Up, close and personal with Mars: ReachBot and the future of space missions

As you're reading this, NASA's Perseverance Rover is busy at work, steering itself across the Jezero crater landscape.

The dried-up river delta could reveal signs of past microscopic life.

Perseverance is collecting rock cores for return to Earth as part of the Mars Sample Return mission with an inbuilt comprehensive and complex sample collection system. "Percy" is rather smart - the rover has an enhanced self-driving ability and can adapt to the terrain - it can get around boulders on its own.

Percy could return to Earth bearing good news. But, there may be more areas with past signs of life on Mars than just river deltas.

Scientists have predicted that massive volcanic caves could exist beneath the surfaces of Mars, Venus, and even our Moon, formed by flowing lava and covered in tiny crystals.

Beyond zeroing in on the best spots to search for life, these lava tubes could bring us closer to developing permanent homes on the Moon and Mars.

Back on Earth, these caves can support complex ecosystems. Scientists with NASA’s Biologic and Resource Analog Investigations in Low Light Environments project (BRAILLE) believe such life could exist or have once existed in Martian caves.

Navigating these surfaces is imperative, with significant geological and astrobiological interest in these caves. However, the current generation of robotic devices cannot venture into this type of uncharted terrain.

Paving the path for the next generation of Mars explorers, perhaps?

ReachBot could change all that. The mobile manipulation platform leverages lightweight, extendable booms to achieve extensive reach with a small footprint, giving it distinct access to steep, vertical, and overhanging surfaces in Martian caves.

While the Mars rovers are great at rolling along the surface and collecting data, ReachBot would be capable of climbing on cliffs and through caves, anchoring itself to rock walls.

Marco Pavone, associate professor, Department of Aeronautics and Astronautics at Stanford University and the lead researcher on ReachBot, was among the five researchers who received Phase II grants in 2022 for NASA Innovative Advanced Concepts (NIAC). 

IE spoke to Ph.D. students Stephanie Scheider and Tony Chen, who are working with Pavone on the concept. They gave us an enhanced picture of how ReachBot was developed, and how it could impact space exploration.

Excerpts.

Simple beginnings

SS: I'm a Ph.D. student at the Autonomous Systems Laboratory that's run by Marco. I was interested in space robotics - motion planning, trajectory optimization, controls and was trying to find a project when Andrew Bylard, a student who has since graduated, came up with the idea for ReachBot. I jumped on it in the development phase itself. He, Tony, and I proposed it to Marco who thought it was a great idea. 

TC: I'm a fifth-year Ph.D. student at the Biomimetics and Dexterous Manipulation Lab, and my PI is Mark Cutkosky. We look for bio-inspired robots and build grippers based on their features. For example, we built grippers based on the adhesive that allows geckos to climb a window. In my lab, we look at nature and the animals that can crawl on rock surfaces, like arthropods. They have sharp hair called spikes that aid them to grab onto the rock surface. Based on that, we use needles and fishing hooks to manufacture micro grippers equipped with these spikes that can claw onto uneven surfaces so that we can assist the robot climb. 

Andrew had asked if we could use these structures that we used to deploy space booms as limbs of robots and then have these grippers at the end of each arm to be able to reach out to rock surfaces. The result would be a much faster rock climbing robot. So my research mainly focuses on designing grippers and prototyping the system architecture for ReachBot.

Mars first, Moon next

SS: The International Space Station has a free-flying robot system called the Astrobee, which is a test platform for a bunch of things. They've tested their gecko gripper on Astrobee on ISS. But tasks like servicing and maintenance involved require forceful manipulation. Andrew wondered if we could anchor on the opposite wall and use that to pull strongly. So, I think ReachBot was mainly born out of station maintenance, microgravity applications, and things like that. Then we started brainstorming - oh, this is a cool robot; it is good for environments where it needs to anchor onto things or overhanging surfaces where anchor points can be sparse. Additionally, in these caves, lava tubes, and other places, you don't know how often you're going to get a good hold.

TC: We had this cool robot concept. But to make it useful, we had to come up with a scenario or environment that it can succeed in. So we brought in Mathieu Lapôtre, Assistant Professor of Geological Sciences at Stanford, and asked him about the environment he could picture the robot being useful to explore. Mathieu talked about these rovers on Mars and how NASA has always been looking for life. He mentioned these underground caves, where a lot of the signs of past water are, and the presence of potential bio-organisms inside the cracks of lava tubes, which is a rich environment. So we were like, okay, that's a good concept. Because right now, there are no rovers or robots that can go into the cave and explore essentially beneath the surface of Mars. I think that's sort of where we came from. We're currently using Mars as the flagship concept, but the same idea could be used in a lava cave exploration on the Moon or even on Europa.

