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SpaceHopper: A Small-Scale Legged Robot for Exploring Low-Gravity Celestial Bodies

Alexander Spiridonov, Fabio Buehler, Moriz Berclaz, Valerio Schelbert, Jorit Geurts, Elena Krasnova, Emma Steinke, Jonas Toma, Joschua Wuethrich, Recep Polat, Wim Zimmermann, Philip Arm, Nikita Rudin, Hendrik Kolvenbach, Marco Hutter

TL;DR

This work introduces SpaceHopper, a compact three-legged robot designed for controlled jumping and attitude reorientation in low-gravity environments. It combines a differential hip drive and three-DOF legs with end-to-end DRL policies trained in IsaacGym to achieve reorientation and jumping in simulated low-gravity (e.g., 0.029 g) scenarios, validated by a hardware gimbal and counterweight setup. Key contributions include a complete mechanical-electrical design, a DRL-based attitude and locomotion controller, and demonstrations of upright reorientation with small attitude errors and end-to-end jumping performance, along with a plan for parabolic-flight validation and improved state estimation for real microgravity missions. The results demonstrate a viable path toward compact, space-qualified legged locomotion for mobile exploration of asteroids and moons, highlighting both the potential and the remaining challenges toward robust, real-world deployment.

Abstract

We present SpaceHopper, a three-legged, small-scale robot designed for future mobile exploration of asteroids and moons. The robot weighs 5.2kg and has a body size of 245mm while using space-qualifiable components. Furthermore, SpaceHopper's design and controls make it well-adapted for investigating dynamic locomotion modes with extended flight-phases. Instead of gyroscopes or fly-wheels, the system uses its three legs to reorient the body during flight in preparation for landing. We control the leg motion for reorientation using Deep Reinforcement Learning policies. In a simulation of Ceres' gravity (0.029g), the robot can reliably jump to commanded positions up to 6m away. Our real-world experiments show that SpaceHopper can successfully reorient to a safe landing orientation within 9.7 degree inside a rotational gimbal and jump in a counterweight setup in Earth's gravity. Overall, we consider SpaceHopper an important step towards controlled jumping locomotion in low-gravity environments.

SpaceHopper: A Small-Scale Legged Robot for Exploring Low-Gravity Celestial Bodies

TL;DR

This work introduces SpaceHopper, a compact three-legged robot designed for controlled jumping and attitude reorientation in low-gravity environments. It combines a differential hip drive and three-DOF legs with end-to-end DRL policies trained in IsaacGym to achieve reorientation and jumping in simulated low-gravity (e.g., 0.029 g) scenarios, validated by a hardware gimbal and counterweight setup. Key contributions include a complete mechanical-electrical design, a DRL-based attitude and locomotion controller, and demonstrations of upright reorientation with small attitude errors and end-to-end jumping performance, along with a plan for parabolic-flight validation and improved state estimation for real microgravity missions. The results demonstrate a viable path toward compact, space-qualified legged locomotion for mobile exploration of asteroids and moons, highlighting both the potential and the remaining challenges toward robust, real-world deployment.

Abstract

We present SpaceHopper, a three-legged, small-scale robot designed for future mobile exploration of asteroids and moons. The robot weighs 5.2kg and has a body size of 245mm while using space-qualifiable components. Furthermore, SpaceHopper's design and controls make it well-adapted for investigating dynamic locomotion modes with extended flight-phases. Instead of gyroscopes or fly-wheels, the system uses its three legs to reorient the body during flight in preparation for landing. We control the leg motion for reorientation using Deep Reinforcement Learning policies. In a simulation of Ceres' gravity (0.029g), the robot can reliably jump to commanded positions up to 6m away. Our real-world experiments show that SpaceHopper can successfully reorient to a safe landing orientation within 9.7 degree inside a rotational gimbal and jump in a counterweight setup in Earth's gravity. Overall, we consider SpaceHopper an important step towards controlled jumping locomotion in low-gravity environments.
Paper Structure (37 sections, 4 equations, 10 figures, 2 tables)

This paper contains 37 sections, 4 equations, 10 figures, 2 tables.

Table of Contents

  1. INTRODUCTION
  2. SYSTEM DESIGN
  3. System Overview
  4. Leg Design
  5. Hip Design
  6. Combined Torque for Jumping
  7. Static Motor Placement
  8. Body Design
  9. Materials
  10. Electrical System
  11. CONTROL SYSTEM OVERVIEW
  12. Control Architecture
  13. Locomotion Controller
  14. ATTITUDE CONTROL
  15. Simulation Study
  16. ...and 22 more sections

Figures (10)

  • Figure 1: The small-scale low-gravity robot SpaceHopper fully assembled with labeled parts and dimensions.
  • Figure 2: Cut section of the hip and knee with annotated transmissions, movements, and components.
  • Figure 3: Dataflow visualization inside SpaceHopper during reorientation test. Green boxes show software modules running on the onboard computer, and yellow boxes show peripheral modules.
  • Figure 4: SpaceHopper reorienting itself to the upright orientation, which is displayed as a black line.
  • Figure 5: SpaceHopper's body orientation (Euler angles $x,y,z$) in simulation during reorientation from random initial attitude to upright (left panel), and a box plot of final orientation errors for twenty random initial orientations (right panel).
  • ...and 5 more figures