Cassini Significant Event Report
For Week Ending 04/17/98
The Cassini spacecraft is presently traveling at a speed of approximately 139,000 kilometers/hour (~86,000
mph) relative to the sun and has traveled approximately 511 million kilometers (~317 million miles) since
launch on October 15, 1997.
As of Wednesday this week, the Cassini spacecraft has been flying for 6 months. Cassini's first planetary
gravity assist, a technique used to increase spacecraft velocity, is approaching; the Venus-1 flyby scheduled
for Sunday morning, April 26th.
The most recent Spacecraft status is from the DSN tracking pass on Thursday, 04/16, over Canberra. The
Cassini spacecraft is in an excellent state of health and is executing the C7 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.
Spacecraft Activity Summary:
On Friday, 04/10, the two of the three components of the second quarterly Periodic Engineering Maintenance
activity were performed. The AACS BAIL Maintenance occurred first and was followed by the Engine
Gimbal Actuator (EGA) exercise. (The third component - a Reaction Wheel Assembly exercise - is scheduled
for early May.)
The AACS BAIL software is stored on EEPROMs for the purpose of providing basic AACS capabilities for
use in the recovery from a deep undervoltage anomaly, should one ever occur. The BAIL maintenance,
planned every 3 months, is intended to detect and repair any Single Bit Errors (SBEs) that may have occurred
on the EEPROMs in the preceding period. The activity will also identify, but not repair, any Double Bit Errors
(DBEs); DBEs would then be repaired by further ground commanding at a later time. The EGA actuator
exercise moves both Cassini main engines through their range of motion to assure that gimbal lubricant remains
evenly distributed. Both activities were accomplished successfully. Telemetry indicated no SBEs or DBEs had
occurred in the BAIL EEPROMs.
On Saturday, 04/11, 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 Sunday, 04/12, and Monday, 04/13, there were no changes in spacecraft configuration.
On Tuesday, 04/14, a maintenance activity was performed on the SSR Flight Software Partitions. This activity
repairs any SSR double bit errors (DBEs) which have occurred in the code-containing portions of the Flight
Software partitions during the preceding period.
On Wednesday, 04/15, there were no changes in spacecraft configuration.
On Thursday, 04/16, the second Periodic Instrument Maintenance activity (PIM) began execution, as
planned. This activity is carried out every three months by 11 of the 12 Orbiter instruments. This activity
concludes Friday evening (04/17). PIM results will be reported in next week's Significant Events Report.
Also on Thursday, the Solid State Recorder (SSR) record and playback pointers were reset, according to
Activities scheduled for the week of 4/17 - 4/23 include: conclusion of the Periodic Instrument Maintenance
(4/17), an SSR pointer reset (4/21), and uplink of the RPWS/Radar Venus-1 Minisequence (4/22).
Venus-1 gravity assist flyby occurs early Sunday morning, 4/26/98.
Over the past week Cassini had 9 scheduled DSN tracks occurring from 04/10 through 4/16. In the coming
week there will be 10 DSN passes.
Huygens Probe Status:
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.