UW–Madison researchers have uncovered a flaw in the way that rover tests are evaluated on Earth, using computer simulation.
Before extraterrestrial rovers are sent to outer space, they undergo rigorous mechanical and mobility testing to ensure they can handle the new terrain. These tests aim to simulate the sand, rocks and slopes of planets, to ensure the rovers can be remotely controlled successfully.
The gravitational pull is six times weaker on the moon than on Earth. Engineers have traditionally accounted for this difference in gravity by creating prototypes that are one-sixth the mass of an actual rover.
Dan Negrut, a professor of mechanical engineering at UW–Madison, and his collaborators have discovered that these past tests have missed one key terrain detail, which impacts a rover’s success – the impact of the Earth’s gravity on sand.
Their findings, first publicized in July 2025 and published in the Journal of Field Robotics, have drawn national attention in the past few months. News outlets like NBC News and ScienceDaily have praised the team for drawing attention to a long-overlooked issue in rover design and testing.
“In retrospect, the idea is simple: We need to consider not only the gravitational pull on the rover but also the effect of gravity on the sand to get a better picture of how the rover will perform on the moon,” Negrut says. “Our findings underscore the value of using physics-based simulation to analyze rover mobility on granular soil.”
Using computer simulation, Negrut and his team have proven that the Earth’s gravity pushes down on the sand beneath a rover much more than previously thought.
The sand on the Earth’s surface is rigid, holding firm underneath the treads of a moving rover. However, on the moon, the weaker gravity leads to a less firm or “squishy” surface of sand, which can cause rovers to lose traction or get stuck.
The study, titled “A Study Demonstrating That Using Gravitational Offset to Prepare Extraterrestrial Mobility Missions Is Misleading,” confirms that decades of rover testing on Earth may have led to overly optimistic performance predictions.
The team’s discovery came from combining two tools.
Negrut and his team used a simulation of the NASA-funded VIPER Rover and a physics-based engine called ProjectChrono. Chrono, developed at UW-Madison in collaboration with scientists from Italy, helped researchers to model the complex situation of a rover moving on an unstable surface like the terrain of the moon.
This discovery has implications beyond improving extraterrestrial rovers, said Negrut.
ProjectChrono, the modeling engine used to simulate the rover on the surface of the moon, has applications in robotics and mechatronic systems, and can even be used to simulate three-dimensional shapes for collision detection.
More importantly, Chrono is free and available to the public, leading to the possibility of more users putting its simulation abilities to good use.
While no official changes to NASA’s rover-testing protocols have been announced, Negrut and his team are committed to improving the software to stay applicable for future challenges.
“It’s very unusual in academia to produce a software product at this level,” Negrut says. “There are certain types of applications relevant to NASA and planetary exploration where our simulator can solve problems that no other tool can solve, including simulators from huge tech companies, and that’s exciting.”