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Cassini Significant Events 12/23/08 – 01/06/09

The most recent spacecraft telemetry was acquired on Jan. 6 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/mission/presentposition/

Tuesday, Dec. 23 (DOY 358):

Activities on the ground have been minimal over the last two weeks due to the holiday period. An interesting twist was the possibility of an unplanned live update as the result of the cancellation of Orbit Trim Maneuver (OTM) 179. Read on for details.

The propellant cost for cancellation of OTM-179 -- due to execute tomorrow, Christmas Eve - was only about 0.46 m/s. Execution would have required the insertion of a bias prior to the OTM in order to eliminate a period of about one hour of low rpm time on one of the reaction wheels. Because of the low cost and the complications associated with the RWA bias, and in order to give the flight team a break and reduce work over the holiday, OTM-179 was cancelled.

Remember that every time Cassini performs a maneuver, the result is an adjustment to the spacecraft trajectory to keep it flying along the route planned for it by the Navigation team. Canceling a maneuver means the trajectory will be slightly different from what was planned. For many prior cancellations, the change in spacecraft pointing as a result of the change in the trajectory was so minimal that there was no impact. In this case, the impact to science pointing was not clear. At the weekly Operations Status and Coordination meeting, Science Planning announced that an unplanned live update might be necessary. Based on the latest orbit determination solution, pointing was a bit off for an Imaging Science (ISS) observation of Tethys due to execute on Jan. 4. With tomorrow being Christmas Eve, a schedule was set by the sequence leads for a Go/No Go meeting on Monday, Dec. 29, and uplink of the files just prior to execution. Science would have to perform their analysis quickly in order to meet the schedule. As it turned out, the input from Science was that this was a high phase Tethys observation at ~500,000 kilometers. It was not high enough priority to recommend unplanned work over the holidays for the uplink people. The update became a No/Go.

A traveling exhibit called “Spectacular Saturn - Images from the Cassini-Huygens Mission” ended a three-month run at the Herbert F. Johnson Museum of Art at Cornell University this week. Over 35,000 visitors attended the exhibit, which will now transfer to the National Air and Space Museum in Washington where it is due to open on February 2.

Saturn is now visible in the eastern evening sky by mid evening. Telescope viewers are in for a real treat in 2009. Since 1995 we have been seeing the southern face of the rings. Saturn’s rings start 2009 nearly edge-on; open slightly showing the south side of the rings through May, tipping back to an edge-on view in September, a view that won’t be seen again for approximately 14.5 years. After Saturn passes conjunction with the Sun in late September, Saturn’s north side of the rings is visible and open slightly through the rest of the year, but you’ll have to get up before dawn to see them!

Part two of the S46 background sequence, which was uplinked to the spacecraft back on Dec. 12, began execution today. Part two will run until Jan. 9 when S47 takes over.

Wednesday, Dec. 24 (DOY 359):

An update to the ISS planning software Imaging Science Subsystem Pre-commanding Tool was installed for operational use last week. ISSPT is used by the Support Imaging team to set the image parameters for support images.

From now through the next 5 days, the spacecraft is the one that did all the work! Here is what went on:

Dec. 24, DOY 359:
Ultraviolet Imaging Spectrograph (UVIS): Observed stellar ring occultations of the stars Delta Centauri and Beta Crucis
Cassini Plasma Spectrometer (CAPS): Pointed the spacecraft for the Magnetospheric and Plasma Science (MAPS) survey of the magnetosphere
ISS: Observed several small satellites as part of a satellite orbit campaign

Dec. 25, DOY 360:
Composite Infrared Spectrometer (CIRS): Mapped the thermal emission of the rings at 60 degrees north latitude and mid-phase
Optical Remote Sensing (ORS) instruments: With UVIS as prime, observed the occultation of Alpha Arae by the rings
ISS: targeted the rings to produce short movies of selected features
CIRS: Scanned the rings at 20 degree north latitude and high phase as one of multiple observations designed to look at both the lit and unlit sides of the rings at similar geometries to study the vertical thermal distribution within the rings

Dec. 26, DOY 361:
ISS: Targeted the icy moon Enceladus to obtain medium resolution images of the plumes at high phase
CIRS: Scanned the rings at 20 degrees south latitude at high phase to study the vertical thermal distribution
Visual and Infrared Mapping Spectrometer (VIMS): Performed a high phase rings composition observation with CIRS and UVIS as riders
CIRS: Followed with a scan of the Encke Gap within the A ring

Dec. 27, DOY 362:
CIRS: The day was filled with a 21 hour CIRS rings observation that continued into DOY 363. This was an excellent opportunity for obtaining thermal azimuthal scans at several radii in each of the major - A, B, and C - ring systems. The instrument looked for variations in the temperatures as a function of local hour angle with emphasis on the rings’ particles as they entered the shadow of Saturn. From this information, scientists learn about how quickly the ring particles cool down and heat up, which in turn tells them about the structural properties of the ring particles.

Dec. 28, DOY 363:
ORS: With ISS as prime, obtained one more of hundreds of distant observations of Titan designed to improve understanding of the clouds on Titan.
Magnetometer (MAG): Performed a rolling calibration, which is done outside of 15 Rs.

Dec. 29, DOY 364:
CIRS: Performed regional mapping of Saturn’s upper troposphere in the far infrared. These are repeated over the course of the mission to reveal seasonal variations.

