Cassini Significant Events 09/14/06 - 09/20/06
September 22, 2006
(Source: Cassini Project)
The most recent spacecraft telemetry was acquired on Wednesday, Sept. 20, from the Goldstone tracking complex. The Cassini spacecraft is in an excellent state of health and all subsystems are operating normally. Information on the present position and speed of the Cassini spacecraft may be found on the "Present Position" web page at http://saturn.jpl.nasa.gov/operations/present-position.cfm .
Thursday, Sept. 14 (DOY 257):
Orbit trim maneuver #72 was performed today. This was the apoapsis maneuver setting up for the Titan 18 encounter on Sept. 23. The main engine burn began at 4:30 AM. Telemetry immediately after the maneuver showed a burn duration of 50.8 seconds, imparting a delta-V of approximately 8.2 m/s. All subsystems reported nominal performance after the maneuver.
An image from Cassini's radar instrument taken during the T17 encounter shows an impact crater with a diameter of 30 kilometers on the surface of Saturn's moon Titan. Cassini data have only revealed three definite impact craters on Titan so far, so each new discovery adds significantly to our body of knowledge. To view the image and access the text, go to: http://saturn.jpl.nasa.gov/photos/imagedetails/index.cfm?imageId=2261
With the Titan 16 (T16) encounter on July 22, the Navigation team has begun to rotate the line of apsides of Cassini's orbit to more favorable geometries for atmospheric observations. The line of apsides is a line which passes through both periapsis, Cassini's closest approach to Saturn in an orbit, and apoapsis, the farthest spot in the orbit from Saturn. Apoapsis had been located on the dark side of Saturn to support magnetotail observations, but it is now being moved toward the lighted side of Saturn via a pi transfer. A pi transfer achieves this goal more quickly than could be done with other non-resonant transfers.
So what is a non-resonant transfer? According to members of the Cassini Navigation Team, to understand non-resonant transfers, you must first understand resonant transfers. With a resonant transfer, the spacecraft encounters a satellite, Titan in this case, at the same place in its orbit as at the previous encounter. In other words, there are multiples of 360 degrees between encounters for both Titan and Cassini, and not necessarily the same number for each. Transfers that encounter Titan with an arbitrary angle - not multiples of 360 degrees - between encounters are referred to as non-resonant transfers. A pi-transfer is a particular type of non-resonant transfer. The spacecraft encounters Titan 180 degrees plus some multiple of 360 degrees from the previous encounter.
Now, back to this particular transfer. From T16 to T24, Titan gravity assists rotate the line of apsides while raising the spacecraft's orbital inclination to approximately 59 degrees, the amount required to perform the pi transfer. The pi transfer occurs between T24 on January 29 and T25 on February 22, 2007, situating the T25 encounter 180 degrees around Saturn from where the T24 encounter takes place. >From T25 to T33 on June 29, 2007, the line of apsides continues to rotate toward Saturn's lighted side as inclination is lowered back into Titan's orbit plane. This phase of the mission offers several low altitude Titan flybys and favorable ring observation geometry to the science community.
Friday, Sept. 15 (DOY 258):
Sept. 15 is the beginning of a series of very important scientific opportunities for the Cassini mission. The trajectory at apoapsis takes the spacecraft behind Saturn such that both the Sun and Earth are obscured. This geometry offers the Radio Science Subsystem (RSS) a very slow occultation of Saturn's rings and atmosphere and allows Optical Remote Sensing (ORS) instruments ample time for nearly 180-degree phase angle ring observations. The backlit rings provide the second most important data set, next to Saturn Orbit Insertion, for rings scientists who are trying to better characterize the structure and composition of Saturn's rings. ORS instruments will use the time during solar occultation to study Saturn's rings at high phase. The Visible and Infrared Mapping Spectrometer (VIMS) instrument will perform a 3-hour high phase planet shadow mosaic, then the Imaging Science Subsystem (ISS) will take a number of images of the unlit side of Saturn and the ring system. The Magnetospheric and Plasma Science (MAPS) instruments will also operate to observe the structure and dynamics of Saturn's magnetotail. Finally, ISS will use the Wide Angle Camera for a 3-hour observation of the right ansa region of the ring system using four different filters.
