Cassini Executes Instrument Checkout

January 15, 1999

Engineers controlling the Saturn-bound Cassini spacecraft have commanded the spacecraft to resume checkout of its onboard instruments at 8 a.m. Pacific time Saturday, January 16, having determined why the spacecraft placed itself in a "safe" mode earlier this week, program officials reported.

Commands were radioed to the spacecraft's low-gain antenna at about 2 a.m. today to reorient its 4-meter-diameter (12-foot) high-gain antenna toward Earth, turn on several science instruments, and resume its instrument checkout scheduled for this weekend.

Saturday's instrument checkout activities include the first operation of Cassini's optical instruments, including the imaging system, visible and infrared mapping spectrometer, and the ultraviolet imaging spectrograph. The instruments will view the bright star Spica, using it as a calibration and pointing target. Data returned will be used to generate a photomosaic of Spica and the surrounding sky and test the spacecraft and optical instruments' pointing accuracy. The instruments will gather additional calibration and scientifically useful data when Cassini makes its second flyby of Venus in June.

Engineers have determined that a particular geometry that the spacecraft experienced during a very slow roll maneuver was responsible for placing it into "safe" mode Monday, January 11. The spacecraft's fault protection software was triggered when the attitude and articulation control system detected a larger-than-expected change in its estimate of the spacecraft's orientation, or attitude. The spacecraft's measured orientation is constantly compared to its predicted orientation, and even a very slight variation for a short period of time will cause the attitude control system to re-establish its attitude. This is accomplished by entering into "safe" mode.

In this particular case, one star was very near the edge of the stellar reference unit's field-of-view. This contributed to a larger-than-normal error estimate and, because of the particular geometry and roll rate, the error persisted for longer than the 50 seconds required to activate the safe-mode response. In safing itself, the spacecraft re-initialized the attitude estimate and placed itself in a thermally safe orientation to await further commands from Earth. Such fault protection software is used on all of NASA's robotic planetary spacecraft, and is designed to ensure that the spacecraft halts non-critical activity and maintains its telecommunications link
with Earth whenever an unexpected event has occurred. The spacecraft was never out of touch with Earth, as communications continued through Cassini's low-gain antenna.

Additional information about Cassini-Huygens is online at

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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