"We're about to begin the final chapter of this remarkable story, the #GrandFinale at Saturn." Cassini's 20-watt transmitter, whose invisible radio beam has been in touch with Earth since 1997, is reverently called "the tweeta." That TWTA (Traveling-Wave Tube Amplifier), though, wasn't the source of this week's momentous social-media "tweet." The Twitter announcement was about a very special news briefing. It concluded with, "Details: go.nasa.gov/2oFzFci .” The four-minute video linked there is a must-see.
At the briefing, NASA's exuberant Director of Planetary Science joined the Cassini Program Manager, the Project Scientist, and one of the spacecraft engineers for 60 minutes on April 4. The news: a grand adventure and new discoveries are in line. After completing the current set of F-ring grazing orbits, 7.2 days each, a flyby of Titan will flip the switch on April 22. Then, Cassini's Grand Finale orbits will begin with a brand-new trajectory, which will take the spacecraft on 22 trips, plunging at very high speed between Saturn's rings and the top of its atmosphere.
Wednesday, March 29 (DOY 088)
The Ion and Neutral Mass Spectrometer (INMS) was in position to make direct measurements of ring particles for two hours during today's plunge southward just outside the F ring. Periapsis passage came right on the heels of INMS's observation, when the spacecraft turned to point the Composite Infrared Spectrometer (CIRS) towards Saturn's small, enigmatic moon Enceladus. Two weeks ago, Cassini had received commands to refine the pointing for this special observation. Those commands were waiting until today, when they ensured that Enceladus would appear right in the instruments' fields of view. All the other optical instruments participated: the Imaging Science Subsystem (ISS), the Visible and Infrared Mapping Spectrometer (VIMS), and the Ultraviolet Imaging Spectrograph (UVIS). They took data that will measure temporal variations in Enceladus's heat, on short time scales and according to the active moon's orbital position.
Not to waste any time, VIMS took command to make four consecutive observations of Saturn's southern latitudes totaling 12.7 hours, with CIRS and UVIS riding along.
Thursday, March 30 (DOY 089)
VIMS and UVIS pointed their solar ports to the Sun while our star became occulted by Saturn's rings over the course of nearly two hours. The observation measured the size and spatial distributions of the smallest particles in the rings. Next, VIMS and CIRS spent four hours mapping Saturn's southern hemisphere.
Friday, March 31 (DOY 090)
Saturn's bright crescent limb was the next target. ISS led UVIS and VIMS for four hours looking back towards the planet. The image linked below, under April 3, is similar to ISS's view today; it shows an eerie, though majestic, swath of rings protruding into the night-side of the giant planet with the brighter, narrow F ring clearly discernible.
Cassini's freshly updated Enceladus vector came into play again today, when all the ORS instruments spent nearly seven hours examining Enceladus's icy plume. While it was backlit by the Sun, the plume's fine material shone brightly in forward-scattered light.
After the observation, Cassini turned its high-gain antenna to Earth for a routine session of two-way communications and tracking with the 70-meter diameter Deep Space Network (DSN) station in Australia. The important Enceladus observations (for which pointing had been updated) were captured as telemetry coming down from Cassini. In a flash, the digital data traveled to JPL, where the ISS images were published online in raw-image form, and were distributed to the Cassini Imaging Science team, which is headquartered at the Space Science Institute in Boulder, Colorado. The plume itself is difficult to see in this sample raw image, given that the data has not yet been validated or calibrated by the science team: https://saturn.jpl.nasa.gov/raw_images/410373 . (The more adventurous reader can download the raw image, use a photo viewing program to zoom in on Enceladus, and then expand the brightness level to quickly reveal the plume.)
Saturday, April 1 (DOY 091)
CIRS spent 10.3 hours staring at one spot on Saturn today, covering a full planet-rotation, to measure the atmosphere's composition; UVIS and VIMS rode along. The spacecraft passed through apoapsis during the observation, which marked the start of Orbit #268. Then, ISS, CIRS, and VIMS monitored the weather on Saturn's planet-like moon Titan for 90 minutes; this observation repeated on Monday.
Sunday, April 2 (DOY 092)
UVIS, CIRS, and VIMS spent nearly 12.5 hours studying Saturn's sunlit northern auroral oval.
Monday, April 3 (DOY 093)
Today, CIRS turned to stare at the sunlit side of the Cassini Division between Saturn's A and B rings, which, despite its name and appearance, is not devoid of ring material. Moderate-wavelength-resolution spectra in the thermal infrared from this activity will help constrain knowledge of the composition and structure of this region of Saturn's main ring system.
Cassini scientists began attending the fourth “Titan Through Time” workshop at the NASA Goddard Space Flight Center. The three-day workshop aimed to bring together a diverse spectrum of researchers working on all aspects of Titan science – Cassini scientists, laboratory investigators, ground-based observers and theoretical modelers – to focus interdisciplinary attention on solving the "big questions" related to Titan's origin, evolution, and fate.
A beautiful image of a crescent Saturn and its rings is featured from early in the F-ring orbits: https://saturn.jpl.nasa.gov/resources/7626 .
NASA’s Astronomy Picture of the Day featured a stunning image of Saturn today: https://apod.nasa.gov/apod/ap170403.html .
Tuesday, April 4 (DOY 094)
ISS spent 8.5 hours acquiring images of Saturn's F ring at medium resolution, to study its dynamics and the evolution of its intriguing and complicated structures; CIRS and VIMS rode along. Those instruments then targeted the red star lambda Velorum, and tracked its 4.7-hour occultation behind Saturn's rings until it disappeared into the planet's limb. Following the VIMS stellar occultation, CIRS targeted the location on Saturn's limb at which another star will be occulted on the following day.
The DSN communicated with and tracked Cassini on six occasions this week, using stations in Australia and California. A total of 51 individual commands were uplinked, and about 1,690 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 F-ring-grazing orbits of Saturn, with a period of 7.2 days in a plane inclined 63.6 degrees from the planet's equatorial plane. The 20 orbits are nearly identical, with Cassini's nearest point at about 150,000 kilometers, and farthest point at about 1.28 million km from Saturn. Speeds relative to Saturn at those points (periapsis and apoapsis), are close to 76,150 km per hour and 9,000 km/h respectively.
The most recent spacecraft tracking and telemetry data were obtained on April 4 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 http://saturn.jpl.nasa.gov/anomalies .
The countdown clock in Mission Control shows 164 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 .