Cassini Significant Events 04/01/09

Cassini Significant Events
04/01/09 - 04/07/09

The most recent spacecraft telemetry was acquired on Apr. 7 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, April 1 (DOY 091)

With the successful execution of Orbit Trim Maneuver (OTM) #186 on Mar. 29,
targeting for the Titan 52 flyby is assured and OTM-186a is not required.
OTM-186a has been cancelled.

An encounter strategy meeting was held today to cover the period between
April 4 and April 20, Titan flybys T52 and T53, and maneuvers 188-190.

Upon a recommendation from Navigation, the Spacecraft Operations Office is
planning to update two Main Engine (ME) maneuver parameters in AACS Flight
Software in order to keep performance better aligned with actual physics.
The first parameter is the Accelerometer (ACC) Scale Factor. This has been
patched four times since launch with the most recent correction occurring in
July 2006. The second parameter is a change to the ME tail-off impulse.
This will correct a small - about 3 mm/sec - but persistent underburn
condition. This parameter has not been updated before. These patches will
go through testing in the Integrated Test Laboratory the week of Apr. 6.
The first OTM targeted to use the new parameters is #192 on Apr. 28, a
fairly large ME maneuver estimated at 2.2 m/sec.

Members of the Spacecraft Telecommunications Team updated Cassini's Radio
Frequency Subsystem Best Lock Frequency (BLF) today. The free running
voltage controlled oscillator frequency had drifted about 3.5 kHz off the
commanded DSN BLF over the past two years. The commanded DSN BLF now is
within 0.5kHz of the free running frequency.

Cassini passed by Titan (T52) at an altitude of 4150 km and a speed of 5.8
km/sec. The latitude at closest approach was 2.8 degrees S and occurred at
approximately 7:58 PM PDT. Unlike other flybys, this encounter was set up
with only one maneuver: a combined post-flyby cleanup/Titan approach
maneuver, executed on March 29. Due to the short interval between T51 and
T52, the Navigation team had scheduled four different opportunities for this
important maneuver to insure its execution.

The T52 egress atmospheric occultation was one of the highest priority RSS
occultations of the extended mission. It was the first Cassini occultation
to probe Titan near-equatorial latitudes, while the ingress occultation
probed mid-northern latitudes. The occultations will shed more light on
latitudinal variability of the electron density profile of the ionosphere,
temperature and pressure profile, extinction profile, and small
scale-structure of the neutral atmosphere. The RSS occultations are the only
Cassini observations able to map the thermal structure of the atmosphere
down to the surface with high spatial resolution. They require
implementation of critical pointing to steer the spacecraft during the
observations to keep track of the radio signal as it bends through the
atmosphere. To maximize chances of success, the pointing design was updated
and radiated to Cassini the week before execution of the experiment.

The T52 inbound bi-static scattering observation probed for the first time
high-northern latitude regions of Titan's surface. The outbound bi-static
measurements probed the mid-southern latitude region. The incidence angle

for both sides was close to the Brewster angle range of likely surface
compositions. Same- and cross-polarized components of the quasi-specular
surface echo will provide information about the dielectric constant and
physical state of the surface region probed.

Both T52 RSS experiments were successful. Real-time bi-static echoes were
detected from two surface regions not probed before by RSS, and high quality
occultation data were acquired.

Solar and stellar occultations by Titan are the most valuable Titan
observations for the Ultraviolet Imaging Spectrograph (UVIS) because they
provide detailed vertical profiles of nitrogen in the Extreme Ultraviolet
channel during solar occultation and hydrocarbons, HCN, and aerosols in the
Far Ultraviolet channel during stellar occultations. In T52, UVIS took
advantage of a very slow occultation, among the slowest in Cassini's
extended mission, to probe Titan's polar vortex region. These profiles probe
altitudes between 300 km and 2400 km, which fill the gap between CIRS and
the Ion and Neutral Mass Spectrometer (INMS) measurements. Much of the
chemistry and aerosol formation occurs in this vertical region. Observations
taken over the course of the mission will collectively provide coverage at
many latitudes and local times and these will be used to study meridional
and local time gradients in the upper atmosphere. Knowledge of these
gradients is important for understanding the meridional circulation and
other dynamical and chemical processes. For more information on the T52
flyby go to:
http://saturn.jpl.nasa.gov/files/20090403_titan_mission_description.pdf and
http://saturn.jpl.nasa.gov/mission/flybys/titan20090403/

Monday, April 6 (DOY 096):

The Target Working Team (TWT)/ Orbiter Science Team (OST) integrated
products for S54, covering orbits 119 through 121, were delivered today. The
integrated products are in their final form and no re-integration is
planned. The next step in sequence development, Science Operations Plan
(SOP) implementation, will kick off on Apr. 20. Between now and then, the
instrument teams will be working on pointing designs for the sequence.
TWT/OST teams deliver integrated sequence products for the extended mission
about every five weeks.

While touring the Saturn system in 2008, Cassini enabled great scientific
studies and observations. Listed below are the top 10 science highlights of
the year as selected by the Cassini science teams.

1. Positive identification of liquid ethane in a lake on Titan
2. Polar storms on Saturn
3. Strong inference of a liquid water layer in Titan's interior
4. The likelihood of dusty rings around Rhea
5. The possibility of plate-tectonic-like spreading in the Enceladus south
polar region
6. Water vapor jets inside the plume of gas leaving Enceladus
7. Moonlet population in and around the F ring
8. New insights into Saturn's aurora
9. Three belts of sub-moonlets in the A ring (propellers)
10. Six month-old lightning storm shatters record for longevity
For all the details link to:
http://saturn.jpl.nasa.gov/news/cassinifeatures/feature20090330/

Tuesday, April 7 (DOY 097):

Due to the excellent accuracy of Orbit Trim Maneuver (OTM) #186 on Sunday,
Mar. 29, OTM-188, the T52 cleanup maneuver scheduled for April 7, 2009, was
cancelled. Since OTM-189 is a fairly large deterministic maneuver of about
7 m/s, it is possible to combine 188 and 189 with a delta V penalty of only
22 mm/sec.

The seventh edition of the Cassini Scientist for a Day contest has begun.
This national essay contest is for U.S. students in grades 5-12. This year
students may select from one of three available targets: Dione,
Prometheus/the Rings of Saturn, or Janus/Epimetheus, and write a 500-word
essay on why they think that studying their selected target will yield the
best science results. Essay contest entries must be received by April 30,
2009, at 12:00 noon Pacific Daylight Time. For more information link to:
http://saturn.jpl.nasa.gov/education/scientistforaday7thedition/intro/

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Cassini Significant Events 04/01/09 - 04/07/09