Saturn's rings

Cassini zoomed in on Saturn's A ring, revealing narrow, detailed structures that get even finer as the cameras' resolution increases. Even at this level of detail, it is still not fine enough to resolve the individual particles that make up the ring. Full image and caption

Those little moons just will not sit still for a photo. Pan races around Saturn in 13.9 hours, going more than 60,000 kilometers per hour relative to the planet. Enceladus and its geysers, somewhat farther out, orbit the gas giant every 32.8 hours. Ever since the S98 command sequence started clocking out on Cassini, in early February, Enceladus has gone dozens of times around Saturn, and Cassini has completed seven orbits of its own. Will the spacecraft's attitude control system still "know" exactly where to point the telescopes for an important Enceladus observation coming up?
 
The spacecraft maintains built-in knowledge of the direction to find Earth, Saturn, and many of its moons over time, but small uncertainties (that can result in getting a picture of empty space!) do creep in. Early in S98 development, five key observations were identified that might need a realtime update of pointing knowledge. Last week's close-up of Pan was one of them, and so was next week's observation of Enceladus. These called for an update process which, until recently, has been happening only about once in any given 10-week command sequence. Months ago, the Navigation team agreed to generate and deliver some extra orbit-determination solutions, based on their ongoing tracking from Earth, to accomplish the updates. Commands for the Enceladus update were sent to the spacecraft on Friday of this week.
 
Wednesday, March 15 (DOY 074)
 
There is a science team somewhere on Earth for each of Cassini's scientific instruments. Yesterday morning, the engineer who leads the S98 Sequence Implementation Process (SIP) team polled the Optical Remote-Sensing (ORS) instrument teams at their various institutions. The question at hand was about the observation of Enceladus coming up next week; will the pointing be accurate enough? To help them decide, the SIP team sent simulated views of Enceladus with and without the update, showing the optical instruments' fields of view superimposed on (or near) the body. Today, based on the science teams' responses, the Attitude and Articulation Control team delivered tested commands to the SIP team, to make the pointing correction. Without this, the prime instrument's view would be off by 1.6 milliradians. While that might seem rather small, it’s about a quarter of the narrow angle camera’s field of view.
 
Meanwhile in Saturn orbit, the Ultraviolet Imaging Spectrograph (UVIS) had Cassini turn and look back towards the planet as it was beginning to recede. UVIS observed Saturn's dark south-polar region for 5.5 hours, while the Composite Infrared Spectrometer (CIRS) and the Visible and Infrared Mapping Spectrometer (VIMS) rode along.
 
Next, the Imaging Science Subsystem (ISS) took advantage of the high-phase (backlit) illumination of Saturn's fainter rings for 5.8 hours, with CIRS and VIMS riding along.
 
Cassini continued to speed away from Saturn's dark side, and later today the Sun rose from behind the planet. For 2.2 hours VIMS and UVIS observed a solar occultation by Saturn's rings. Finally, the Magnetospheric Imaging Instrument (MIMI) took control of spacecraft's orientation to make 6.2-hour observation of plasma flow within the local electron field.
 
Fizzy ethane-nitrogen slush is the topic of today's news feature. It pertains to Titan's cold lakes: https://go.nasa.gov/2mtNdpx .
 
Thursday, March 16 (DOY 075)
 
Members of the flight team held a Command Approval Meeting (CAM) this morning for the updated Enceladus vector. Each ORS science team telephoned in to the CAM, and the participants reviewed the commanding scheme using an electronic Command-Request Form (eCRF) visible to everyone. The file, containing a total of two individual commands, was approved, and its associated form was signed and placed in line for radiating to the spacecraft on the following day.
 
Out at Saturn, UVIS began an 8.9-hour observation of the planet's dark south-polar region; CIRS and VIMS rode along. Next, after an engineering activity, ISS led CIRS and VIMS in a 90-minute Titan monitoring observation to watch the weather there. The latter would be repeated on Saturday, and again on Sunday.
 
