Surface Mobility on Small Bodies

Hedgehog_on_Phobos

We’re developing a mission architecture that will allow the systematic and affordable in-situ exploration of small Solar System bodies, such as asteroids, comets, and Martian moons, where the gravity levels are too low to use traditional rovers. Our architecture relies on the novel concept of spacecraft/rover hybrids, which are surface mobility platforms capable of achieving large surface coverage with attitude-controlled hops, fine mobility by tumbling, and instrument pointing by yawing and other small maneuvers in the micro-g to milli-g gravity environments of small bodies. The hybrids maneuver by spinning three internal flywheels, a minimalistic design that allows all subsystems to be packaged in a single sealed enclosure, reducing the overall cost of the mission architecture

Why explore small bodies anyway?

Asteroids and comets are some of the oldest objects in our Solar System, dating back to the early formation of the planets over 4.5 billion years ago. Scientists believe we can learn a lot about the origins and evolution of the Solar System by studying these ancient small bodies. Recent observations also suggest that some asteroids may be rich in resources that could be used in space for construction and spacecraft refueling, or may be economically viable to mine for precious metals. NASA and other space agencies also recognize that visiting asteroids may be a useful stepping-stone for manned-missions to Mars and beyond.


67P_Rosetta

A challenging environment to traverse

On Earth and Mars, the traditional way of moving around is with wheels – they are robust and simple to control. On small bodies, however, where gravity is 1,000 to 100,000 times lower and the surfaces are extremely rocky and irregular, traditional mobility approaches are ineffective: wheels are likely to slip, legs need anchor points, and gas thrusters create hazardous debris. Our hybrid rovers use these difficult environments to their advantage, exploiting the low gravity through their simplistic and symmetric design.


Mission Architecture

In one potential architecture, a single mother spacecraft would deploy one or more spacecraft/rover hybrids to the surface of a target body. Once deployed, the hybrids would snap images and take measurements, from which scientists would choose targets to investigate. The hybrids would then use large hops to maneuver towards each target, followed by small tumbles to position sensors for in-situ science measurements. The mother spacecraft would act as a communication relay to Earth and would aid the hybrids with tasks such as localization and navigation.

Mission concept


General Features

General Features


Basic Concept

The hybrid’s chassis is rotated by applying torques to the three internal flywheels. With its spikes resting on the surface, the rover pushes forwards and upwards into a hopping trajectory. For long-range hops, higher torques are provided by slowly accelerating the flywheels and then stopping them quickly using mechanical brakes.

Basic Concept


Dynamic Model

Dynamics

Studying the dynamics of hybrid rovers in a theoretical framework allows us to better understand how their motion is affected by design and actuation. Our dynamic analyses have implemented various models that lend complementary insights into the small body mobility problem. Highly simplified models (e.g. left figure) can yield analytical results that are useful for predicting behavior and designing control algorithms, whereas more complex 3D models can better capture the physics and are useful for high-fidelity numerical simulations.


Control

So how can we utilize the mobility subsystem (flywheels, motors, and brakes) to move from one point on the surface to another in a deliberate way? This problem is well understood for wheeled rovers, however remains largely unexplored for internally-actuated platforms, especially in microgravity. Using an iterative approach, we have studied the hybrid’s dynamics, created simulations, and performed experiments, to refine our control techniques and achieve targeted mobility.


Motion Primitives

Hybrid Control


Having three orthogonal flywheels allows the rover to produce torques and manipulate angular momentum around arbitrary axes. This enables a wide variety of possible maneuvers, or motion primitives. Hopping is the primary maneuver for covering large distances, where the direction can be controlled using a combination of flywheels. Tumbling and pointing maneuvers, on the other hand, produce small movements for precise control and pointing. We also inadvertently discovered a “tornado” escape maneuver that allows the rover to leap vertically, to escape from sandy sinkholes or fissures.

While hybrid rovers can reliably execute these motion primitives, some randomness and uncertainty remains in their final landing position, mostly due to uneven terrain and surface properties. Golf provides a nice analogy; large hops drive the rover down the fairway towards a target, while small tumbles are used like a putter on the green (the tornado maneuver is like a sand wedge.) Like a golfer, our hybrid control strategy compensates for these random errors while maneuvering.


