Cassini executed another close encounter with Titan this week: T-123 was one of the few remaining in the mission. At some 5,000 kilometers in diameter, this largest of Saturn's moons surpasses the planet Mercury in size. In addition to having its surface and thick atmosphere scrutinized again at close range, massive Titan lent exactly the gravity-assist bump the mission planners had prescribed.
Wednesday, Sept. 21 (DOY 265)
Cassini began the week by observing Saturn's planet-like moon Titan for 90 minutes while the Imaging Science Subsystem (ISS), the Composite Infrared Spectrometer (CIRS) and the Visible and Infrared Mapping Spectrometer (VIMS) monitored Titan's weather from a distance of 2.3 million km. This observation was repeated on Friday. Next, ISS made a 50-minute satellite orbit observation, looking for small objects near the planet. Without missing a beat, ISS and VIMS then looked over to the planet to make a two-minute storm-watch observation. When this was done, VIMS, CIRS and the Ultraviolet Imaging Spectrograph (UVIS) observed the sunlit side of Saturn's rings for six hours, adding to Cassini's overall coverage of the rings from varying latitudes and lighting phase angles. This observation was repeated during 7.6 hours of observations on Friday. Finally today, CIRS made a four-hour observation of the ring system from high above them; the viewing angle is simulated here: http://go.nasa.gov/2d4ZAWk .
Thursday, Sept. 22 (DOY 266)
ISS watched Saturn's narrow, dynamic F ring for 15 hours, which was enough time for any one F-ring particle to whip completely around its 880,000 km-long racetrack just outside the main rings; CIRS rode along.
This evening, the Cassini Project Scientist and the Program Manager joined forces to present a public lecture at JPL in the Von Kármán series, titled "Revealing Saturn: Cassini Science Highlights and the Grand Finale." A video replay of this excellent talk is linked from this page: http://go.nasa.gov/2d52TNv .
Friday, Sept. 23 (DOY 267)
During a routine tracking and communications session with the Deep Space Network, Cassini received commands that the flight team created near real time, to execute Orbit Trim Maneuver (OTM)-460. In response, the spacecraft turned and fired its small hydrazine-fed rocket thrusters for 21.5 seconds. This provided the change in the spacecraft's velocity of 24 millimeters per second to make the required fine adjustment to Cassini's flight path on approach to the upcoming Titan encounter.
Using OTM-460 and with buy-in from across the flight team, the Navigation team implemented a small change to the previously planned target for the T-123 flyby. The change amounted to only a few kilometers at Titan, and would be almost insignificant for the T-123 encounter itself. Nonetheless Cassini's science teams evaluated the new plans to see whether any of their observations would need to be updated.
The payoff was well worth the effort. By meticulously planning ahead, navigators determined that it would mean significant downstream improvement by keeping Cassini's actual trajectory closer to the "reference trajectory," or flight plan. This would conserve Cassini's precious velocity-changing capability by nearly 1,000 millimeters per second overall, considering the OTMs required for the upcoming T-124, T-125, and T-126 targeted Titan flybys.
Cassini resumed its science-data taking after the OTM. VIMS occupied the driver's seat for the next 7.5 hours, first to observe a stellar occultation by the rings, as the bright red star Alpha Scorpii, also known as Antares, appeared to move behind the entire ring system, going from the planet outwards. This helped measure the structure of the rings on a fairly small scale. All of the above-mentioned optical remote-sensing (ORS) instruments rode along with VIMS.
Next, VIMS led ORS, targeting the bright spot at the "zero-phase point" directly opposite the Sun, as it passed across the rings. This kind of observation is diagnostic of the small-scale structure of the ring particles.
Saturday, Sept. 24 (DOY 268)
VIMS watched the red star X Ophiuchi cut behind the rings while the spacecraft traveled southward, just after crossing through the ring plane. ISS and CIRS rode along. This stellar occultation was ideal for highlighting structures in relatively faint rings such as the C ring.
Next, ISS took 105 minutes to image Saturn's small moons Polydeuces and Prometheus, with the other ORS instruments riding along. When this was completed, ISS led UVIS and CIRS for eight hours, examining the outer edge of Saturn's broad A ring, the narrow F ring, and the region in between them. Cassini coasted through periapsis in its Saturn orbit #243 during the observation.
Yet another stellar occultation presented itself, as the red star R Cassiopeiae crossed behind Saturn's whole main ring system and the planet's atmosphere. VIMS led the 3.5-hour observation, with ISS and CIRS riding along.
Next, ISS turned to Saturn's A ring to track some "propellers" (http://go.nasa.gov/17oqTWF) for 1.6 hours, with CIRS riding. CIRS then picked up control, and spent four hours scanning the vicinity of Saturn's shadow on the rings. By measuring the thermal emission in this part of the rings, CIRS can determine how ring-particle temperatures respond when Saturn's shadow abruptly cuts them off from the light and heat of the Sun.
Sunday, Sept. 25 (DOY 269)
CIRS made a four-hour observation of the Cassini Division between Saturn's A and B rings to study the composition of dust that it contains. ISS then turned towards Saturn's small but active moon Enceladus at high-phase illumination angles, which highlight the fine particles in the body's south-polar plume. VIMS and UVIS participated in the five-hour observation.
Finally, VIMS observed a solar occultation. Catching the Sun as it reappeared from behind Saturn's atmosphere, VIMS then tracked the Sun as it passed behind the main rings. UVIS rode along. Solar observations like this help study the size and spatial distribution of the smallest particles in Saturn's rings.
A beautiful view of Saturn was selected to be NASA's Astronomy Picture of the Day today: http://apod.nasa.gov/apod/ap160925.html .
Monday, Sept. 26 (DOY 270)
By mid-day today, Cassini had come closer to Titan than Earth is to our own Moon, and all of the spacecraft's "eyes" turned to Saturn's largest satellite to begin the "train" of encounter observations.
This week's featured image shows a highly detailed view of Saturn, with the rings close to edge-on, and shadows of the inner rings: https://saturn.jpl.nasa.gov/resources/7506 .
Tuesday, Sept. 27 (DOY 271)
During its "elastic collision" with Titan today, Cassini connected with the huge moon's orbital momentum using gravitation, passing at high southern latitudes and slightly ahead of the natural body in its 16-day orbit of the gas giant. The exchange shortened Cassini's own orbit of Saturn from 12 to 9.6 days, and the spacecraft's orbital inclination increased by another 4.9 degrees, to a plane that's now 57.9 degrees off the equatorial ring plane. There will be only three more targeted, orbit-shaping flybys before Titan gives Cassini its final "nudge to destruction" next September. Today's T-123 encounter is described here: https://saturn.jpl.nasa.gov/news/2937/titan-t-123-studying-titan-with-two-kinds-of-eyes .
This week's news feature compiles a summary of everything Cassini has learned about Titan during the mission so far: https://saturn.jpl.nasa.gov/science/titan .
The Deep Space Network communicated with and tracked Cassini five times this week, using stations in Australia. A total of 136 individual commands were uplinked, and about 1,630 megabytes of telemetry data were downlinked and captured at rates as high as 124,426 bits per second.
Cassini is orbiting Saturn with a period of 9.6 days in a plane inclined 57.9 degrees from the planet's equatorial plane. The most recent spacecraft tracking and telemetry data were obtained on Sept. 28, using the 70-meter diameter DSN station 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.
Cassini's path up to mid-day Sept. 27 is illustrated here; notice that Cassini has just passed by Titan in the illustration: http://go.nasa.gov/2cEsL21 .
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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