From: Jet Propulsion Laboratory
Posted: Tuesday, September 2, 2014
Cassini is orbiting Saturn with a 31.9-day period in a plane inclined 44.6 degrees from the planet's equatorial plane. The most recent spacecraft tracking and telemetry data were obtained on Aug. 26 using one of the 34 meter-diameter Deep Space Network (DSN) stations at Canberra, 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. Information on the present position of the Cassini spacecraft may be found on "Eyes on the Solar System":
This week's highlight was the T-104 encounter with Saturn's enigmatic moon Titan early Thursday morning. During the close encounter, all the spacecraft's rotations and scientific observations executed with precision, commanded by the on-board S85 command sequence. The DSN captured every bit of the long-distance telemetry playback on the following day. Thanks to the DSN's exquisitely sensitive radiometric tracking, the Cassini Navigation team determined that the spacecraft had flown within 90 meters of its target. With such accurate targeting, Monday morning's opportunity to execute a post-encounter trajectory-cleanup rocket firing did not have to be used.
Wednesday, Aug. 20 (DOY 232)
Dutifully following Newton's and Kepler's laws, Cassini began slowing a bit while coasting up and away from yesterday's Saturn-orbit periapsis. Today the spacecraft rotated so that its telescopic instruments could begin their observations of Titan across the spectrum. From now until Friday, Titan came under the close scrutiny of the Visible and Infrared Mapping Spectrometer (VIMS), the Imaging Science Subsystem (ISS), the Composite Infrared Spectrometer (CIRS), and the Ultraviolet Imaging Spectrograph (UVIS). At times, more than one instrument "rode along," acquiring data while another instrument had command of the ship's attitude.
In addition to the remote-sensing instruments' observations, Cassini's direct-sensing instruments made essentially continuous observations during this segment as well: the Magnetometer, the Magnetospheric Imaging Instrument, the Radio and Plasma Wave Science instrument, and the Ion and Neutral Mass Spectrometer (INMS).
Inbound to Titan today, CIRS focused on mid- and far-infrared nadir integrations to measure the abundances of carbon monoxide, water, and cyanide. ISS made a mosaic of southern latitudes on Titan's sub-Saturnian hemisphere, and VIMS took low resolution images while riding along with ISS. Observations covered latitudes between the equator and 60 degrees south, and included monitoring the south polar vortex.
Thursday, Aug. 21 (DOY 233)
RADAR operated in altimetry mode, and later made high-priority SAR-mode observations. INMS served as the spacecraft's primary instrument inbound near closest approach, commanding the best spacecraft attitude for ingesting and analyzing the content of Titan's ionosphere. Subsequently, INMS rode along while RADAR controlled pointing.
On the outbound leg after RADAR finished, VIMS became prime. It monitored the northern great lake Kraken Mare at the point where a specular reflection of infrared sunlight might be glimpsed. Catching one would help assess whether liquids are still present, and if still liquid whether winds create waves on the surface. VIMS also looked for clouds at high northern latitudes and mapped the north polar area to monitor the evolution of its lakes and seas. ISS acquired a mosaic of northern latitudes on Titan's trailing hemisphere.
Cassini departed Titan having taken more than images, spectra, and other science data -- it also took a course change via gravity assist. Without changing the spacecraft's orbital period, the encounter decreased the orbit's inclination from 48 degrees to 44.6 degrees. Subsequent Titan encounters will each further reduce Cassini's inclination until it is basically equatorial again next March. The T-104 encounter is further described here:
Friday, Aug. 22 (DOY 234)
After Titan observations were sent to DSN stations on Earth, Saturn again commanded attention, beginning a 27 day period of observing the giant planet. First was a nearly nine-hour VIMS mosaic of Saturn's northern hemisphere; CIRS and ISS rode along.
Saturday, Aug. 23 (DOY 235)
UVIS began its observations of Saturn with a slow scan across the planet's illuminated hemisphere to form spectral images. CIRS and ISS rode along.
Sunday, Aug. 24 (DOY 236)
ISS interrupted the Saturn monopoly to first conduct a 90 minute Titan cloud monitor observation, with VIMS and CIRS riding, and then to take images of the dwarf planet Pluto against background stars. This was an optical navigation frame providing ephemeris data for use by navigators of the New Horizons Mission.
Monday, Aug. 25 (DOY 237)
The S85 command sequence entered a planned gap on the spacecraft today so that commands could be uplinked to execute Orbit Trim Maneuver (OTM) #389; normally such OTM commands would be designed based on the latest available navigation data and uplinked near real time. None were needed today, though, because Thursday's encounter targeting had been so accurate. When S85 resumed, CIRS executed a 13.5 hour mid-infrared map to determine temperatures in Saturn's upper troposphere and tropopause; ISS and VIMS rode along.
A Cassini image featured today shows material orbiting within the Enke gap in Saturn's A ring. This material, along with the small satellite Pan (which does not appear in the image), completes an orbit around the gas giant once every 13.9 hours:
Much of Cassini's instrumentation was designed to address questions raised when the two Voyager spacecraft flew quickly through the Saturnian system a few decades ago. Today's NASA Astronomy Picture of the Day celebrates a different Voyager achievement: today marks the 25th anniversary of the Voyager-2 encounter with Neptune's moon Triton -- its final planetary encounter:
Tuesday, Aug. 26 (DOY 238)
CIRS preformed an 11 hour mid-IR temperature map of Saturn, with VIMS again riding. CIRS then occupied the last nine hours of the day staring at one location on the planet to derive composition. UVIS and VIMS rode along.
This week, the DSN communicated with and tracked Cassini on six routine occasions, using stations in Australia and Spain. A total of 5,805 individual commands were uplinked, and about 2,360 megabytes of telemetry data were downlinked at rates as high as 142,201 bits per second. Coherent Doppler-shift data and ranging data were obtained for a total of about 32 hours. As always, unprocessed ISS images are available right away for browsing:
Visit the JPL Cassini home page for more information about the Cassini Project: http://saturn.jpl.nasa.gov/
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