At an elevation of around 6,000 feet near Mount Hood, located roughly 70 miles east of Portland, an interdisciplinary team comprising members from the University of Pennsylvania, the University of Southern California, Texas A&M University, Georgia Institute of Technology, Oregon State University, Temple University, and NASA, embarked on a field mission. The group of engineers, cognitive scientists, geoscientists, and planetary scientists was there to conduct field tests with a four-legged robot named Spirit, part of the Legged Autonomous Surface Science in Analog Environments (LASSIE) Project.
Over five days in the summer of 2023, Spirit was put through its paces, navigating diverse and challenging terrain that included loose soil, melting snow, and rocky paths. According to the team, the robot’s metal legs, designed for agility, allowed it to traverse these varied landscapes, occasionally stumbling but largely succeeding in its task. These trials sought to better understand the underlying terrain properties and refine the robot’s ability to sense and move through such environments. The data collected from Spirit’s experiences are intended to inform the development of future robots for exploration on extraterrestrial bodies, such as the moon and, some day, Mars.
"What we realized, pretty early on, is that a legged robot has the ability to interact with soil in ways that wheels cannot,” says Douglas Jerolmack, a principal investigator on the LASSIE Project. “This interaction isn’t just about mobility; it’s about questioning and understanding the environment it moves through, in real time." He explains that legged robots like Spirit can assess and adapt to the varying friction and stability of the ground beneath them, much like how humans test questionable surfaces like a frozen-over river by doing a few foot taps before placing their full body weight onto it.
LASSIE’s origins
The collaboration that eventually led to the LASSIE project started around 2015 when the School of Engineering and Applied Science’s Daniel Koditschek, known for his work in developing legged robots, reached out to Jerolmack, a geoscientist in the School of Arts & Sciences and Penn Engineering with expertise in landscapes and granular materials. The partnership sought to find a way to tie in their labs’ expertise.
“The initial idea was to use these legged robots to carry scientific instruments into hazardous or difficult-to-reach areas or places too dangerous to send research assistants and graduate students,” Jerolmack jests. “But it quickly snowballed from there once we saw the potential in having a robot learn information about the physics of its environment and how it operates in it. This was particularly relevant in the study of soil mechanics, where the robots’ movement and interaction with the ground could provide valuable data on soil properties.”
Jerolmack and Koditschek later jointly recruited Feifei Qian as a postdoctoral researcher for their labs with an extensive background in both granular materials science and robotics. She “played a crucial role in bridging the gap between the two fields,” Jerolmack says. “Feifei’s involvement truly helped to integrate the scientific and engineering aspects of the collaboration, allowing for more sophisticated experiments and research questions, which is why we’re thrilled so to see her as the lead on LASSIE.”
Qian is now an assistant professor of electrical and computer engineering at the University of Southern California Viterbi School of Engineering and School of Advanced Computing. Her lab is leading the NASA-founded project. Their approach to robotics draws much inspiration from nature by using information such as how ants change their posture to crawl through tiny spaces or how dogs move differently over patchy, uneven surfaces.
“When the robot leg slips on ice or sinks into soft snow, it inspires us to look for new principles and strategies that can push the boundary of human knowledge and enable new technology,” Qian says. “We learn and improve from the observed failures.”
The more machines the merrier
With a two-year $2 million grant from NASA that extends on LASSIE insights, researchers at Penn Engineering hope to expand on this work to develop new approaches to robotic navigation of challenging lunar-like environments. Led by Cynthia Sung, an expert in robot design and multi-robot systems, the goal of the TRUSSES Project: Temporarily, Robots Unite to Surmount Sandy Entrapments, then Separate is to enable multi-robot teams to use measured ground conditions for planning on risky terrain. The project also draws expertise from Mark Yim, who specializes in modular self-reconfigurable robots, as the robots may need to physically connect into larger structures during very complex maneuvers.
“Very little of the moon’s surface has actually been explored,” Sung says, “so it makes sense to have robot teams that can both sense and adapt to unexplored terrain. A key enabling idea for this project is that the robot teams can sense ground interactions on the fly as they are walking around, which means that they can adjust their existing knowledge of the environment and use it to plan their missions safely.”
“They would sense how the ground conditions are,” Qian says, “and then exchange that information with one another and collectively form a map of locomotion-risk estimation. The team of robots can then use this traversal-risk map to inform their planetary explorations. There is an extremely soft sand patch that might be high-risk for wheeled rovers. Come over here, this might be a safer area,” she says.
The team envisions other robots working alongside Spirit: A wheeled rover (great for payload and long distances), a Hexapedal robot (intermediate payload but better mobility than the wheeled), and doglike ones like the rugged version of Spirit (highest mobility, shorter distances). The researchers say this synergistic robot network would be able to aid Spirit’s mission. They note that if one got in a jam, made immovable by loose dirt or a rock or a ravine, its bot-mates would arrive and link together and form a bridge, or a pyramid, to hoist their pal to safety. And then back to work.
“When they plan for the strategy to pull the robot up,” Quian says, “they’ll decide what force to exert and what position the robot should go to, while also compiling the terrain information. That’s the key idea of how to use these capabilities: to both prevent and recover from locomotion failures in extreme terrain.”
Back to Mount Hood
The multi-university team field tested Spirit at a number of sites. The robot gets around a variety of natural environments, to learn how to better move on challenging terrains. Qian has let it off its leash on Southern California beaches and in the grassy hills of the San Diego Zoo Safari Park. The team has also tested the robot in the soft granules of White Sands National Park in New Mexico. A video shot at Mount Hood, however, shows just how otherworldly that landscape can be in these planetary-analogue environments. The tests provide Spirit with plenty of opportunities to learn on Earth, before potentially exploring other planets.
“It would be very hard to drive up this,” Ryan Ewing, a geologist from NASA Johnson Space Center, says. “But as a legged being, as humans, we can step around it easily. A dog could walk around it easily. So, this project is the proving ground that we can enable new science and new mobility on environments that are like other planets.”
Then proving the point, a German shepherd appears within frame frisking about. The dog, Howard, belongs to Christina Wilson, cognitive scientist and former postdoctoral researcher in the Koditschek Lab, and he wanders about with an agility Spirit could only dream of. “We are going to observe how Howard moves in different types of snow and ice conditions,” Qian says. “What exactly, out of those combined motions, allows him to succeed on challenging terrain?”
The LASSIE Project calls for two more test trips for Spirit: to White Sands and back to Mount Hood. The TRUSSES team, from Penn and USC, also plans to visit White Sands next year with Spirit and other new, multitasking robots. Imagine WALL-E with friends.
Landon Hall of the University of Southern California contributed to this story.
This research was supported by the National Aeronautics and Space Administration, Planetary Science and Technology from Analog Research program (80NSSC22K1313) and by the NASA Lunar Surface Technology Research program (80NSSC24K0127).
Douglas Jerolmack is a professor in the Department of Earth and Environmental Science in the School of Arts & Sciences and in the Department of Mechanical Engineering and Applied Mechanics in the School of Engineering and Applied Science at the University of Pennsylvania.
Cynthia Sung is the Gabel Family Term Assistant Professor at Penn Engineering’s Department of Mechanical Engineering and Applied Mechanics.
Feifei Qian is a former postdoctoral researcher at Penn Engineering and is currently the WiSE Gabilan Assistant Professor and assistant professor of electrical and computer engineering and aerospace and mechanical engineering at the Viterbi School of Engineering at the University of Southern California.
