Cassini Significant Events 08/10/06 - 08/16/06
August 18, 2006
(Source: Cassini Project)
The most recent spacecraft telemetry was acquired on Tuesday, August 15, 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, August 10 (DOY 222):
The Radio Science S-band equipment was powered on from DOY 219 to 221 during solar conjunction to aid in an experiment being run at Arecibo.
August 9 was the last day of the Cassini command moratorium during conjunction. Separation angle is now at three degrees and climbing, the command loss timer has been set to the nominal value of 85 hours, and normal commanding has resumed. Today the spacecraft played back data acquired during the 11 days of conjunction. Although this conjunction was deeper than last year's, spacecraft telemetry was off lock for shorter periods, indicating quieter solar activity. Superior conjunction will officially end this Sunday, August 13.
Today was the final day of the 2006 Operations Readiness Test (ORT). The training obtained by the flight team was invaluable. The test coordinator described it as "¿ a really hard ORT because it had serious Navigation recovery, long term Spacecraft and Mission implications AND a major Science objective coming in less than a week." When the test was planned, management was looking for both strategic planning and tactical recovery. In the final analysis, it was felt that the OTM teams really did well - this should make the real thing - if it ever happens - look easy!
Science activities on board the spacecraft this week included an Imaging Science Subsystem (ISS) 17-hour ansa movie of the F ring at very high phase, the completion of several slow scans across Saturn's visible hemisphere to form spectral images by the Ultraviolet Imaging Spectrograph (UVIS), and the Visual and Infrared Mapping Spectrometer (VIMS) observed Saturn to assemble a methane fluorescence mapping, and tracked many of Saturn's atmospheric features. The Magnetospheric and Plasma Science (MAPS) instruments simultaneously performed low-rate outer magnetospheric surveys to observe the variability of magnetospheric boundaries at a variety of radial distances.
The AACS Z Sigma Ratio error monitor unexpectedly exceeded the threshold on Aug. 10, 2006. The High Water Mark (HWM) value reached 13. The nominal threshold value is 10. It remained above the threshold for 10 seconds. This ratio compares the measured star brightness to the expected. The last occurrence was Christmas Eve, 2005, when it reached 11 for 5 seconds, because Rhea's interference was interpreted as an unexpected bright body.
Since this monitor triggered two of the four Cassini safing events, the Spacecraft Team has been following a strategy of masking and unmasking to prevent it from responding to bright bodies in the Stellar Reference Unit field of view. This strategy prevents a call to safing, and logs the event for future analysis. This time there was no bright body problem so the AACS team is investigating the cause.
Friday, August 11 (DOY 223):
The DSN has examined the Goldstone 70m elevation bearings at DSS-14. The bearings show normal wear and should have several more years of life left in them. With this information, DSN personnel will now have a stable environment for resource planning and track allocation. Now the only remaining uncertainty affecting the allocation of stations to support Cassini is related to the launch of STEREO, currently scheduled for August 31
Monday, August 14 (DOY 226):
Today was a very busy day for sending commands and files to the spacecraft. A Dione/Mimas/Saturn/Helene/Rhea Live Inertial Vector Propagator (IVP) update sequence, and a RADAR Dione/Rhea instrument expanded block (IEB) trigger update movable block sequence were uplinked. Both files have registered and activated on-board. The IVP update sequence will begin clocking out on DOY 227 while the RADAR sequence will begin execution on DOY 228.
Additional files uplinked today included S23 IEBs for ISS, Composite InfraRed Spectrometer (CIRS), VIMS, Ion and Neutral Mass Spectrometer (INMS), UVIS, Cassini Plasma Spectrometer (CAPS), and Optical Navigation. The remaining IEBs will be sent up tomorrow, and the background sequence will follow on Thursday.
Attitude and Articulation Control Subsystem (AACS) gyro calibration parts A & B were executed today piggybacking on an Inertial Reference Unit calibration.
Tuesday, August 15 (DOY 227):
Background sequence Reaction Wheel Assembly (RWA) biases were performed on DOY 224 and today to counter the low RWA RPM rates that otherwise would have occurred during the execution of this sequence segment. In addition, Periodic Engineering Maintenance (PEM) was performed today.
The final development process for the S25 background sequence and associated files kicked off today. Stripped subsequence files have been provided to the teams to populate, and a set of integrated merged products was produced and made available to the team.
Wednesday, August 16 (DOY 228):
The final sequence approval meeting for S23 was held today. Although all planned and required activities have been accommodated, there is concern regarding DSN allocations. Passes are still in heavy contention between September 2 and September 20 based on the new Stereo launch date of August 31. Should the situation change, real-time action would be required and may cause loss of data, and could affect the uplink of files in mid September to support S24. The allocation schedule is being worked on a week-by-week basis with upper project and lab management involvement. S23 will be uplinked to the spacecraft over the Cassini DSN pass tomorrow.
As of today, real-time commands will need to be sent to deal with DSN station allocation changes to passes on September 2, 4 and 5. The commands are needed to preserve the planned data playback. Changes to the DSN allocation for these days came too late for the sequence leads to accommodate them in the background sequence.
The sequence leads for S24 reported that an allocation file for the sequence is still not available. Development work is proceeding on the assumption that all requested passes will be obtained. Unfortunately, this is unlikely given the delayed return to service of DSS-63 and the needs of the STEREO and MRO missions.
As Cassini approached periapsis today, Cosmic Dust Analyzer (CDA) was on a campaign to obtain Tethys orbit crossing data and E ring measurements. In addition, the live update files mentioned on DOY 226 of this report are intended to update this period of periapsis observations - Dione, Mimas, Helene and Rhea observations are among the planned activities.