Cassini Significant Events
10/29/08 - 11/04/08
The most recent spacecraft telemetry was acquired on Nov. 4 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, Oct. 29 (DOY 303)
Orbit Trim Maneuver (OTM) #169 was performed today. This is the
apoapsis maneuver setting up for the Enceladus 6 and Titan 46
encounters on Oct. 31 and Nov. 3. The reaction control subsystem
(RCS) burn began at 10:59 AM PDT. Telemetry immediately after the
maneuver showed the burn duration was 191 seconds, giving a delta-V
of 0.23 m/s. All subsystems reported nominal performance after the
maneuver. Due to large downstream delta-v costs associated with
executing the maneuver over the backup pass, this OTM was uplinked a
day early to help ensure success over the primary pass. Thruster
performance was slightly off nominal during this OTM. This resulted
in a 7 mm/sec under burn due to increased thruster chamber pressure
roughness. This roughness has been higher for the last few RCS OTMs.
Small variations in chamber pressure roughness and thrust are normal
for hydrazine thrusters, and this recent increase is within the
historical range. More analysis will be performed and a correction
for this will be applied at OTM-171.
Thursday, Oct. 30 (DOY 304)
Uplink Operations has heard from Science Planning, Imaging Science
(ISS), the Cassini Plasma Spectrometer (CAPS), and Composite Infrared
Spectrometer (CIRS) teams. All are a no-go for the Live Update
planned for DOY 313, Nov. 8.
Friday, Oct. 31 (DOY 305)
Non-targeted flybys of Methone, Pandora, Daphnis, Atlas, and
Epimetheus occurred today.
On Oct. 31, Cassini flew by Enceladus at an altitude of 174 km, a
speed of 17.7 km/sec, and latitude 28 degrees S. This was the second
close flyby of Enceladus this month with the Optical Remote Sensing
(ORS) instruments primary for the majority of observations, and with
the Radio Science Subsystem (RSS) taking Enceladus gravity data both
before and after closest approach to help measure the gravity field
by estimating the mass separate from other gravitational harmonic
coefficients. This was the closest non-targeted flyby of the mission
to date. The previous closest was an Enceladus flyby back in orbit
#3 of the Prime Mission at a little under 1300 km.
In close coordination with the other ORS instruments, ISS held prime
pointing control of the spacecraft at closest approach for E6.
Images were obtained of the south polar terrain and active tiger
stripes at very high spatial resolution. This observation complements
coverage obtained in E4, the last ORS Enceladus flyby in mid-August
2008.
ISS imaged active vent regions in unprecedented detail to identify
geological structure modifications of the tiger stripe rifts that
accompany ongoing eruptions. High resolution imaging of surface
features in between the tiger stripes will provide details that could
be used to characterize the surface modification effects of fallback
from the plumes adjacent to eruption centers. ISS also obtained
high-resolution multi-spectral mosaics of the tiger stripes to search
for temporal changes in composition associated with possible time
variability of the eruptions. By combining the results of E4 and E6
closest-approach imaging, ISS obtained data for stereo coverage of
the tiger-stripes to produce high-definition three-dimensional
topographic maps of both active and inactive eruption centers.
Data from the RSS gravity passes in the wings of this Enceladus flyby
will be used in conjunction with the passes on orbits 80 and 130 to
determine the Enceladus gravity field up to degree 2. This
observation consisted of two segments; an inbound, covered by the
Madrid station, and an outbound, covered by Canberra and Madrid.
Saturday, November 1 (DOY 306)
This will be the last report on the Cassini Main Engine (ME) cover
until next year. For the last several months there has been a great
deal of ME cover activity as the flight team used this device to
protect the spacecraft from potential dust hazards. The cover was
deployed on Oct. 30 at the end of the Orbit Trim Maneuver (OTM) 169
backup pass for a dust hazard at E6, and reopened on Nov. 1 for the
50th cycle of the cover. The next predicted hazard does not occur
until October 2009.
Monday, November 3 (DOY 308)
On Sunday the CIRS instrument team reported that their Bus Interface
Unit (BIU) had been marked "sick". A reboot of the instrument
re-established communications between CIRS and the CDS, but the end
result was the loss of CIRS' Titan (T46) data. This particular
anomaly occurs occasionally for CIRS with no harm done other than a
brief data outage. As reported by members of the CIRS team, "The
CIRS instrument has fully recovered from the BIU anomaly that
occurred on Sunday morning. All telemetry is as expected and packets
came flying across the BIU indicating that the instrument was going
about the business of collecting Deep Space Calibration (DSCAL) data.
We'll spend the next few days poring over the telemetry, memory
readouts, and instrument packets to see if we can further our
knowledge of the anomalous state. Meanwhile, the instrument will
continue executing commands from the background sequence." Back to
business as usual.
On Monday, Nov. 3, Cassini flew by Titan at an altitude of 1105 km, a
speed of 6.3 km/sec, and operating under RCS control. Latitude at
closest approach was 3.5 degrees S, and the encounter occurred during
orbit number 91. T46 was the tenth in a series of outbound
encounters and the second Titan encounter in the Cassini Equinox
Mission.
RSS observations of Titan included ionospheric and atmospheric
occultations and bistatic surface scattering on both the ingress and
egress sides. The egress atmospheric occultation was the first to
probe the mid northern-latitudes of Titan at ~33 deg. N. The ingress
occultation probed mid-southern latitudes at ~33 deg. S. Combined
with results from eight other latitudes probed during the Cassini
prime mission, the occultations will shed more light on latitudinal
variability of the electron density profile of the ionosphere,
temperature and pressure profile, microwave absorption profile, and
small scale-structure of the neutral atmosphere including gravity
waves and turbulence.
In addition, bistatic surface scattering was observed at low southern
latitudes on the ingress side from 24-31 deg. S, and low northern
latitudes on the egress side from 25-30 deg. N. The incidence angle
for both sides was close to the Brewster angle range for likely
surface compositions. Same- and cross-polarized components of
mirror-like surface echoes observed at the ground receiving stations,
if detectable, will provide valuable information about the dielectric
constant and physical state of the surface region probed.
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