7 min read

Cassini Significant Events 03/11/2015 – 03/17/2015

Cassini is currently orbiting Saturn with a 28-day period in a plane inclined 0.3 degrees from the planet's equatorial plane. The most recent spacecraft tracking and telemetry data were obtained on March 18 using one of the 34-meter diameter Deep Space Network 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/news/significantevents/anomalies/ .

On board Cassini, commands from the 10-week long S88 sequence orchestrated most of the activities during Monday's Titan encounter T-110. Some of the important events for the flyby were commanded closer to real time, though, as noted below. The flyby page offers more detail: http://saturn.jpl.nasa.gov/mission/flybys/titan20150316 .

Wednesday, March 11 (DOY 070)

The Imaging Science Subsystem (ISS) still had control of spacecraft pointing today while it completed a 13-hour series of observations that had begun the day before, to make a low-resolution movie of Saturn's F-Ring. The Visible and Infrared Mapping Spectrometer (VIMS) also took data while riding along. After completing this, and a nine-hour communication session via the Deep Space Network (DSN), the Composite Infrared Spectrometer (CIRS) took the lead. With VIMS riding along, it performed a six-hour far-infrared stare at the sunlit side of Saturn's B-Ring, collecting spectral data to study the ring particles' chemical composition.

Thursday, March 12 (DOY 071)

Three of Cassini's Optical Remote-Sensing (ORS) instruments -- VIMS, CIRS, and the Ultraviolet Imaging Spectrograph (UVIS) -- jointly observed the rings for 6.5 hours, adding to overall coverage from varying latitudes and illumination phase angles. This was repeated on the following day for six hours. Both before and after today's observation, ISS took a two-minute storm-watch look at Saturn. Next, ISS turned to Saturn's largest satellite for 90 minutes to make a Titan monitoring observation, with CIRS and VIMS riding along. Titan was 2.4 million kilometers away.

Friday, March 13 (DOY 072)

ISS Studied Saturn's dense B ring for six hours, with the other ORS instruments riding along. The results will be assembled into a movie.

The flight team sent a special set of commands to Cassini today, and confirmed later via telemetry that they were stored aboard for execution late the next day, in preparation for the T-110 encounter. The commands would configure the spacecraft to use its Low-Gain Antenna (LGA) for communications so that the Radio Science team could make measurements of Titan's gravity field, even though the ORS instruments would be controlling Cassini's orientation. This culminated many weeks of planning, including preparations for contingency actions in case any problems were to develop.

In addition to the special LGA commanding, the flight team also sent instructions to perform a more routine activity, Orbit Trim Maneuver (OTM) 406, which executed later in the day. The 18-second rocket thruster burn provided a change in velocity of 23 millimeters per second, fine-tuning the approach to Titan for the T-110 encounter.

Late in the day, ISS made another 90-minute Titan monitoring observation. The distance to Titan had shrunk to 1.8 million kilometers by today, and by Monday's encounter it would be more than a thousand times closer. Next, the Cosmic Dust Analyzer (CDA) began a 12-hour observation to collect dust during Cassini's southbound pass through Saturn's ring plane the next day, at a distance just outside the denser parts of the wide, sparse E ring. The CDA is capable of determining many characteristics of each dust particle that enters the instrument.

Saturday, March 14 (DOY 073)

With CDA's observation complete, VIMS led a watch of the red star X Ophiuchi for nearly three hours, while it was occulted by Saturn's rings -- probing them with starlight as Cassini's motion in orbit caused the star to pass behind. ISS and CIRS rode along. Shortly after the stellar ring-occultation experiment, Cassini passed through periapsis in its orbit about the gas giant. It came within the distance of the moon Helene's orbit, 314,000 kilometers above Saturn's visible limb.

Sunday, March 15 (DOY 074)

Now that Cassini had passed south of the ring plane and the rings were sunlit from behind, ISS spent 4.6 hours making high-resolution images of the narrow but bright F ring with CIRS and VIMS riding along. It then spent 5.7 hours on close-up images of the backlit D ring -- the innermost ring nearest Saturn. VIMS and UVIS rode along on the latter.

Responding to the previous day's commanding, motorized wave-guide switches changed the microwave "plumbing" configuration in Cassini's onboard uplink path, then later its downlink path, to the LGA. The LGA provides a much larger beam width than the normally used High Gain Antenna (HGA). The DSN then maintained a coherent two-way link while the spacecraft was twisting and turning to aim the ORS instruments toward various targets on Titan. This would allow, for the first time ever, the Radio Science team to obtain measurements of Titan's gravity field via the LGA, to study the giant moon's interior.

Monday, March 16 (DOY 075)

Cassini's closest approach to Titan occurred near the middle of the day, while VIMS and then CIRS led the ORS observations, and the DSN remained in contact via Cassini's LGA using its 70-meter diameter stations in California and Australia. The flyby page offers more detail: http://saturn.jpl.nasa.gov/mission/flybys/titan20150316 .

Since the LGA is much less capable than the HGA, neither telemetry data for science and engineering, nor ranging data for navigation, were in the plan for the flyby. This left a pure X-band (8 GHz) radio carrier signal for the Radio Science team to make a very successful Titan gravity investigation.

Thanks to earlier work by the mission planners, today's gravity-assist encounter with Titan achieved a reduction in both the period and the inclination of Cassini's orbit about Saturn. The period went from 31.9 to 28 days, and the inclination decreased from 8.5 to 0.3 degrees. Essentially equatorial now, which is optimal for the icy satellite flybys later this year, it will remain close to equatorial until the T-115 Titan encounter next January begins a final series of inclination-raising flybys.

Saturn's hazy-edged shadow stretches across the ring system in an image featured today. Many of the rings' details are clear to see: the A ring with its Keeler Gap and Enke Gap; structure within the Cassini Division; the bright, dense B ring; and the strands of the C ring near the central planet. Just a trace of the F ring is discernible outside of it all, with a familiar object paying it an orbital visit:
/resources/16172 .

At the end of the busy day, Friday's commands successfully reconfigured Cassini's trusty Radio-Frequency Subsystem back to normal, restoring use of the HGA for regular telecommunications.

Tuesday, March 17 (DOY 076)

With the ORS instruments still trained on Titan, now receding, CIRS finished up mapping in the far-infrared and the mid-infrared parts of the spectrum, gathering data on the thermal structure of Titan's stratosphere. ISS, UVIS, and VIMS rode along at various times.

After the DSN began capturing high-rate telemetry data that Cassini was playing back from the encounter observations, the flight team sent up a few more realtime commands to complete the spacecraft's nominal configuration, as well as wrap up the precautionary contingency measures that had been put in place.

During the past week, the DSN communicated with and tracked Cassini on eight occasions, using stations in California and Australia. A total of 190 individual commands were uplinked, and about 2,122 megabytes of telemetry data were downlinked and captured at rates as high as 142,201 bits per second.

Milestones spanning the whole orbital tour are listed here: http://saturn.jpl.nasa.gov/mission/saturntourdates .

Information on the present position and speed of the Cassini spacecraft may be found on the "Present Position" page at:
http://saturn.jpl.nasa.gov/mission/presentposition/ .