Between phases

SS: Initially, we applied for NIAC's Phase I in 2019 but did not get it. We then tailored the mission to be more aligned with what NASA is interested in looking at. We received the Phase I grant for 2021, during which we looked at a lot of the feasibility concepts - we claimed that this robot is beneficial because it has a large reachable workspace, but was it possible to use these booms to get that reachability? Questions like that. And so we did a lot of modeling and simulation that showed that these booms exist and they can reach out this far. We wondered if we could come up with a control strategy to use that technology to move down a cave or a lava tube. I work on those controls in my lab. 

TC: ReachBot is a complicated concept. For it to work in 3D, it needs to have many arms, potentially, to be able to secure itself. And that was too complicated of a problem to start [with] because we don't even know the design space in 3D. So we considered simplifying the model from 3D to 2D - a planar ReachBot. So essentially, we found a piece of really large flat ground and built a planer prototype of ReachBot, a square robot, and we put a bunch of ball bearings on the bottom so that it could fly around on the flat ground. The prototype has these forearms at each corner made of motorized tape measures, as 3D space booms are too expensive to use as arms. We then mounted a gripper that could grab onto rocks. An environment with lava rocks was simulated for the prototype wherein it was able to pull at a heavy object while anchored to the lava rock. That's how we proved the concept and knew that it would work in 2D.

We learned a bunch of lessons from the 2D prototype - like needing to minimize the weight of the gripper that will be extended. So that's mainly what I focused on during Phase I. We gained enough confidence, and we're sure it will work as a three-dimensional robot.

SS: There are still a lot of feasibility questions that need to be answered, which will be done in Phase II. For example, we're using tape measures on this prototype, which have much lower fidelity than you would have in the real thing. If you've ever held up [an extended] tape measure, it's going to flop over. The buckling and the bending are problems that will take place with these space booms too. We need to consider that. 

Also, in this simulation, ReachBot is crawling down a cave. We've made assumptions about the booms being strong enough. But what does strong enough mean? How can we use intelligent control strategies to leverage the booms' strengths, and overcome their weaknesses? These finer details are important to prove the feasibility of the concept.

TC: On my part, I need to look into optimizing the microscopic grippers and understanding the design principle. Now we have built a lightweight, robust microspikes gripper for 3D rock surfaces, the potential interesting surface that we want to grasp. And at the end, we want to be able to build at least one prototype of an arm. And we're hoping to field test it in a lava cave in New Mexico or Hawaii. Another issue that we did not deal with in Phase I was selecting a grasp site. In Phase II, we're investigating the selection of a good grasping site.

SS: Another thing that we'll be examining in Phase II, is the perception and localization of ReachBot. Again, we're looking at the perception of the gripper, looking at a rock, and figuring out where it's good to grab.

Challenges and limitations

TC: No one knows what these caves on Mars look like, though we can have a pretty good guess at it. So a big limitation would be the difference in rock surface - what if it isn't ideal for our microspikes? It's hard for us to separate the limitation of the concept and the limitation of the technology right now. 

SS: One thing that comes up a fair amount is the tether, which is essential when a question regarding power and data comes up. Our answer so far has been, well, we'll have a tether to something on the surface. We're kind of assuming and hoping that is a viable solution. But again, not knowing the geometry of these caves could complicate that a lot. And I know that other people have looked into that, but we haven't. 

Abstract

Robots are widely deployed in space environments because of their versatility and robustness. However, adverse gravity conditions and challenging terrain geometry expose the limitations of traditional robot designs, which are often forced to sacrifice one of mobility or manipulation capabilities to attain the other. Prospective climbing operations in these environments reveal a need for small, compact robots capable of versatile mobility and manipulation. We propose a novel robotic concept called ReachBot that fills this need by combining two existing technologies: extendable booms and mobile manipulation. ReachBot leverages the reach and tensile strength of extendable booms to achieve an outsized reachable workspace and wrench capability. Through their lightweight, compactable structure, these booms also reduce mass and complexity compared to traditional rigid-link articulated-arm designs. Using these advantages, ReachBot excels in mobile manipulation missions in low gravity or that require climbing, particularly when anchor points are sparse. After introducing the ReachBot concept, we discuss modeling approaches and strategies for increasing stability and robustness. We then develop a 2D analytical model for ReachBot’s dynamics inspired by grasp models for dexterous manipulators. Next, we introduce a waypoint-tracking controller for a planar ReachBot in microgravity. Our simulation results demonstrate the controller’s robustness to disturbances and modeling error. Finally, we briefly discuss the next steps that build on these initially promising results to realize the full potential of ReachBot.