Vision and Voyages for Planetary Science in the - Solar System ...

atmospheric probe and another a separate KBO-flyby spacecraft), as well as a “high performance” orbiter.
All flyby options relied exclusively on chemical propulsion, while all other options included a solarelectric
propulsion (SEP) system. The most complex of the simple orbiters and the high-performance
orbiter would insert themselves into orbit about Neptune via 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
Advanced Stirling Radioisotope Generators (ASRGs), depending on 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.
Mission Challenges
All concepts studied had moderate reliability risk due to long mission durations. Furthermore,
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 the fly-by missions to the simple and high
performance orbiters.
The point design was terminated before full evaluation of risk, cost and schedule as it was
deemed less technically feasible than a comparable Uranus mission (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.
Conclusions
The flyby mission architectures were deemed to achieve significant science progress since
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 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.
PREPUBLICATION COPY—SUBJECT TO FURTHER EDITORIAL CORRECTION
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