Tests with a miniature model of NASA's Resource Prospector 15 (RP15) rover have demonstrated the rover's ability to escape unpredictably flowing granular terrain without sinking deep into the material, thanks to its peculiar crawling gait.
This work, published May 13 in Science Robotics, represents a step forward in ongoing efforts to meet NASA's expanded lunar and Martian exploration goals. These include traversing the dark craters and treacherous deposits of the Moon and Mars that may contain valuable and mineable resources.
Traversing sloped granular terrain — where soft soils can readily avalanche in response to the slightest disturbance — is one of a rover's greatest challenges. The mini rover's success in digging itself out of grainy beds suggests next-generation rovers informed by RP15's unique design could finally access sites like the icy slopes of the lunar poles without getting stuck.
"We want to make a rover able to drive into dangerous terrain, gather data and samples, survive the experience and then get back to safety to tell the tale," said co-author Robert Ambrose, division chief of the software, robotics and simulation division at NASA Johnson Space Center.
In May 2009, engineers on Earth scrambled to rescue the NASA rover Spirit from a bed of loose soil called regolith near the Gusev crater on Mars. Regolith — layers of dust, broken rock, and other particles found on planets, moons and asteroids — acts like a solid when compact, but can flow unpredictably like a fluid when disturbed.
The more Spirit tried to maneuver the regolith, the further it sunk into the terrain, unable to gain traction. After repeated failures to release Spirit from the soft soil trap, NASA scientists reoriented the rover's objectives toward collecting data while stationary, and they eventually ended the mission in 2011.
To continue venturing into uncharted lands, researchers now knew they needed a rover that could escape entrapment by granular media. They tackled this objective, among many others, in the Resource Prospector project, launched by NASA JSC in 2014. In 2015, they developed a rover with new legs that coupled conventional wheel spin motion with a lifting and sweeping legged motion.
Due to shifting budgetary constraints, the Resource Prospector project was cancelled in 2018, but RP15's design concepts were re-absorbed into today's expanded lunar mission called Volatiles Investigating Polar Exploration Rover (VIPER). With heads now turned to the VIPER mission, "the Resource Prospector team stayed together and used the existing rover prototype RP-15 for testing and to make some engineering advances," said Ambrose.
He approached Daniel Goldman, professor of physics at Georgia Institute of Technology, whose team has been studying robotic hopping motions on granular material. Upon Ambrose's request, Goldman and his team adopted a new research question — attempting to discern what generates "good" versus "bad" rover locomotion.
They proceeded to test RP15 by constructing a miniature version out of 3D-printed plastic. They chose an easy flowing granular bed of poppy seeds to replicate the fluid-like states of regolith.
Activating just the wheels made the model slip and sink in the seed bed, Goldman's team found. But when the robot's legs began their sweeping motions, the mini rover was able to lift and paddle against the terrain, agitating the seeds to its advantage and pushing itself up the hill. Even with one disabled limb, the mini rover successfully navigated the grainy bed using this motion, albeit at a lower speed than four-legged trials.
"The robot agitated the media to avalanche into climbable 'steps,'" said Andras Karsai, a graduate student working in Goldman's lab at Georgia Tech and a co-author of this paper. In other words, the rover could use the terrain's collapsing motion as an advantage to movement rather than a hindrance, added Karsai.
Additional tests with the full-sized RP15 rover at NASA JSC validated the findings with the mini rover. However, because of technical issues and limited resources, the researchers could only conduct RP15 tests with three fully functioning appendages on a flat and moist sand bed.
"I never dreamed that we would succeed in discovering principles for new kinds of soft terrain locomotion and form the basis for better rover propulsion," said Goldman. "Sadly, our grant with NASA has ended but we continue to discover cool physics of seemingly simple interactions; for example, why wheels lose traction with sand."
At NASA, VIPER mission scientists intend to build rovers that can survive multiple dark periods and extreme temperatures and pursue longer, more diverse paths across the Moon's poles. To do this, the engineers will have to continue investigating RP15's peculiar gait on steeper slopes formed out of various homogeneous and mixed granular terrain, while adding new capabilities such as local terrain sensing — all to ensure future rovers will never again meet Spirit's unfortunate end.
"Exploring is about the unknown," said Ambrose. "Like human explorers, these robots will experience an exciting adventure."
[Credit for associated image: Shrivastava et al., Sci. Robot. 5, eaba3499 (2020)]