Cassini Significant Events 05/23/07 - 05/29/07

June 1, 2007

(Source: Cassini Project)

The most recent spacecraft telemetry was acquired on Tuesday, May 29, from the Madrid 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 located at

Wednesday, May 23 (DOY 143):

The spacecraft team performed a Memory Read Out (MRO) today to downlink the SSR hardware single/double bit error counters for all 128 submodules in each SSR. This activity, scheduled once each month, executes every hour for the duration of one downlink track. Analysis of the results is underway.

The Extended Mission Segmentation Working Group (SWG) met last week to work any remaining periapsis allocation conflicts and the apopasis allocations. One of the first steps in getting a background sequence up to the spacecraft with commands that allow science acquisition is to divide the entire tour into science segments. Once assigned, the appropriate Rings, Saturn, X-Discipline, or MAG Target Working Team (TWT) along with the Titan and Satellite Orbiter Science Teams (OST) become the custodians and stewards of their allocated segments, and lead them through the next step of integration.

The members of the Working Group included all of the TWT/OST leads, Investigation Scientists, Science Planners, and the Deputy Project Scientist. It was the aim of the SWG to take this effort as far as possible so that the proposed segmentation plan could be reviewed and perhaps accepted at the Project Science Group (PSG) meeting next week.

Current major issues still to be addressed are the conflict between the Saturn deep atmosphere observations and the icy satellite targeted and non-targeting flybys in orbits 119-128, and accommodating Saturn gravity passes and Saturn atmosphere occultations. These conflicts will need to be discussed and addressed at the upcoming PSG meeting.

Thursday, May 24 (DOY 144):

Uplink Operations sent commands to the spacecraft today for a Live Inertial Vector Propagator (IVP) update to be performed on DOY 146, and for a Radio Science (RSS) Live Update Block to be performed on DOY 148.

Orbit Trim Maneuver (OTM) #112, the Titan 31 approach maneuver set for Friday, May 25, has been cancelled. A reaction wheel (RWA) bias in lieu of the maneuver will be uplinked May 25 to execute May 26.

The decision to cancel centered on the trade between the risk of accurately executing a very small maneuver and the impact on an important Titan occultation radio science experiment. Using the tracking data from the Wednesday night pass, and the updated value for the RWA bias, the OTM had a magnitude of 12.8 mm/sec. When the deadband tightening delta-V of 5 mm/sec is subtracted, the 7.8 mm/sec would have been the smallest OTM attempted to date.

RSS team members evaluated a number of different trajectories. After evaluations and discussions during the final OTM review, the consensus was that while there was a preference for executing the maneuver, when considering the uncertainties in the execution of the small OTM and the attendant growth in the delivery errors, maneuver cancellation was an acceptable option.

The S32 sequence leads reported this week that the RSS Titan Bistatic activity would be tested in the Integrated Test Laboratory in early June. In addition, DSN allocations have been finalized for all but the last four days of the sequence. As a result, a Science Allocation Panel meeting will be held next week to distribute any available sponge bits for science use.

Saturday, May 26 (DOY 146):

As Cassini approached Saturn and flew through periapsis, the Optical Remote Sensing (ORS) instrument teams observed some of Saturn's larger icy satellites. As non-targeted flybys of Tethys and Epimetheus were occurring, the instruments observed Tethys, Mimas, Enceladus, Dione and Rhea as part of this campaign. Additionally, RADAR turned its attention to a scatterometry observation of Rhea.

A Radio and Plasma Wave Science Whistler activity was performed, this time with the spacecraft under RWA control. The previous two observations were executed with the spacecraft in thruster (RCS) control to eliminate interference from the reaction wheels.

Monday, May 28 (DOY 148):

Today Cassini encountered Titan for the second time in the S30 sequence, traveling at a speed of 6.1 km/sec, and at an altitude of 2300 km. The highlight of this fly-by was the Radio Science instrument's atmospheric occultation. This was the last of four Titan occultations planned for Prime Mission, and was distinguished by being the only one that used reaction wheels for pointing control instead of thrusters. The exceptional pointing performance that is expected with this strategy will be critical for accurate measurement of the intensity extinction profiles for X-, S-, and Ka-band in the neutral atmosphere. This is particularly important for the highly pointing sensitive Ka-band observations.

The observations should provide the large and small structure of Titan's neutral atmosphere in exquisite detail. This occultation covered both the ingress and egress sides, probing latitudes of 75 degrees south and 73 degrees North, respectively, the highest achieved in the nominal mission. Together with previous occultations, these observations will be critical for characterization of atmospheric dynamics close to Titan's polar regions, in particular Titan's polar vortex.

Tuesday, May 29 (DOY 149):

An informal discussion about lowering the altitude for the Titan 70 flyby, the last flyby in the proposed extended mission, was held at the Mission Planning Forum today. Spacecraft (SCO), Mission Planning, and TOST representatives reviewed the status of T70 analysis work and the current thinking about minimum altitude selection. Both SCO and the science teams will address this topic at the PSG meeting next week.

The final build for the Cassini Command and Data Subsystem (CDS) flight software (FSW) version 10 was delivered May 30, 2007. Functional and regression testing have begun, with uplink planned for September 24, 2007. The changes to the FSW provide the capability for an SSR submodule to be "turned off" by the flight team in the event of a permanent failure of a submodule in the Dynamic Random Access Memory (DRAM). Prior to launch, the commercial DRAM had been expected to fail at a much higher rate than has been experienced in flight. As a result, this submodule isolation capability originally planned to be uplinked in 2003 as part of version 9 was deemed low priority at that time. Each submodule makes up less than one percent of the memory capacity of each of the two SSRs.

Since the Significant Events report is written on Wednesday, we are happy to report that a RADAR image from the Titan 30 flyby is Astronomy Picture of the Day on Wednesday, May 30.