Saturn and its majestic rings

distance. The Saturn system has been Cassini's home for 13 years. › Full image and caption

Cassini made its final orbit of Saturn this week. It climbed, slowing through apoapsis on Tuesday Sept. 5. From there it plunged more than a million kilometers to speed through the ring plane and periapsis on Saturday. The spacecraft then coasted all the way back up to apoapsis again by Tuesday Sept. 12, a day after its final Titan flyby.

It's all downhill from there: Cassini will burn into Saturn's atmosphere three days later, before ever crossing the ring plane again.

Wednesday, Sept. 6 (DOY 249)

Having passed apoapsis on the previous day, Cassini once again had an astounding view of Saturn, its ring system, and moons from a good distance away; this graphic illustrates the viewing geometry: https://go.nasa.gov/2h29ddw. For the final time, the Imaging Science Subsystem (ISS) examined Saturn's narrow F ring, which is the constantly changing strand just outside the main rings. All the other telescopic instruments participated: The Ultraviolet Imaging Spectrograph (UVIS), the Visible and Infrared Mapping Spectrometer (VIMS), and the Composite Infrared Spectrometer (CIRS). Their observation lasted 14 hours 55 minutes, which is just enough time for an F-ring particle to make one whole revolution around Saturn.

Thursday, Sept. 7 (DOY 250)

Cassini used its reaction wheels to rotate and point to Titan for 90 minutes to monitor the weather on this planet-like moon of Saturn. While ISS tracked, Titan was moving closer, towards one final encounter with Cassini several days later. CIRS and VIMS rode along.

Next, CIRS took control of pointing to observe Saturn's rings for eight hours, with all the other telescopic, Optical Remote-Sensing (ORS) instruments also observing. ISS then spent 2.2 hours looking at propellers in the rings (https://saturn.jpl.nasa.gov/resources/7611). CIRS and UVIS rode along this time.

Friday, Sept. 8 (DOY 251)

While Cassini was closing in on Saturn, halfway down from apoapsis, it made a clever "one-two punch" pair of observations. First, CIRS stared straight down at a particular latitude on the planet, and took data for two hours. About 11 hours later, VIMS observed the bright star Gamma Crucis while it was being occulted by that same patch of Saturn's atmosphere, this time after it had rotated onto the planet's limb; CIRS rode along for the 1.3-hour stellar occultation of Saturn. In between these, VIMS had taken 6.5 hours to watch Saturn's north pole one final time, while the other ORS instruments rode along. It also observed Gamma Crucis for 1.6 hours as the star passed behind the rings on its way into the atmosphere; again, CIRS rode along.

Throughout this period, the Magnetospheric and Plasma Science (MAPS) instruments continuously collected valuable data about the Saturn environment. The Radio and Plasma Wave Science instrument (RPWS) observed the inner magnetosphere, the auroral magnetosphere, and Saturn Kilometric Radiation (SKR) source regions while Cassini flew in near Saturn's north pole.

Late in the day, the spacecraft switched its attitude control from the fine-control Reaction Wheel Assembly (RWA) to the stronger Reaction Control System (RCS) thrusters, in order to maintain control authority against the drag from Saturn’s upper atmosphere.

Saturday, Sept. 9 (DOY 252)

Cassini successfully completed its 20-second and final ring-plane passage between Saturn and the D ring today; it sailed through periapsis about 5 minutes later. This was the fifth of Cassini’s “Final Five” ring plane crossings where, near periapsis, the spacecraft briefly passes through the upper reaches of Saturn’s atmosphere. The spacecraft was oriented to favor INMS for direct atmosphere sampling. Cassini’s Radar observed using its passive microwave-radiometer mode. Atmospheric torques were easily handled by the attitude control system.

Just as during last week's proximal encounter, as soon as the spacecraft switched back to the RWAs for attitude control, UVIS tracked Saturn's southern aurora for a little more than eight hours; ISS rode along for the second half of the observation.

When Cassini turned its high-gain antenna dish to Earth for a routine session of radiometric tracking and two-way digital communications with the Deep Space Network (DSN), it happened to be the final time a Madrid DSN station would ever support Cassini. At the end of this support, the Cassini Real-time Operations Team acknowledged the Madrid DSN crew for their accurate and dedicated contributions over the past decades of Cassini's flight, and thanked them.

Sunday, Sept. 10 (DOY 253)

For two hours, ISS monitored Titan’s atmosphere again, with CIRS and VIMS riding. CIRS then led an 8.6-hour study of the composition of Saturn’s atmosphere, with UVIS and VIMS riding along.

Mid-day today, VIMS took the reins and pointed the spacecraft. It used the nine-hour opportunity, with CIRS riding, to create a mosaic of the full Saturn disk. Mostly the planet's night side was visible, crowned by a thin, bright crescent in the north.

