Among the various types of Cassini's scientific observations, two well-rehearsed Radio Science experiments took the spotlight this week. One of them probed Saturn's rings while the other was conducted during a close flyby of Saturn's largest moon; this would be the final "bistatic" observation in the mission, actively probing Titan's surface by reflecting radio-frequency energy off the surface and on towards Earth. The routine types of data from Cassini, which are the familiar telemetry and command, were turned off during these two Radio Science observations. Shutting off the modulation of symbols that represent the ones and zeros of digital data provided a pure, clean signal with which the Cassini Radio Science team could accurately probe the rings, and Titan's surface.
Wednesday, Nov. 9 (DOY 314)
Cassini's Visible and Infrared Mapping Spectrometer (VIMS) spent seven hours controlling the spacecraft's pointing to observe the sunlit side of Saturn's rings, with the Composite Infrared Spectrometer (CIRS) riding along. Next, the Imaging Science Subsystem (ISS) took the reins to observe Saturn's largest moon Titan for 90 minutes. CIRS and VIMS rode along. The planet-like moon was 2.1 million kilometers away from the spacecraft. This observation was repeated on the following day, though without CIRS's participation. Finally today, the Ultraviolet Imaging Spectrograph (UVIS) conducted a Saturn north-polar auroral observation for 3.3 hours with CIRS and VIMS riding along. This observation was repeated on the following day for just over five hours.
Thursday, Nov. 10 (DOY 315)
The stars aligned very fortuitously for UVIS today. Using its high-speed photometer, UVIS tracked the bright blue star Alpha Lupi as it moved behind the rings for 3.4 hours. Then it tracked another star having similar spectral characteristics, Lambda Scorpii, for 3.3 hours while it did the same. But these were no ordinary stellar ring occultations. The apparent motion of the stars, due to Cassini's motion in orbit, came very close to matching the enormous speed of the particles' natural motion in the rings as they race around Saturn. This made for exquisite spatial-resolution observations of ring material, down to meter-level precision. CIRS rode along.
When UVIS let go of the reins, ISS led CIRS, VIMS, and UVIS in a one-hour-long examination of Saturn's big icy moon Tethys, to further study the red streaks seen on its surface in earlier images.
"We had to go back" is the title of an article published today, discussing the compelling scientific questions about Saturn, its rings, its huge moon Titan, smooth Enceladus, and other phenomena. The feature may be found here: https://saturn.jpl.nasa.gov/news/2962/saturn-we-had-to-go-back .
Friday, Nov. 11 (DOY 316)
Having now passed below the ring plane, UVIS turned the spacecraft to begin nearly 20 hours of tracking and observing Saturn's south-polar auroral region, with CIRS, ISS, and VIMS riding along. At the same time, the Magnetometer (MAG) led an Enceladus-auroral footprint observation, with the other Magnetospheric and Plasma Science (MAPS) instruments participating. The sensible "footprint" on Saturn results from electrical interaction with Saturn's small, active moon Enceladus.
Saturday, Nov. 12 (DOY 317)
Thirty-six years ago today the Voyager-1 spacecraft made its closest approach to Saturn during a high-speed flyby before heading out on its long journey towards interstellar space. The data it returned answered many questions about the planet, its rings, moons and magnetosphere. But it opened up many more, which Cassini would be designed to address.
Cassini's unique Magnetospheric Imaging Instrument (MIMI) began a 10-hour observation of the Saturn night-side flux-tube in two parts. "Flux tube" refers to Saturn's electrical interaction with Enceladus. In between the two parts of MIMI's observation, Cassini passed behind Saturn's rings as seen from Earth. The Radio Science team took advantage of this by conducting a ring-occultation experiment similar to the one described in detail on Nov. 2. Today's experiment, though, had the added advantage of support from the European Space Agency's 35-meter diameter radio telescopes in addition to the DSN's.
