its shape, sidereal rotation period, and pole-axis orientation. Kiviuq is unusual among Saturn's irregular
moons because its light-curve has a large amplitude, indicating that Kiviuq is a prolate object, and possibly a contact-binary or even a binary moon.
Here is a remarkable time-lapse view of Saturn's south polar region and wisps of aurora, moving as the planet rotates. The dark planet is seen in the upper half of the image, and background stars streak by in the bottom part, which
is "above" the planet: https://saturn.jpl.nasa.gov/resources/7720.
A Saturn "noodle" of images was selected as today's NASA Astronomy Picture of the Day: https://apod.nasa.gov/apod/ap170728.html.
Saturday, July 29 (DOY 210)
Early today Cassini coasted through apoapsis, the highest and slowest part of its orbit of Saturn. This marked the start of its Saturn orbit #286.
As soon as the long ISS observation of Kiviuq was done, it was time for Kappa Orionis to re-emerge from behind Saturn's atmosphere, this time in the sunlit north. UVIS watched it for another 70 minutes as it re-appeared. This blue
star is well known as the lower-left star in the constellation Orion's main quadrangle. It can be seen beginning to egress from Saturn occultation, in this simulated view from Cassini: https://go.nasa.gov/2uS8j6b.
The two stellar occultation observations allowed Cassini to measure vertical profiles of temperature, and some hydrocarbon species, in Saturn's largely unexplored thermosphere. They also allowed the use of latitude/pressure distribution
of photochemical tracers to assess the circulation and photochemical processes in the thermosphere.
ISS, CIRS and VIMS performed another 90-minute observation in the Titan monitoring campaign, this time from a range of 1.8 million km.
Sunday, July 30 (DOY 211)
Cassini coasted north of Saturn's equatorial ring plane, and UVIS began a 6.5-hour observation of the planet's northern aurora, concentrating on parts of the aurora on Saturn's night side. Meanwhile, the Magnetospheric and Plasma
Science (MAPS) instruments were making in-situ measurements in the vicinity of magnetic field lines that were feeding the auroral dynamics.
Following this, CIRS observed the Cassini Division, which is between Saturn's broad A ring and the optically dense B ring. This four-hour observation studied azimuthal variations in the thermal emission from particles in that area.
The other telescopic, Optical Remote-Sensing (ORS) instruments, ISS, UVIS, and VIMS, rode along.
Monday, July 31 (DOY 212)
Continuing inbound from apoapsis, ISS spent 2.3 hours observing known "propellers" (https://go.nasa.gov/2mMff0f) in the broad, sunlit A ring. Propellers' orbits are known to change on many timescales. Repeatedly measuring their positions
helps understand the causes of this behavior. Propellers in Saturn’s rings are the only objects ever to have had their orbits tracked while embedded in a disk, which is a configuration analogous to planetesimals in early solar systems.
Cassini rocked back and forth for nearly 15 hours today, slewing UVIS's field of view across Saturn's northern auroral zone. These observations provided a clear view of the auroras at close range while the spacecraft continued gathering
speed and soaring ever closer to Saturn. The results were some of Cassini's highest-resolution Saturn auroral observations, capturing multiple arcs and spots of ultraviolet emissions; they included a search for satellite "footprints" where the magnetic field
lines connect some of Saturn's moons with the planet.
Last Saturn summer, which began over 30 Earth-years ago, the two Voyager spacecraft had already made their fleeting observations of the ringed gas giant. This summer, it's Cassini's turn, while sunlight bathes Saturn's northern hemisphere.
Today's featured image shows off the planet's bright northern hemisphere: https://saturn.jpl.nasa.gov/resources/7719.
Astrobiologists are intrigued with the discovery of acrylonitrile molecules, also known as vinyl cyanide, in the atmosphere of Saturn's moon Titan. Today's news feature has the story: https://saturn.jpl.nasa.gov/news/3095/nasa-finds-moon-of-saturn-has-chemical-that-could-form-membranes.
Tuesday, Aug. 1 (DOY 213)
Another close-in ring-plane crossing and periapsis passage occurred today, while the spacecraft maintained a "cold" attitude to benefit infrared observations. Cassini was not scheduled to be in communication via the DSN during this
16th proximal plunge, but it would be tracked again some 19 hours later. Just before periapsis, VIMS began a 4.3-hour observation of Saturn's southern aurora, with CIRS and UVIS riding along. This was the mission's closest auroral observation for VIMS; it
provided important coverage of the long-term variability of Saturn's aurora. Also during this passage, the Radio and Plasma Wave Science instrument (RPWS) made observations to characterize the Saturn Kilometric Radiation source region.
Three hours past the periapsis plunge, CIRS led the other ORS instruments for nearly seven hours in an observation of the sunlit face of Saturn's small but active moon Enceladus. The special observation provides CIRS its final opportunity
to determine Enceladus' endogenic heat flow, and to monitor the spatial and temporal variability of the enigmatic south-polar plume of ejecta.
Next, ISS took control of the spacecraft's pointing, and turned to Saturn's large icy moon Dione for the next five hours. The other ORS instruments rode along. All the telemetry data from observations that began before today's periapsis
passage waited aboard Cassini's solid-state recorders, to be played back early the following day.
The DSN communicated with and tracked Cassini on six occasions this week, using stations in California, Spain, and Australia. A total of 1,155 individual commands were uplinked, and about 1,720 megabytes of science and engineering
telemetry data were downlinked and captured at rates as high as 142,201 bits per second.
Wrap up:
Cassini is executing its set of 22 Grand Finale Proximal orbits, which have a period of 6.5 days, in a plane inclined 61.8 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 July 31 using the 70-meter diameter DSN 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 https://saturn.jpl.nasa.gov/anomalies.
This illustration shows Cassini's path up to mid-day Aug. 1: https://go.nasa.gov/2r7Y0dI.
The countdown clock in Mission Control shows 44 days until the end of the Mission.
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This page offers all the details of the Mission's ending: <https://saturn.jpl.nasa.gov/mission/grand-finale/overview >
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Milestones spanning the whole orbital tour are listed here:
< https://saturn.jpl.nasa.gov/mission/saturn-tour/tour-dates/>
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Information on the present position and speed of the Cassini spacecraft may be found on the "Present Position" page at:
<https://saturn.jpl.nasa.gov/mission/saturn-tour/where-is-cassini-now/>
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