The Last Orbit: Planning Cassini’s Plummet into Saturn

Abstract

Cassini's final orbit around Saturn will culminate in a dramatic ending as the spacecraft plunges into the ringed planet's atmosphere, never to escape or be heard from again. The last hours of the mission prior to the final loss of signal have some of the most unique and valuable science to date. Cassini will take a unique trajectory to dive deep into the atmosphere on its approach to final disposal and no spacecraft, Cassini included, has entered these depths of Saturn's atmosphere. The science community has placed heavy emphasis on this once-in-a-lifetime opportunity to inspect these deeper regions of Saturn's atmosphere. The Cassini project specifically aims to collect the very last bits of data during the final plunge to get samples of the deepest regions before the spacecraft is lost forever. The desire to collect the final bits of data presents several challenges. Cassini's Mission Planning (MP) team has developed an End of Mission (EOM) scenario to tackle these demands. The EOM scenario outlines the framework for the entire last orbit of the mission and details the strategy for data collection and transmission. Attaining near real-time transmission is key for the acquisition of the very last bits of data. The Cassini spacecraft will use a new mode of operations to successfully achieve this real-time transmission. In addition to this primary investigation and planning for telecommunications, key risks have been studied within the realm of the last orbit. Ultimately, this paper shows how the Cassini Project plans to ensure the return of every last bit of data before the spacecraft is consumed by Saturn forever. Nomenclature MP= Mission Planning EOM = End of Mission RTG = Radioisotope Thermoelectric Generator CDS = Command and Data System SSR = Solid State Recorder DST = Deep Space Transponder TWTA = Traveling Wave Tube Amplifier AACS = Attitude and Articulation Control Subsystem RCS = Reaction Control System RWA = Reaction Wheel Assembly MEA = Main Engine Assembly SFP = System Fault Protection CDA = Cosmic Dust Analyzer CIRS = Composite Infrared Spectrometer INMS = Ion and Neutral Mass Spectrometer MAG = Magnetometer MIMI = Magnetospheric Imaging Instrument RPWS = Radio and Plasma Wave Science UVIS = Ultraviolet Imaging Spectrograph CAPS = Cassini Plasma Spectrometer 2 RADAR = Cassini Radio Detection and Ranging VIMS = Visible and Infrared Mapping Spectrometer S101 = Sequence 101 DSN = Deep Space Network OTM = Orbit Trim Maneuver Rev = Revolution LOS = Loss of Signal SCET = Spacecraft Event Time T126 = Titan Flyby 126, the last targeted Titan flyby kbps = kilobits per second DSS = Deep Space Station 70-m = 70-meter DSN antenna 34-m = 34-meter DSN antenna DOY = Day of Year SP = Science Planning UTC = Coordinated Universal Time FSDS = Flight Software Development System SCO = Spacecraft Operations