Monday, Dec. 29 (DOY 364)

Back to work!

With S47 beginning execution on Jan. 9, the process has now begun for assessment of the first live update for that sequence, scheduled to execute Jan. 12-14. Saturn and Rhea are the targets, with observations planned by ISS, CIRS, and VIMS. Science Planning has deferred the Go/No go decision to the instrument teams. The kick off meeting is scheduled for Jan. 5 with the Go/No go on the 6th.

Tuesday, Dec. 30 (DOY 365):

The Command Loss Timer setting was increased in the background sequence to 5 days on Dec. 30 for the time range Dec. 31, 2008 to Jan. 4, 2009. It will be restored to 90 hours on January 4, 2009. The command loss timer is part of system fault protection on board the spacecraft. If the craft does not hear from the ground within a specified period of time between commands, it automatically takes steps to protect itself. Since the Cassini Program had scheduled DSN tracks approximately every other day from Dec. 29 through Sunday, Jan. 4, it made sense to increase the CLT duration for this period.

More science:

Dec. 30, DOY 365:
CIRS: With UVIS riding along, stared at Saturn for over twelve hours to measure oxygen compounds in the stratosphere. Each such study focuses on aparticular latitude. Today the sub-spacecraft latitude was 14 degrees
UVIS: Made several slow scans across Saturn’s visible hemisphere to form spectral images in both the far ultra-violet and the extreme ultra-violet

Dec. 31, DOY 366:
ORS: Performed a long range, low phase observation of Dione
VIMS: Performed an in-flight calibration of the solar port

Jan. 1, DOY 001:
MAPS: Took over pointing control of the spacecraft to direct activity to monitoring the plasma and field characteristics
CIRS: As Cassini headed back toward Saturn, the distance from the planet was again appropriate for regional mapping by CIRS of the upper troposphere in the far infrared. Repeating these observations provides measurements that can be compared for temporal variations
VIMS: Observed the Pleiades to provide in-flight characterization of the pixel look directions within the instrument. This supports geometric calibration of images

Jan. 2, DOY 002:
VIMS: Began the first of several lengthy studies of the polar regions to study the structure and dynamics of the polar vortices and their variability over time, including seasonal changes.

Jan. 3, DOY 003:
UVIS: An occultation of Beta Cru by Saturn provided an opportunity for UVIS to measure the vertical profile of aerosol and gas opacity as well as temperature at the highest altitudes of Saturn’s atmosphere
VIMS: With ISS and UVIS participation, returned to the polar dynamics study

The north polar region is experiencing sunlight for the first time since Cassini has been at Saturn. Images of the north pole from VIMS, as well as ISS and UVIS, will reveal the structure and microphysical nature of upper tropospheric clouds that help form the bizarre hexagonal feature there.

Jan. 4, DOY 004:
Several Cassini remote sensing instruments - in particular ISS, UVIS, and VIMS – made a collective, concerted effort to reveal the multifaceted nature of polar aurorae. All three instruments imaged the aurorae using a variety of wavelengths, thus quantitatively mapping their power over the polar regions.

Multiple images acquired regularly over short periods of time spanning minutes to hours will characterize the transient nature of auroral phenomena. Correlations of auroral activity with underlying hazes will help our understanding of the role aurorae play in generating polar hazes and clouds.

ORS: Imaged Tethys from 415,000 kilometers away at a dark phase angle of 162 degrees.

MAPS: Engaged in a campaign to measure the vertical structure and dynamics of the inner magnetosphere as Cassini crossed Saturn’s equatorial plane nearly tangent to the L-shell set of planetary magnetic field lines.

Jan. 5, DOY 005:
The polar dynamics study of the last several days continued with attention shifted to the south pole. The south polar region is about to enter more than a decade of polar winter. One goal of these observations is to track the winds and determine vertical cloud structure in the dynamic polar regions. Simultaneous observations by ISS and VIMS provided the opportunity to solve for cloud heights using two independent measurements: by absorption by methane gas above the clouds from ISS, and by thermal emission at various wavelengths from VIMS.

Friday, Jan. 2 (DOY 002):

Seven Instrument Expanded Block files (IEB) were uplinked to the spacecraft in preparation for the start of S47 execution next week. An additional IEB will be sent up tomorrow, and the background sequence itself will go up on the 7th.

Monday, Jan. 5 (DOY 005):

An Uplink Readiness Review for the propulsion system fuel-side repressurization activity was held Jan. 5. The command files have been tested, procedures have been finalized and approved, and the Spacecraft Team is prepared for the activity. The first commands will be uplinked to the spacecraft tomorrow.

Update: Activities began on Jan. 6, with the lowering of fault protection thresholds for the Overpressure-1 and Overpressure-2 System Fault Protection algorithms. This was successfully done and brings the fault protection thresholds to levels consistent with the remaining mission bi-propellant levels.

The final sequence approval meeting for S47 was held today along with the kickoff meeting for the Saturn/Rhea live update. The ORS instruments have given their Go so the teams will now begin to build the files for uplink on Jan. 10.

Tuesday, Jan. 6 (DOY 006):

Port 2 products were delivered today as part of the S50 Science Operations Plan (SOP) Implementation process. A kickoff meeting was also held today for S51 SOP.

Most of today was devoted to the first of three compositional studies of Saturn’s atmosphere led by CIRS with UVIS and VIMS participating. These measurements require lengthy integration times at a single point on Saturn.