In anticipation of the occultation event, a Feature was designed to give background information and to alert the public that a significant milestone was about to be reached. The feature, titled "Saturn's Rings To Shine As Never Before," appeared on Sept. 15, and may still be seen at http://saturn.jpl.nasa.gov/news/features/feature20060915.cfm
Sunday, Sept. 17 (DOY 260):
From Friday, Sept. 15, through Sunday, Sept. 17, Cassini Radio Science conducted the longest Saturn rings and atmospheric occultation planned in the tour. The ring occultation will yield important information about radial ring structure, dynamics, and physical properties. Similarly, the atmospheric occultation will yield the small- and large-scale structure of the southern atmospheric region, including the temperature-pressure profile and a profile of the abundance of microwave absorbing species.
All three DSN complexes supported the occultation experiment. S- and X-band support was provided by the 70-m antennas at Goldstone and Canberra and by the 34-m HEF at Madrid. X- and Ka-band support was provided by the beam wave guide antennas at all three complexes.
Overall, the rings and atmospheric occultations went well, the experiments were completed as planned, and good, high quality data were collected at all three observation frequencies of S, X, and Ka.
Perhaps the most exciting part of this long RSS observation period came during its last couple of hours when Cassini emerged from behind Saturn at about 8 pm PDT on Saturday night. As usual, the S/X/Ka levels slowly climbed to steadily increasing values as they cleared the neutral gas along the path. Suddenly, huge fluctuations in the S-band signal sent team members watching in the operations room gasping. X- band followed with similar fluctuations, although somewhat more muted in level, while Ka-band showed little variations from its steady free- space value. The huge dispersive behavior is a classic signature of plasma and is another example of how rich is the data the three-frequency radio observations collectively provide. In all likelihood, a substantial ionosphere characterizes the mid-southern latitude region probed, and it should be interesting to compare the corresponding electron density profiles with the many other near-equatorial ones obtained from previous occultations.
Monday, Sept. 18 (DOY 261):
This afternoon the project made the decision to cancel Orbit Trim Maneuver (OTM) #73, the Titan 18 minus 3 day maneuver, because the delta-v of 8 mm/sec required was below the Spacecraft Office minimum threshold of 10 mm/sec, and no adverse effects would occur because of the very slight deviation from the previously planned trajectory. OTM-74 is currently scheduled for DOY 269.
Tuesday, Sept. 19 (DOY 262):
A news release was issued today on the discovery of a new ring surrounding Saturn. Images were obtained during the longest solar occultation of Cassini's four-year mission. Sunday's occultation allowed Cassini to map the presence of microscopic particles that are not normally visible across the ring system. As a result, Cassini saw the entire inner Saturnian system in a new light. Over 100 media stories ran based on this press release. Images of this discovery along with a complete description can be found at: http://saturn.jpl.nasa.gov/news/press-release-details.cfm?newsID=691
Members of the Spacecraft Propulsion Module Subsystem presented "Cassini N2H4 Mass used to Date: High Time for a Model Update!" at the Mission Planning Forum today. A re-examination of the hydrazine consumption model indicates that an additional 6 kg of hydrazine may be available. The project will incorporate this change in future reporting of hydrazine usage.
Wednesday, Sept. 20 (DOY 263):
As one of the last activities for S23, a Reaction Wheel Assembly Bias was uplinked today in lieu of the cancelled OTM-73.
A Live Inertial Vector Propagator (IVP) update is planned for Saturn and Janus from DOY 268-272. At the Go/No-go meeting today it was confirmed that the update would only be required for the Janus vector. AACS provided the file to Uplink Operations immediately after the meeting for processing. Teams will have until the approval meeting on Friday to review the file.
The keys to the spacecraft were handed off from the S23 to the S24 leads today. S24 began execution at 2006-263T20:22 and will conclude 35 days later. During this sequence, six OTMs numbered 74 through 79 are scheduled to be performed, targeted Titan flybys 18 and 19 will occur at altitudes of 960 km and 980 km respectively, the main engine cover will be closed as part of nominal dust hazard avoidance for approximately four days, and one live IVP update is planned for execution during the last week of September.