Saturn and its rings

The light of a new day on Saturn illuminates the planet’s wavy cloud patterns and the smooth arcs of the vast rings. Full image and caption

The Cassini flight team continues working at full speed, while knowingly going headlong towards a very sudden interruption. On Friday Sept. 15, the spacecraft will become a meteor high in Saturn's hydrogen atmosphere. This is not the first abrupt ending that some of Cassini's team members have encountered. In 1994, when the Magellan Spacecraft was intentionally destroyed in Venus's thick atmosphere after its successful mission, the local newspaper echoed the words from HAL in Arthur C. Clarke's 2001 A Space Odyssey, when the fictional self-conscious computer was intentionally being destroyed. The article "quoted" Magellan when a person named Dave sent the destructive commands: "Just what do you think you're doing, Dave?" Later, the Galileo Spacecraft was driven into Jupiter's atmosphere, in 2003. Galileo's computers were highly capable (but of course not conscious), and the mission's demise made a splash in the headlines. Today, though, the Cassini flight team enjoys enormous support from millions of members of the public who have been following the mission's scientific discoveries and engineering accomplishments. So, Cassini's Grand Finale will affect everyone who is connected around our blue planet via the internet and social media. This featured article considers the final sequence of commands recently uplinked to Cassini: /news/13078/halfway-home.

Though Cassini will be gone forever, there is no lack of possibility for the next generation of spacefaring robots. Thanks to Cassini, Saturn's moons Enceladus and Titan are now known to be compelling destinations, though no new missions have yet been funded. But thanks largely to the Galileo Mission, Jupiter's moon Europa – another ocean world – awaits a new visitor, as indicated on that mission's website: https://www.jpl.nasa.gov/missions/europa-clipper.

Wednesday, July 5 (DOY 186)

Cassini's Imaging Science Subsystem (ISS) began a week in which it would capture a fairly typical total of 767 images. First, ISS had the spacecraft turn to view Saturn's largest moon. The Composite Infrared Spectrometer (CIRS) and the Visible and Infrared Mapping Spectrometer (VIMS) also observed during this 90-minute installment in the campaign to monitor Titan's atmosphere.

CIRS then spent three hours leading the three other Optical Remote-Sensing (ORS) instruments, which are ISS, VIMS, and the Ultraviolet Imaging Spectrograph (UVIS). Their target was Saturn's ring system, and the primary objective was to see how the rings' thermal emission varies with phase angle and viewing elevation. UVIS then spent nearly four hours recording the occultation of the blue-white star Kappa Canis Majoris as it passed behind the main rings. CIRS took advantage of the opportunity, and rode along mapping the rings at thermal-infrared wavelengths. Finally, late today ISS began a series of ring observations. The first two hours were devoted to a high-resolution color image of a selected part of the rings; CIRS and VIMS rode along.

Thursday, July 6 (DOY 187)

Continuing its ring observations, ISS reacquired and tracked some vaguely S-shaped propeller-like features (https://go.nasa.gov/2pzWU7G) for another two hours. Next, while flying in over the northern, sunlit side of Saturn and its rings, ISS targeted the edge of the broad A ring, the narrow F ring, and the area in-between, studying structures and dynamics in this region for 90 minutes. Images from ISS's narrow-angle camera enjoyed resolutions better than two kilometers per pixel. CIRS rode along with this, and with the propeller observation.

The red background star Gamma Crucis, which is one of the brightest stars in the southern sky, gradually passed behind the whole ring system. and (from Cassini’s perspective) into Saturn's atmosphere, by virtue of the spacecraft's motion. VIMS watched the occultation for 2.5 hours, accompanied by ISS and CIRS.

When the day began, Cassini was little more than halfway through its million-kilometer inbound leg to Saturn. Ever gathering speed, only 9.5 hours later it would whip through periapsis in its Orbit #282. During the remaining two hours before periapsis, UVIS took advantage of the proximity to Saturn's C ring to slowly scan across it, obtaining ultraviolet spectra at high spatial resolution. This will allow for compositional studies of small-scale structures in the C ring. CIRS and ISS rode along.

Four minutes before periapsis, Cassini plunged through Saturn's ring plane for the 12th time, on the planet side of the ring system. During the brief period when impacts were considered most likely, the Radio and Plasma Wave Science instrument (RPWS) "listened" for the impacts of ring particles, which RPWS detects via the plasma clouds generated upon impact.

