Occultations offer huge advantages for learning about an object. When a source of light passes behind Saturn's rings, for example, observing the light's behavior means learning about many characteristics of the rings. If the light is from a star, its pinpoint can reveal minute details in the rings' structure as the star seemingly blinks off and on, grows dimmer and brighter. In a similar situation, watching the Sun light up Saturn's atmosphere from behind, or the atmosphere of its largest moon Titan, provides rich data, including spectral evidence of the atmospheres’ chemical constituents, and measurements of other properties. Cassini has observed stellar occultations and solar occultations at various wavelengths -- ultraviolet, visible, infrared, probing the planet's upper atmosphere, its moons, its rings, and even the icy plume emanating from Enceladus. Cassini itself often takes an active role in occultation experiments, by using its own radio transmitters as sources of "light." This was the case in the typical Radio Science Occultation experiment described here: https://saturn.jpl.nasa.gov/resources/5816 .
The orbits that Cassini is flying now, and the 22 Grand Finale orbits beginning next week, provide opportunities for more and more occultations. These science experiments are populating a library's worth of exquisite needlepoint probes of the Saturn system.
Wednesday, April 12 (DOY 102)
Today's occultation experiment occurred when the bright red star Gamma Crucis went behind Saturn's rings, thanks to Cassini's motion. The Visible and Infrared Mapping Spectrometer (VIMS) observed it for three hours, with the Composite Infrared Spectrometer (CIRS) riding along.
Next, the Imaging Science Subsystem (ISS) targeted the region between Saturn's C and B rings. ISS spent two hours observing, using multiple color filters. CIRS, VIMS, and the Ultraviolet Imaging Spectrograph (UVIS), the rest of Cassini's Optical Remote Sensing (ORS) instruments, participated. The results should help constrain models of how Saturn's rings evolve over time.
Following this, UVIS scanned across Saturn's northern polar region to image the aurorae there; CIRS rode along. Then, just 20 minutes before speeding through periapsis, Cassini flew within 12,000 kilometers of Saturn's diminutive moon Atlas -- it was less than one Earth-diameter away. ISS led all the ORS instruments observing Atlas for 2.6 hours. In all its years at Saturn, this approach was by far the closest to the little moon, which orbits Saturn in the space between the broad A ring and the bright, narrow F ring. The Magnetospheric and Plasma Science (MAPS) instruments were making in-situ measurements this whole time, which included the ring-plane crossing. The Radio and Plasma Wave Science (RPWS) instrument increased its sampling frequency to record the signature of dust impacts on the spacecraft, and to study plasma waves near Saturn's magnetic equator. With high-quality Atlas images stored aboard, ISS then turned back to view the receding A ring from its dark side, and led the ORS instruments for 1.3 hours of close-up observations of structures that scientists have dubbed "propellers" (http://go.nasa.gov/17oqTWF).
Studies of Saturn's aurora continued for another 3.7 hours, as VIMS stared at the planet's dark southern polar regions. The lack of sunlight there made it easier to image aurorae; CIRS rode along. Scientific scrutiny then turned back towards some more propellers, with ISS, CIRS, and UVIS observing. Finally, CIRS began a six-hour study of the unlit side of the A ring, with UVIS riding along. Ring temperatures drop abruptly when the particles pass through Saturn's shadow. In perpetual sunlight now that Saturn is near solstice, particles in the A ring are no longer passing through the shadow, so CIRS scanned along the A ring looking for more subtle variations in thermal emission.
Thursday, April 13 (DOY 103)
The next occultation experiment was jointly observed by VIMS and UVIS for 1.2 hours today, when the Sun set behind the A ring. Shortly after passing behind the Cassini Division, the Sun disappeared behind Saturn from Cassini's vantage point. With the Sun safely blocked, ISS turned to image the faint rings. Such geometries are conducive to studying ring structures that are dominated by small, micron-sized particles, such as Saturn's innermost D ring, and the outlying G and E rings.
The spectacular images of Atlas went right to the Cassini Raw Images website as soon as they came down to Earth (as do all Cassini's images). Here are some of them, with standard captions: https://saturn.jpl.nasa.gov/news/3025/cassini-sees-flying-saucer-moon-atlas-up-close .
News from Cassini about Saturn's moon Enceladus, and related news from the Hubble Space Telescope about Jupiter's moon Europa were the subjects of today's NASA press conference: https://saturn.jpl.nasa.gov/news/3024/nasa-missions-provide-new-insights-into-ocean-worlds-in-our-solar-system .
