Cassini Mission-to-Saturn Spacecraft overview & CDS preparations for end-of-mission Proximal Orbits

In November 2016, the Cassini Mission-to-Saturn Spacecraft will enter a new mission regime, bringing the orbiter closer to Saturn's gaseous surface than ever before. This unique orbital profile will call for the vehicle to fly at very high speeds in a ballistic trajectory with shorter orbits than previously flown, within the vicinity of the “F” and “D” Rings of the Saturnian system. This mission phase is called the “F-Ring/Proximal Orbit” segment where a close flyby of Saturn's moon Titan will place the spacecraft in an orientation which resides near the outside edge of Saturn's F-Ring, positioning the vehicle in the proper flight path to complete the final 42 orbits of Cassini's mission. After 20 F-Ring orbits, a second close flyby of Titan will place the spacecraft between the innermost D-Ring and Saturn, where it will continue to collect new and unique science data for the remaining 22 orbits, before a third and final distant flyby of this moon alters Cassini's flight path to an impact trajectory with Saturn's surface, plunging the vehicle into the planet's atmosphere and ending the spacecraft's very successful 20-year mission, on September 15, 2017. Environmental assessments indicate that the D-Ring contains a significantly higher dust level than previously encountered by the spacecraft, while the F-Ring contains relatively little dust. Although Cassini was built to withstand dust and micrometeoroid impacts, high dust levels still pose a hazard to the vehicle during ring flyby events, and also cause a higher incidence of “bit flips” on the spacecraft's two Solid State Recorders (SSR) within the Command & Data Subsystem (CDS). Since these devices contain copies of the computer and instrument flight software (FSW), in addition to stored (recorded) science data, bit flip occurrences are autonomously repaired where possible. A high incidence of flipped bits could corrupt the SSR's stored science data significantly, and cause autonomous repairs in the FSW region, which if activated often enough, can interrupt science data collection to a significant degree and inhibit instrument FSW load retrieval from the SSR when required, as well as contribute to the likelihood of SSR memory failures. Additionally, cosmic ray-induced bit flips can occur on uplinked commands sent from the Ground to the spacecraft, such as the November 2010 Safing event where Cassini executed its Spacecraft Safing Response upon receipt of a corrupted uplink command, causing the autonomous Fault Protection (FP) to activate, which consumes a portion of Cassini's carefully budgeted propellant residual. Also of concern for the Proximal Orbits is the possibility of high radiation levels which contribute to Solid State Power Switch trips on the 192 switches aboard Cassini; several of which activate FP. In the presence of this adverse environment where very short orbits require minimized fault recovery time to maximize science capture, preparatory measures have been taken to address anomalous events quickly and are detailed in this paper.