Cassini Significant Events -- 08/31/05 - 09/07/05

September 9, 2005

(Source: Cassini Project)

Wednesday, August 31 (DOY 243):

A Science Operations Plan Update (SOPU) Project Briefing and Wrap-up meeting was held today for S16. The final development process for this sequence is scheduled to begin next week.

An ACS flight software (FSW) parameter patch was uplinked to the spacecraft using the OTM-30 backup window. This patch is designed to avoid possible thruster branch swapping due to atmospheric torques that may be induced during the T-7 flyby.

The Ion and Neutral Mass Spectrometer (INMS) performed its first prime inner magnetospheric measurements near the S14 periapsis pass last week. This observation allowed INMS to measure the composition of particles moving along magnetic field lines and look for a wide range of particles. The results will be used to fine tune similar observations in subsequent passes.

Thursday, September 1 (DOY 244):

A Reaction Wheel Assembly (RWA) friction test was performed today. Test results showed little change relative to tests performed six months and one year ago.

Cassini Outreach gave a "One Year Cassini Highlights" talk to several hundred attendees at the annual Oregon Star Party in the high desert Ochoco National Forest of central Oregon. Cassini bookmarks were handed out to all attendees, including participants in the children's science activity tent each day. A total of 800 bookmarks found new homes with event participants.

A Titan 7/Hyperion 1 Encounter Strategy meeting was held today. The strategy covers maneuvers 32 through 34.

Nine sequence change requests (SCR) were dispositioned at the S15 Preliminary Sequence Integration and Validation SCR approval meeting. Eight of the nine were approved and one withdrawn. So what types of changes are requested at these meetings you ask? Here's a sample: changes to the duration of a rolling downlink, the time of execution for a Cosmic Dust Analyzer (CDA) decontamination event, Imaging Subsystem (ISS) re-targetables, data volume, and pointer directives. 

Friday, September 2 (DOY 245):

Instrument Operations and the Multi-mission Image Processing Laboratory (MIPL) supported downlink of an Optical Navigation Image (OPNAV) designated as "critical" by the Navigation Team. This involved special staffing by both Cassini and multi-mission personnel to monitor the automated system and perform a parallel contingency process. The automated system performed flawlessly and the four OPNAV images were delivered within seconds of arrival at the Telemetry Delivery Subsystem (TDS).

So what is meant by "critical?" Well, in this case it depends on the team using the term and the context in which it is used. "Critical OPNAVs" aren't critical in the sense that something really terrible will happen if we lose them. If we lose them, we'll end up with somewhat larger trajectory control errors than we otherwise would. It turns out that there's a need to minimize the Titan 7 and Hyperion 1 trajectory errors as much as possible; T7 because of the low Titan flyby and H1 because we've determined that the pointing requirements levied on Navigation are just marginally being met.

OPNAVs are sometimes declared critical by Navigation due to a heightened need for these image files to be delivered at a particular time. This is due to defining a data cutoff so that processing can begin for a spacecraft ephemeris delivery. Declaring an OPNAV "critical" provides greater confidence that the files will be available for the orbit determination process, improving the orbit estimate and ultimately allowing us to design a maneuver that will cause the target miss distance to be smallest. For MIPL, a "critical OPNAV" means that someone will be around to check and make sure that the OPNAV gets delivered at the agreed upon time. 

A few days ago, Cassini received a request from the Mars Global Surveyor (MGS) Project to release its DSN coverage at the 70 m station #14 on the DOY 245 and DOY 247 tracks. At this time MGS was in safe mode and asked for the tracks to help recover the spacecraft. Unfortunately, when a request like this comes in after the sequence has been developed, it means a data loss for one or more of the Cassini instrument teams. The Program did agree to give up the pass on DOY 245. As a result, the spacecraft was commanded to lower telemetry rates to get back what data was possible over the 34 m station #25. 

The pass on DOY 247 could not be released as it is the backup OTM-31 uplink window and could not be released till Friday night 9/2 20:00 PDT at the earliest. Navigation needs the full track time for radiometric data in preparation for the Titan 7 flyby, and there are several unique and valuable science observation activities occurring between DOY 245 and DOY 247 that would be lost if DSS-14 were taken away. CIRS observations during this time are the only data obtained in the entire mission of the kind planned at this time. The geometry to do what is planned in rev 14 is never available at any other time in the tour. In addition, RADAR has some rings radiometry observations that are unique - this observation is the second of only three such activities in the Cassini mission. With this rational, MGS agreed to obtain additional tracks at other times that did not impact Cassini.

Saturday, September 3 (DOY 246):

OTM-31 was performed today. This maneuver, also called the T7 approach maneuver, refines Cassini's trajectory for the 1,075 km flyby of Titan on September 7. The reaction control system (RCS) burn began at 11:52 a.m. PST. A "quick look" immediately after the maneuver showed the burn duration was 66.8 seconds, giving a delta-V of approximately 63.1 mm/s. ACS reported the burn termination was a "nominal complete" with a "virtual accelerometer" cutoff, and all subsystems reported nominal performance after the maneuver.

Sunday, September 4 (DOY 247):

The main engine cover was closed today to avoid potential dust hazards on Monday. The cover will be reopened on Tuesday, September 6, and will remain open until Friday, September 23.

Uplink Operations sent commands to the spacecraft today for an INMS FSW patch to execute after the T7 encounter, and a RADAR T7 Immediate/Delayed Action Program trigger. Both files are absolutely timed and will execute on DOY 251T12:57:58 and 250T07:25:06 respectively. Both files have been successfully registered on board.

Monday, September 5 (DOY 248):

The flight team celebrated Labor Day today by performing the eighth and final Radio Science Occultation Experiment, the RSS Live Movable Block Saturn occultation sequence, and by performing non-targeted flybys of Pandora, Prometheus, and Methone.

Tuesday, September 6 (DOY 249):

A striking image of the Tiger Stripes on Enceladus was Astronomy Picture of the Day today.

ISS performed a high resolution radial scan from one ansa to the other to take a snapshot of the rings before the co-orbital satellites, Janus and Epimetheus, swap orbits in early 2006. The intent is to look for changes in the resonances they raise in the rings by comparing the before and after views. We also searched for other temporal changes and looked for new azimuthal structures in the rings and ring edges and gaps.

Wednesday, September 7 (DOY 250):

Cassini's seventh targeted encounter with Titan occurred today.

The S16 Science and Sequence Update Process Kickoff meeting was held today.

RADAR performed ring radiometry in Rev 14. This is low-resolution, low-inclination mapping of the rings to bring out the ring global properties, such as the scattering function of the ring particles and possible azimuthal variations in the ring brightness.

A news release has been issued as a follow-up to a news briefing held at the 37th Annual Meeting of the Division for Planetary Sciences meeting that concluded last week in Cambridge, England. The briefing announced the discovery of an unexpected grouping of clouds lurking in the depths of Saturn's complicated atmosphere. These clouds are deep in the atmosphere, about 30 kilometers underneath the upper clouds usually seen on Saturn. They also behave differently from those in the upper atmosphere and are made of different materials. They are made of either ammonium hydrosulfide or water, but not ammonia -- generally thought to comprise the upper clouds. For more information go to

Wrap up:

The most recent spacecraft telemetry was acquired Tuesday, September 6, from the Goldstone tracking complex. The Cassini spacecraft is in an excellent state of health and all subsystems are operating normally. Check out the Cassini web site at for the latest press releases and images.