Cassini Significant Events 07/20/06 - 07/26/06

July 28, 2006

(Source: Cassini Project)


The most recent spacecraft telemetry was acquired on Wednesday, July 26, 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 located at http://saturn.jpl.nasa.gov/operations/present-position.cfm .

Thursday, July 20 (DOY 201):

The Flight Operations Lead from the Cassini Mission Support & Services Office presented "Cassini Real-time Operations" as part of JPL's Office of Communications and Education 2006 von Kármán Lecture Series. The lecture was given both Thursday and Friday nights and provided the audience a look behind the scenes as Cassini Mission Controllers send commands to the distant spacecraft. A virtual visit to Cassini real-time operations was part of the event.

At an additional presentation, a member of the Outreach team gave a Cassini Mission talk to 60 summer students currently in residence at JPL.

Friday, July 21 (DOY 202):

On July 21 Cassini flew past Titan for the 17th targeted encounter with the lowest altitude yet, at 950 km. This high-latitude flyby increases Cassini's Saturn-relative orbit inclination from near equatorial to 14.9 deg. Inclination will now continue to increase with Titan encounters through February 2007, reaching a maximum of 59.8 degrees. ACS performed nominally, but with a higher than predicted thruster duty cycle during the +/- 15 minutes around closest approach. The higher duty cycle implies a greater atmospheric density than expected. Discussions are underway between the various teams to resolve whether the Titan 16 atmospheric results, if applied to Titan 17 on September 7, indicate that changes should be made to that flyby.

During this very close encounter, Cassini passed by Titan at approximately 6.0 kilometers per second as the RADAR instrument performed observations to obtain high-resolution coverage over Titan's high northern latitudes.

Additional observations for this flyby included an Ultraviolet Imaging Spectrograph (UVIS) stellar occultation to provide a detailed profile of Titan's thermosphere, Composite Infrared Spectrometer (CIRS) observations emphasizing the far-infrared part of the spectrum to map Titan's composition, limb sounding, and mapping of Titan's surface temperatures, Ion and Neutral Mass Spectrometer (INMS) observations of the atmospheric and ionospheric thermal structure, and a continuous survey of Titan's interaction with Saturn's magnetosphere by the Magnetospheric and Plasma Science (MAPS) instruments.

All the closest approach data from T16 and the scheduled dual playbacks of INMS, Magnetometer Subsystem (MAG), and ACS data were downlinked on the DOY 203 DSN track. There was a two minute outage observed during the DOY 203 track but this occurred after all of the closest approach data had been downlinked.


Saturday, July 22 (DOY 203):

Beginning today the Cassini Radio Science team will acquire open loop tracking, Doppler, and ranging data during the 2006 solar conjunction period. The window for the Solar Corona Experiment (SCE) is centered on conjunction day where the Sun-Earth-Probe or SEP angle is the smallest.

The experiment started today, will continue until August 21, and is covered by approximately one DSN pass a day. The tracks are mostly over Goldstone with only four passes over Madrid. X- and Ka-band data will be acquired throughout the experiment, and S-band over seven passes only. Unlike the SCEs performed during the cruise phase of the mission, this is not a continuous experiment. That is, we don't have continuous downlink and continuous DSN coverage for 30 days.

Data from this experiment will be analyzed in order to investigate the solar corona's electron density as a function of SEP angle and combine the data set with sets from previous experiments to correlate the data with the 11-year solar cycle. The data and resulting models will be used to further calibrate Radio Science experiments throughout the mission, since Radio Science data quality is highly dependent on solar and interplanetary plasma effects. In addition, the precision radio system of the DSN and Cassini will be used to investigate spatial and temporal structures in the solar corona.


Sunday, July 23 (DOY 204):

The portion of S22 executing today contained a Rings segment concentrating on several periapsis observations, and included non-targeted flybys of both Telesto and Tethys. Additional science included INMS and MAG inner magnetosphere observations, a Visual and Infrared Mapping Spectrometer (VIMS) stellar occultation of the star Alpha Ori as it cuts through both the F and A rings, and a CIRS observation of the thermal infrared at zero phase as it crosses the rings. As a precaution, ACS masked, then unmasked the z sigma ratio Fault Protection (FP) monitor for the Tethys flyby. This prevented FP from potentially responding to a bright body in the Stellar Reference Unit field of view.


Monday, July 24 (DOY 205):

The Imaging Science Subsystem (ISS) obtained a 7-hour movie of the Maxwell Gap within the C Ring for satellite detection today. 

Due to the cancellation of OTM-068 last week, Spacecraft Operations Office personnel uplinked the usual reaction wheel bias today. In addition, on the recommendation of the Navigation team, an ACS accelerometer scale factor update patch was also uplinked. This patch improves OTM performance prediction and increases the scale factor by 0.06% to better correspond to the Navigation team values. There was no rush to get the patch on board, but both teams wanted to have it in place prior to the execution of OTM-069 on August 1. Since there are no more OTMs scheduled for August, the teams would have had to wait till September to use the patch if it did not go up now.

