From: Jet Propulsion Laboratory
Posted: Tuesday, February 7, 2012
The most recent spacecraft tracking and telemetry data in this reporting period were acquired on Feb. 1 from the Deep Space Network 34 meter Station 26 at Goldstone, California. The Cassini spacecraft is in an excellent state of health with all subsystems operating normally except for the known issues with the Cassini Plasma Spectrometer and the Ultrastable Oscillator. 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/.
This week, command sequence S72 started its science observations with a Titan Meteorological Campaign, in which the Imaging Science Subsystem (ISS) looked for cloud events. The Ultraviolet Imaging Spectrograph (UVIS) then executed slow scans in the extreme- and far-ultraviolet across Saturn's illuminated hemisphere. The Visual and Infrared Mapping Spectrometer (VIMS) concentrated on the remains of the storm in Saturn's northern hemisphere, which has been raging since December 2010. Scientists think the String of Pearls (http://saturn.jpl.nasa.gov/news/newsreleases/newsrelease20061011/) may be hidden beneath the storm, and after the storm wanes will try to discover whether the feature may have been destroyed by the tempest. ISS then acquired images over a range of latitudes at low, medium, and high emission angles as the planet rotated. (Emission angle is the angle between the camera boresight and a line normal to the surface point being imaged; straight down is zero degrees.)
The Composite Infrared Spectrometer (CIRS) mapped Saturn's prime meridian from the north pole to the equator to determine the upper troposphere and tropopause temperatures with spatial resolutions of about two degrees of latitude and longitude. Following this, VIMS acquired a high spatial resolution map of the dynamics of Saturn's deep atmosphere. VIMS imaged the same area twice, measuring winds via motions of the clouds. Since these observations were made while the spacecraft was near periapsis, clouds as small as 300 kilometers wide were detectable.
RADAR mapped the same deep region at and around periapsis, obtaining radiometry data at microwave wavelengths much longer than those seen by VIMS. These RADAR maps looked for ammonia gas to reveal the variability of this condensable constituent over the same area where VIMS mapped clouds (formed either via chemical reaction with hydrogen sulfide, thus forming ammonia hydrosulfide clouds, or via direct condensation into ammonia clouds). Unfortunately, this carefully collected RADAR data set and most of the corresponding VIMS data were lost when heavy Australian summer rainstorms drowned out Cassini's signal while it was arriving at Earth on Sunday, January 29.
Targeted Titan encounter T-81 then executed perfectly on January 30, as detailed below.
Wednesday, Jan. 25 (DOY 025)
The S72 background command sequence went active on the spacecraft today. The Cosmic Dust Analyzer instrument turned on heaters to perform a routine 15-hour long decontamination activity.
Thursday, Jan. 26 (DOY 026)
JPL flags flew at half-mast as a NASA-wide Day of Remembrance was observed in honor of the Apollo 1, Challenger, and Columbia crews, as well as other members of the NASA family who have died supporting NASA's mission.
The S74 Sequence Implementation Process kick-off meeting was held today.
Attitude and Articulation Control Subsystem (AACS) engineers read out the spacecraft's Reaction Wheel Assembly revolution counter, and then reset it to 0 counts. This is done about every six months to maintain an accurate accounting of wheel usage and avoid rollover of the counter.
It was discovered that the Cassini remote Science Operations Planning Computers in the ground system were unable to obtain data from some of the servers at JPL that were upgraded last week. Additional server upgrades have been postponed, and the previous version of software was re-installed to restore service.
Friday, Jan. 27 (DOY 027)
Orbit Trim Maneuver 308, the Titan T-81 cleanup maneuver, is being prepared this week to execute on Feb. 2 local time.
Saturday, Jan. 28 (DOY 028)
Cassini passed through periapsis this morning, about 206,000 kilometers above Saturn's cloud tops, which is the closest point to the planet for this orbit. Relative to Saturn, Cassini was traveling 58,187 kilometers per hour.
Sunday, Jan. 29 (DOY 029)
AACS executed a Reaction Wheel Bias maneuver while Cassini was not being tracked from Earth. Using thrusters to maintain attitude, the reaction wheel speeds were set to the required values.
Monday, Jan. 30 (DOY 030)
During the high-altitude Titan T-81 encounter, ISS performed high-resolution observations around closest approach (31,131 kilometers) along Titan's leading hemisphere at high southern latitudes, including a late view of Ontario Lacus before the Sun sets for southern winter. ISS also "rode along" (took advantage of instrument pointing) with CIRS inbound high-phase angle observations and outbound observations over Titan's anti-Saturnian hemisphere at low phase angles. This encounter provided one of Cassini's last views of Titan's high southern latitudes. VIMS rode along with ISS looking for lakes in the south polar region.
Inbound and outbound UVIS observations obtained an image cube of Titan's atmosphere at extreme- and far-ultraviolet wavelengths by sweeping its slit across the disk. These cubes provided spectral and spatial information on nitrogen emissions, hydrogen emission and absorption, absorption by simple hydrocarbons, and the scattering properties of haze aerosols. This was one of many such cubes gathered over the course of the mission to provide latitude and seasonal coverage of Titan's middle atmosphere and stratosphere.
During the flyby, the Radio and Plasma Wave Science instrument (RPWS) 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.
More details about this encounter are available at: http://saturn.jpl.nasa.gov/mission/flybys/titan20120130/.
Back on Earth, Cassini's 56th Project Science Group meeting convened at JPL today; about 130 scientists are in attendance at various meetings and workshops throughout the week.
Near the end of the day, AACS executed a Reaction Wheel Bias maneuver while Cassini's high-gain antenna was off Earth-point.
Tuesday, Jan. 31 (DOY 031)
Starting at midnight Pacific time, Deep Space Station 14, the 70 meter aperture at Goldstone, California, began capturing realtime tracking data (Doppler and range for navigation), and playback of the telemetry that had been stored on Cassini's Solid-State Recorder during the Titan T-81 encounter. All of the data were safely down by 11:00 a.m. local. For background on telecommunications in general, see: http://www.jpl.nasa.gov/basics/bsf10-1.php.
Since approach science began in January, 2004, there have been 258,340 ISS images planned, commanded, acquired, and downlinked. Of these, only 8 percent have suffered any missing data. VIMS has acquired 128,946 cubes in the same time frame, with a similar rate of loss. Raw images from ISS are always posted online soon after receipt at: http://saturn.jpl.nasa.gov/photos/raw/.
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