Cassini Significant Events — 04/06/06 – 04/12/06

Cassini Significant Events -- 04/06/06 - 04/12/06

April 14, 2006

(Source: Cassini Project)

The most recent spacecraft telemetry was acquired Wednesday, April 12, 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 .

The editors of Aviation Week & Space Technology magazine honored the
Cassini-Huygens mission team, which has captured the most detailed views
ever of Saturn and its myriad moons, with an Aerospace Laurel award.
Aviation Week presented the award for the successful landing of the European
Space Agency's Huygens probe on Saturn's moon Titan, and for the science
return and inspiring images from NASA's Cassini orbiter.


Thursday, April 6 (DOY 096)

A new Cassini observation of Saturn shows that the 'A' ring contains more debris than once believed, according to a new University of Colorado at Boulder study. Previous observations made in the early 1980s found the ring
to be more nearly transparent, indicating less material. New calculations
based on observations made with Cassini's Ultraviolet Imaging Spectrograph
indicate the opacity of the ring is up to 35 percent higher than previously
reported. A newly released image shows the distribution of the ring
material. To obtain the text of the full release and view the image, link
to: http://saturn.jpl.nasa.gov.

Reading, Writing, and Rings was presented at the National Science Teachers'
Association annual convention in Anaheim, California, that ran from April
6-9. The Cassini Outreach Team presented a workshop to 30 people as part of
convention events.

Friday, April 7 (DOY 097)

Radio and Plasma Wave Science (RPWS) performed an antenna calibration today.

The Rings, Saturn, and Titan Orbiter Science Team (TOST) groups have held
meetings this past week to continue their evaluation of possible variations
in the extended mission tour. For the TOST meeting, participants were
requested to come armed with criteria that could be used to rank the various
tours under evaluation, and to consider how long an extended mission should
be in order to observe seasonal changes in the Saturnian system. Overviews
of eight variations of the tour were presented along with a description of a
new tour analysis tool, and discussion of tour products that are currently
available.

Saturn and the moon are the featured telescopic objects in Pasadena and
Monrovia, California, tonight and tomorrow at events hosted by Cassini
Outreach and the Old Town Astronomers. The next local viewing opportunities
will be May 5 and 6. Contact Cassini Outreach (jane.h.jones@jpl.nasa.gov)
for other viewing opportunities.

Outreach published some Internet statistics for Cassini for the month of March:

Recent highlights to the mission website include:

- NASA's Cassini Discovers Potential Liquid Water on Enceladus -- March 9

- Cassini Flies by Titan, Studies Atmosphere -- March 18

- Cassini's Best Maps of Jupiter -- March 27

- Locating the Propellers -- March 29


Stats for the mission website include:

- Total number of raw images in database: 68745

- Total number of raw images for month of March 2006 (March 1 - March 28):
4258

- Total number of videos in the database: 107

- Total number of press images in the database and accessible in the
Cassini Content Management System (CMS): 1320

- Total number of news items (press releases and significant event reports)
in database and accessible in the CMS: 632

Monday, April 10 (DOY 100):

The Monopropellant Tank Assembly (MTA) (aka the hydrazine tank) recharge
activity was successfully completed on the spacecraft today. Propulsion,
Thermal, Power/Pyro, Command Data System, and System Fault Protection all
reported a nominal event, with predictions met. The recharge brings the
thrusters up to full thrust level to provide control authority for the
planned lower altitude Titan flybys, starting with T16 on July 22. The
thruster levels are now 0.98 N for the A branch and 1.05 N for the B branch.

As part of the MTA recharge procedures, the B Branch thruster values were
updated in the prime ACS Flight Computer (AFC) prior to the firing of pyro
valve 40 to ensure that the spacecraft was robust in the event of a leaky
"A" branch thruster after a successful MTA recharge. The next step was the
patching of ACS FSW to account for the change in physical thruster
magnitudes by updating the default FSW thruster magnitude values. After the
update, the FSW version on board will be A8.7.4.

