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Trident was one of the four Discovery-class Step-1 mission proposals selected by NASA in 2020 for further development and study; however, in 2021, the Step-2 proposal was not down-selected to transition into the next phase of mission development, i.e., a mission for flight.Neptune’s largest moon, Triton, was the primary focus of study for Trident. Triton’s physical and orbital characteristics make it a unique planetary target for scientific exploration, providing opportunities for investigations in a wide variety of scientific fields, including geomorphological, atmospheric, geophysical, magnetospheric, and ionospheric studies. The science objectives of the Trident mission encompassed an in-depth interior-to-exterior set of objectives, focused on multiple outstanding questions resulting from the 1989 encounter of Voyager 2, and subsequent analysis.Ball Aerospace Corp. was tasked with building the Trident spacecraft, with JPL responsible for providing Engineering Support (Mission Design & Navigation, Mission Planning, Flight Operations, Ground Data Systems, Systems Engineering) and leading Project Management. The observatory would carry a wide-ranging suite of scientific instruments onboard, including an Infrared Spectrometer (IRS) and Narrow Angle Camera (NAC) to be provided by Ball Aerospace Corp., a Wide Angle Camera (WAC) from JPL, a Magnetometer from UCLA, a contributed Plasma Science Suite from IRF (Sweden), and a contributed Radio Science instrument from ASI (Italy). All of these instruments would be used to collect unique datasets during the Triton encounter. Trident would have taken advantage of an ~13-yr, nearly-ballistic trajectory to Triton, utilizing a timely Jupiter Gravity Assist, to execute a 10-day long encounter in the Neptunian system. Launch was planned for October 2025, with Triton arrival scheduled for December 2038. The timeline for this mission would have been sub-divided into seven major phases: Launch, Commissioning, Inner Planet Cruise, Outer Planet Cruise, Approach, Encounter, and Science Data Return. Multiple planetary flybys were planned to be performed during the cruise, including three Earth flybys and one Venus flyby in the Inner Planet Cruise phase, and one Jupiter flyby in the Outer Planet Cruise phase. Along with conventional (Range and Doppler) tracking data, Delta-DOR and Optical Navigation data were also to be acquired to assist with spacecraft navigation during the Approach and Encounter phases. A 3 meter X-Band High Gain Antenna would allow playback of all science data at 1 kbps within 1 year after the Triton Encounter. The Mission Planning element on Trident encompassed and informed multiple aspects of this proposal, ranging from science observation planning during the Triton Encounter phase, to generation of activity timelines for all mission phases; performing ground coverage analysis for science observations to be acquired by all instruments and tracing them to science requirements; evaluation of spacecraft resources including data volume stored onboard, power/energy consumption, telecom (commanding/telemetry) requirements, and overall, working at the interface of science and engineering teams on the mission. All of these functions that were performed by the Mission Planning team on this proposal are discussed in this paper.

Jupiter’s icy moon Europa is a prime target in our exploration of potentially habitable worlds beyond Earth. The combination of a subsurface liquid water layer in contact with a rocky seafloor may yield an ocean rich in the elements and energy needed for the emergence of life, and for potentially sustaining life through time. Europa may hold the clues to one of NASA’s long-standing quests – to determine whether or not we are alone in the universe. The Europa Clipper mission will characterize Europa’s habitability as the first step in the search for potential life at Europa by conducting approximately four dozen flybys. The project is preparing for a System Integration Review in November of 2021. This paper will summarize changes from the mission’s Preliminary Design Review (PDR) baseline, including science objectives, launch vehicle, interplanetary trajectory, science tour, flight system, payload and mission phases. Since PDR several of the Level 1 science requirements were descoped and the science requirement flow was realigned, in order to control cost growth. The realignment deemphasized science redundancy, which had resulted in an over-constrained science tour, in favor of a more targeted linkage to the primary responding instrument. Additionally, changes to the PDR flight system design baseline include changes to the sounding radar antenna design, the magnetometer architecture, and the heat redistribution system pumps. All changes have been incorporated into a reference science tour used to demonstrate that the requirements and design of the flight system and payload are mature enough to support flight builds.