NASA selected Cassini's images of Mimas in Saturn-light for today's Astronomy Picture of the Day: https://apod.nasa.gov/apod/ap170316.html .
 
If ever there were reason to don those red-blue 3-D goggles, it’s this anaglyph of Pan: https://saturn.jpl.nasa.gov/resources/7616 .
 
Friday, March 17 (DOY 076)
 
While Cassini was passing by the orbit of Saturn's largest moon Titan, the Cosmic Dust Analyzer (CDA) spent 10.4 hours studying the population of E-ring particles there. This should help reveal any effects on streams of dust that might be caused by the massive moon.
 
One of the Deep Space Network's (DSN) stations in Australia finished turning its 34-meter diameter dish toward the eastern horizon this morning, just as the first waves of Cassini's continuous radio signal arrived. Then, 25 minutes later, the Cassini Realtime Operations controller at JPL used his command system to send the new vector on an 83-minute journey at the speed of light. Another 83 minutes after that, incoming telemetry showed that the spacecraft had received the commands, and that they were properly poised to take effect on March 29. This very careful process has become quite routine for all sorts or commanding; as of today, there have been 8,140 eCRFs processed.
 
Saturday, March 18 (DOY 077)
 
Reaching the top of its million-mile-high roller-coaster ride (apoapsis), the view from Cassini was stunning. Simulated here, note the familiar background stars of the constellation Orion; bright Sirius is seen south of Saturn: https://go.nasa.gov/2o5w1qY .
 
UVIS turned to Saturn today to observe the planet's northern aurora, starting 26 hours of observations; CIRS rode along. During the ORS observations, the in-situ Magnetospheric Imaging Instrument (MIMI) and the Radio and Plasma Wave Science instrument made a survey of Saturn's outer magnetosphere.
 
Sunday, March 19 (DOY 078)
 
While UVIS conducted a hydrogen survey, RPWS observed the acceleration region of Saturn's auroral effects on the magnetosphere and also made measurements of the Saturn kilometric radiation.
 
Monday, March 20 (DOY 079)
 
From Cassini’s vantage point today, a portion of Saturn's northern regions are in darkness. UVIS, along with ride-along support from CIRS, started a three-hour observation of that dark area. Next, it was time for a non-targeted, distant (870,000 km) encounter with Titan (no propellant is expended to achieve a non-targeted encounter).
 
A close-up of part of Saturn's A ring was the featured image today: https://saturn.jpl.nasa.gov/resources/7617 .
 
Tuesday, March 21 (DOY 080)
 
A little while after closest approach to Titan, having waited until Titan and its atmosphere were basically backlit by the Sun, ISS, CIRS, and VIMS observed the planet-like moon's mid-northern latitudes on its sub-Saturn hemisphere.
 
UVIS sometimes rides along to make an interplanetary hydrogen survey when the spacecraft is rotating for some other reason. Today this occurred during calibration activities for other instruments. Next, the Ion and Neutral Mass Spectrometer (INMS) made observations as part of a campaign to determine atmospheric and ionospheric thermal structure. Finally, UVIS went back to viewing Saturn, and began 7.8 hours of observing the planet's northern auroral oval. At the end of UVIS's activity, Cassini was poised for another close-in ring-plane crossing and periapsis passage early the following day.
 
The DSN communicated with and tracked Cassini on five occasions this week, using stations in Australia and California. A total of 47 individual commands were uplinked, and about 1,290 megabytes of telemetry data were downlinked and captured at rates as high as 124,426 bits per second.
 
Wrap up:
 
Cassini is executing its set of F-ring-grazing orbits of Saturn, with a period of 7.2 days in a plane inclined 63.6 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 March 22 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.
 
The countdown clock in Mission Control shows 178 days until the end of the mission.

Cassini position on March 21, 2017 An illustration of Cassini's path up to mid-day March 21, 2017. Cassini's orbit looks the same again this week, but the positions of spacecraft and moons and the Sun are different.