Cassini Significant Event Report

For Week Ending 06/05/98

Spacecraft Status:


The Cassini spacecraft is presently traveling at a speed of approximately 129,881 kilometers/hour (~81,000
mph) relative to the sun and has traveled approximately 673 million kilometers (~417 million miles) since
launch on October 15, 1997.

The most recent Spacecraft status is from the DSN tracking pass on Saturday, 05/30, over Goldstone. The
Cassini spacecraft is in an excellent state of health and is executing the C8 sequence nominally. The speed of the spacecraft can be viewed on the "Where is Cassini Now?" web page (http://saturn.jpl.nasa.gov/operations/present-position.cfm)


Inertial attitude control is being maintained using the spacecraft's hydrazine thrusters (RCS system). The
spacecraft continues to fly in a High Gain Antenna-to-Sun attitude. It will maintain the HGA-to-Sun attitude,
except for planned trajectory correction maneuvers, for the first 14 months of flight.


Communication with Earth during early cruise is via one of the spacecraft's two low-gain antennas; the antenna
selected depends on the relative geometry of the Sun, Earth and the spacecraft. The downlink telemetry rate is
presently 40 bps.


For the next several months, due to increasing range from Earth and relatively high angles with respect to the
Low Gain Antenna boresight, the DSN tracking passes for Cassini will be dedicated either to command and
telemetry (for spacecraft activities and health monitoring) or to Navigation ranging data (for orbit
determination). This approach manages the available telecommunications signal strength, directing it to either
command/telemetry or Navigation, according to plan. Telecommunications link performance will improve
again this Fall.


Spacecraft Activity Summary:


On Friday, 5/29, an MRO was performed to collect additional data stored in memory when the backup CDS
computer (CDS-A) underwent a reset at the end of April. See last week's report for backgound information.
If additional readouts are needed to pursue the matter, they will be performed over presently scheduled DSN
tracking coverage.


Also on Friday, a housekeeping activity was performed which reads out a set of ACAS Attitude Estimator
(ATE) measurements not available in regular engineering telemetry. This ATE telemetry allows ground
controllers to track the normal functioning of the attitude estimator software on the spacecraft. The readout is
scheduled approximately every 2-4 weeks, over an available DSN telemetry pass.


Lastly, on Friday and Saturday, 5/29 & 5/30, the second and third data playbacks occurred for the Huygens
Probe AGC Test (which took place on Thursday, 5/28). As described in last week's report, this test turned
the Orbiter 12 degrees off its normal HGA-to-Sun line and powered on the Probe electronics to investigate
unexpectedly low AGC values measured across the Probe-to-Orbiter umbilical link during the first and
second inflight Probe checkouts. The data played back confirmed the hypothesized Solar effect, i.e. that the
Probe hardware is, indeed, healthy and the depressed AGC values in the inflight checkouts were the result of
solar noise.


From Sunday, 05/31 through Tuesday 06/02, there were no changes to spacecraft configuration.


On Wednesday, 06/03, the Solid State Recorder (SSR) record and playback pointers were reset, according
to plan. This housekeeping activity, done approximately weekly, maximizes the amount of time that recorded
engineering data is available for playback to the ground should an anomaly occur on the spacecraft.


On Thursday, 06/04, there were no changes to spacecraft configuration.


Upcoming events:


Activities scheduled for the week of 6/05 - 6/11 include an SSR Pointer Reset (06/10).


DSN Coverage:


Over the past week Cassini had 4 scheduled DSN tracks. Passes occurring on 5/29 and 5/30 were for
telemetry and command. Passes occurring on 5/31 and 6/02 were ranging only. In the coming week there will
be 2 DSN track periods.


The Cassini Project Science Group (PSG) met in Pasadena from June 1 through 5. The primary topics
covered included formation of an Orbiter Science Working Group, discussion of possible new formats for
future PSG Meetings, summary of Cassini findings during the 4/26/98 flyby of Venus, discussion of
interactions between the Science Teams and MSO and the Science Office (Sci). There was also a review of
Science Planning preliminaries carried out by Sci using a sample Saturn Tour ("T18-5") to show the times that
various desired geometries occur. This was the 17th meeting of the PSG since its inception in 1990.



Additional information about Cassini-Huygens is online at http://saturn.jpl.nasa.gov.


Cassini will begin orbiting Saturn on July 1, 2004, and release its piggybacked Huygens probe about six months later for descent through the thick atmosphere of the moon Titan. Cassini-Huygens is a cooperative mission of NASA, the European Space Agency and the Italian Space Agency. JPL, a division of the California Institute of Technology in Pasadena, manages the mission for NASA's Office of Space Science, Washington, D.C.


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