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Cassini Significant Events 07/22/09 – 07/28/09

The most recent spacecraft telemetry was acquired on July 28 from the Deep
Space Network tracking complex at Madrid, Spain. 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, July 22 (DOY 203)

The results from data collected by the Ion and Neutral Mass Spectrometer
(INMS) during Enceladus flybys in July and Oct. 2008 were released in the
July 23 issue of the journal Nature. One of the chemicals definitively
identified by INMS was ammonia. In space, the presence of ammonia provides
strong evidence for the existence of at least some liquid water. For the
full details on this release link to: http://saturn.jpl.nasa.gov/news/cassinifeatures/feature20090722/.

Thursday, July 23 (DOY 204):

The S51 sequence concluded and S52 began execution today at 2009-204T21:51
SCET. The sequence will run for 33 days and conclude on August 25. During
that time there will be one targeted encounter with Titan and five
non-targeted flybys – one each of Prometheus, Pandora, Janus, Tethys, and
Atlas. Six maneuvers are scheduled, numbered 209 through 214. Science at
the beginning of S52 included an Imaging Science (ISS) night side
observation of Titan and Composite Infrared Spectrometer (CIRS) far-infrared
vertical scanning.

Friday, July 24 (DOY 205):

On Friday a meeting was held to discuss the options – reaction wheel (RWA)
versus thruster control (RCS) - for the Enceladus 9 flyby in late April
2010. Personnel from Science, Science Planning, the Science and Uplink
Operations Office, and the Spacecraft Office attended. E9 is the first
planned opportunity for Cassini to obtain close flyby Radio Science (RSS)
gravity measurements during a 100 km flyby over the south pole. Two other
observations during the proposed extended-extended mission would complete
the set of three flybys designed to reveal the interior structure of
Enceladus. Gravity at Enceladus in general -- and over the south pole in
particular -- is high priority for Cassini Science.

The gravity experiment requires the flyby to be performed on reaction wheels
since thruster firings would cause spacecraft motions which would corrupt
the radio signal which carries the gravity information being sought.
However, prior AACS data is necessary to verify that it is acceptable to fly
on wheels through the plume at this altitude. The necessary AACS data won't
be obtained until the Enceladus 7 flyby on Nov. 2 of this year. By that
time, development would already have begun for the E9 flyby. The Satellite
Orbiter Science Team (SOST) has suggested a plan in which two parallel E9
observation timelines would be developed, one assuming that E9 is performed
on wheels and planning for the RSS gravity measurement, and another timeline
that assumes E9 must be done on thrusters and plans a different set of
Enceladus observations.

The proposal for two timelines puts significant extra work on the SOST,
Science Planning, and AACS teams. In order to assist with the AACS workload
associated with keeping the wheel timeline as an option, meeting attendees
agreed to keep the wheel timeline design as simple as possible by planning
the E9 encounter observation period as Earth-pointed. There will be no
turning to Enceladus or other targets for other observations during this

The project approved the plan to develop the two options. A decision will
be made after the similar E7 flyby, which is done on thrusters, to verify
that the RWAs will have the control authority needed to fly through E9 at
the specified altitude and attitude, and not violate wheel speed

On July 24, Cassini flew by Titan for the T59 targeted encounter. The
spacecraft passed at an altitude of 955 km and a speed of 6 km/sec. The
latitude at closest approach was 62 degrees S and the encounter occurred on
orbit number 115. On this flyby, Titan was in the pre-midnight region of
Saturn’s magnetosphere, and the spacecraft's trajectory crossed over the
moon's south pole. The peak duty cycle for the thrusters was 43%, using 373
grams of hydrazine, very close to the pre-encounter prediction.

For only the third time during a Titan flyby, the Cassini Plasma
Spectrometer (CAPS) was the primary instrument. During the flyby CAPS
collected a full sampling of the inbound interaction region inside of 12
Titan radii. CAPS also took part in a combined observation with INMS at
closest approach to Titan's ionosphere.

INMS performed observations on the night side of Titan at high southern
latitudes. This is one of two high southern latitude passes that will help
fill in the latitude coverage of Titan's atmosphere.

In addition, RADAR obtained a Synthetic Aperture Radar (SAR) swath running
parallel to observations obtained in the T55-58 southern hemisphere mapping
sequence covering south polar terrain, and Magnetometer measurements
provided a description of the draping and pileup of the external magnetic
field around Titan on the night side hemisphere.

Following closest approach, CIRS closed out the flyby with VIMS riding along
with observations of the same area that had been observed at T58. For the
full description of all science activities for this flyby, link to: http://saturn.jpl.nasa.gov/mission/flybys/titan20090724/.

Sunday, July 26 (DOY 207):

Non-targeted flybys of Prometheus, Pandora, Janus, and Tethys occurred on
Sunday, July 26.

At periapsis today – the time when Cassini is closest to Saturn in its orbit
– ISS performed an azimuthal scan of a strange ringlet to get vertical
relief information. This was followed by the Janus flyby, ~94,000 km at
closest approach, 60 deg phase. The secondary pointing attitude of the Janus
observation was selected by members of the Cosmic Dust Analyzer instrument
team in order to collect ring plane crossing data. Then ISS turned to
Enceladus to capture the plumes while Enceladus was eclipsed by Rhea.
Finally, the day concluded with VIMS gathering compositional data on the
dark side of the rings at low phase angle.

Tuesday, July 28 (DOY 209):

Orbit Trim Maneuver (OTM) #209 was performed today. This is the cleanup
maneuver following the Titan 59 encounter on July 24. The main engine burn
began at 9:45 AM PDT. Telemetry immediately after the maneuver showed the
burn duration was 37.08 seconds, giving a delta-V of 6.28 m/s. All
subsystems reported nominal performance after the maneuver.

An encounter strategy meeting was held today to cover the period between
Aug. 9 and Aug. 25, Titan flybys T60 and T61, and maneuvers 212-214.

Last Friday Science Planning delivered the handoff package for S54 to Uplink
Operations. Today a kickoff meeting was held for the final sequence
development process prior to uplink to the spacecraft in September.

The Cassini-Huygens Analysis and Results of the Mission (CHARM)
teleconference for July occurred today and covered part 2 of the Cassini
Fifth Anniversary at Saturn. Presentations included Highlights of Rings and
Dust Science, and Titan and Icy Satellite Science. A copy of the
presentation may be obtained at: http://saturn.jpl.nasa.gov/video/products/MultimediaProductsCharm/.