Rovers Are So Yesterday. It's Time to Send a Snakebot to Space
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science
Feb 2023
If the boxy Opportunity rover could elicit years of anthropomorphized love and goodwill, then surely Earthlings will warm to the idea of sending a snake-shaped robot to the moon. This robot--the brainchild of students at Northeastern University--is meant to wiggle across difficult terrain, measure water in the pit of craters, and bite its own tail to become a spinning ouroboros tumbling down the side of a lunar cliff.
NASA's annual Big Idea Challenge presents a new query each year that's geared toward an engineering problem the agency needs to solve. In fall 2021, students from universities across the United States set out to design a robot that could survive extreme lunar terrain and send data back to Earth. The winning team, of students from Northeastern's Students for the Exploration and Development of Space club, took home the top prize in November and now hope to turn their winning design into an advanced prototype that could actually be sent to the moon.
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Using $180,000 of NASA funds, the students focused on designing a robot that could navigate Shackleton Crater--a 13-mile-wide basin near the lunar south pole where NASA confirmed the presence of water ice in 2018. Water is plentiful on Earth but a high-value commodity outside our atmosphere. Humans require water to survive, but it's extremely heavy, and lugging it 240,000 miles from home is cost-prohibitive. So local water in ice form would be an enormous boon for NASA's Artemis mission as it seeks to establish a lunar base.
Before the agency can rely on this ice for crewed missions, however, it needs to confirm just how much is located in different regions of the lunar surface and what its chemical composition is. But there are a few challenges to getting data from a 2-mile-deep crater. One: The floor is in permanent shadow, which means temperatures hover hundreds of degrees below freezing. Two: The angle of incline from the rim to the floor is 30.5 degrees, steeper than Mount Everest. Three: The moon is sandy. Any robot attempting to traverse this terrain is going to have to survive bone-chilling temperatures, a precipitous descent, and a gritty environment.
The students considered hopping, legged, and rolling robots, like the wheeled rovers already on Mars. But rolling robots would sink in the regolith and couldn't safely navigate terrain as steep as the Shackleton rim. Legged robots also sink and are less stable in sandy environments. Hopping robots would have a difficult time launching and landing without sustaining damage or getting stuck. "We looked at this whole suite of different robot designs and thought, is there any way we could combine different locomotions?" recalls Yash Bhora, a physics major who helped build software for the team.
Bhora and his teammates considered a tumbling robot, one that could harness the partial gravity of the moon to propel itself down the crater more efficiently. But once it arrived at the floor, it would need a different type of functionality. "A tumbling robot by itself cannot really manipulate a big science instrument or maneuver as precisely as a walking robot," says Matthew Schroeter, the team's lead, who graduated from Northeastern in 2022 and now works at Honeybee Robotics.
The key, they decided, was to mimic the movement of an Earth creature that has to deal with a grainy, hilly environment: the sidewinder. "The regolith and sand have similar properties. They're both very porous. We looked at real snakes that use this locomotion called sidewinding to go up slopes using the friction of the sand, and we eventually came up with the design," says Schroeter.
They dubbed it Cobra, which stands for Crater Observing Bio-inspired Rolling Articulator. The students first built a "Mini Cobra," which at just under 2 feet long and 5 pounds is about a third the size of the final design. It is made of 11 linked carbon fiber and nylon units. Each houses a battery-powered actuator--essentially a motor--that can transform commands from a Raspberry Pi in the snake's head into motion. Because it is modular, it can be manipulated into a sidewinding position to navigate flat, sandy areas like the bottom of a crater, and into a hexagonal wheel that can roll down steep slopes.
Courtesy of Northeastern University
Cobra's tail is designed to house a mini neutron spectrometer, which can measure changes in the energy of neutrons on the moon's surface and identify hydrogen, and thus water, deep within Shackleton Crater. The team also built in the capacity for the robot to be fitted with radar sensors and an inertial measurement unit so operators on the ground can keep tabs on Cobra's motion, speed, and location as it rolls and winds its way around.
To test these functions, the team sent the Mini Cobra flying down loading docks and through parking lots around Northeastern's downtown Boston campus. One challenge was finessing the latching mechanism that connects Cobra's head and tail when it switches into tumbling mode. It would sometimes latch too strongly, creating the potential for damaged wires or a lost connection. Bhora worked up to the last minute troubleshooting the issue in the robot's software and eventually landed on a two-step process that prevented the robot from wobbling and created a secure latch.
In November, the team traveled to California's Mojave Desert to demonstrate Cobra in terrain that resembles what the bot would have to navigate on the moon. They faced off against six other teams, which had brought legged robots, wheeled robots, a robot that lowered itself down steep terrain on a cable, and a Lego-like design from MIT that could be reconfigured into several shapes. When it was Cobra's turn to test its mettle, it latched itself seamlessly into a circle and propelled itself down a steep hill, with the team cheering it on from behind. It side-wound its way into some sagebrush, but the operators wriggled it out of the prickly scrub and sent it on its way. The team was able to successfully demonstrate all of Cobra's modes of locomotion and took home the Artemis Award, the competition's top honors.
Courtesy of Northeastern University
Past winners have occasionally gone on to further develop their concepts, and a couple of those are even being considered for integration into upcoming NASA missions. Other times, projects languish after team members graduate. According to Kevin Kempton of NASA's Game Changing Development Program, one of the competition's lead judges, it depends on the motivation of team members. "I try to tell the teams, the next step is to look for announcements of opportunity," Kempton says. "NASA is always looking for low-cost payloads."
In Cobra's case, most of the team's members are undergraduates still active in the space exploration club, and they want to ready the concept for an actual moon mission. That will take a bit of work. Most of Cobra's components are 3D-printed materials that wouldn't survive the harsh thermal gradients at the lunar poles, where sun-baked crater rims give way to ice-cold depths near the floor. To make the system space-ready, Cobra's components will have to be built from sturdy metals, like titanium, which can withstand dramatic temperature and pressure changes and resist corrosion.
And in the California desert, students commanded the robot from only a few steps away. But signals take about three seconds to travel from the Earth's surface to the moon and back again, a lag that requires lunar systems to have some autonomous decision-making capabilities.
"I always say to my students, if something is trivial on Earth, it doesn't mean that it's trivial on the moon or Mars," says Alireza Ramezani, the team's faculty advisor and a professor of engineering at Northeastern. But Ramezani says that a team of doctoral candidates is currently looking into the autonomy requirements for commanding the Cobra system, and that they have received queries from private robotics companies interested in partnering to further develop the project. The students will also enlist help from the university's Institute for Experiential Robotics to develop Cobra into a completely space-ready system.
Ramezani specializes in bio-inspired robots and designed the Leonardo robot in 2019. The birdlike creation both walks and hovers--and can even skateboard--taking advantage of two modes of locomotion to stabilize itself over rough terrain. He says he is excited to see NASA endorse new, multimodal robotic designs, such as Ingenuity, the first helicopter deployed on Mars, which was carried there in the belly of the Perseverance rover and has since flown dozens of its own missions.
"All of this shows that we are seeing a new era of space robot design, systems that can switch from one mode of mobility to another to accommodate all the tasks of their mission," he says. "I think we will see more interesting robots down the road."
