Cassini Significant Events 07/29/09 – 08/04/09

The most recent spacecraft telemetry was acquired on Aug. 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, July 29 (DOY 210)

A kickoff meeting was held today for the S57 Science Operations Plan (SOP)
process. The process runs for approximately 15 weeks and will conclude on
Oct. 26 when it will be handed off to Uplink Operations for final
development and execution.

Thursday, July 30 (DOY 211):

As Cassini approaches Saturn equinox on Aug. 11, science observations have
become more tailored to this specific geometry. This week, imaging (ISS)
looked for moon shadows on the rings, and performed a continuation of a
12-hour long movie of a streamer-channel feature raised in the F ring by
Prometheus. The Visual and Infrared Mapping Spectrometer (VIMS) had an
11-hour activity to observe ring spoke formation at high phase, Imaging
Science took 14 hours of data for a movie of the unlit ring face at
high-phase to look for spoke features, and the Composite Infrared
Spectrometer (CIRS) observed the rings at orthogonal pointing attitudes to
look for polarization effects.

Also this week ISS performed a first-time observation of Titan's shadow as
it crossed Saturn. This data will be useful for Titan aerosol science. For
this observation Cassini is at yet another unique geometry that occurs only
a few times in the Extended Mission and did not occur at all in Prime
Mission.

Continuing activities this week included campaigns to obtain data by the
Magnetosphere and Plasma Science instruments on magnetospheric boundaries,
and ongoing Optical Remote Sensing observations for satellite orbit
determination and Titan cloud monitoring.

Friday, July 31 (DOY 212):

New meteorological data indicate the value for Saturn's rotation period
could be more than five minutes shorter than previously believed - and that
Saturn is more like its larger neighbor Jupiter than previously considered.
The rate at which Saturn spins provides important data for planetary
scientists interested in the ringed world. Obtaining an accurate fix on that
number is critical to enhancing scientists’ understanding of the planet's
evolution, formation, and meteorology. The report on this finding is
published in the July 30 issue of the journal Nature. A summary may be seen
at http://saturn.jpl.nasa.gov/news/cassinifeatures/feature20090729/.

Saturday, Aug. 1 (DOY 213):

Orbit Trim Maneuver (OTM) #210 was performed today. This is the apoapsis
maneuver setting up for the Titan 60 encounter on Aug. 8. The Reaction
Control Subsystem burn began at 4:59 pm PDT. Telemetry immediately after the
maneuver showed the burn duration was 14.75 seconds, giving a delta-V of
22.84 mm/s. All subsystems reported nominal performance after the maneuver.

Monday, Aug. 3 (DOY 215):

The Attitude Control flight software on board Cassini uses an algorithm to
allocate the 165 milli-Newton-meters of torque available for the Reaction
Wheel Assembly (RWA) into certain categories. One of the categories is
torque available to compensate for friction experienced by the wheels as
they spin. In the past, Spacecraft Operations (SCO) has used a conservative
number for friction torque because large friction spikes were not expected,
and it was considered preferable to have an autonomous swap to the RCS
thrusters than to let the wheels continue to run in an unexpected condition.
However, as the wheels have aged and the lubricant has become less
efficient, the size of the friction torque spikes has understandably
increased, and it is preferred to avoid an unnecessary swap to thrusters
because of a well understood condition, and the attendant hydrazine usage
that would accompany that. SCO increased the original flight parameter
values in early 2001, after a large friction spike caused a transition from
RWA control to thruster control. It was increased again in 2006 after
larger friction spikes were identified. It was recently decided to increase
the amount of torque available to any wheel for friction spikes to be the
maximum available after compensating for all other parameters. This patch
took effect Monday, Aug. 3, 2009, while the spacecraft was on thruster
control during an RWA bias.

Navigation published a new Cassini Reference Trajectory labeled 090721.
This is an update to the trajectory published in August of 2008, and goes
through the end of the proposed 7-year extended-extended mission.

Tuesday, Aug. 4 (DOY 216):

A dust hazard update was presented today at the Mission Planning Forum. A
slightly updated model of the G ring based on data collected by Cassini to
date has given some better insight into dust conditions in the Saturn
environment for the remainder of the extended mission and on into the
proposed Solstice Mission. This update will be factored into any protection
or avoidance strategies that the flight team implements. Over all, most
hazard assessments did not change; only the identified hazard in February of
2010 has measurably increased. In addition, rings centered on the orbits of
some of the Saturnian satellites are about as dense as the G ring at
~180,000 km.

The Rhea 2 flyby in March of 2010 was included in the discussion today as a
possible dust hazard. The current planned flyby trajectory is 100 km over
the north pole. Although dust is believed to have been detected by the
Magnetospheric Imaging Instrument in the vicinity of Rhea, the results are
hotly debated. No known large-only dust populations exist anywhere in the
Solar System; there is always some size distribution. It is also the case
that dust particles orbiting outside of about ±40 deg inclination would have
their orbital parameters affected by Saturn’s gravity sufficiently over time
that they would collide with Rhea, and small dust should be lost regardless
of inclination via solar radiation pressure and charging effects. The
conclusion at the end of the forum was that there is no plausible
explanation for dust sufficient to constitute a risk to Cassini over Rhea’s
poles, and that the Rhea-2 flyby will be safe at the 100 km altitude.