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On Monday of this week, Cassini successfully completed its 18th ringplane passage between Saturn and its innermost rings, out of a total of 22. This passage was the first of these crossings where the spacecraft briefly
passed through the upper reaches of Saturn’s atmosphere. The spacecraft's distant flyby of Titan on Aug. 10 had caused Monday's periapsis to be lowered by about 1,200 kilometers. The Ion and Neutral Mass Spectrometer (INMS) was prime during the passage, and
it performed the first-ever direct sampling of Saturn’s atmosphere. As expected, the spacecraft had to utilize its thrusters to offset the torque imparted by atmospheric drag. The torque was higher than predicted, which was good news about atmospheric density
for INMS, but it remained well within the attitude control system’s control authority.
Cassini's five final proximal passages, plus the final atmospheric entry, are characterized in this news feature: https://saturn.jpl.nasa.gov/news/3098/cassini-to-begin-final-five-orbits-around-saturn.
Wednesday, Aug. 9 (DOY 221)
Cassini spent all day slowly rotating to keep the Imaging Science Subsystem (ISS) trained on one of Saturn's small irregular moons, Kiviuq. This very dark-surfaced object is only about 14 km in diameter; its 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.
Thursday, Aug. 10 (DOY 222)
With the observation of Kiviuq complete, ISS spent half an hour observing the planet Neptune, which happened to be nearby in the sky as Cassini turned from Kiviuq. Its actual distance from Cassini was more than 29 times the average
Sun-Earth distance (this happens to be about the same as Neptune's distance directly from the Sun, due to the orbital positions of the planets). Though Neptune would appear as barely more than one pixel in ISS's narrow-angle camera, this sighting, plus another
one next week, would complete Cassini's survey of the planets in our solar system (excluding Mercury, which Cassini has never imaged). Cassini has even observed the dwarf planet Pluto.
Saturn's largest moon Titan was the next target. ISS led an observation for one hour, with the Composite Infrared Spectrometer (CIRS) and the Visible and Infrared Mapping Spectrometer (VIMS) riding along. CIRS then began a 6.6-hour
observation of that planet-like moon, with VIMS and ISS riding. During this observation Cassini coasted through apoapsis, marking the start of its orbit #288 of Saturn.
After the CIRS-controlled period, ISS took the reins for another hour's observing time on Titan, with CIRS and VIMS riding.
Some of Cassini's serendipitous findings are the subject of today's news feature: https://saturn.jpl.nasa.gov/news/3090/nine-ways-cassini-matters-no-7.
Friday, Aug. 11 (DOY 223)
The distance between Cassini and Titan shrank today to minimum for this orbit, coming within 195,000 km of each other. Taking advantage of the proximity, CIRS and ISS traded control of pointing, watching Titan for another 24.7 hours;
when one led the observation, the other rode along, with additional participation from VIMS and the Ultraviolet Imaging Spectrograph (UVIS).
Even at the relatively large distance to Titan, the gravitational connection provided just the right exchange of orbital momentum to make a small but valuable planned change in the spacecraft's orbit. Flying ahead of Titan in that
body's path around Saturn, the exchange ensured that on Cassini's next periapsis passage, on Monday Aug. 14, the spacecraft would come 1,200 km closer to Saturn than it had on its previous proximal plunge. This would ensure that Cassini's in-situ science instruments
would have a chance to sample the upper atmosphere and ionosphere of the gas-giant planet.
Today's news features an account of Titan as a resource for shaping Cassini's orbits, as well as its compelling and ongoing scientific interest: https://saturn.jpl.nasa.gov/news/3099/cassini-prepares-to-say-goodbye-to-a-true-titan.
Saturday, Aug. 12 (DOY 224)
When ISS finished its Titan observing today, it was time to turn the telescopic instruments to Saturn as it grew closer. The Magnetospheric and Plasma Science (MAPS) instruments continued measuring the local environment, and the optical
instruments spent a total of 4.6 hours viewing Saturn's crescent limb and northern hemisphere, including the aurora. The direct-sensing instruments INMS and the Cosmic Dust Analyzer (CDA) conducted surveys in this region of space as Cassini continued along
its inbound leg.
