Some of the most appealing science targets for future exploration missions in our solar system lie in terrains that are inaccessible to state‐of‐the‐art rover technology. This paper discusses the challenges and constraints of designing a robotic explorer for these “extreme'' terrains and then examines the trade‐offs among different mobility architectures. To address many of these challenges, we have developed the Axel family of rovers. The Axel rover is a two‐wheeled tethered robot capable of rappelling down steep slopes and traversing rocky terrain. The DuAxel rover, which is a four‐wheeled vehicle formed from two Axel rovers, provides untethered mobility to get to extreme terrains. We describe the basic design principles of these rovers and review our efforts to implement the key features of the Axel concept. Limitations found during experiments with prior Axel rover prototypes led to the design of a third‐generation Axel rover. We describe the features of this new rover and then present a thermal analysis conducted to assess the feasibility of exploring lunar cold traps, which are expected to have a temperature range of 40 to 70 K. We conclude the paper with results showcasing Axel and DuAxel's extreme‐terrain performance as evaluated during two recent field tests in a steeply sloped Southern California rock quarry and at a location in Arizona that closely resembles rugged Martian terrain. We also summarize lessons learned during the Axel development program. © 2012 Wiley Periodicals, Inc.

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An evaluation of the feasibility and mission performance benefits of using advanced space storable propellants for outer planet exploration was performed. For the purpose of this study, space storable propellants are defined to be propellants which can be passively stored without the need for active cooling. A secondary purpose o f this study was to provide guidance as to the limits, benefits, and possible methods of passively storing such mild cryogenic propellants for deep space missions. The study was composed of four distinct efforts. First, candidate propellants were defined and their relevant properties determined. Second, a propellant combination analysis using the Two Dimensional Kinetics 1997 (TDK97) program was conducted. Third, a thermal storage design was analyzed. Lastly, a mission and systems analysis was performed for three outer planetary missions. The missions are representative in complexity, duration, and requirements for a variegated set of outer planet exploration missions currently being considered by NASA. Nonetheless, the analysis conducted and outlined in this paper determined that outer planet exploration using advanced storable propellants was feasible and offered a significant benefit in delivered payload compared to previous design studies.

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