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Cassini Significant Events 9/13/17 – 9/19/17

Cassini's path into Saturn's upper atmosphere, with tick marks every 10 seconds
Cassini's path into Saturn's upper atmosphere, with tick marks every 10 seconds.

While millions of people viewed the events at JPL via the Web, including the Cassini Mission's many international partners, the Cassini Spacecraft plunged into Saturn's atmosphere on Friday Sept. 15. The spacecraft was sending back unique science data all the way up to its demise. Thus ended Cassini's enormously successful, 19.9-year mission.

Wednesday, Sept. 13 (DOY 256)

Cassini turned and pointed its Imaging Science Subsystem (ISS) telescopes to Saturn. It spent 2.5 hours making one final mosaic of the planet and its rings. From the viewing geometry shown here,

, one can imagine the spacecraft's final path, consisting of a daring dive over the planet's northern hemisphere, which would end on Saturn's sunward- and earthward-facing side.

Next, ISS turned to Saturn's small, active moon Enceladus for 55 minutes, viewing as the little geyser-world set behind Saturn’s illuminated north pole. When this was done, ISS viewed Titan one last time, observing the weather on Saturn's huge planet-like moon for two hours; the Visible and Infrared Mapping Spectrometer (VIMS) and the Composite Infrared Spectrometer (CIRS) rode along to make their own observations. Titan was at a distance of 730,000 kilometers from Cassini.

Finally today, the Ultraviolet Imaging Spectrograph (UVIS) began one final observation of the aurora on Saturn's north-polar region. This activity would last nearly 19 hours, with CIRS riding along.

Today’s news release offers an illustrated account of the final approach to Saturn:


Thursday, Sept. 14 (DOY 257)

Peggy was ISS's first subject today, for a 40-minute observation; this object is a moonlet that appears to be forming within the outer edge of Saturn's A ring. After this, the spacecraft turned back to Saturn's northern latitudes so that UVIS could spend another 3.5 hours completing its study of the aurora. Next, ISS turned to Saturn's A ring for a two-hour propeller (

) retargeting observation, with CIRS riding along.

For half an hour, VIMS led the other Optical Remote-Sensing instruments, ISS, CIRS, and UVIS, as their telescopes pointed to the area on Saturn which would rotate into position to become the place where Cassini's plunge would end.

These became the final images and other data that Cassini would store on its recorder and then play back to Earth. The spectacular results may be viewed here:


For the final time after nearly 7,300 days in flight, Cassini turned its High-Gain Antenna (HGA) dish towards Earth. The spacecraft's dependable telecommunications subsystem spent nearly 11 hours playing back all the stored data.

During the playback, the Magnetospheric Imaging Instrument (MIMI), took its last energetic neutral images of Saturn’s north pole and inner radiation belts. After a short roll, while continuing to downlink, the CIRS and UVIS fields of view passed across Saturn's main rings, allowing one last look at them.

Telemetry from Cassini propagated through interplanetary space for 83 minutes, and, as usual, the Deep Space Network (DSN) captured every bit. First to view Cassini's signal today was the 70-meter-diameter DSN station at Goldstone, California, and as the hours passed the data rate went up to 124,401 bits per second.

Friday, Sept. 15 (DOY 258)

As Saturn set in the western California sky, the DSN stations in Australia locked onto the spacecraft's signal as Saturn rose above their eastern horizon. And Cassini locked onto the frequency-reference signal coming from the DSN, which (as usual) enabled accurate, continuous radiometric data: the spacecraft radio's Doppler shift (for velocity measurements) and range (distance measurements). About six hours later, Cassini's downlink data rate changed to 27,650 bits per second. It was 1:37 in the morning Pacific Daylight Time. The lower data rate ensured that not only the 70-meter diameter DSN station there in Canberra, Australia, would be able to capture Cassini's telemetry - the ones and zeroes of digital downlink - but also that the 34-meter diameter station there could capture the data, and serve as a backup to the large aperture.

There was another change at the same time, as Cassini continued to faithfully follow the commands it had received months before. Instead of sending telemetry from Cassini's data recorder, the data packets were now being sent directly from the in-situ science instruments. Cassini collected and transmitted home in real time the data from MIMI, the Ion and Neutral Mass Spectrometer (INMS), the Magnetometer (MAG), the Radio and Plasma Wave Science (RPWS) instrument, and the Cosmic Dust Analyzer (CDA). One after another, the live packets of data reported on conditions right outside the spacecraft, as they sensed Saturn's upper atmosphere. The instruments reported on the atmosphere's constituents, and on the changing magnetic field, and the particle environment, while Cassini reached lower and lower altitudes into the gas giant itself. While still functioning normally, Cassini survived to a density level at least twice that of the preceding proximal orbits.

Data continued to flow, and every bit of telemetry was captured at the Australian DSN stations. The data went immediately to JPL, and thence to the science teams and engineering teams. In addition, the European Space Agency's 35-meter diameter antenna, situated in New Norcia, Australia, tracked Cassini's carrier signal. All three of the Australian antennas recorded the signals' Doppler shift, which held further information about the density of Saturn's atmosphere. At the same time, the Radio Science team took the opportunity to conduct an in-situ Saturn ionosphere-occultation observation, using all three of Cassini's radio-frequency bands.

At a Pacific time of 4 hours 55 minutes 46 seconds Cassini's signals were gone. The aerodynamic torques imparted to Cassini from Saturn's atmosphere, growing denser at lower altitudes, had overcome the spacecraft's thrusters' ability to hold the HGA facing towards Earth. As it tumbled out of control, within minutes Cassini had come apart, melted, and vaporized into its host planet.

Saturday, Sept. 16 (DOY 259)

NASA selected Cassini's final image to be the Astronomy Picture of the Day:


The Mission's ending is the subject of this feature:


Here is a feature article on some of the members of the Cassini's flight team as the mission's end nears:


A freely available electronic book of Cassini's results, "The Saturn System through the Eyes of Cassini" has been published:


Monday, Sept. 18 (DOY 261)

This image of Saturn's two-tone moon Iapetus, taken last May, was featured today:


Tuesday, Sept. 19 (DOY 262)

The DSN communicated with and tracked Cassini on six occasions this week, using stations in California and Australia. One individual command was uplinked, and about 1,000 megabytes of science and engineering telemetry data were downlinked and captured at rates as high as 124,426 bits per second. The European Space Agency tracked Cassini on Cassini's final day.

Wrap up:

35 years after the joint mission was first proposed and after 19.9 years in flight, the Cassini-Huygens Mission to Saturn and Titan has concluded.

The most recent spacecraft tracking and telemetry data were obtained on Sept. 15 using the 70-meter diameter DSN station at Canberra, Australia, one of the 34-meter diameter stations there, and the European Space Agency's 35-meter station at New Norcia, Australia.

Until the very end, the spacecraft remained in an excellent state of health with all of its subsystems operating normally except for the instrument issues described at


This page offers all the details of the Mission's ending: <https://saturn.jpl.nasa.gov/mission/grand-finale/overview/>
Milestones spanning the whole orbital tour are listed here:
Information on the present position and speed of the Cassini spacecraft may be found on the "Present Position" page at:
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