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NASA Cassini Significant Events 07/22/09 - 07/28/09

Status Report From: Jet Propulsion Laboratory
Posted: Sunday, August 9, 2009

image 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 period.

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 constraints.

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/.

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