• Develop an understanding of the spatial distribution of surface composition and how the composition is coupled to geologic processes on a given KBO.
The RMA study examined seven flyby architectures with varying degrees of complexity and focus on Triton, or KBOs: a “minimal” orbiter of Neptune, five simple orbiter concepts (one including a shallow atmospheric probe, and another separate KBO-flyby spacecraft), as well as a “high-performance” orbiter. All flyby options relied exclusively on chemical propulsion; all other options included a solar-electric propulsion system. The most complex of the simple orbiters and the high-performance orbiter would insert themselves into orbit around Neptune by means of aerocapture. The remaining orbiter concepts employed chemical propulsion for this purpose. Most architectures included a 25-kg or 60-kg primary instrument payload (predominantly based on New Horizons heritage). The high-performance architecture allocated up to 300 kg in payload mass. All missions called for the use of three to six ASRGs, depending on the mission architecture.
The follow-on point-design, full mission study focused on an orbiter mission with limited payload and a shallow atmospheric probe (1- to 5-bar terminal pressure). The studies were limited to trajectories without Jupiter gravity assists in order to assess the difference between identical Uranus and Neptune missions without narrow launch window constraints.
All of the concepts studied had moderate reliability risk due to the long duration of the missions. Furthermore, a failure of multiple ASRGs was deemed a moderate risk for all of the simple-orbiter concepts. The availability of plutonium-238 and other logistical issues associated with ASRGs also incurred moderate implementation risks for all options. For the most elaborate of the simple orbiters and the high-performance orbiter, the use of aerocapture was identified as necessitating further technology development and therefore posed a moderate programmatic and technical risk. Scheduling constraints were identified for all but the high-performance option by the availability of a Jupiter gravity assist maneuver, which would favor a launch between 2016 and 2018, with reduced-performance opportunities sporadically thereafter. Cost increases proportionally from that for the flyby missions to that for the simple and high-performance orbiters.
The point design was terminated before a full evaluation of risk, cost, and schedule was completed, as it was deemed less technically feasible than a comparable Uranus mission (see Appendix C). A Neptune mission without a Jupiter-flyby gravity assist requires aerocapture for orbit insertion. Aerocapture itself not only adds complexity and risk but also makes probe delivery and orbits that allow Triton encounters more challenging. Even with a SEP system, a mission to Neptune has a long duration and thus higher risks for instruments and spacecraft components.
The flyby mission architectures were deemed capable of achieving significant science progress beyond that from Voyager 2’s visit of Neptune and offer the potential for new KBO science. Even the simplest of the flyby missions exceeded the cost cap of a New Frontiers mission and offered low science return relative to its cost; it was deemed not compelling. More complex missions and orbiters provided a vast gain in science objectives that would be unavailable to flyby missions, but at increased cost; the highest-performance option yielded a modest increase in estimated science value for its higher cost. More detailed design work of a “sweet spot” mission design identified technical risks that make a Uranus mission more favorable for the coming decade. Technology development will increase the feasibility of a future Neptune orbiter mission.