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Cassini has now completed 20, out of 22, of its daring plunges through the empty space between Saturn and its inner rings. It was about seven years ago when the project decided to make use of this extraordinary opportunity.
Mission designers discovered the option and presented it as an elegant way to end the mission at a time when most of the spacecraft's propellant would be used up. Among the many options studied, the Cassini Project Science Group judged the Solstice Mission
as by far the most scientifically rewarding. The proximal plunges are proving to be an exciting, and very scientifically productive time, an almost new (though short-lived) mission for Cassini. All the commands are now on board to squeeze out the best science
right up to the final seconds, after the Sept. 11 distant Titan flyby puts the spacecraft on a collision course with Saturn. This week, Cassini utilized many of its best remaining observational opportunities.
Wednesday, Aug. 23 (DOY 235)
Cassini's Imaging Science Subsystem (ISS) spent three hours today doing some astronomy that happened to be convenient, by making a mosaic of the Large Magellanic Cloud, a small galaxy that orbits our own Milky Way. The "Cloud" was
named before it was known that separate galaxies exist in nature. Ferdinand Magellan sighted the object on his voyage in 1519, and his writings brought it into common knowledge in the Western world.
Next, ISS tracked one of Saturn's irregular moons. Thrymr is the name given to this small, very dark-surfaced body, which is about 6 kilometers across. It revolves around Saturn backwards, or retrograde, compared to most of Saturn's
other moons, in a highly elliptical, thousand-day orbit that takes it as far as 30,000,000 km from the planet. The name comes from a frost-being in Norse mythology. ISS's goal during this 12-hour observation was to measure Thrymr's rotation rate, which is
already known to be slower than Earth's.
Following the Thrymr observation, the Composite Infrared Spectrometer (CIRS) and the Visible and Infrared Mapping Spectrometer (VIMS) joined ISS to monitor the weather on Saturn's huge, planet-like moon Titan. Shortly after this was
done, Cassini floated through apoapsis, the slowest and most distant part of its orbit, relative to Saturn. This marked the start of Cassini's Orbit #290.
Commands went up to Cassini today to make a small timing adjustment to the block of observations near the upcoming ring-plane crossing. After a round-trip time of 2 hours 42 minutes at the speed of light, telemetry confirmed that
the commands had been received.
The artists who created most of the artwork on the Cassini website over the years have produced a light-hearted series of posters about Cassini's final days: https://saturn.jpl.nasa.gov/resources/7763.
Thursday, Aug. 24 (DOY 236)
Looking "down" to Saturn from a million km away, the Ultraviolet Imaging Spectrograph (UVIS) began a 21-hour study of the planet's northern auroral zone; VIMS rode along.
Friday, Aug. 25 (DOY 237)
Late in the day, CIRS began mapping Saturn’s northern hemisphere at far-infrared wavelengths, to determine temperatures in the upper troposphere and tropopause; UVIS rode along.
Today marks the 28th anniversary of the Voyager-2 Spacecraft's close encounter with the planet Neptune. Cassini paused in its observations two weeks ago to capture this much more distant view of the beautiful blue planet that Voyager
saw: https://saturn.jpl.nasa.gov/news/3107/neptune-from-saturn.
A unique movie has been made from images that Cassini's ISS took during its 19th close-in dive through Saturn's ring plane last week. From inside, the planet-side, the scene shows what Cassini saw while looking outward across the
vast ring system: https://saturn.jpl.nasa.gov/resources/7751.
Saturday, Aug. 26 (DOY 238)
In today's first science activity VIMS, CIRS and UVIS spent eight hours creating a map of Saturn’s equatorial region. Next, CIRS took control for 5.5 hours, and led an observation of Saturn's dark limb at 10 degrees south latitude.
This high-priority activity was the mission's final CIRS limb observation of Saturn. The goal was to derive a vertical profile of temperature from the altitude of the 10-microbar down to 10-millibar level in Saturn's atmosphere; all of the other Optical Remote-Sensing
(ORS) instruments rode along. The viewing geometry is illustrated in this simulation:https://go.nasa.gov/2vsQNsR.
