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Cassini Significant Events for 09/14/06 - 09/20/06



Cassini Significant Events
for 09/14/06 - 09/20/06

The most recent spacecraft telemetry was acquired Wednesday, September 20,
from the Goldstone tracking stations. The Cassini spacecraft is in an
excellent state of health and is 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 .

Thursday, September 14 (DOY 257):

Orbit trim maneuver #72 was performed today. This was the apoapsis maneuver
setting up for the Titan 18 encounter on September 23. The main engine burn
began at 4:30 AM. Telemetry immediately after the maneuver showed a burn
duration of 50.8 seconds, imparting a delta-V of approximately 8.2 m/s. All
subsystems reported nominal performance after the maneuver.

An image from Cassini's radar instrument taken during the T17 encounter
shows an impact crater with a diameter of 30 kilometers on the surface of
Saturn's moon Titan. Cassini data have only revealed three definite impact
craters on Titan so far, so each new discovery adds significantly to our
body of knowledge. To view the image and access the text, link to: 

http://saturn.jpl.nasa.gov/multimedia/images/image-details.cfm?imageID=2261 

With the Titan 16 (T16) encounter on July 22, the Navigation team has begun
to rotate the line of apsides of Cassini's orbit to more favorable
geometries for atmospheric observations. The line of apsides is a line which
passes through both periapsis, Cassini's closest approach to Saturn in an
orbit, and apoapsis, the farthest spot in the orbit from Saturn. Apoapsis
had been located on the dark side of Saturn to support magnetotail
observations, but it is now being moved toward the lighted side of Saturn
via a pi transfer. A pi transfer achieves this goal more quickly than could
be done with other non-resonant transfers.

So what is a non-resonant transfer? According to members of the Cassini
Navigation Team, to understand non-resonant transfers, you must first
understand resonant transfers. With a resonant transfer, the spacecraft
encounters a satellite, Titan in this case, at the same place in its orbit
as at the previous encounter. In other words, there are multiples of 360
degrees between encounters for both Titan and Cassini, and not necessarily
the same number for each. Transfers that encounter Titan with an arbitrary
angle - not multiples of 360 degrees - between encounters are referred to as
non-resonant transfers. A pi-transfer is a particular type of non-resonant
transfer. The spacecraft encounters Titan 180 degrees plus some multiple of
360 degrees from the previous encounter.

Now, back to this particular transfer. From T16 to T24, Titan gravity
assists rotate the line of apsides while raising the spacecraft's orbital
inclination to approximately 59 degrees, the amount required to perform the
pi transfer. The pi transfer occurs between T24 on January 29 and T25 on
February 22, 2007, situating the T25 encounter 180 degrees around Saturn
from where the T24 encounter takes place. From T25 to T33 on June 29, 2007,
the line of apsides continues to rotate toward Saturn's lighted side as
inclination is lowered back into Titan's orbit plane. This phase of the
mission offers several low altitude Titan flybys and favorable ring
observation geometry to the science community.

Friday, September 15 (DOY 258):

September 15 is the beginning of a series of very important scientific
opportunities for the Cassini mission. The trajectory at apoapsis takes the
spacecraft behind Saturn such that both the Sun and Earth are obscured. This
geometry offers the Radio Science Subsystem (RSS) a very slow occultation of
Saturn's rings and atmosphere and allows Optical Remote Sensing (ORS)
instruments ample time for nearly 180-degree phase angle ring observations.
The backlit rings provide the second most important data set, next to Saturn
Orbit Insertion, for rings scientists who are trying to better characterize
the structure and composition of Saturn's rings.ORS instruments will use the
time during solar occultation to study Saturn's rings at high phase. The
Visible and Infrared Mapping Spectrometer (VIMS) instrument will perform a
3-hour high phase planet shadow mosaic, then the Imaging Science Subsystem
(ISS) will take a number of images of the unlit side of Saturn and the ring
system. The Magnetospheric and Plasma Science (MAPS) instruments will also
operate to observe the structure and dynamics of Saturn's magnetotail.
Finally, ISS will use the Wide Angle Camera for a 3-hour observation of the
right ansa region of the ring system using four different filters.