Three additional articles featured Enceladus on the Cassini website today:
Hydrothermal activity https://saturn.jpl.nasa.gov/news/3023/hydrothermal-activity .
Free samples https://saturn.jpl.nasa.gov/news/3022/free-samples .
The enormous, puzzling E ring https://saturn.jpl.nasa.gov/news/3021/put-a-ring-on-it .
Friday, April 14 (DOY 104)
Today is the birthday of Christiaan Huygens, the polymath who correctly surmised the configuration of Saturn’s rings, and discovered its largest moon Titan in 1655, when he was 26 years old.
CIRS spent three hours with the campaign to create a library of moderate wavelength-resolution spectra of various regions in Saturn's rings; VIMS and UVIS rode along. Later, today's occultation experiment involved VIMS tracking the bright red star Alpha Orionis, also known as Betelgeuse; CIRS rode along. The star made a full radial egress behind Saturn's main rings. This was the last science activity stored in the 10-week-long S98 sequence of commands. The command sequence S99, whose uplink last week was captured on video (https://go.nasa.gov/2oq3jnM), took over control of Cassini for the next seven weeks.
The MAPS instruments continued their direct-sensing surveys as the S99 sequence took effect; the Cosmic Dust Analyzer (CDA) looking at dust, the Ion and Neutral Mass Spectrometer (INMS) measuring the atmospheric and ionospheric thermal structure of Saturn, and the Magnetometer (MAG) and the RPWS performing magnetospheric measurements.
ISS undertook the first of three three-hour observations of Saturn's bright crescent limb today, with UVIS and VIMS riding along. The second one took place on the next day, and the third on Sunday. After today's, there was a 11.2-hour VIMS observation of the Saturn globe as it rotated through one Saturn day; CIRS participated. Cassini's view of the looming gas giant is simulated here: https://go.nasa.gov/2pSJQdd .
Saturday, April 15 (DOY 105)
During today's communications session with the Deep Space Network, the spacecraft was slowly rolling about its longitudinal axis, so that UVIS could conduct an interplanetary hydrogen survey, while CIRS performed a calibration with cold deep space in its field of view. Next, CIRS spent 12 hours making a mid-infrared map of Saturn’s northern hemisphere, to determine upper troposphere and tropopause temperatures.
Sunday, April 16 (DOY 106)
During the CIRS activity, the spacecraft coasted through apoapsis. This marked the start of Cassini's Orbit #270, which will be the final one in its series of 20 F-ring grazing orbits.
None other than Cassini's view of Enceladus was the subject of NASA's Astronomy Picture of the Day today: https://apod.nasa.gov/apod/ap170416.html .
Monday, April 17 (DOY 107)
CIRS began the day with far-infrared mapping of Saturn’s northern hemisphere, again to measure upper troposphere and tropopause temperatures; VIMS rode along with this 22.3-hour observation, taking images about once every hour.
Late in the day, UVIS and VIMS began 12 hours of daytime observations of Saturn's northern auroral region, with CIRS riding along.
Saturn cloud patterns were the subject of today's featured image: https://saturn.jpl.nasa.gov/resources/7645 .
Tuesday, April 18 (DOY 108)
Cassini's final targeted Titan encounter, T-126, is coming up on April 22. This page offers details of the flyby: https://saturn.jpl.nasa.gov/news/3019/titan-flyby-t-126-final-close-encounter-gateway-to-the-grand-finale .
The DSN communicated with and tracked Cassini on six occasions this week, using stations in California and Australia. A total of 153 individual commands were uplinked, and about 1,718 megabytes of science and engineering telemetry data were downlinked and captured at rates as high as 124,426 bits per second.
Cassini is executing its set of F-ring-grazing orbits of Saturn, with a period of 7.2 days in a plane inclined 63.5 degrees from the planet's equatorial plane. The 20 orbits are nearly identical, with Cassini's nearest point at about 150,000 km, and farthest point at about 1.28 million km from Saturn. Speeds relative to Saturn at those points (periapsis and apoapsis), are close to 76,150 km per hour and 9,000 km/h respectively.
The most recent spacecraft tracking and telemetry data were obtained on April 19 using one of the 34-meter diameter DSN stations in California. 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 .
The countdown clock in Mission Control shows 150 days until the end of the Mission.
This page offers all the details of the Mission's ending: https://saturn.jpl.nasa.gov/mission/grand-finale/overview .