On Friday night PDT, Cassini passed slightly ahead of Titan. While the spacecraft connected with the huge moon via gravitation, the two made a little exchange: Cassini gave up a precisely planned amount of its orbital momentum, and Titan gained a little from Cassini (but not enough to measure). That slow-down, while the spacecraft was near apoapsis (the high point in its orbit over 1.2 million kilometers from Saturn), meant that its next periapsis (low point in the inclined elliptical orbit) would occur at a much lower altitude above Saturn than ever before; it will fly even closer to the planet than the rings themselves.
The flyby went fine; Cassini's Radar instrument made its final swath of high-resolution synthetic-aperture imaging of Titan's extraordinary surface, and played it and other data back to Earth. The new lower periapsis, and ring-plane crossing, will occur early on Wednesday April 26. The first radio contact following that historic passage will occur just after midnight Thursday morning via the largest Deep Space Network (DSN) station in California.
Not only did the gravity-assist flyby kick off Cassini's 22 Grand Finale proximal orbits, but it also ensured that the fine spacecraft will burn up like a meteor flying into Saturn, on Sept. 15, satisfying international planetary-protection requirements. The change in Cassini's orbital trajectory is clearly represented by the green arc here: http://go.nasa.gov/2mKUBxh . Milestones and resources pertaining to this new phase of the mission are presented here: https://saturn.jpl.nasa.gov/mission/grand-finale/overview/ .
Wednesday, April 19 (DOY 109)
Cassini careened through periapsis in its orbit #270, and then through Saturn's ring plane today. This completed the Mission's series of 20 F-ring-grazing orbits, at 7.5 days each. On the way in, super-high-resolution optical observations accompanied investigations by the Magnetospheric and Plasma Science (MAPS) instruments. First, the Composite Infrared Spectrometer (CIRS) observed Saturn's A ring for five hours, with the Imaging Science Subsystem (ISS) and the Ultraviolet Imaging Spectrograph (UVIS) riding along.
Next, ISS turned and studied structures in Saturn's C ring known as plateaus, where photometric properties are distinct from adjacent regions. ISS achieved better resolution than 1 km per pixel during the 1.4-hour activity; also participating were the other Optical Remote Sensing (ORS) instruments: CIRS, UVIS, and the Visible and Infrared Mapping Spectrometer (VIMS).
One hour's worth of super-high resolution imaging of the rings came next, as Cassini sped southward towards the ring plane, and ISS led pointing for all the other ORS telescopes. At the time of crossing, the Ion and Neutral Mass Spectrometer (INMS) enjoyed the favored spacecraft orientation for 1.2 hours, so it could make direct-sensing measurements of stray ring dust.
The spacecraft then returned to its task of making super-high-resolution studies of the rings, while the ORS instruments observed, for 0.9 hours. When this was completed, VIMS spent 3.7 hours observing Saturn's dark southern auroral region, with CIRS riding along. ISS finished the day by starting a two-hour observation of A-ring "propellers" (https://go.nasa.gov/2mMff0f); CIRS rode along.
Thursday, April 20 (DOY 110)
CIRS spent six hours leading the other ORS instruments in watching the C-ring plateau. Next, Cassini turned to point its High-Gain Antenna (HGA) towards Earth, and locked onto a continuous frequency-reference radio signal that the DSN had begun transmitting some 79 minutes earlier. For the next six hours, DSN stations in California and Australia, plus a European Space Agency station in Argentina, participated in a Radio Science Occultation experiment watching the spacecraft pass behind Saturn's rings and atmosphere. While the spacecraft was behind Saturn, it kept its HGA pointing to the planet's limb, so that the spacecraft's continuous radio signal would refract through Saturn's upper atmosphere on its way to being recorded on Earth.
Following the Radio Science activity, ISS turned to Titan for two hours and monitored its weather, with CIRS and VIMS riding along.
A little world was captured between Saturn's A ring and F ring in an image featured today. On that world there are saltwater oceans and freshwater rivers and lakes, oxygen in its shallow nitrogen atmosphere, and a high likelihood that intelligent beings exist there. At the same time, some from that world were certainly looking through telescopes of all sorts toward Saturn and its rings. This is the image: https://saturn.jpl.nasa.gov/news/3028/nasa-image-shows-earth-between-the-rings-of-saturn .
Friday, April 21 (DOY 111)
VIMS led CIRS in a one-hour observation of the bright red star Alpha Orionis, also known as Betelgeuse, while the star slipped behind Saturn's atmosphere. After the stellar occultation, ISS turned to image Saturn's innermost ring, the D ring, while it was sunlit from behind. The other ORS instruments rode along on this 1.5-hour observation.
Next, all of Cassini's ORS "eyes" turned to Saturn's largest moon, which by mid-day had come closer to the spacecraft than our own Moon is to Earth. Fifteen hours of continuous Titan observations then ensued.
Saturday, April 22 (DOY 112)
The activities during Cassini's final flyby of Titan, T-126, are described on this page: https://go.nasa.gov/2nFHaTo as well as on this page, which was published following the flyby: https://go.nasa.gov/2q7dzQA .
NASA selected Thursday's featured image of Earth as the Astronomy Picture of the Day today: https://apod.nasa.gov/apod/ap170422.html .
Sunday, April 23 (DOY 113)
Cassini coasted through apoapsis early in the day, having climbed not quite as high from Saturn as it had on its previous twenty orbits. With optics still trained on Titan, CIRS led ORS observations of the atmosphere-enshrouded, planet-like moon for 11.5 more hours.
Cassini scientists began attending the European Geophysical Union General Assembly Meeting this week in Vienna, Austria. Highlights included a press conference on Cassini’s Grand Finale.
Monday, April 24 (DOY 114)
Cassini acted on realtime commands to turn and burn its small rocket thrusters for 165 seconds. The resulting Orbit Trim Maneuver-470 provided a change in velocity of 155 millimeters per second to clean up the spacecraft's trajectory following the T-126 encounter. After the OTM, ISS led CIRS and VIMS for 50 minutes of Titan monitoring.
An image of Saturn's icy moon Tethys, which Cassini collected in January, was featured today: https://saturn.jpl.nasa.gov/resources/7655 .
Tuesday, April 25 (DOY 115)
For 16.5 hours today, the ORS instruments observed Saturn's northern hemisphere. The spacecraft was heading toward its closest periapsis ever, to pass interior to the ring system and soar past the planet's upper atmosphere at close range, on the following day. Some of the sharpest images and spectral observations to date were included in this period. The MAPS instruments continuously collected unique and valuable data about this newly entered region of the Saturn environment. Of note, the Magnetometer (MAG) obtained unique observations of Saturn’s internal magnetic field. Playback of these and other important observations, as well as confirmation that the spacecraft has successfully navigated the new region near Saturn, must wait until early on April 27.
The DSN communicated with and tracked Cassini on 13 occasions this week, using stations in California, Spain, and Australia. A total of 215 individual commands were uplinked, and about 2,275 megabytes of science and engineering telemetry data were downlinked and captured at rates as high as 124,426 bits per second.
Cassini has finished its set of F-ring-grazing orbits of Saturn, and has begun the 22 Proximal orbits of its Grand Finale, which have a period of 6.4 days in a plane inclined 62.4 degrees from the planet's equatorial plane.
The most recent spacecraft tracking and telemetry data were obtained on April 25 using the 70-meter diameter DSN 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/anomalies .
The countdown clock in Mission Control shows 143 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 .