From: Jet Propulsion Laboratory
Posted: Saturday, July 18, 2009
The most recent spacecraft telemetry was acquired on July 14 from the Deep Space Network tracking complex at Goldstone, California. 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" page at: http://saturn.jpl.nasa.gov/operations/present-position.cfm
Wednesday, July 8 (DOY 189)
Part two of the S51 background sequence began execution in the early hours this morning. S51 will conclude on July 23.
A kickoff meeting was held today for the S56 Science Operations Plan (SOP) process. The process runs for approximately 15 weeks and will conclude on Oct. 12 when it will be handed off to Uplink Operations for final development and execution.
On Wednesday, July 8, Cassini flew by Titan at an altitude of 965 km and a speed of 6 km/sec. Closest approach for T58 occurred at 11:26 AM PDT at a latitude of 52.2 degrees S. T58 is the seventh flyby in a series of eleven inbound encounters and the fourteenth Titan encounter in Cassiniís Equinox (extended) Mission.
During T58, the Ultraviolet Imaging Spectrograph (UVIS) observed a long stellar occultation and a solar occultation. The two observations probe different parts of the atmosphere and are the most valuable Titan observations for UVIS because they provide detailed vertical profiles of nitrogen using the Extreme Ultraviolet (EUV) channel during solar occultations, and hydrocarbons, HCN, and aerosols using the Far Ultraviolet (FUV) channel during stellar occultations.
The solar occultation sampled the northern polar vortex region from about 900 kilometers altitude up to about 2,300 kilometers. This range overlaps the atmospheric region sampled by the Ion and Neutral Mass Spectrometer (INMS) and by Cassiniís attitude control system, or AACS. Solar occultation measurements give a measure of the density profile of N2, the main constituent of the atmosphere, and the rate of change of the N2 density with altitude gives information on the temperature. There has been a long-running controversy about the density of the high atmosphere. AACS consistently gets a higher value than INMS and UVIS. This is one of the questions to address with the data from these observations.
Both the solar and stellar occultations show a complex picture of the upper atmosphere. Density profiles and mixing ratios cannot be described as a simple function of latitude and longitude. There is more going on, perhaps gravity wave activity, perhaps some other phenomena, which make the upper atmosphere more variable than simple models would predict. Continued observations like the ones in T58 will help us sort out these issues.
With INMS riding along, the RADAR instrument observed the western edge of Xanadu to study the boundary with Shangri-La using Synthetic Aperture Radar (SAR). The swath ran parallel to the T55/56/57 mapping sequence and covered Ontario Lacus. Altimetry observations of this same area will be obtained by RADAR during the T60 flyby.
The Composite Infrared Spectrometer (CIRS) performed a surface temperature scan and disk integration to search for new gases in far-IR, Imaging Science (ISS) acquired full-disk, global-mapping, and regional-mapping mosaics of the region southwest of Senkyo and northeast of Tsegihi at low phase angles, and rode along with the Visual and Infrared Mapping Spectrometer (VIMS) for high resolution imaging and cloud monitoring. VIMS ride-along observations provided information on the composition of Titan's atmosphere, and along with UVIS, observed during a stellar occultation that will provide information on the composition of Titan's atmosphere.
For the Magnetospheric and Plasma Science (MAPS) instruments, the Magnetospheric Imaging Instrument (MIMI) measured energetic ion and electron energy input to Titanís atmosphere, and the Radio and Plasma Wave Science (RPWS) instrument measured thermal plasmas in Titan's ionosphere and surrounding environment, searched for lightning in Titan's atmosphere, and investigated the interaction of Titan with Saturn's magnetosphere. As in T55, T56 and T57, Magnetometer measurements provided a description of the draping and the pileup of the external magnetic field around Titan on the night side hemisphere. This data set will be a good complement to those from T52, T53, T54, T55, T56 and T57 to characterize the background field for similar local times with respect to Saturn and different Saturn Kilometric Radiation longitudes.
Thursday, July 9 (DOY 190):
The Cassini Satellite Orbiter Science Team (SOST) has requested the Project to perform the Enceladus 9 flyby, scheduled for April 2010, on reaction wheels instead of thrusters for more accurate acquisition of Radio Science (RSS) gravity data. AACS is evaluating the request and will decide if the wheels have sufficient control authority after the very similar E7 flyby on Nov. 5 of this year. SOST has proposed to perform parallel sequence development of the wheels/thrusters options so that products due for port 1 of the S59 SOP process will not be delayed. If the decision is that E9 will remain on thrusters, SOST will alter the observations to a MAPS flyby rather than an RSS flyby.
Friday, July 10 (DOY 191):
Weeks 35-36 and part of week 37 have been negotiated for the S53 DSN station allocations. One issue affects the Titan 61 dual playback activity in August. The DSS-43 track scheduled on DOY 239 has been lost due to a bearing maintenance activity scheduled for the same time. An extension of the DOY 239 DSS-14 track plus a shadow DSS-63 track on DOY 241 to make up for some of the lost data volume has been negotiated. An SSR overage waiver will still be needed and work on this waiver will proceed once the allocation file has been released.
The first science activity today was one observation in a CIRS campaign to study how and how quickly ring temperatures change as the Sun heats one side of the rings and then the other as Saturn passes through equinox. This was followed by two ISS observations: an AZSCAN to track azimuthal variations in the rings and an EQXSHADOW intended to utilize the low solar elevation prior to equinox to look for shadows cast by vertical structure within the rings. Later in the week, ISS will be performing MNRNGSHAD observations looking for shadows cast by various moons of Saturn as they shepherd the rings. While all this was occurring, the MAPS instruments surveyed the inner portion of the Saturnian magnetosphere.
Saturday, July 11 (DOY 192):
A non-targeted flyby of Dione occurred today.
Sunday, July 12 (DOY 193):
Orbit Trim Maneuver (OTM) #206 was performed today. This is the cleanup maneuver following the Titan 58 encounter on July 7. The main engine burn began at 10:38 PM PDT. Telemetry immediately after the maneuver showed the burn duration was 20.75 seconds, giving a delta-V of 3.51 m/s, as planned. All subsystems reported nominal performance after the maneuver.
Monday, July 13 (DOY 194):
Files were due today for the Port 2 delivery as part of the SOP process for S55. Spacecraft Operations and Science Planning are currently evaluating hydrazine use for conducting the Enceladus 8 flyby Ė in November - on thrusters or reaction wheels. The decision will be made before the Port 3 delivery.
The S55 Science Operations Plan process Port 2 delivery was due today. The merge will be performed this evening with files distributed to the team for review tomorrow. Files for the S54 Port 3 delivery are also due tomorrow.
The Integrated Test Laboratory performed a simulation of the Titan 61 flyby this week to enable RADAR to verify their observation plans. RADAR is currently reviewing the test results and c-kernel, and will report back with their findings.
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