Motion Planning

Communication latencies and blackouts are unavoidable in deep space, and with limited windows for interaction with the ground, traditional teleoperation approaches are inefficient. Thus, increased autonomy is desirable to ensure efficient deployments. Hybrid rovers are designed to perform motion planning onboard, enabling navigating without interaction with the ground. We are designing computationally efficient algorithms that, combined with recent advances in microprocessor technology, can plan sequences of maneuvers to navigate hybrid rovers towards a target, while avoiding obstacles and considering uncertainty in our dynamic and environmental models.


Simulations

Simulations are invaluable for studying many aspects of mobility, from controlling motion primitives on a local scale, to the execution of planning algorithms on a global scale. The video on the left shows one of our simulations; an interactive tool that models hybrid rovers on the surface of small bodies (Asteroid 25143 Itokawa is shown). The user can directly input torques to the flywheels or define targeted destinations and let the motion planning algorithms determine how to actuate the platform.
Note:  (1) the surface color reflects the local slope of the terrain and (2) the simulation is run at 100 times real-time (motion on small bodies is slow!)


Experiments

So how can we actually test a rover specifically designed for microgravity here on Earth? There are three ways in which we have approached this problem: (1) over-designing a prototype so that it can operate in a 1g environment, (2) emulating microgravity in the lab with gravity-offloading test beds, and (3) reduced gravity parabolic flights. No single approach can exactly replicate the asteroid environment, but each approach has complementary benefits.


Unconstrained Experiments

It turns out that the transfer of energy from the flywheels into a hop is a terribly inefficient process (~1%), which is fine on a small body with barely any gravity to overcome, but highly restrictive on Earth. Nonetheless, we have developed a prototype optimized for efficient mobility, which can indeed perform short hops and tumbles in 1g. For comparison, the maneuvers you see in this video would translate into hops over a football field on Phobos!


Test Beds

Testing on Earth does not truly capture the dynamic interactions we would expect in a microgravity environment. In order to “turn down the gravity knob” in the lab, we have developed a first-of-a-kind 6 DoF gravity-offload test bed, capable of emulating microgravity down to 0.0005g and within a 1.5m x 3m x 1m workspace. The active gantry control together with passive compliance allows smooth motion tracking even during impulsive force inputs (as seen during hopping and ground collisions).


Parabolic Flights

Test beds allow for iterative testing of control algorithms in a controlled pseudo-microgravity environment, but they inevitably introduce exogenous dynamics. A more accurate microgravity analog can be achieved in a “reduced-gravity airplane,” which flies in parabolic trajectories to provide 20 second periods of near-zero g’s inside the cabin. We took our prototypes aboard 4 of these flights, for a total of nearly 200 parabolas, in which we tested various maneuvers on several surfaces within a sealed experimental chamber. Of particular interest were the experiments on granular media (i.e. sand), which behaves very differently in microgravity.


Publications


In the press

Aerospace America, The Economist, San Francisco Chronicle, Forbes, Reuters, ABC, NBC, Stanford News, Popular Science, The Verge, Slashdot, Huffington Post, The Register, Times of India, NASA OCT Website, Universe Today


Team

This project brings together a strong team of experts in astronautics, human-space flight, science, and engineering from Stanford, JPL and MIT, and engages graduate students at both Stanford and MIT.


Marco Pavone
Assistant Professor, Principal Investigator
Stanford University
Department of Aeronautics and Astronautics


Julie C. Castillo-Rogez
Planetary Scientist, Co-I and JPL lead
NASA Jet Propulsion Laboratory


Andreas Frick
Systems Engineer, Co-I
NASA Jet Propulsion Laboratory


Benjamin Hockman
Graduate Student, GN&C lead
Stanford University
Department of Aeronautics and Astronautics


Jeffrey A. Hoffman
Professor (and former NASA astronaut), Co-I
Massachusetts Institute of Technology
Department of Aeronautics and Astronautics


Issa A. D. Nesnas
Technical group supervisor, Co-I
NASA Jet Propulsion Laboratory
Robotic Software Group


Robert Reid
Research Technologist, Localization lead
NASA Jet Propulsion Laboratory
Robotic Software Group


Additional collaborators at Stanford: Ross Allen, Daniel Washington, Lawrence Leung, Adam Koenig, Nicholas Cheung, Ben Kirshner, John McMordie
Additional collaborators at JPL: Mark Amash, Gareth Meirion-Griffith, Elizabeth Carey, Jacklyn Green, Loris Roveda, Christine Fuller, Chris McQuin, Tam Nguyen (JPL/MIT)