Monday, Sept. 11 (DOY 254)

Titan came around to give Cassini its fatal nudge today. Though the distance between them was as much as 119,000 km, their gravitational interaction made all the difference. The "elastic collision" decreased Cassini's orbital momentum, imparting a decrease in the spacecraft's velocity (relative to Saturn) of 29 meters per second. With Cassini out near apoapsis, this was enough of a nudge to ensure atmospheric entry, and destruction, on its next periapsis.

This illustration shows Cassini having just passed Titan for the final time (motions are counterclockwise in this view). The fact that the spacecraft passed Titan on that moon's leading side in its orbit of Saturn meant that the encounter would remove momentum from Cassini: https://go.nasa.gov/2qCSkoN.

Cassini's instruments took ultimate advantage of this final Titan flyby, and spent more than 40 hours examining the atmosphere-enshrouded moon. In addition to the suite of ORS instruments, the Cassini’s Radar used its active altimeter mode in search of Arecibo-like specular reflections from the surface. Raw images of the lakes from the final flyby are now available at Cassini’s raw image site: https://saturn.jpl.nasa.gov/galleries/raw-images.

The Cassini Project Science Group meeting #73 began in Pasadena today; it will continue through Friday, Sept. 15.

There were several items of interest published on the Cassini website today and on subsequent days:

A story about the last nudge from Titan: https://saturn.jpl.nasa.gov/news/3116/cassini-makes-its-goodbye-kiss-flyby-of-titan.

An essential guide to every part of Cassini's final plunge: https://saturn.jpl.nasa.gov/news/3119/cassini-spacecraft-makes-its-final-approach-to-saturn.

Enceladus matters: https://saturn.jpl.nasa.gov/news/3084/nine-ways-cassini-matters-no-1.

Though it may not be possible to see Cassini's meteoric entry into Saturn on Sept. 15 from Earth, some serious telescopes will be watching, just in case they can catch any evidence from the event: https://saturn.jpl.nasa.gov/news/3114/how-two-ground-based-telescopes-support-nasas-cassini-mission.

For anyone else who intends to train any sort of telescope on Saturn, here's how to find the milky-white planet in the sky: https://saturn.jpl.nasa.gov/resources/7768.

NASA's Astronomy Picture of the Day for today features a 2.5-minute video compilation of Saturn and some of its moons, taken by Cassini, and set to the well-known Adagio in G minor: https://apod.nasa.gov/apod/ap170911.html.

Tuesday, Sept. 12 (DOY 255)

Cassini finished up its flyby observations of Titan, and then spent the remaining days of the Mission making its final observations of Saturn, Enceladus, Titan, and the rings. On Sept. 15 Cassini would switch into a special real-time mode to send data from the in-situ science instruments to Earth continuously until the last second.

Today's featured image shows a natural-color view of some beautiful vertical structure in Saturn's clouds at close range: https://saturn.jpl.nasa.gov/resources/7778.

There's a new party game. It's called “Pin the Cassini on Saturn." Along with some other such games, it was invented by a young fan for his "Goodbye Cassini" party: https://saturn.jpl.nasa.gov/news/3115/meet-the-6-year-old-hosting-a-goodbye-party-for-cassini.

The story of Cassini's launch is being recounted in a series of stories: https://saturn.jpl.nasa.gov/news/3118/t-minus-10-years-til-liftoff.

The DSN communicated with and tracked Cassini on six occasions this week, using stations in California, Spain, and Australia. A total of 365 individual commands were uplinked, and about 1,800 megabytes of science and engineering telemetry data were downlinked and captured at rates as high as 124,426 bits per second.

Wrap up:

Cassini has executed the last of 22 Grand Finale Proximal orbits, which had a period of 6.5 days, in a plane inclined 61.7 degrees from the planet's equatorial plane. Each orbit stretched out to an apoapsis altitude of about 1,272,000 km from Saturn, where the spacecraft's planet-relative speed was around 6,000 km/hr. At periapsis, the distance shrinks to about 1700 km above Saturn's visible atmosphere (for reference, Saturn is about 120,660 km in diameter), and the speed was around 123,000 km/hr.

The most recent spacecraft tracking and telemetry data were obtained on Sept. 13 using the 70-meter diameter DSN station at Canberra, 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 athttps://saturn.jpl.nasa.gov/anomalies.

This illustration shows Cassini's path. Aside from the close-up for Sept. 11 (https://go.nasa.gov/2qCSkoN), this week's path is very similar to last week's: https://saturn.jpl.nasa.gov/news/3112/cassini-significant-events-83017-90517.

The Grand Finale Toolkit is an extensive resource: https://saturn.jpl.nasa.gov/mission/grand-finale/overview.

The countdown clock in Mission Control shows two 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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