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JET PROPULSION LABORATORY
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The Cassini operations team remained in contact with the Saturn-bound spacecraft after Cassini entered a "safe" mode Monday, January 11, as commanded by the spacecraft's built-in fault protection software. The software, used on all of NASA's robotic planetary spacecraft, is designed to insure that the spacecraft halts non-critical activity and maintains its telecommunications link with Earth whenever an unexpected event has occurred.

Cassini engineers said that at about 3 p.m. PST on Monday, the spacecraft was in the midst of an instrument checkout exercise when it sensed a potential error in its orientation. Attitude and articulation control system software sent this information to Cassini's central computer, the command and data subsystem, which executed fault protection routines to "safe" the spacecraft and reinitialize its orientation. The spacecraft was put in a minimum power configuration and thermally safe environment, pointing its 4-meter-diameter (12-foot) antenna toward the Sun to shade the rest of the spacecraft. Communication with the spacecraft is being conducted with the low-gain antenna.

Cassini Program Manager Bob Mitchell said engineering data from the spacecraft are being transmitted to Earth to help engineers pinpoint the initiating incident that caused Cassini to enter the safe mode. Engineers currently suspect that during a tracking maneuver, Cassini's star scanner may have viewed a patch of sky devoid of the bright stars the spacecraft uses to orient itself in space. Attitude control software would have alerted the command and data subsystem when the scanner had spent sufficient time searching for but not spotting familiar stars.

Mitchell said he expects the team to take Cassini out of its safe mode later this week after the initiating event has been identified and all engineering data have been thoroughly analyzed. In the meantime, the spacecraft remains in good health as it awaits new commands from controllers on Earth.