The Cassini grand finale mission: Planning for a new mission environment

The Cassini F-Ring & Proximal Orbits (FRPO) is a new and unique mission; to ensure the highest priority science gets implemented, the POST (Proximal Orbit Science Team) was created to pre-allocate the time around periapse for all 22 proximal orbits. The F-ring orbits, and proximal time outside of POST, were handled similar to Cassini's Solstice Mission using the Pre-Integrated Event (PIE) process. The new and unique properties of the spacecraft's trajectory required much forethought to be flown safely while still planning for the most and best science return possible. Some ring-plane crossings (RPX) will be protected against dust impacts by turning the high gain antenna (HGA) to the dust RAM direction (HGA2RAM). If on the first proximal RPX higher than expected dust readings are seen then the Project Office may choose to require more (all) subsequent RPX to be HGA2RAM, implemented via a real-time command overlay for uplinked sequences. The pointing uncertainties will be larger than usual after the final targeted flyby; some of the process changes to address this include adding extra orbit trim maneuvers (OTMs) (fuel permitting) to resync to the reference trajectory and reduce pointing uncertainties; and movable blocks of commands to be used for some periapses where atmospheric drag may cause large timing shifts Changes made for FRPO to address perceptions that these sequences will be hard to implement include requiring early pointing designs (during integration) for certain types of observations, requiring teams to check early on that they can turn to and from their observation attitude, and that their attitude is safe, and adjusting the Implementation process to give more time for science observation designers.