A Minimally Actuated Hopping Rover for Exploration of Celestial Bodies Eric Hale, Nathan Schara, Joel Burdick Mechanical Engineering California Institute of Technology Pasadena, California 91125 Paolo Fiorini Jet Propulsion Laboratory California Institute of Technology Pasadena, California 91109 Abstract: This paper describes a minimalist hopping robot that can perform basic exploration tasks on Mars or other moderate gravity bodies. We show that a single actuator can control the vehicle’s jumping and steer- ing operations, as well as the panning of an on-board camera. Our novel thrusting linkage also leads to good system efficiency. The inherent minimalism of our hopping paradigm offers interesting advantages over wheeled and legged mobility concepts for some types of planetary exploration. The paper summarizes the evo- lutionary development of the system, issues relevant to the design of such jumping systems, and experimental results obtained with system prototypes. 1 Introduction and Motivation With the recent success of the Pathfinder mission to Mars [10], there is an increasing interest in robotic exploration of Mars and other celestial bodies such as moons, asteroids, and comets. These bodies are charac- terized by a low to medium gravitational environment. The space exploration community has spent consider- able effort and has significant ongoing interest in the development of mechanical mobility systems that are capable of supporting long-range scientific exploration of such bodies. The most successfully deployed paradigm, as seen in the Pathfinder mission’s Sojourner vehicle [10], is a multi-wheeled rover. This concept is currently being extended to both larger and smaller sized rovers. Most 6-wheeled rover designs can traverse obstacles that are at most about 1.5 times their wheel diameter. Inflat- able wheels may be able to overcome somewhat pro- portionally larger obstacles. Smaller rovers can be ef- fectively used in tandem with larger rovers to increase exploration range, in spite of their limited size, by ex- ploring difficult areas, such as mountain cliffs, in a teth- ered configuration. Legged rovers have previously been proposed for Lunar and Martian exploration [1] in or- der to overcome the limited traversability of wheeled vehicles in rugged terrain. These approaches to surface mobility have two sig- nificant drawbacks. First, even exotic types of wheeled rovers can only drive over obstacles that are at best a fraction of the vehicle’s body length. Thus, some terrains are not accessible to wheeled vehicles. While legged robots can potentially access rough terrains, they are mechanically complex, requiring numerous joints, actuators, and linkages. Even wheeled rover ve- hicles use a significant numbers of actuators and com- plex suspension linkages. For example, the Sojourner mobility system used 10 motors, while prototypes for the 2005 Mars mission have 12 independent actuators [17]. Hence, most actively explored paradigms for plan- etary mobility are based on a large number of actua- tors. There are a number of obvious drawbacks to using many motors and their associated linkages: an inherent risk in system failure; a need for larger power supplies and/or solar cells; a need for complex power electron- ics; and increased system weight (which reduces the weight that can be allocated to science payloads). Reducing the number of actuators is an attrac- tive goal for planetary rover design, since such de- signs are like to be smaller and lighter, with lower risk of failure. Furthermore, with significantly reduced size/mass, there is a greater likelihood that several such rovers could be deployed in a single rocket launch pay- load. However a truly minimally actuated device may not have the functionality necessary to carry out mean- ingful tasks. The research presented in this paper ex- plores the trade-offs between functionality and com- plexity in the context of the design and development of a single-actuator hopping robot, capable of moving a camera and a small science package by jumping. Our hopper’s operation, which is described below, is more akin to the movement of a frog, rather than the oscilla- tory behavior of typical hopping robots [12]. We show that a single actuator is enough to propel, steer, and self-right a simple hopper. The same actuator can also pan an on-board camera as well as manage a science package. Furthermore, the entire system weighs less 1