Cassini Significant Events 08/05/09 – 08/11/09

The most recent spacecraft telemetry was acquired on Aug. 11 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, Aug. 5 (DOY 217)

The Spacecraft Operations Office (SCO) held an engineering activities review
for S58 today. At the review the team discussed all engineering events to
execute from March 1 to April 5, 2010.

According to the Navigation Team, the target miss distance at T60 resulting
from cancelling Orbit Trim Maneuver (OTM) #211 would be less than 1 km, and
the downstream consequences are not significant. In addition, when comparing
pointing performance with and without the maneuver, Science Planning
determined that there was very little change. Because of this, OTM-211, due
to execute on Aug. 6, has been cancelled.

Thursday, Aug. 6 (DOY 218):

All participating teams have submitted files for the first input port of the
S56 Science Operations Plan process. On Monday, Aug. 10, Science Planning
will host an S56 Science Highlights meeting. The presentation will focus on
what observations will be unique and highest priority during that sequence,
and will include presentations from the Target Working Teams and Orbiter
Science Team leads, with comments from Investigation Scientists and other
instrument team representatives.

Friday, Aug. 7 (DOY 219):

A new moonlet, situated about 500 kilometers inward from the outer edge of
the B ring, was found by detection of its shadow which stretches 41
kilometers across the rings. The shadow length implies the moonlet is
protruding about 200 meters above the ring plane. If the moonlet is orbiting
in the same plane as the ring material surrounding it, it must be about 400
meters across. This type of observation is only possible around the time of
the Saturn equinox crossing. The illumination geometry that accompanies
equinox lowers the sun's angle to the ring plane and causes out-of-plane
structures to cast long shadows across the rings.

Sunday, Aug. 9 (DOY 221):

Cassini completed its 61st targeted flyby of Titan on Aug. 9, passing the
moon at a distance of 970 kilometers above the surface and at a speed of 6.0
kilometers per second. The onboard sequence clocked out as expected and all
data was waiting to be transmitted to the DSN. As unique, high-priority
data from Saturn's equinox crossing was to be acquired soon after the flyby,
there was no redundant playback scheduled. In addition, a redundant
playback of AACS data over a DSS-15 pass tomorrow had been intentionally
removed within the last few weeks in order to implement a Y-thruster RWA
bias demonstration on Monday. The Titan and AACS data were lost during
transmission due to a problem at the Goldstone Deep Space Network facility.
The Antenna Logic Controller (ALC) at DSS14 went down just prior to the
start of the Cassini track. Goldstone was able to provide limited support
at DSS-25 in order to supply radiometric data for Navigation, and to support
some real-time commanding. However, the downlink data rates sequenced on
the spacecraft in anticipation of 70m support could not be acquired by the
34m antenna.

Monday, Aug. 10 (DOY 222):

As a result of the very accurate flyby at T60 and an upcoming 13 m/s
deterministic maneuver, there is a delta V savings to cancel OTM-212 and
accomplish all of the correction at OTM-213. Since the orbit determination
solution had converged, there was no reason to wait until the planned final
Navigation review tomorrow. Therefore OTM-212 has been cancelled.

An RWA momentum bias demonstration using only the B-branch Y facing
thrusters, conducted today, was completely successful. The use of
Y-thruster only biases is expected to reduce hydrazine consumption as well
as more evenly balance hydrazine throughput between the Y and Z thrusters. Y
thruster biases are necessarily done off Earth-line, and hence complicate
the navigation process due to the lack of visibility, which is the primary
reason why they haven’t been used to this point in the mission. The benefit
of more balanced hydrazine throughput is the lessened use of the Z thrusters
and the corresponding presumed mitigation of the problems recently
experienced on the A-branch thrusters. The intention is to implement Y
thruster biases as a regular part of operations beginning with S60 in May of
2010. Sequence development for S60 begins in November of this year.

Tuesday, Aug. 11 (DOY 223):

A non-targeted flyby of Atlas occurred today.

A real-time command was sent to the spacecraft today to power Ka-band
transmission on, then off, in support of a test at the tracking complex at
Narrabri, Australia.

Since Saturn's axis is tilted relative to its orbit about the sun, Saturn
has seasons -- seasons that last for over seven Earth years. One result of
this is a varying exposure angle of the rings to the sun. When this angle
goes to zero, that is, an equinox crossing, sunlight will hit Saturn's thin
rings edge-on. The light reflecting off this extremely narrow band is so
small that for all practical purposes the rings simply vanish. The solar
ring plane crossing which occurred today provides a unique opportunity for
Cassini scientists. The sunlight hitting the rings edge-on can illuminate,
or throw shadows, revealing vertical ring structures and oddities previously
unseen. This phase of Cassini’s Equinox Mission was designed with just this
opportunity in mind.

Because Saturn's position in the sky as seen from Earth is so close to the
sun now, it is extremely difficult to see from Earth. But Cassini is there,
ready and waiting. Around equinox, Cassini's thermal instrument is tasked
with measuring the temperature of both sides of the rings to see how the
rings cool as they go through this seasonal change. The spacecraft's cameras
are looking for topographic features in the rings, like tiny moons and
possible ring warps, which are only visible at equinox, while the
near-infrared and ultraviolet instruments will be on the hunt for signs of
seasonal change on the planet.
For more details, go to: http://saturn.jpl.nasa.gov/news/cassinifeatures/feature2009087/.