The preliminary port occurred today as part of the Science Operations Plan Update process for S25. When all inputs are received, they will be merged and a report sent out to the teams. Final inputs are due next Friday.

The Cassini spacecraft, using its radar system, has discovered very strong evidence for hydrocarbon lakes on Titan. Dark patches, which resemble terrestrial lakes, seem to be sprinkled all over the high latitudes surrounding Titan's north pole. For the complete story link to: http://saturn.jpl.nasa.gov


Tuesday, July 25 (DOY 206):

A Cassini-Huygens Analysis and Results of the Mission (CHARM) teleconference covering Saturn Atmosphere Results from VIMS was held today.


Wednesday, July 26 (DOY 207):

Back around the end of June, DSS-63, the 70-meter antenna at Madrid, began scheduled downtime for maintenance and would not return to service until October 1, 2007. Cassini Mission Planning knew in advance about the downtime, so sequences that would execute before that date were built without using this antenna. Cassini would again begin use of DSS-63 during the execution of S24,

At the start of the downtime, the hydraulic oil supply for the elevation bearings was analyzed and a higher than normal metal particle count was found. The covers were removed and two of the four bearings were found to be damaged. No spares are held in stock so replacements would have to be obtained from outside vendors. Replacements are not common as these particular bearings are rather large. Replacements have been found, but the downtime has now been extended to January of 2007.

Cassini is heavily dependent on preplanned sequences that are designed, built, validated,uploaded, and then executed on the spacecraft. One of the key elements in a sequence is the plan for getting the data collected by the instruments and spacecraft subsystems transmitted to the ground through DSN resources. The need to know what antennas will be available, as far in advance as possible, is one of the factors affecting sequence development. Development is a complex process that involves a large number of parties. Adequate time needs to be available to accomplish this work, and uncertainties or last minute changes cause much disruption of this process.

Additional factors are now playing into this situation. The STEREO launch has been delayed to the period of August 20 to September 4, with spacecraft commissioning and Earth orbit operations continuing until mid October. The DSN resources that are required to support STEREO are also required to support several other missions that are planning critical events, Cassini being one of them. When a launch is planned, DSN passes are assigned to that mission and are not available to other projects. If the launch is moved, new passes are taken away from projects they were previously assigned to. Sometimes the old passes can be used by other projects, and sometimes not depending on where they are in their development cycle.

Finally, the oil is now being analyzed at the other 70-meter antennas at the Canberra and Goldstone facilities to see if there may be bearing problems there as well. During a scheduled down time from July 7 through July 21, Canberra antenna personnel removed the DSS-43 inboard and outboard elevation bearing covers. All four bearings were inspected and found to be in good shape and in no apparent danger of failing. The rollers are smooth as are the bearing surfaces.

An opportunity arose in July to inspect one of the bearings at DSS-14 in Goldstone, California. The good news is that this bearing shows normal wear and should have a few more years of service. A planned downtime for the Goldstone antenna is scheduled for August 6 - 10 to look at the other three bearings. We will have a better understanding of the status of the bearings and the antenna at that time.

Ok. So where are we now? Cassini sequences in development that are impacted by the bearing issues and the STEREO launch are S23, S24, S25, and S26. From October to December, Titan flybys 20, 21, and 22 are now affected. These flybys occur in sequences S25 and S26, which are currently in the Science Operations Plan Update process. 

The worst problem in S25 occurs between the Rings_31 and Saturn_31 segments. Rings had planned to wrap-up their segment using DSS-63, but coverage has now been downgraded to a 34-meter array. This has resulted in a lot of data coming into the Saturn segment and a subsequent overflow of the SSR of 1352 Mbits. 

For S25 and S26, Science Planning has been working to determine how much science return must be cut in order to fit the downlink we now "think" we have available for each sequence. The Target Working Teams and Orbiter Science Teams are working on "sharing the pain" of the cuts for the instruments and trying to keep an eye on unique observations or high value science as they do it.  

S23 and S24 are currently in the Science Sequence Update Process. S23 goes active on board the spacecraft on August 19 so this represents an area of great concern. Waivers and sequence change requests to the S23 sequence were just approved this week that were generated as a result of the DSN issues. There are numerous contentions for passes and impacts for DOY 240, 246, 252, 256, 257, 258, 259, 260, 262, and 265 due to the Stereo launch slip. Many of these passes are during critical activities for S23. These can be worked as soon as Cassini receives an impact analysis and options report from the DSN schedulers, probably on July 27.

An additional wrinkle for S23 is that there now exists the possibility that the Titan 17 altitude may need to be raised based on data from the Titan 16 flyby. A Titan Atmospheres Working Group meeting will be held on July 27 where some of the T16 results will be presented.

With all of this, the DSN schedulers are stretched to the limit as they try to juggle their various customers among the flight projects against reduced antenna availability and changes in launch dates that impact already assigned stations. The resource planning personnel are working with the project schedulers to find the best match of the resources available on the ground and the requirements of the science goals of the projects. Stay tuned.