Sunday, Nov. 13 (DOY 318)
Today's close encounter with Saturn's planet-like moon Titan is described here: https://saturn.jpl.nasa.gov/news/2945/titan-flyby-124-t-124-nov-13-2016 . Of note, this was the Cassini Mission's final Radio Science bistatic observation of the mission. During this experiment, Cassini beamed continuous radio signals towards Titan's surface, where they produced specular reflections that were received successfully on Earth, providing information about that alien surface. Punga Mare was probed for the first time.
As planned, the gravity-assist that Cassini obtained during the encounter reshaped the spacecraft's orbit of Saturn. It cranked up the orbit's inclination from 57.9 to 61.4 degrees off the equatorial plane. It also reduced the spacecraft's orbital period from 9.6 to 8.0 days.
Monday, Nov. 14 (DOY 319)
A train of observations continued outbound from Titan closest approach: ISS acquired global-scale mosaics of Titan's trailing hemisphere at mid-northern latitudes. CIRS made thermal maps to monitor seasonal changes in global temperatures, as well as a surface-temperature map to determine the seasonal changes in sunlight reaching the surface, and the response of the surface to this insolation. In addition, atmospheric limb sounding was performed, allowing measurements of the vertical profile of trace constituent gases, such as hydrocarbons and nitriles.
Before leaving Titan, VIMS rode along with CIRS to look for specular reflections of the Sun on Titan's lake Kraken Mare, to monitor the evolution of the liquid hydrocarbon reservoirs. UVIS rode along with CIRS to measure aerosol scattering and gaseous absorption features in the atmosphere.
Saturn's enigmatic north-polar region, fully in daylight now, appears in this week's featured image, taken through ISS's near-infrared filter: https://saturn.jpl.nasa.gov/resources/7552 .
The Cassini Project Manager, Project Scientist, Spacecraft Operations Manager, and Radio Science Operations Team Lead visited the DSN complex in Canberra Australia today. They gave presentations there describing the Cassini Grand Finale, and at the Australian National University (also in Canberra) during the week.
The Cassini Outreach team has launched the last edition of the “Cassini Scientist for a Day” essay contest. For further information, go to https://solarsystem.nasa.gov/kids/scientist-for-a-day/ .
Tuesday, Nov. 15 (DOY 320)
ISS and VIMS monitored Titan for more than nine hours today as it receded, tracking the evolution of clouds in the body's northern hemisphere, looking for possible seasonal changes as northern summer arrives on Titan. During this activity, Cassini coasted through apoapsis, marking the start of Saturn Orbit #249. It had reached an altitude of 1.33 million km and slowed to 10,286 km per hour with respect to the planet.
Late in the day, one of the 34-meter diameter DSN stations in California was carrying out routine two-way digital communications, and gathering radiometric tracking data for navigation, across the span of 1,639,000,000 km. Suddenly and unexpectedly, Cassini's radio signal disappeared. After checking to rule out any ground-system problem, it was clear that Cassini had anomalously stopped transmitting.
While hurried hallway conferences convened, the voice-net soon crackled with the welcome news that the DSN station had "telemetry back in lock" from the distant robot. Upon analysis, it became apparent that the spacecraft's Solid-State Power Switch (SSPS) had experienced an upset, probably caused when a cosmic-ray particle slammed through one of its transistors. This had caused electrical power to the spacecraft's transmitter to shut off. On-board, a fault-protection algorithm had faithfully recognized the condition, brought the switch to the "RESET" state, paused, and then set it back to the "ON" state, restoring communications. This amounted to the 38th time an SSPS trip occurred on Cassini; only three of them, though, ever have affected this transmitter.
The Deep Space Network (DSN) communicated with and tracked Cassini 13 times this week, using stations in Spain, Australia and California. European Space Agency stations in Argentina and Australia participated in the Radio Science ring occultation experiment on Saturday. A total of 9,563 individual commands were uplinked, and about 1,690 megabytes of telemetry data were downlinked and captured at rates as high as 110,601 bits per second.
Cassini is orbiting Saturn with a period of eight days in a plane inclined 61.4 degrees from the planet's equatorial plane. The most recent spacecraft tracking and telemetry data were obtained on Nov. 16, using one of the 34-meter diameter DSN stations in 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 at http://saturn.jpl.nasa.gov/anomalies .