The Cassini Solstice Mission (CSM) is the second extended mission phase of the highly successful Cassini/Huygens mission to Saturn. Conducted at a much-reduced funding level, operations for the CSM have been streamlined and simplified significantly. Integration of the science timeline, which involves allocating observation time in a balanced manner to each of the five different science disciplines (with representatives from the twelve different science instruments), has long been a labor-intensive endeavor. Lessons learned from the prime mission (2004-2008) and first extended mission (Equinox mission, 2008-2010) were utilized to design a new process involving PIEs (Pre-Integrated Events) to ensure the highest priority observations for each discipline could be accomplished despite reduced work force and overall simplification of processes. Discipline-level PIE lists were managed by the Science Planning team and graphically mapped to aid timeline deconfliction meetings prior to assigning discrete segments of time to the various disciplines. Periapse segments are generally discipline-focused, with the exception of a handful of PIEs. In addition to all PIEs being documented in a spreadsheet, allocated out-of-discipline PIEs were entered into the Cassini Information Management System (CIMS) well in advance of timeline integration. The disciplines were then free to work the rest of the timeline internally, without the need for frequent interaction, debate, and negotiation with representatives from other disciplines. As a result, the number of integration meetings has been cut back extensively, freeing up workforce. The sequence implementation process was streamlined as well, combining two previous processes (and teams) into one. The new Sequence Implementation Process (SIP) schedules 22 weeks to build each 10-week-long sequence, and only 3 sequence processes overlap. This differs significantly from prime mission during which 5-week-long sequences were built in 24 weeks, with 6 overlapping processes.

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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

The Cassini spacecraft consists of 12 instruments: 4 Optical Remote Sensing Instruments (ORS), 6 In-situ observation instruments to study Magnetospheric and Plasma Science (MAPS), one Radar instrument, and one Radio Science (RSS) instrument. When this complex mission was initially architected, much of the early emphasis was placed on the spacecraft function and design, rather than operations. The spacecraft and mission design posed significant challenges to the science and sequence development process for the four-year tour of the Saturnian system.The science planning and sequence development process produces a comprehensive set of commands for all science and engineering activities for an approximate 40 day time period. The end-to-end sequence design process consists of five phases: 1) Integration of the Science Operations Plan (SOP), a high-level plan of science and engineering activities, detailing their timing, power, thermal, data volume, and pointing profiles 2) SOP Implementation, in which resource conflicts are resolved and activities constraint checked 3) Aftermarket and SOP Update, in which the SOP is updated while in tour using the latest information on the navigation ephemeris, and the spacecraft's and instruments' performance 4) Science and Sequence Update Process, which results in an integrated, validated, uplinkable, and flyable distributed sequence 5) Execution, which includes system-level and instrument-internal real-time commands, anomaly response, and sequence pointing and timing adaptation using the latest ephemeris information Each phase of the sequence development process had to overcome many operational challenges due to the immense complexity of the spacecraft, tour design, pointing capabilities, flight rules and software development. This paper will address the specific challenges related to each of those complexities and the methods used to overcome them during operations.

We describe the problem of regular small telemetry losses incurred during coherency mode transitions in Cassini's telecommunication. The project did not originally plan any corrective steps for avoiding these data losses, because of 1) the disparity between the small durations of the transitions (1-2 min) and large playback capability losses (15 min) needed for bracketing the transition time spans and 2) the unpredictable content of data downlinking during the transitions. However, as the intense science data return from the tour began, it became apparent that the impact of these small losses can sometimes be significant. We provide two examples of the impact on Radar-dedicated Titan flybys. In general, the impacts are larger for high-rate data and for data acquired during a targeted flyby of Titan and other icy satellites. Although the content of data during a transition for every downlink pass is unpredictable, we are certain that some important data will be lost on downlink passes dedicated to transmit the flyby data and it does not matter what part of the data will be hit by the transitions. We collected more than 200 days of data from Cassini tour operations between June 2004 and February 2005 to analyze the distributions of the start time and duration of the transitions. We found that the occurrence of a transition can be predicted within a 5-min window, with 95 percent confidence. Given that, it is possible to eliminate the data losses by pausing playback at the beginning of a transition for 5 minutes and resuming playback after transition completion. We briefly describe three operational fixes as to how to implement the playback pause, with the pros and cons for each method. Finally, we report the results of the method chosen by the project and implemented on the spacecraft for several Titan and icy satellites flybys between September and October, 2005.