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Posted by baalke on June 1, 2007, 9:49 pm
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Cassini Significant Events
for 05/23/07-05/29/07
The most recent spacecraft telemetry was acquired on Tuesday, May 29,
from
the Madrid tracking complex. 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" web page located at
http://saturn.jpl.nasa.gov/operations/present-position.cfm.
Wednesday, May 23 (DOY 143):
The spacecraft team performed a Memory Read Out (MRO) today to
downlink the
SSR hardware single/double bit error counters for all 128 submodules
in each
SSR. This activity, scheduled once each month, executes every hour
for the
duration of one downlink track. Analysis of the results is underway.
The Extended Mission Segmentation Working Group (SWG) met last week to
work
any remaining periapsis allocation conflicts and the apopasis
allocations.
One of the first steps in getting a background sequence up to the
spacecraft
with commands that allow science acquisition is to divide the entire
tour
into science segments. Once assigned, the appropriate Rings, Saturn,
X-Discipline, or MAG Target Working Team (TWT) along with the Titan
and
Satellite Orbiter Science Teams (OST) become the custodians and
stewards of
their allocated segments, and lead them through the next step of
integration.
The members of the Working Group included all of the TWT/OST leads,
Investigation Scientists, Science Planners, and the Deputy Project
Scientist. It was the aim of the SWG to take this effort as far as
possible
so that the proposed segmentation plan could be reviewed and perhaps
accepted at the Project Science Group (PSG) meeting next week.
Current major issues still to be addressed are the conflict between
the
Saturn deep atmosphere observations and the icy satellite targeted and
non-targeting flybys in orbits 119-128, and accommodating Saturn
gravity
passes and Saturn atmosphere occultations. These conflicts will need
to be
discussed and addressed at the upcoming PSG meeting.
Thursday, May 24 (DOY 144):
Uplink Operations sent commands to the spacecraft today for a Live
Inertial
Vector Propagator (IVP) update to be performed on DOY 146, and for a
Radio
Science (RSS) Live Update Block to be performed on DOY 148.
Orbit Trim Maneuver (OTM) #112, the Titan 31 approach maneuver set for
Friday, May 25, has been cancelled. A reaction wheel (RWA) bias in
lieu of
the maneuver will be uplinked May 25 to execute May 26.
The decision to cancel centered on the trade between the risk of
accurately
executing a very small maneuver and the impact on an important Titan
occultation radio science experiment. Using the tracking data from
the
Wednesday night pass, and the updated value for the RWA bias, the OTM
had a
magnitude of 12.8 mm/sec. When the deadband tightening delta-V of 5
mm/sec
is subtracted, the 7.8 mm/sec would have been the smallest OTM
attempted to
date.
RSS team members evaluated a number of different trajectories. After
evaluations and discussions during the final OTM review, the consensus
was
that while there was a preference for executing the maneuver, when
considering the uncertainties in the execution of the small OTM and
the
attendant growth in the delivery errors, maneuver cancellation was an
acceptable option.
The S32 sequence leads reported this week that the RSS Titan Bistatic
activity would be tested in the Integrated Test Laboratory in early
June.
In addition, DSN allocations have been finalized for all but the last
four
days of the sequence. As a result, a Science Allocation Panel meeting
will
be held next week to distribute any available sponge bits for science
use.
Saturday, May 26 (DOY 146):
As Cassini approached Saturn and flew through periapsis, the Optical
Remote
Sensing (ORS) instrument teams observed some of Saturn's larger icy
satellites. As non-targeted flybys of Tethys and Epimetheus were
occurring,
the instruments observed Tethys, Mimas, Enceladus, Dione and Rhea as
part of
this campaign. Additionally, RADAR turned its attention to a
scatterometry
observation of Rhea.
A Radio and Plasma Wave Science Whistler activity was performed, this
time
with the spacecraft under RWA control. The previous two observations
were
executed with the spacecraft in thruster (RCS) control to eliminate
interference from the reaction wheels.
Monday, May 28 (DOY 148):
Today Cassini encountered Titan for the second time in the S30
sequence,
traveling at a speed of 6.1 km/sec, and at an altitude of 2300 km.
The
highlight of this fly-by was the Radio Science instrument's
atmospheric
occultation. This was the last of four Titan occultations planned for
Prime
Mission, and was distinguished by being the only one that used
reaction
wheels for pointing control instead of thrusters. The exceptional
pointing
performance that is expected with this strategy will be critical for
accurate measurement of the intensity extinction profiles for X-, S-,
and
Ka-band in the neutral atmosphere. This is particularly important for
the
highly pointing sensitive Ka-band observations.
The observations should provide the large and small structure of
Titan's
neutral atmosphere in exquisite detail. This occultation covered both
the
ingress and egress sides, probing latitudes of 75 degrees south and 73
degrees North, respectively, the highest achieved in the nominal
mission.
Together with previous occultations, these observations will be
critical for
characterization of atmospheric dynamics close to Titan's polar
regions, in
particular Titan's polar vortex.
Tuesday, May 29 (DOY 149):
An informal discussion about lowering the altitude for the Titan 70
flyby,
the last flyby in the proposed extended mission, was held at the
Mission
Planning Forum today. Spacecraft (SCO), Mission Planning, and TOST
representatives reviewed the status of T70 analysis work and the
current
thinking about minimum altitude selection. Both SCO and the science
teams
will address this topic at the PSG meeting next week.
The final build for the Cassini Command and Data Subsystem (CDS)
flight
software (FSW) version 10 was delivered May 30, 2007. Functional and
regression testing have begun, with uplink planned for September 24,
2007.
The changes to the FSW provide the capability for an SSR submodule to
be
"turned off" by the flight team in the event of a permanent failure of
a
submodule in the Dynamic Random Access Memory (DRAM). Prior to
launch, the
commercial DRAM had been expected to fail at a much higher rate than
has
been experienced in flight. As a result, this submodule isolation
capability originally planned to be uplinked in 2003 as part of
version 9
was deemed low priority at that time. Each submodule makes up less
than one
percent of the memory capacity of each of the two SSRs.
Since the Significant Events report is written on Wednesday, we are
happy to
report that a RADAR image from the Titan 30 flyby is Astronomy Picture
of
the Day on Wednesday, May 30.
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.
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