The most recent spacecraft tracking and telemetry data were collected April 18 using the Deep Space Network's 34 meter Beam-Waveguide Station 55 at Madrid, Spain. Beyond the issues in work with the Ultrastable Oscillator (see the January 5, 2012 Significant Events) and the Cosmic Dust Analyzer, the Cassini spacecraft is in an excellent state of health. Its subsystems are operating normally as the spacecraft orbits, nearly in Saturn's equatorial plane, once every 17.8 days. Information on the present position of the Cassini spacecraft may be found on the "Present Position" page at: http://saturn.jpl.nasa.gov/mission/presentposition/.
E-18, the targeted flyby of Enceladus on Saturday, was this week's highlight. The Ion and Neutral Mass Spectrometer (INMS) and the other Magnetospheric and Plasma Science (MAPS) instruments were prime for closest approach and flight through the plume. View the E-18 web page here: http://saturn.jpl.nasa.gov/news/cassinifeatures/feature20120416/
Among Cassini's routine activities this week were three observations in the Titan monitoring campaign to look for cloud activity over a long time baseline. There were also two observations in the Satellite Orbit Campaign, in which the Imaging Science Subsystem (ISS) measures positions of small satellites, improving knowledge of their orbits. One reason for these frequent observations is that over the mission's duration it may be possible to measure small changes in their orbits. Such changes could potentially provide an independent determination of the mass of Saturn's rings or of certain of the icy satellites such as Mimas.
ISS also undertook searches at the L4 and L5 Lagrange points in Rhea's orbit about Saturn, looking for any small satellites that might be trapped in these stable points 60 degrees ahead and behind Rhea, respectively. So far, none has been discovered.
Wednesday, April 11 (DOY 102)
Orbit Trim Maneuver (OTM) 316, the Enceladus 18 approach maneuver, was performed today. As usual, the maneuver was designed based on the latest navigation data, and translated into commands that were transmitted to the spacecraft only a few hours before they would execute. The RCS thrusters fired for 25.5 seconds, providing Cassini 31 mm/s of delta-V. In the target plane, the aim-point was intentionally biased by about 4 kilometers off the nominal trajectory because doing so would save propellant and simplify implementation.
Realtime commands were uplinked today that will turn on the Cosmic Dust Analyzer (CDA) shortly before the E-18 encounter on Saturday. Commands in the background sequence (S73) turned on CDA heaters for roughly ten hours today to perform a routine decontamination of the instrument.
Thursday, April 12 (DOY 103)
The Navigation Team took five images of Saturn's moon Dione against the background stars for optical navigation purposes, then the Ultraviolet Imaging Spectrograph (UVIS) began a 13 hour observation of Saturn's aurora.
The main engines' protective cover was deployed to its closed position.
Friday, April 13 (DOY 104)
When UVIS completed its aurora observation, the spacecraft turned to point its high-gain antenna (HGA) to Earth, and downlinked all the telemetry data that had been stored on the Solid-State Recorders (SSRs). The Canberra, Australia, 70 meter diameter Deep Space Network (DSN) station captured every bit. This was one of seven routine DSN tracking activities for the week.
The E-18 flyby filled up the SSRs with new data. It began with long Enceladus plume observations led by the Visible and Infrared Mapping Spectrometer (VIMS) and ISS at a low latitude and at a variety of spatial scales, to be used for morphological studies and context.
Saturday, April 14 (DOY 105)
As the E-18 encounter continued, the Composite Infrared Spectrometer (CIRS) led Optical Remote-Sensing (ORS) viewing of Enceladus's night-side anti-Saturn-facing hemisphere, with dark sky behind, to search for possible hot spots away from the "tiger stripe" crevasses. At this time, the commands uplinked on Wednesday took effect, powering CDA on for the encounter.
Closing in on the icy target, the spacecraft turned to give INMS prime status for closest approach, going 27,000 km/h with respect to Enceladus. The purpose of this series of observations was to understand variability in Enceladus's activity and to map the plume's three-dimensional structure. As the spacecraft then proceeded outbound from Enceladus (though still inbound to Saturn), CIRS made observations to complete its coverage and to search for additional hot spots on the day-side Saturn-facing hemisphere.
Following the E-18 encounter, VIMS performed a stellar occultation experiment. This means that VIMS kept its telescope trained on the bright star Procyon (alpha Canis Minoris) and watched it for 2 hours 40 minutes while Cassini's motion caused the star to set slowly into Saturn's atmosphere, to gain information about the atmosphere.
Before the stellar occultation was finished, Cassini passed through periapsis, the low point in its orbit about Saturn, going 68,123 km/h. (Compare this to its Saturn-relative speed of 5,496 km/h at the high point in its orbit last week.)
Next came a series of observations of the moon Tethys from a close-approach distance of 9,053 kilometers by various ORS instruments. These observations will provide data to extend the mapping of Tethys's geologic features, and to understand the interaction of the E ring and energetic electrons with the icy moon's surface. The observations from this non-targeted flyby should help refine mapping of Tethys's "pacman" feature, similar to that seen on Mimas, caused by the varying thermal inertia across the surface. For reference, see http://saturn.jpl.nasa.gov/photos/imagedetails/index.cfm?imageId=3919.
Sunday, April 15 (DOY 106)
Today's task was to downlink all the E-18 telemetry data at rates up to 142,201 bits (binary digits) per second. Thanks to the 70 meter diameter DSN stations in Canberra, Australia and Madrid, Spain, the data were captured flawlessly.
After the E-18 flyby, CDA stopped producing data. It was turned off by real-time command today after members of the flight team held an anomaly meeting.
Cassini's ORS instruments made an observation of Saturn's icy moon Rhea as a small dark crescent with dark sky behind.
The main engine cover was stowed to its open position. This was the 73rd in-flight cycle.
Saturn comes closest to Earth today as Saturn and Earth orbit the Sun. The ringed planet is at opposition, so it rises in the East at sunset, and sets near dawn. This highlights a season of great viewing opportunities for anyone who has access to even a small telescope. See "Viewing Saturn in 2012": http://saturn.jpl.nasa.gov/education/saturnobservation/viewingsaturn/
Monday, April 16 (DOY 107)
CIRS began a 12 hour Saturn observation to measure oxygen compounds (H2O, CO2) in the stratosphere, then the magnetometer executed a calibration while rolling the spacecraft about its Z-axis, keeping the HGA facing Earth all the while for communication.
The DSN station 34 in Canberra, Australia, acquired the Ka-band (32 GHz) signal from Cassini to perform a calibration of monopulse, the fine-pointing capability used in Radio Science experiments. This occurred at the same time as X-band (8 GHz) communications and tracking were in progress.
Sequence Implementation Process team members held a Science Forum meeting for development of the S75 command sequence, which is scheduled to be uplinked in August.
OTM-317, the E-18 clean-up maneuver that was scheduled for April 18, was cancelled today. This was because the propellant cost of making the maneuver implementable was found to be about equal to the downstream cost of canceling it.
An image of Enceladus in front of Titan and Saturn's rings is featured today: http://saturn.jpl.nasa.gov/photos/imagedetails/index.cfm?imageId=4497
Tuesday, April 17 (DOY 108)
The Navigation Team took five images of Saturn's moon Iapetus against the background stars for optical navigation purposes.
JPL software developers identified a fix in an upcoming delivery of ground software for a problem that prevented remote Science Operations Planning Computers from querying the JPL-based Telemetry Delivery System.