The in-situ exploration of small Solar System bodies (such as asteroids or comets) is becoming a central objective for future planetary exploration. Such bodies are characterized by very weak gravitational fields, which make hopping mobility platforms one of the preferred mobility strategies for microgravity surface exploration, as recognized by space agencies worldwide. However, little is known about the dynamical behavior of hopping platforms in low gravity environments, where small bodies' rotational dynamics can have a critical effect. Accordingly, the objective of this paper is to study in detail the “dynamic envelope” of hopping microgravity rovers, with a focus on internal actuation. Specifically, we first perform a static analysis with the goal of determining regions of a small body where an internally-actuated hopping rover can stably remain at rest. Then, we perform a dynamic analysis and discuss the actuation and instrument pointing performance of hopping microgravity platforms as a function of a number of system and environmental parameters (e.g., rover shape, body rotation rate). Finally, we tailor our analysis to a potential mission to Mars' moon Phobos. Collectively, our results show that internally-actuated rovers, from an actuation standpoint, are a viable mobility solution for a vast class of small Solar System bodies. Also, our analysis represents a key first step to develop path planning algorithms for microgravity explorers to safely explore dynamically feasible regions.

The future exploration of small Solar System bodies will, in part, depend on the availability of mobility platforms capable of performing both large surface coverage and short traverses to specific locations. Weak gravitational fields, however, make the adoption of traditional mobility systems difficult. In this paper we present a planetary mobility platform (called “spacecraft/rover hybrid”) that relies on internal actuation. A hybrid is a small (~ 5 kg), multi-faceted robot enclosing three mutually orthogonal flywheels and surrounded by external spikes or contact surfaces. By accelerating/decelerating the flywheels and by exploiting the low-gravity environment, such a platform can perform both long excursions (by hopping) and short, precise traverses (through controlled “tumbles”). This concept has the potential to lead to small, quasi-expendable, yet maneuverable rovers that are robust as they have no external moving parts. In the first part of the paper we characterize the dynamics of such platforms (including fundamental limitations of performance) and we discuss control and planning algorithms. In the second part, we discuss the development of a prototype and present experimental results both in simulations and on physical test stands emulating low-gravity environments. Collectively, our results lay the foundations for the design of internally-actuated rovers with controlled mobility (as opposed to random hopping motion).