Cassini Significant Events for 07/14/05 - 07/20/05 The most recent spacecraft telemetry was acquired Wednesday, July 20, from the Madrid tracking stations. The Cassini spacecraft is in an excellent state of health and is 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 . Activities this week: Thursday, July 14 (DOY 195): Cassini passed by Rhea today for a non-targeted encounter. The RADAR instrument performed scatterometry and radiometry observations of that moon to constrain the bulk density of the top decimeter of the surface, and to help constrain the surface thermal properties. The main event today was Cassini's second targeted flyby of Enceladus. A non-targeted encounter took place on February 17, 2005 at a range of 1172 km. At that time the Dual Technique Magnetometer (MAG) saw a remarkable signature in Saturn's magnetic field in the vicinity of Enceladus suggesting the presence of ions surrounding the icy moon. Gases may be originating from the surface or possibly the interior of Enceladus. The first targeted encounter was on March 9, 2005 at a distance of 500 km. The encounter this week had originally been designed to be flown at an altitude of 1000 km. Because the February and March flybys suggested intriguing magnetic and atmospheric signatures, the Cassini science community was very interested in obtaining more detailed data. After investigating the situation, the Program approved the release of a new reference trajectory lowering the flyby altitude to 175 km. This is Cassini's lowest-altitude flyby of any object to date. For this encounter the entire suite of Magnetospheric and Plasma Science instruments took high data-rate science. MAG monitored the interaction of Enceladus with the Saturnian magnetospheric plasma to determine whether it generates an internal magnetic field, and engaged in measurements that will yield unique observations of Saturn's internal magnetic field. The Cosmic Dust Analyzer took measurements to help determine the density of the E Ring, and of the dust surrounding Enceladus. The Radio and Plasma Wave Science instrument (RPWS) executed many observations in the immediate vicinity of Enceladus that will help characterize the plasma wave spectrum, and searched for evidence of pickup ions. All the Optical Remote Sensing instruments participated in a hectic campaign to obtain high-resolution mosaics and spectra of Rhea and Enceladus during their respective flybys. Of primary interest to the Ultraviolet Imaging Spectrograph is an occultation by Enceladus of the star Bellatrix during Cassini's closest approach which will help constrain Enceladus' atmospheric species and density. The Composite Infrared Spectrometer obtained its only temperature measurement of Enceladus' winter north polar night region, which has been in darkness for more than eight years, and also looked for active surface sources that may contribute material to Saturn's E Ring. Further icing on the cake today included additional non-targeted flybys of the Saturnian moons Prometheus, Methone, and Epimetheus. Friday, July 15 (DOY 196): Following yesterday's very successful Enceladus flyby, the Radio Science Subsystem (RSS) performed its fifth occultation experiment intended to measure the properties of Saturn's rings and atmosphere. The orbit 11 RSS Saturn/Rings Egress occultation completed successfully over the DSS stations at Canberra for the first segment of the occultation. Then these stations overlapped with the Madrid stations for 45 minutes before Madrid was left to cover the second segment. This was the first occultation implemented within a Ground Movable Block (GMB) and did not require a Live Inertial Vector Propagator Update shortly prior to the occultation. Out of the eight diametric occultations taking place between May and September of this year, this occultation occurred over the lowest ground antenna elevation angles, not exceeding 30 degrees of elevation. This made it very challenging in terms of developing an optimum pointing strategy for Ka-band. Monday, July 18 (DOY 199): Cassini enters Solar Conjunction today with a separation angle of 4 degrees. During conjunction, communications with the spacecraft become degraded due to interference from the sun. For the next nine days the Spacecraft Operations and Mission Support and Services offices will participate in a campaign where multiple non-operational commands are sent to the spacecraft on a daily basis to obtain link characterizations. In addition, Commands will be sent to the spacecraft to modify the System Fault Protection Command Loss Timer (CLT) strategy during the conjunction period. Part of the fault protection system monitors communications with the ground. If for any reason a CLT command is not received by the spacecraft before the expiration of the time limit, the spacecraft will assume there is a "problem," call fault protection and put Cassini into safe mode until the "problem" is resolved. This is a common feature on all spacecraft and CLT commands are sent each time Cassini has a DSN pass to prevent the expiration of the time limit. As was mentioned in the report last week, the Program approved raising the T7 flyby altitude from 1025 to 1075 km to avoid the risk of atmosphere-induced torques that could possibly result in spacecraft loss of attitude control and safing. Today the Navigation team released a new reference trajectory 050207 to accommodate these changes. The final sequence products for S13 were made available in the Program file repository for team review. Uplink Operations sent commands to the spacecraft to disable RPWS sounder operations, turn the Ka-band ON for an RSS Solar Conjunction demo, load the ISS version 1.4 flight software in to the cameras, and repair the SSR partitions for CDS, ACS and the instruments. A workshop on the surface of Titan was hosted by the USGS in Flagstaff, Arizona on July 18 and 19. The goal of the workshop was to develop an integrated picture of Titan from data sets returned over the last year regarding primarily the surface, but also including the atmosphere, especially as it relates to the surface, and use this as an aid to planning the remainder of the Cassini prime mission. Items that were discussed included a summary of the main findings from each team, opportunities for integrated data interpretation, the development of cooperative observations and the TITEWAD web-based GIS planning tool, ground-based support measurements, aftermarket changes to the science operations plan, and extended mission planning. Tuesday, July 19 (DOY 200): An Atmospheric Science Preview meeting was held today to highlight interesting Saturn observations being conducted in Revs 9-15. In the October time frame, another preview meeting will be held to cover the Equatorial Orbits, i.e. Revs 15-24ish. Wednesday, July 20 (DOY 201): An S15 Science Operations Plan Update process project briefing and waiver disposition meeting was held today. It was reported in the Significant Events last week that an encounter strategy meeting for Enceladus2 through Titan5 was held on July 7. Well, I goofed. The notice was distributed on the 7th. The actual meeting does not occur until July 22. Sorry about that! The RADAR data previously reported as delivered to the archive is now on-line at the PDS Imaging Node/USGS. To access the data link to http://pdsimg.jpl.nasa.gov/Admin/resources/cd_cassini.html. The data will also be placed on-line at the Imaging Node/JPL so that users can search for products based upon geometry. Wrap up: Check out the Cassini web site at http://saturn.jpl.nasa.gov for the latest press releases and images. The Cassini-Huygens mission is a cooperative project of NASA, the European Space Agency and the Italian Space Agency. The Jet Propulsion Laboratory, a division of the California Institute of Technology in Pasadena, manages the Cassini-Huygens mission for NASA's Science Mission Directorate, Washington, D.C. JPL designed, developed and assembled the Cassini orbiter. --- To unsubscribe from Cassini Spacecraft Updates, send a message to xxx@xxxxxxxxxxxxxxxxx --- Visit the JPL Cassini home page for more information about the Cassini Project: <http://saturn.jpl.nasa.gov/>