Sunday, Aug. 13 (DOY 225)
UVIS surveyed the abundance of hydrogen in interplanetary space today while Cassini had its high-gain antenna trained on Earth for tracking and communications. UVIS often rides along to do these surveys while Cassini is in contact
with the Deep Space Network (DSN).
Halfway along its inbound leg now, VIMS, and then CIRS, controlled pointing for a total of 8.6 hours to observe Saturn's northern hemisphere while ISS and UVIS took turns riding along. Some of the benefits of these close-in observations
include measuring wind speeds in the polar vortex and its surrounding hexagonal jet stream, about as far down into Saturn's atmosphere as the 2-bar level.
The Radio and Plasma Wave Science (RPWS) instrument conducted another observation in its campaign to observe the auroral magnetosphere and the source of Saturn's kilometer-wavelength radio emissions.
Monday, Aug. 14 (DOY 226)
First off today came 20 minutes of dead time in Cassini's sequence of stored commands being executed. Commands sent from Earth last Tuesday took effect, and using this dead-time, shifted a six-hour block of observations based on the
latest navigation solution that provided a more accurate estimate of Cassini’s position relative to Saturn and the rings. This Live Moveable Block (LMB) technique is used on proximal passages to reduce timing uncertainties and ensure that the instruments are
observing as precisely as possible during each rapid plunge near Saturn.
Turning to Saturn's fast-approaching rings, and benefitting from the LMB, ISS observed puzzling streaks present in the C ring; the other optical instruments rode along for this 2.5-hour activity. INMS then took the lead for two hours
to cover periapsis passage at this new, lower altitude. Dipping into the upper reaches of Saturn's atmosphere, Cassini's thrusters were employed to maintain attitude control, overcoming torque from the atmospheric drag to hold a steady orientation. INMS measured
the densities of molecular hydrogen, hydrogen and deuterium, helium, and other neutral species and ions.
Cassini's Radar instrument rode along with INMS during the LMB, watching Saturn to obtain high-resolution observations of 2-cm wavelength thermal emission across the various latitudes, looking to spot variations in ammonia concentration
in the atmosphere.
CDA measured dust in the ringplane, and dust that is lofted toward the south by Saturn's magnetic field. RPWS listened for "whistler" signals that reveal lightning. RPWS also counted dust hits on the spacecraft, and obtained high-resolution
measurements of plasma waves at the magnetic equator.
Following the flurry of observations crossing the ringplane and periapsis, UVIS turned and began a 15.5-hour observation of Saturn's southern aurora, with CIRS, VIMS, and ISS riding along. This observation was a rare opportunity to
spatially resolve multiple auroral arcs, observe discrete spots of emission, and search for satellite footprints.
Cassini's instruments are sensitive to wavelengths of light that the human eye cannot discern. Making sense of these invisible colors, of course, relies on the use of false color: https://saturn.jpl.nasa.gov/resources/7721.
Tuesday, Aug. 15 (DOY 227)
UVIS watched as the westernmost star in Orion's belt, Delta Orionis, was occulted by Saturn's F ring, then the A ring, the Cassini Division, and then most of the B ring. In this "tracking occultation," Cassini's speed nearly matched
the motion of the ring particles as they passed in front of the star, resulting in extremely fine resolution of ring material in the azimuthal direction.
Following the stellar occultation, CIRS ended the day by targeting the ansa of Saturn's F ring for 6.6 hours to look for variations in its physical properties, as its particles revolved through the instrument's field of view.
An essay about the complexity of Saturn's rings was featured today: https://saturn.jpl.nasa.gov/news/3089/nine-ways-cassini-matters-no-6.
The DSN communicated with and tracked Cassini on five occasions this week, using stations in California and Australia. A total of 10 individual commands were uplinked, and about 1,920 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 62.0 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. 16 using one of the 34-meter diameter DSN stations 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. 15: https://go.nasa.gov/2qCKH1K.
The countdown clock in Mission Control shows 30 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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