Sunday, Aug. 27 (DOY 239)
Cassini's attitude control system transitioned from fine reaction-wheel control to the stronger rocket-thruster control, for the duration of today's 20th proximal plunge. This was Cassini's third of five occasions in which Cassini's
highly elliptical orbit carries it so low that the spacecraft passes briefly through Saturn’s atmosphere; today's was the deepest passage yet. Ringplane crossing occurred early in the day, four minutes before periapsis. As usual Cassini was coasting, except
that the thrusters did impart some slight changes in the spacecraft's velocity; estimates and measurements of these were carefully modeled by the navigation team, and factored in to their orbit solutions.
In a unique observation, Cassini's Radar instrument operated in its active mode, sending pulses of radio into Saturn's atmosphere during closest approach. Previously, Radar had observed Saturn only in its mode as a passive radiometer.
Today's observation took measurements of Saturn’s natural 2-cm wavelength emission, which is an indication of ammonia concentrations in the atmosphere just below the ammonia cloud base. This enables study of the small-scale structure of Saturn’s atmosphere,
as opposed to regional averages, and it gives insight into weather occurring in and below Saturn’s ammonia clouds.
While Radar was observing during the two hours surrounding periapsis passage, the Ion and Neutral Mass Spectrometer (INMS) was also collecting high priority data, measuring densities of molecular hydrogen (H2), hydrogen and deuterium
(HD), and helium (He) in the neutral exospheres of Saturn and the rings. It may have also measured oxygen-bearing species, depending on their densities. INMS is also sensitive to the hydrogen-ion species H3+ in Saturn’s ionosphere, which is deemed important
to studying Saturn's atmosphere.
Next, having passed into the dark southern hemisphere, UVIS spent seven hours observing Saturn's southern aurora. Finally today, ISS, CIRS and VIMS observed Titan’s atmosphere for 105 minutes.
Monday, Aug. 28 (DOY 240)
Cassini and Titan came within 681,000 km of each other today. This was not close enough to cause any meaningful gravity-assist change to Cassini's orbit, but it did afford time for extensive optical remote sensing. First, CIRS led
ISS and VIMS for 4.2 hours to characterize the buildup of hydrocarbons and nitriles in Titan's atmosphere. Next, ISS led CIRS and VIMS in another 105-minute stare at Titan.
By now, Enceladus had moved into position for a good view of its southern plume of fine ejecta, backlit just right at high-phase. ISS led the other ORS instruments in a 14.3-hour observation. This was the final view of Enceladus's
plume in the Cassini Mission.
The image of the week is a view of Saturn’s turbulent clouds: https://saturn.jpl.nasa.gov/resources/7750.
When the Enceladus observation was done, ISS turned back to Titan to lead CIRS and VIMS in another 5.5-hour examination of its atmosphere.
The long-term perspective has been important in the study of this ringed world in the outer solar system, according to today's feature: https://saturn.jpl.nasa.gov/news/3087/nine-ways-cassini-matters-no-4.
Tuesday, Aug. 29 (DOY 241)
Today, VIMS mapped the full disk of Saturn in an eleven-hour observation, with CIRS riding along.
Viewers of today's Astronomy Picture of the Day are at risk of experiencing considerable nostalgia. It's Cassini's image of Saturn from a decade ago, when it was winter in the north: https://apod.nasa.gov/apod/ap170829.html.
"Cassini has changed the paradigm of where we might look for life," says the Project Scientist in this 4.5-minute video, looking at the Cassini-Huygens Mission from the perspective of its ending. (The music makes it worth the effort
to put on some good headphones.): https://saturn.jpl.nasa.gov/news/3109/cassini-a-saturn-odyssey.
Here is a step-by-step guide to the last days, hours, and minutes of Cassini's mission at Saturn: https://saturn.jpl.nasa.gov/news/3108/saturn-plunge-nears-for-cassini-spacecraft.
The DSN communicated with and tracked Cassini on five occasions this week, using stations in California, Spain, and Australia. A total of 256 individual commands were uplinked, and about 1,950 megabytes of science and engineering
telemetry data were downlinked and captured at rates as high as 124,426 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 61.7 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. 30 using one of the 34-meter diameter DSN stations 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:https://saturn.jpl.nasa.gov/anomalies
This illustration shows Cassini's path up to mid-day Aug. 29: https://go.nasa.gov/2qD5emG
The Grand Finale Toolkit is an extensive resource: https://saturn.jpl.nasa.gov/mission/grand-finale/overview
The countdown clock in Mission Control shows 16 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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