Cassini’s Grand Finale – Attitude Control Subsystem Performance During Proximal Ring Plane Crossings

On April 22nd, 2017, the Cassini spacecraft made its final close targeted flyby of Titan,
placing Cassini in the gap between Saturn and its innermost D-ring for the first time ever of
its mission at Saturn. Cassini proceeded to fly through this region 22 times, and on
September 15th, 2017, Cassini plunged into Saturn’s atmosphere, concluding a remarkable,
nearly 20-year mission. The attitude control subsystem performance during these 22
“proximal” orbits exceeded expectations. Prior to the proximal orbits mission, the effects of
dust hazards, Saturn atmospheric drag torque on Cassini’s control authority, radiation on
the sensitive instruments, bright body interferences to the star tracker, pointing
discrepancies caused by trajectory deviations, and possible fault protection scenarios were
assessed. This paper discusses the results of these risks experienced by the attitude control
subsystem over the proximal orbits mission, any interesting telemetry observed, as well as
any last minute procedural changes implemented along the way.