If you were a fly on the wall at the Rockhall concert hall in Luxembourg in September 2022, you’d be greeted by a strange sight: no bands or raucous crowds in sight, but instead a 220-ton area of lava and rock, with teams of robots crawling across the dusty floor in search of resources.
It was the second and final round of the European Space Agency’s (ESA) Space Resources Challenge, which invited research teams to bring their prototype robots and test their abilities to prospect for resources in a simulated lunar environment. The idea was to search for bright ideas for the next generation of robotic space explorers that would help locate and map lunar resources, such as water, essential for future crewed missions.
One of the winning teams in the challenge consisted of three legged robots that roamed the arena, climbed, poked, and worked together to map, identify, and collect scientific samples. Although this challenge focused on the Moon, the same principles could be applied to robots destined for Mars and other planetary bodies—and this approach would allow robots to explore new environments never seen before.
To find out more, we spoke to Hendrik Kolvenbach of the Robotic Systems Laboratory at ETH Zurich, the team that developed the legged robots.
An idea with legs
Robots for lunar exploration
Curious moon bots aren’t created from scratch; They start as commercially available robots at ANYbotics, which makes legged robots like Boston Dynamics. These models are often used for industrial studies, but they may also have the potential for off-world studies.
The Robotic Systems Lab has customized the robots’ hardware and software, including adding a robotic arm to pluck rocks from the Martian surface and experimenting with different walking patterns to cope with the changing terrain. About the size of a medium dog, the version of the robots used in the ESA challenge was able to carry a small payload of up to 33 pounds, but new systems can carry up to 110 pounds, Kolvenbach said.
That’s enough for a legged robot to carry scientific instruments like cameras, spectrographs or a small drill. Robots are trained using reinforcement learning, so they learn to navigate difficult environments using simulations. While they use their fourth limb to propel and manipulate objects in their environment, they can also learn to use their three legs to stand.
The ultimate goal of such research is to develop modular hardware systems. Instead of creating space explorers from scratch for each mission, future missions will use a basic configuration that can be modified with different instruments and software as needed by developing robotics technology on Earth.
Exploring the unknown
such as wheeled robots The Curiosity and Perseverance rovers currently exploring Mars are good at a few things. They travel relatively fast over wide open terrain and can maneuver around or over surprisingly large rocks and other obstacles. As was the case with Curiosity, years of bouncing around on the Martian surface inevitably damaged their wheels, but they could keep going as long as the drivers were careful.
So why are foot robots necessary? It all depends on the environments the expeditions want to explore. Both Mars and the Moon have surfaces covered in dusty material called regolith, which rovers are designed to travel on, for example.
But both places have intriguing subterranean areas like lava tubes, cave-like structures beneath the surface created by the passage of a hot volcano long ago.
These lavas are of great scientific interest, and there is also practical interest in using them as shelters for future crewed missions, as astronauts build underground bases in them to protect them from dangerous surface radiation. But no one knows exactly what these environments will look like, so any robot that wants to explore them needs to be versatile and able to handle unexpected challenges.
Legged robots are ideal for this type of environment. They are best suited for handling steep slopes found in ditches. This is particularly convenient for regions such as the Moon’s south pole, a hotspot of current exploration activities, which are permanently shadowed and offer craters that could provide important water ice resources.
Slopes can be tricky. „With wheeled robots, we always have a traction problem,” Kolvenbach explained. „You have many cases where regolith is dry, granular material and rovers get stuck.”
Legged robots are „generally more mobile, but that comes at a cost.” For flat areas where there are no major challenges, wheeled robots are more efficient and do not require more complex legged robots. To get a wider overview of a larger area, there is the option of aerial surveying, as demonstrated by the Mars Ingenuity helicopter. But when it comes to unpredictable and unstructured terrain, legged robots are „very agile and robust,” Kolvenbach said. „This is where the unique value of these robots lies.”
Work as a team
Another way to approach the challenges of robotic exploration is to consider the possibilities of teamwork. Because each legged robot is much smaller than the current Mars rover, several of them would be needed to carry the same payload as a single-wheeled robot. But that can be an advantage because robots can work individually and in teams.
For the ESA challenge, the lab used a team of three-legged robots, although a team could theoretically be larger or smaller. By changing how different payloads are distributed among team members, you can create robot experts. For example, one robot can carry tools to quickly map a large area, while another has scientific tools to explore specific points of interest in detail.
This trend now goes to more challenging environments because you can do more interesting science.
It also brings the benefits of redundancy, as the most important activities can be shared across team members. If one robot fails for any reason, others can continue to operate and take over most of the tasks of the failed robot.
As for how a group of robots can communicate with each other, several approaches are being considered in the robotics community. One is to have a central platform that coordinates the actions of each robot. This can be ideal for exploring large, open areas, as robots can be sent in different directions to perform tasks such as collecting samples, which can then be brought back to the site for analysis. As the robots act as couriers, large and heavy equipment stays inside the central base unit.
Another approach is to use communication nodes, where robots act as relays to send commands. It is ideal for exploring underground areas where communications are limited. Robots can drop a Sensors like Breadcrumbs can relay commands, allowing communication even in unknown environments.
The real advantage of this approach is flexibility. What payloads each robot carries, how many robots there are in a team, and how that team is structured are adjustable based on the needs of a particular task or environment.
The legged robots the researchers are working with are commercial hardware, so while they show the technology’s promise for space exploration, they’re far from space-worthy. Placing hardware in spaceflight has stringent requirements, such as the absence of extraterrestrial repair shops, the ability to withstand a wide range of temperatures, handling vibrations and shocks of launch, and the need for extremely high reliability.
So in preparation, the team is working on creating a spaceflight-ready legged robot Space Hopper. „It’s a relatively small-scale robot,” Kolvenbach said. It weighs less than 22 pounds and uses off-the-shelf spaceflight hardware. It was a practical first step because „going from a research prototype to a real space probe is a lot of engineering work. So we decided to do something small.
Spacehopper: The Next Leap in Space Robotics | Launch presentation 2022
They hope the hopper will be ready to fly as a technical demonstration in the next few years, by the time the space agency and commercial missions to the moon begin. Hopefully, this will open the door to further development of larger and more complex legged robots to explore new environments.
Kolvenbach describes the future use of legged robots as „brainless.” There are many instances where a wheeled robot still makes sense for planetary exploration, but when it comes to exploring more challenging and scientifically interesting environments, he sees legged robots as the future.
„There is a clear need for this,” he said. „We’ve done a lot of work in the past on other celestial bodies for relatively flat, easy environments. But now the trend is going to more challenging environments because you can do really interesting science. From the scientific community, there’s a clear need to go there. And get us there to do that science.” Legged robots are one of the emerging technologies to come.
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