NASA Cassini Significant Events 10/29/08 – 11/04/08
The most recent spacecraft telemetry was acquired on Nov. 4 from the Deep Space Network tracking complex at Goldstone, California. 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” page at: http://saturn.jpl.nasa.gov/operations/present-position.cfm.
Wednesday, Oct. 29 (DOY 303)
Orbit Trim Maneuver (OTM) #169 was performed today. This is the apoapsis maneuver setting up for the Enceladus 6 and Titan 46 encounters on Oct. 31 and Nov. 3. The reaction control subsystem (RCS) burn began at 10:59 AM PDT. Telemetry immediately after the maneuver showed the burn duration was 191 seconds, giving a delta-V of 0.23 m/s. All subsystems reported nominal performance after the maneuver. Due to large downstream delta-v costs associated with executing the maneuver over the backup pass, this OTM was uplinked a day early to help ensure success over the primary pass. Thruster performance was slightly off nominal during this OTM. This resulted in a 7 mm/sec under burn due to increased thruster chamber pressure roughness. This roughness has been higher for the last few RCS OTMs. Small variations in chamber pressure roughness and thrust are normal for hydrazine thrusters, and this recent increase is within the historical range. More analysis will be performed and a correction for this will be applied at OTM-171.
Thursday, Oct. 30 (DOY 304)
Uplink Operations has heard from Science Planning, Imaging Science (ISS), the Cassini Plasma Spectrometer (CAPS), and Composite Infrared Spectrometer (CIRS) teams. All are a no-go for the Live Update planned for DOY 313, Nov. 8.
Friday, Oct. 31 (DOY 305)
Non-targeted flybys of Methone, Pandora, Daphnis, Atlas, and Epimetheus occurred today. On Oct. 31, Cassini flew by Enceladus at an altitude of 174 km, a speed of 17.7 km/sec, and latitude 28 degrees S. This was the second close flyby of Enceladus this month with the Optical Remote Sensing (ORS) instruments primary for the majority of observations, and with the Radio Science Subsystem (RSS) taking Enceladus gravity data both before and after closest approach to help measure the gravity field by estimating the mass separate from other gravitational harmonic coefficients. This was the closest non-targeted flyby of the mission to date. The previous closest was an Enceladus flyby back in orbit #3 of the Prime Mission at a little under 1300 km.
In close coordination with the other ORS instruments, ISS held prime pointing control of the spacecraft at closest approach for E6. Images were obtained of the south polar terrain and active tiger stripes at very high spatial resolution. This observation complements coverage obtained in E4, the last ORS Enceladus flyby in mid-August 2008.
ISS imaged active vent regions in unprecedented detail to identify geological structure modifications of the tiger stripe rifts that accompany ongoing eruptions. High resolution imaging of surface features in between the tiger stripes will provide details that could be used to characterize the surface modification effects of fallback from the plumes adjacent to eruption centers. ISS also obtained high-resolution multi-spectral mosaics of the tiger stripes to search for temporal changes in composition associated with possible time variability of the eruptions. By combining the results of E4 and E6 closest-approach imaging, ISS obtained data for stereo coverage of the tiger-stripes to produce high-definition three-dimensional topographic maps of both active and inactive eruption centers. Data from the RSS gravity passes in the wings of this Enceladus flyby will be used in conjunction with the passes on orbits 80 and 130 to determine the Enceladus gravity field up to degree 2. This observation consisted of two segments; an inbound, covered by the Madrid station, and an outbound, covered by Canberra and Madrid.
Saturday, Nov. 1 (DOY 306)
This will be the last report on the Cassini Main Engine (ME) cover until next year. For the last several months there has been a great deal of ME cover activity as the flight team used this device to protect the spacecraft from potential dust hazards. The cover was deployed on Oct. 30 at the end of the Orbit Trim Maneuver (OTM) 169 backup pass for a dust hazard at E6, and reopened on Nov. 1 for the 50th cycle of the cover. The next predicted hazard does not occur until October 2009.
Monday, Nov. 3 (DOY 308)
On Sunday the CIRS instrument team reported that their Bus Interface Unit (BIU) had been marked “sick”. A reboot of the instrument re-established communications between CIRS and the CDS, but the end result was the loss of CIRS’ Titan (T46) data. This particular anomaly occurs occasionally for CIRS with no harm done other than a brief data outage. As reported by members of the CIRS team, “The CIRS instrument has fully recovered from the BIU anomaly that occurred on Sunday morning. All telemetry is as expected and packets came flying across the BIU indicating that the instrument was going about the business of collecting Deep Space Calibration (DSCAL) data. We’ll spend the next few days poring over the telemetry, memory readouts, and instrument packets to see if we can further our knowledge of the anomalous state. Meanwhile, the instrument will continue executing commands from the background sequence.” Back to business as usual.
On Monday, Nov. 3, Cassini flew by Titan at an altitude of 1105 km, a speed of 6.3 km/sec, and operating under RCS control. Latitude at closest approach was 3.5 degrees S, and the encounter occurred during orbit number 91. T46 was the tenth in a series of outbound encounters and the second Titan encounter in the Cassini Equinox Mission.
RSS observations of Titan included ionospheric and atmospheric occultations and bistatic surface scattering on both the ingress and egress sides. The egress atmospheric occultation was the first to probe the mid northern-latitudes of Titan at ~33 deg. N. The ingress occultation probed mid-southern latitudes at ~33 deg. S. Combined with results from eight other latitudes probed during the Cassini prime mission, the occultations will shed more light on latitudinal variability of the electron density profile of the ionosphere, temperature and pressure profile, microwave absorption profile, and small scale-structure of the neutral atmosphere including gravity waves and turbulence.
In addition, bistatic surface scattering was observed at low southern latitudes on the ingress side from 24-31 deg. S, and low northern latitudes on the egress side from 25-30 deg. N. The incidence angle for both sides was close to the Brewster angle range for likely surface compositions. Same- and cross-polarized components of mirror-like surface echoes observed at the ground receiving stations, if detectable, will provide valuable information about the dielectric constant and physical state of the surface region probed.
