Cassini Significant Events
for 8/13/08 - 8/19/08
The most recent spacecraft telemetry was acquired on Aug. 19 from the
Deep Space Network tracking complex at Goldstone, Calif. 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. 13 (DOY 226):
The final integrated S48 sequence segments for orbits 103-106 are due
today to Science Planning. The Science Operations Plan (SOP)
implementation for the S48 sequence begins today with scientists
working on the pointing designs for their observations. This
sequence begins execution on the spacecraft on Feb. 17, 2009.
An encounter strategy meeting was held today to cover the period
between Aug. 11 and Oct. 9, Enceladus flybys E4 and E5, and maneuvers
164, 164A, 165 and 166.
On Tuesday, Aug. 12, Radio Science (RSS) completed the orbit 80
Enceladus gravity observation. Gravity passes in the wings of the
Enceladus 4 flyby will be used in conjunction with the passes on
orbits 91 and 130 to determine the Enceladus gravity field up to
degree 2. The observation consisted of two segments: an inbound,
covered by Madrid, and an outbound, covered mostly by Canberra and
Narrabri, and partially by Madrid. The experiment completed as
planned, and all data have been successfully acquired
Thursday, Aug. 14 (DOY 227):
As part of the Science Operations Plan (SOP) process, a sequence in
development goes through a Reaction Wheel Assembly Bias Optimization
Tool (RBOT) sub-process. The tool was developed by Spacecraft
Operations to help keep the wheels’ RPM rates out of regions that are
more stressful on the bearings in order to extend their useful
lifetime as much as possible. When the wheels are found to fall into
an undesirable region during RBOT analysis, a reaction wheel (RWA)
bias may be inserted to alter that range. The cost of this is that
RWA biases use hydrazine, another limited resource, and one that is
managed by the project. A plan to limit biases while still preserving
the health of the wheels has recently been developed and is now being
implemented in S46.
Friday, Aug. 15 (DOY 228):
A non-targeted flyby of Titan occurred today. While cruising by at
an approximate distance of 300,000 km, Imaging Science (ISS) took
snapshots for about ten hours to improve the global coverage of the
large moon.
JPL has put out a news release with more detail from the close flyby
of Enceladus last week. New carefully targeted images reveal
exquisite details in the prominent south polar “tiger stripe”
fractures from which icy jets emanate. The images show that the
fractures are about 300 meters deep, with V-shaped inner walls. The
outer flanks of some of the fractures show extensive deposits of fine
material. For more information link to:
http://saturn.jpl.nasa.gov/news/press-release-details.cfm?newsID=864
In the Significant Events report last week - and also in the press
release referenced above - the term “skeet shoot” was used in
relation to the imaging technique used to obtain images during the
Enceladus flyby. On Earth, skeet shooting is an outdoor shotgun sport
that simulates shooting game birds in flight. A small Frisbee-like
ceramic disk, called a clay pigeon, is launched through the air,
usually diagonally across in front of a shooter armed with a shotgun.
The skill in successfully hitting the moving clay target with the
birdshot is for the shooter to point a little ahead of the clay
pigeon, and match its angular velocity when the trigger is pulled.
The clay pigeon then passes into the bird shot at exactly when the
shot arrives at its destination in the path of the moving target. So
far so good. At and just after closest-approach on the Enceladus 4
flyby, relative to Cassini and Optical Remote Sensing boresight
directions, Enceladus was streaking too quickly across the sky for
the spacecraft to be able to stably target and track any geological
feature on the surface. Borrowing from the firearms sport, the trick
was to turn the spacecraft as fast as possible in the same direction
as Enceladus’ path across the sky. The plan was to be leading
Enceladus and match its angular velocity at the exact times when our
targets of interest passed into our camera’s field of view.
Apparently ISS is a very good shot.
Spacecraft Operations provided this report on Cassini attitude during
the E4 flyby. While on reaction wheel control, Cassini was commanded
to slew only about the z axis, but saw small attitude control
excursions about the X and Y axes, approximately 0.05 mrad and 0.02
mrad, respectively. These were caused by the plumes of gas emanating
from Enceladus. AACS calculated the peak density to occur at closest
approach (CA) + 20 seconds to CA + 25 seconds, and reports a
preliminary estimated peak density value of 6.07E-12 kg/m3. This
compares closely with the E3 peak density range of 5.8 - 6.3E-12
kg/m3, but still less than the atmospheric densities experienced
during the close Titan flybys.
Monday, Aug. 18 (DOY 231):
Today was the 9th anniversary of the Cassini flyby of Earth in August
of 1999.
The Cassini outreach program “Reading, Writing and Rings” (RWR) has
been given a SciLinks award from the National Science Teachers
Association (NSTA). This means key textbook subjects will be linked
to RWR as an NSTA approved web page so teachers and students can
access real-time information on the topics they study in the
classroom. SciLinks has registered more than 156,000 teachers and
nearly 640,000 students at no cost to use the site, and countless
more use it as “guests.” In addition, an evaluation of Reading,
Writing and Rings will be published in the September issue of the
NSTA publication “Science and Children.”
Science activity today focused on an eleven-hour observation of the
rings of Saturn by the ISS. The observation was designed to target
both the Enke gap and the icy satellite Daphnis. ISS will utilize an
“azimuthal scan” or AZSCAN technique, which allows the camera to
track the rings along their circumference.
Tuesday, Aug. 19 (DOY 232):
Non-targeted flybys of Pallene and Pan occurred today.
The Live-IVP-Update for DOY-232 is a “NO GO.” The S43 sequence leads
have heard from VIMS, ISS and CIRS and all have indicated that this
update is not required.
Today the Radio Science Subsystem (RSS) performed a ring chord
occultation experiment. Every so often, orbital geometry allows
Cassini to point its radio transmitter to send radio signals through
the rings of Saturn and have them picked up by the Deep Space Network
antennas. This technique allows scientists to characterize the fine
details of the ring structure. The experiment was supported by
station DSS-43 at Canberra and DSS-47 at Narrabri. This was the
final scheduled support for Cassini by the Narrabri complex. The
experiment completed nominally and as planned.