In anticipation of the occultation event, a Feature was designed to give
background information and to alert the public that a significant milestone
was about to be reached. The feature, titled "Saturn's Rings To Shine As
Never Before," appeared on September 15, and may still be seen at:

http://saturn.jpl.nasa.gov/news/features/feature20060915.cfm

Sunday, September 17 (DOY 260):

>From Friday, September 15, through Sunday, September 17, Cassini Radio
Science conducted the longest Saturn rings and atmospheric occultation
planned in the tour. The ring occultation will yield important information
about radial ring structure, dynamics, and physical properties. Similarly,
the atmospheric occultation will yield the small- and large-scale structure
of the southern atmospheric region, including the temperature-pressure
profile and a profile of the abundance of microwave absorbing species.

All three DSN complexes supported the occultation experiment. S- and X-band
support was provided by the 70-m antennas at Goldstone and Canberra and by
the 34-m HEF at Madrid. X- and Ka-band support was provided by the beam wave
guide antennas at all three complexes.

Overall, the rings and atmospheric occultations went well, the experiments
were completed as planned, and good, high quality data were collected at all
three observation frequencies of S, X, and Ka.

Perhaps the most exciting part of this long RSS observation period came
during its last couple of hours when Cassini emerged from behind Saturn at
about 8 pm PDT on Saturday night. As usual, the S/X/Ka levels slowly climbed
to steadily increasing values as they cleared the neutral gas along the
path. Suddenly, huge fluctuations in the S-band signal sent team members
watching in the operations room gasping. X- band followed with similar
fluctuations, although somewhat more muted in level, while Ka-band showed
little variations from its steady free- space value. The huge dispersive
behavior is a classic signature of plasma and is another example of how rich
is the data the three-frequency radio observations collectively provide. In
all likelihood, a substantial ionosphere characterizes the mid-southern
latitude region probed, and it should be interesting to compare the
corresponding electron density profiles with the many other near-equatorial
ones obtained from previous occultations.

Monday, September 18 (DOY 261):

This afternoon the project made the decision to cancel Orbit Trim Maneuver
(OTM) #73, the Titan 18 minus 3 day maneuver, because the delta-v of 8
mm/sec required was below the Spacecraft Office minimum threshold of 10
mm/sec, and no adverse effects would occur because of the very slight
deviation from the previously planned trajectory. OTM-74 is currently
scheduled for DOY 269.

Tuesday, September 19 (DOY 262):

A news release was issued today on the discovery of a new ring surrounding
Saturn. Images were obtained during the longest solar occultation of
Cassini's four-year mission. Sunday's occultation allowed Cassini to map the
presence of microscopic particles that are not normally visible across the
ring system. As a result, Cassini saw the entire inner Saturnian system in a
new light. Over 100 media stories ran based on this press release. Images of
this discovery along with a complete description can be found at: 

http://saturn.jpl.nasa.gov

Members of the Spacecraft Propulsion Module Subsystem presented "Cassini
N2H4 Mass used to Date: High Time for a Model Update!" at the Mission
Planning Forum today. A re-examination of the hydrazine consumption model
indicates that an additional 6 kg of hydrazine may be available. The project
will incorporate this change in future reporting of hydrazine usage.

Wednesday, September 20 (DOY 263):

As one of the last activities for S23, a Reaction Wheel Assembly Bias was
uplinked today in lieu of the cancelled OTM-73.

A Live Inertial Vector Propagator (IVP) update is planned for Saturn and
Janus from DOY 268-272. At the Go/No-go meeting today it was confirmed that
the update would only be required for the Janus vector. AACS provided the
file to Uplink Operations immediately after the meeting for processing.
Teams will have until the approval meeting on Friday to review the file.

The keys to the spacecraft were handed off from the S23 to the S24 leads
today. S24 began execution at 2006-263T20:22 and will conclude 35 days
later. During this sequence, six OTMs numbered 74 through 79 are scheduled
to be performed, targeted Titan flybys 18 and 19 will occur at altitudes of
960 km and 980 km respectively, the main engine cover will be closed as part
of nominal dust hazard avoidance for approximately four days, and one live
IVP update is planned for execution during the last week of September.

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 Spsce 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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