The patching of default FSW thruster magnitudes is necessary as a
contingency for an AFC reset occurrence. If ever an AFC reset occurs, the
active thruster magnitudes would be lost and the default magnitudes
restored. Before FSW A8.7.4, the default thruster values were set to
post-probe values of 0.70 N and 0.75 N for A and B- branch thrusters,
respectively. In order to minimize the mismatch between default FSW values
and current physical values of around 0.98 N and 1.05 N, the FSW values were
patched to 0.94 N and 1.01 N for A and B-branch thrusters, respectively.
These values were proven to be robust and have taken into account the
thruster decay for the next ten months, before another patch is needed for
the next phase of the tour mission. The A8.7.4 ACS FSW patch has been
successful.

The final step will be a reaction wheel assembly (RWA) bias on April 13,
which will be the first use of thrusters after the MTA recharge, and will
provide an opportunity to perform an in-flight calibration of the higher
thrust levels. The thrust levels will also be calibrated in a future
thruster-based OTM.

The preliminary port occurred as part of the Science Operations Plan update
process for S22. Analysis of the merge has been delivered to the teams for
review. The official port is scheduled to occur Friday, April 14.

Cassini outreach gave a "Cassini Mission's first year highlights and more"
presentation to the Monrovia League's monthly "Authors and Others Series"
held at the Monrovia Library in California.

Tuesday, April 11 (DOY 101):

Today the Magnetometer Subsystem (MAG) performed an instrument calibration,
and the Magnetospheric and Plasma Science teams wrapped up their distant
magnetotail campaign.

The Aftermarket Process for the S24 sequence began today. This 5-week
process will address proposed Cassini Information Management System changes
that require re-integration of the segments contained in the S24 sequence.
Inputs for this process were due on April 7. The science planning leads for
this sequence published an assessment package and distributed it to the
team. It appears that all of the requested changes can fit within the
available resources. Unless the Target Working Teams (TWTs) and Orbiter
Science Teams (OSTs) recommendations change over the next couple of weeks,
it is likely that the decision meeting scheduled in two weeks will be
canceled.

The course of sequence development never does move smoothly. Or, at least
hardly ever. It was identified during the first merge in the final sequence
development process for S20 that only about 720 words of memory were
remaining. It was anticipated that as development continued more memory
would be required and consume the remaining 720 words, and possibly more.
At that time, the leads for the sequence began exploring options to split
the sequence for uplink and execution. S07 is the only sequence so far in
the tour where the size made splitting necessary. At that time, each team
was asked to submit their products in multiple parts, one for the first
"half" of the sequence, and other for the second "half". Then the parts
were uplinked separately. Since that time, a new technique has been
developed to take one sequence and split it as necessary for storage onboard
the spacecraft.

A test was performed in the Integrated Test Laboratory (ITL) to verify the
splitting technique for S20. Errors were discovered in the seq_gen and
seqtran software causing them to incorrectly specify the last instance of
each cyclic program. As a result, seqtran was not properly specifying the
total number of programs in a variable called "Group_ID". Because of this
mismatch, the primary program in the background sequence did not activate
during the test.

Meanwhile the sequence development process continued. With the most recent merge, it was determined that the remaining 720 words of memory had been
used. In addition, the sequence required an additional approximately 13,000
words of memory, and splitting the sequence would now be necessary.

Meetings were held to discuss the various options and workarounds for this
problem. Some were more arduous than others. Some were more error prone
than others. Some required the loss of science data. All involved an
element of risk.

Now the good news. Analysis of the ITL test results showed that the
software error appeared as a result of the way one of the seqtran directives
was specified to support the test. This is different than the way it will
be specified when building operational flight products. By specifying this
directive as for flight, the error does *NOT* manifest itself, and the
Group_ID variable *DOES* specify the appropriate number of programs, and the
background sequence *WILL* activate as planned.

Uplink Operations would like to see seq_gen modified to enable testing for
the next time a sequence split is required. This will most likely occur for
S24 and S25. However, the workaround that is being employed for S20 can be
used for S24 and S25 if necessary.

Uplink of the S20 background sequence products is due to begin on April 17.

Wednesday, April 12 (DOY 102):

The ITL ran a quick test last night to load S19, S20 part 1 and S20 part 2.
The test was a success.

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.