nuclear power and propulsion systems being developed under the aegis of Project Prometheus. These missions include the following:
Cassini Extended. The logical extension to the observations of the saturnian system following the end of Cassini’s 4-year prime mission. By definition, an operating spacecraft does not require new power and propulsion technologies.
Mars Upper Atmosphere Orbiter. Missions of this type do not, in general, require nuclear power or propulsion for their operation because solar power is adequate and cost-effective inside 2 AU.
Kuiper Belt-Pluto Explorer. Currently being implemented as New Horizons, this is the first of the New Frontiers line of medium-class, principal-investigator (PI)-led missions. It is making use of the last remaining radioisotope thermoelectric generator (RTG) available from the Department of Energy (DOE) and therefore cannot directly benefit from Project Prometheus and related technological developments.
NASA’s current ground rules for several continuing lines of small, PI-led missions specifically exclude the use of radioisotope power systems (RPSs). These lines, given high priority in the SSE decadal survey, include the following:
Mars Scout. These missions are designed to address priorities outside the principal objectives of the Mars Exploration Program and, as such, provide the planetary science community with a means to respond to discoveries and technological advancement. Because these missions are selected via an open competition, the exact scope of each such mission is unknown at this time. If RPSs were allowed, the availability of a range of small to medium-size power sources (i.e., ones providing an electrical output in the range from ~10 mW to ~10 W) would likely enhance capabilities.2
Discovery. Missions in this line are suitable for exploration of Mercury, Venus, near-Earth and main-belt asteroids, comets, and the Moon. Some future missions could potentially benefit from use of RPSs if they were permitted.
Currently viable missions described in the SSE decadal survey that might be significantly enhanced by RPS technologies include the following:
Venus In Situ Explorer. This mission is designed to make compositional and isotopic measurements of the atmosphere (especially the lower atmosphere) and the surface of Venus to characterize the geochemistry, mineralogy, and past tectonic history of Earth’s sister planet. The battery-powered concept in the SSE decadal survey takes samples from the lower atmosphere and surface of Venus and lofts them to a higher and cooler altitude for further analysis. However, the high-temperature and high-pressure environment near the surface of Venus makes such a mission extremely challenging. Refrigeration using an RPS and pressure equalization of the Explorer while keeping it cool would greatly simplify the tasks of both obtaining the samples and analyzing them in situ. A concept for a refrigerated, long-lived Venus lander is described in a subsequent section.
South Pole-Aitken Basin Sample Return. This mission is intended to collect rock and soil samples from one or more locations in the largest and deepest impact basin in the solar system. The capability of this cost-capped, New Frontiers mission to select optimal samples will be limited by its reconnaissance capabilities and by the design of the landers.a If this mission is less rigorously cost capped, one or more rovers with drilling capability could be added to carry out long-term exploration of this very large region. For elaboration of such a mission concept see the discussion of the Lunar Polar Rover/Driller in Appendix C.