Fault Protection Detects Minor Orientation Discrepancy
March 26, 1998
Cassini's fault protection system worked as planned Tuesday after the spacecraft detected a very small orientation difference between its two stellar reference units, according to preliminary data received from the spacecraft yesterday. The event has had no impact on the mission and all planned activity for Cassini's Venus flyby next month remains on schedule.
The second stellar reference unit, part of Cassini's attitude and articulation control subsystem, was in the process of taking over while the first stellar reference unit was to undergo a routine maintenance "bake-out" heating to remove normal post-launch contaminants from its aperture. This was the first use of the second stellar reference unit, and as it began operating, Cassini's attitude control software found a very small difference in the orientation of the units. Sensing this difference, the computer that controls the attitude control subsystem executed pre-programmed commands that brought the spacecraft into a low-activity state to await instruction from ground controllers. The spacecraft executed this response exactly as designed and the spacecraft remains healthy,
project officials said. Cassini has remained in contact with ground controllers throughout, and engineering data on the event and the spacecraft's overall operations were received yesterday. Preliminary analysis indicates that the discrepancy was within specifications, but that control limits were set too tightly, triggering the preprogrammed commands that set the spacecraft in its low-activity state. This problem is expected to be solved easily with an adaptation made to the spacecraft's attitude and articulation control software.
By early this afternoon, ground controllers will have sent commands to return the spacecraft to normal operations. Cassini remains on course for its April 26 flyby of Venus, with the last fine-tuning of the flight path, if it is needed, scheduled for early April. During its Venus flyby, Cassini's radio and plasma wave science instrument will take advantage of the opportunity to search for lightning in Venus's atmosphere.
Today, the spacecraft is approximately 17 million kilometers (about 10.5 million miles) from Venus and is traveling at a speed of about 143,000 kilometers (about 88,800 miles per hour). It has traveled about 440 million kilometers (about 273 million miles) on its Saturn-bound trajectory since launch on October 15, 1997.
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.