Cassini turned to point its high-gain antenna to Saturn's rings, and for two hours carried out its third, active, Synthetic-Aperture Radar (SAR) scan of the main rings. SAR observations are sensitive to centimeter-scale and larger particles. Next, the spacecraft turned slightly to face the HGA toward Earth. For the following eight hours, the Radio Science Subsystem (RSS) team used stations of the Deep Space Network (DSN) and the European Space Agency (ESA) to observe while Saturn's rings were occulted by Cassini's three continuous radio signals. This occultation covered the F ring, A ring, Cassini Division, the B ring and the outer portion of the C ring, before tracing back out past the F ring again.

NASA's Astronomy Picture of the Day featured Cassini's images of three of the larger moons that orbit within the confines of Saturn's rings; each one has a belt, presumably of "snow" from the rings, around its equator: https://apod.nasa.gov/apod/ap170706.html.

Friday, July 7 (DOY 188)

With Cassini now south of the ring plane, CIRS spent 6.7 hours observing the region of Saturn's A ring that lies between the 325-km wide Encke gap and the outer edge of the ring. ISS and UVIS rode along while CIRS measured the thermal-infrared emissivity of this unique region.

The direct-sensing instruments were active as well as the remote-sensing, telescopic instruments. The Cosmic Dust Analyzer (CDA) continued a dust survey, and the Magnetometer (MAG) continued a low-rate magnetospheric survey. While outbound from Saturn again, RPWS started an outer magnetosphere survey, the Ion and Neutral Mass Spectrometer (INMS) started an atmospheric and ionic thermal structure survey, and the Magnetospheric Imaging Instrument (MIMI) started another activity.

UVIS took the reins for 5.2 hours, first viewing Saturn's unlit southern auroral oval, then tracking the star bright blue star Epsilon Orionis as it became occulted behind Saturn's unlit limb. After another watch of Saturn’s southern auroral oval, another ingress occultation took place, this time as star Zeta Orionis went behind the planet's unlit limb. UVIS recorded this occultation for 80 minutes. By now the spacecraft had climbed back out to the vicinity of Titan's orbit, wherein CDA began an 11.2-hour survey of E-ring particles.

During a routine DSN communications and tracking session today, the spacecraft received its final sequence of commands. After a communications lag of 76 minutes, telemetry confirmed that the S101 sequence was registered on-board. It will be controlling Cassini's activities for the remainder of the mission.

Saturday, July 8 (DOY 189)

Epsilon Orionis came out of occultation from behind Saturn early today, and UVIS was on point to watch its egress for 1.3 hours. Next, while Cassini kept its HGA pointed to Earth for another routine, seven-hour DSN session, UVIS spent the time making a survey of interplanetary hydrogen. Finally today, ISS began a 10.3-hour observation of Saturn's irregular moon Bebhionn. This object, about 6 km in diameter, orbits Saturn in an inclined ellipse that reaches as far as 25.1 million km from the planet. It might have a binary or contact-binary nature. Bebhionn was named after a goddess in early Irish mythology.

Sunday, July 9 (DOY 190)

Upon gliding through apoapsis today, Cassini began its Orbit #283. INMS started an atmospheric and ionic thermal structure survey, and MIMI began an inbound survey. RPWS also began an outer magnetosphere survey. These were the final commands in the S100 sequence, which had orchestrated Cassini's science and engineering activities for the past seven weeks.

Monday, July 10 (DOY 191)

The first observations commanded from Cassini's on-board, 10-week-long S101 sequence were of Titan, while the spacecraft and the planet-like moon came within 264,000 km of one another. ISS began a period of nearly 29 hours observing Titan, trading control back and forth, to and from CIRS, during the period. When ISS was in control, CIRS rode along; when CIRS was controlling, ISS was riding. VIMS rode along the whole time.

Morning sunlight on Saturn is the featured image this week: /resources/17701.

Tuesday, July 11 (DOY 192)

UVIS interrupted the ongoing Titan observations today for 90 minutes, to observe a stellar occultation of Saturn while the blue star Beta Canis Majoris passed behind the planet's upper atmosphere.

The DSN communicated with and tracked Cassini on 11 occasions this week, using stations in California and Australia. The European Space Agency also contributed ground antenna support one time from Australia. A total of 6,175 individual commands were uplinked, and about 1,425 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 61.9 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 (by comparison, 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 11 using the 70-meter diameter DSN station in Spain. 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.

The countdown clock in Mission Control shows 65 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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<http://saturn.jpl.nasa.gov/news/mailinglistsignup/>
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For comments and questions, please contact Cassini Public Engagement at:
<http://saturn.jpl.nasa.gov/feedback/>
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