Human Exploration of the Solar System by 2100
It has been suggested that the U.S., in concert with private entities and international partners, set itself on a course to accomplish human exploration of the solar system by the end of this century. This is a strikingly bold vision intended to revitalize the aspirations of HSF in service to the security, economic, and scientific interests of the nation. Solar system distance and time scales impose severe requirements on crewed space transportation systems, however, and fully realizing all objectives in support of this goal will require a multi-decade commitment employing radically advanced technologies - most prominently, space habitats capable of sustaining and protecting life in harsh radiation environments under zero gravity conditions and in-space propulsion technologies capable of rapid deep space transits with earth return, the subject of this paper. While near term mission destinations such as the moon and Mars can be accomplished with chemical propulsion and/or high power SEP, fundamental capability constraints render these traditional systems ineffective for solar system wide exploration. Nuclear based propulsion and alternative energetic methods, on the other hand, represent potential avenues, perhaps the only viable avenues, to high specific power space transport evincing reduced trip time, reduced IMLEO, and expanded deep space reach. Here, very long term HSF objectives for solar system wide exploration are examined in relation to the advanced propulsion technology solution landscape including foundational science, technical/engineering challenges, and developmental prospects.