Mechanical engineers at the University of Wisconsin–Madison have discovered a problem with how rovers are tested on Earth using computer simulations. The study was published in the Journal of Field Robotics.
Mechanical engineering professor Dan Negrut poses with a space rover used for testing. Image Credit: Joel Hallberg
Earth-based engineers act as a virtual tow truck when a multimillion-dollar alien vehicle becomes stuck in soft sand or gravel, as with the Mars rover Spirit in 2009. They send a series of commands that move the vehicle's wheels or reverse its course in a delicate, time-consuming attempt to free it and resume its exploratory mission.
Better terrain testing here on Earth could prevent these celestial catastrophes in the future, but Spirit remained permanently stranded.
This mistake leads to unduly optimistic predictions about the behavior of rovers when they are sent on missions to other planets.
A crucial component of mission preparation is a precise understanding of how a rover will move across alien surfaces in low gravity to avoid becoming stuck in rocky or soft terrain.
The Moon's gravitational attraction is six times less than that on Earth. For many years, scientists testing rovers have taken this disparity into consideration by building a prototype that is just a sixth of the real rover's mass. To learn more about how these lightweight rovers will function on the moon, they test them in deserts and see how they travel over sand.
However, the pull of Earth's gravity on the desert sand was a seemingly small feature that this conventional testing method missed.
Dan Negrut, a mechanical engineering professor at UW-Madison, and his colleagues discovered through modeling that Earth's gravity pushes down on sand far more powerfully than Mars or the moon. Sand on Earth is more solid and supportive, making it less likely to move beneath a vehicle's wheels. However, the moon's surface is “fluffier” and hence moves more easily, giving rovers less traction, which might limit their movement.
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. Our findings underscore the value of using physics-based simulation to analyze rover mobility on granular soil.
Dan Negrut, Professor, Mechanical Engineering, University of Wisconsin–Madison
The researchers' finding stemmed from their work on a NASA-funded effort to emulate the VIPER rover intended for a lunar trip. The researchers used Project Chrono, an open-source physics simulation engine created at the University of Wisconsin-Madison in conjunction with Italian experts. This program enables researchers to rapidly and precisely model complicated mechanical systems, such as full-size rovers operating on “squishy” sand or dirt surfaces.
While simulating the VIPER rover, they discovered discrepancies between Earth-based test results and their simulations of the rover's movement on the moon. Further investigation with Chrono simulations identified the testing problem.
The benefits of this discovery extend well beyond NASA and space flight. On Earth, Chrono has been utilized by hundreds of organizations to better comprehend complicated mechanical systems, ranging from precise mechanical timepieces to U.S. Army vehicles and tanks operating off-road.
Negrut added, “It’s rewarding that our research is highly relevant in helping to solve many real-world engineering challenges. I’m proud of what we’ve accomplished. It’s very difficult as a university lab to put out industrial-strength software that is used by NASA.”
The UW–Madison team works hard to create, maintain, and support Chrono, yet it is freely available to the public for unrestricted usage anywhere in the globe.
“It’s very unusual in academia to produce a software product at this level. 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,” Negrut stated.
Negrut and his team are committed to continuously improving and upgrading Chrono because it is open source to remain relevant.
“All our ideas are in the public domain and the competition can adopt them quickly, which is drives us to keep moving forward,” he says. “We have been fortunate over the last decade to receive support from the National Science Foundation, U.S. Army Research Office and NASA. This funding has really made a difference, since we do not charge anyone for the use of our software,” further added Negrut.
The US Army Research Office (W911NF1910431 and W911NF1810476), the National Science Foundation (OAC2209791), and NASA STTR (80NSSC20C0252) provided funding for this study.
Journal Reference:
Hu, W., et al. (2025) A Study Demonstrating That Using Gravitational Offset to Prepare Extraterrestrial Mobility Missions Is Misleading. Journal of Field Robotics. doi.org/10.1002/rob.22597.