Cassini Significant Events 03/25/09 – 04/03/09

The most recent spacecraft telemetry was acquired on Mar. 31 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, March 25 (DOY 084)

Teams delivered files for Port 2 of the S52 Science Operations Plan process
today.

As reported in the Cassini Significant Events report for March 9, following
the execution of OTM-183, a red alarm was detected in association with one
of the temperature sensors on the prime main engine combustion chamber. It
appeared that one of the two redundant sensors had failed. Temperature
transducer failures are not uncommon and have been seen previously on
several missions. This is the first temperature transducer failure on
Cassini in its 11-year history. Although this event will have no effect on
the operation of the main engine, it does mean that there is now one less
level of redundancy in monitoring the performance of the prime main engine.
Because this sensor was used both in System Fault Protection and Attitude
Control Fault Protection, real time commands were sent to the spacecraft
this week to mark this sensor “dead”. This means that readings from this
sensor will no longer be incorporated into autonomous fault protection
responses.

Sequence leads uplinked a patch today to the Composite Infrared Spectrometer
(CIRS) instrument flight software. The patch is designed to test a
capability to reduce or remove an internally generated noise spike in CIRS
data by changing how the flight software checks the status of the motion of
the scanning mirror. The test will execute on Mar. 31 with the patch
automatically removed at the end of the activity.

Thursday, March 26 (DOY 085):

The S48 sequence concluded and S49 began execution today at 2009
085T10:05:00 SCET. The sequence will run for 40 days and conclude on May 5.
During that time there will be three targeted encounters and two
non-targeted encounters, all with Titan. Eight maneuvers are scheduled, 186
through 193.

Friday, March 27 (DOY 086):

On March 27, Cassini flew by the south polar region of Titan at an altitude
of 963 km, and a speed of 6.3 km/sec. Closest approach occurred at 9:53 PM
PDT, at a latitude of 30.9 degrees S. Titan 51 (T51) was the first Titan
encounter performed using the B-branch thrusters.

The Ion and Neutral Mass Spectrometer (INMS) was prime for this flyby,
controlling spacecraft pointing on the outbound leg. Filling in science
coverage of mid to high latitudes, the INMS observations were designed to
measure the dayside ionosphere and the wake/outer flank of the
magnetospheric interaction region. VIMS looked for changes in the south
polar area as it becomes less and less illuminated. The Magnetospheric
Imaging Instrument measured energetic ion and electron energy input to
Titan’s atmosphere.

Following closest approach, the Radio Science Subsystem (RSS) performed a
bi-static scattering observation of the surface of Titan, probing the
mid-northern latitude surface region at ~30-40 deg North latitude, ~190-200
deg West longitude. Cassini transmitted a right circularly polarized
signal, and both the right and left circularly polarized components of
potential surface echoes were monitored at the ground receiving stations of
the DSN. If detected, the relative power in each echo component provides
unique information about the dielectric constant and physical state of the
surface region probed. The spectral shape characterizes the surface
roughness. The T51 observation geometry was optimized to maximize chances of
detecting both right and left polarized echo components. Monitoring of the
real-time spectra suggested no immediately detectable surface echoes, but
the data are still being processed.

Sunday, March 29 (DOY 088):

Orbit Trim Maneuver (OTM) #186 was performed today. This is the clean-up
maneuver following the Titan 51 encounter on Mar. 26. The main engine burn
began at 7:15AM PDT. Telemetry immediately after the maneuver showed the
burn duration was 4.437 seconds, giving a delta-V of 0.75 m/s. All
subsystems reported nominal performance after the maneuver.

If you are wondering what happened to OTM-185, this maneuver number was
deleted from the sequence to maintain a numbering convention. All apoapsis
maneuver numbers up to this point have been divisible by three. Because the
spacecraft is now in the middle of a pi-transfer, OTM-187 will disappear as
well. It made sense to keep this convention for simplicity, data query, and
other reasons. The next scheduled maneuver is OTM-188 on Apr. 6. For more
information on pi-transfers see the Cassini Significant Events report for
09/14/06 - 09/20/06, Thursday, September 14 (DOY 257).

Members of the New Hampshire Astronomical Society gave a presentation on the
Cassini Equinox Mission as part of the Super Stellar Friday evening lecture
series at the McAuliffe-Shepard Discovery Center. This was followed by a
Saturn sky watch event.

Monday, March 30 (DOY 089):

Spacecraft Operations began a weeklong end-to-end dry run of the AACS A8.7.7
flight software in the Integrated Test Laboratory today. Planned for uplink
at the end of May, AACS flight software version A8.7.7 will cover the time
period from June 2009 to July 2010, and will update the default safing
attitude and the default thruster magnitudes.

Science Planning hosted a forum today of key science to occur during the
execution of S52. Meetings of this kind are valuable to keep the flight
team apprised of important events and when they will occur.

Tuesday, March 31 (DOY 090):

The International Year of Astronomy's cornerstone project "One Hundred Hours
of Astronomy" takes place April 2-5. Saturn is well placed for observing
during this celebration. Join your local astronomy club or get in touch
with one of the 400 Cassini Saturn Observation Campaign volunteers to view
Saturn or participate in other celebrations during the four days. See http://saturn.jpl.nasa.gov/Education/saturnobservation/viewing2009/ for
details.

The Science Operations Plan process for S51 concluded today with all
products being collected and handed over to Uplink Operations. The kick-off
meeting for the final sequence development process – Science and Sequence
Update - will be held Thursday, Apr. 2.

The Cassini-Huygens Analysis and Results of the Mission teleconference for
March presented the topic: "Spin: A new window on Titan's winds and
internal ocean." An audio recording of this event and the PowerPoint and
PDF versions will be available next week.