University of Wisconsin–Madison mechanical engineers, working with funding from NASA and the U.S. Army Research Office, have determined that a misunderstanding of gravity explains the difficulty mission operators face when trying to help rovers trapped on a distant planet free themselves from the alien soil, as happened to the Mars Spirit Rover in 2009.
Previous efforts have successfully simulated the differing effects of gravity on a Mars rover on Earth versus its planned destination by lowering the mass of model rovers tested on Earth by one-sixth, corresponding to the lower Martian gravitational pull.
In new simulations originally designed for another NASA mission, the UW-Madison team found that failures to account for the effects of reduced gravity on the simulated Martian soil led to extraction plans that were effective in the Earth-based simulated Martian environment but failed on Mars.
According to a statement announcing the discovery of a misunderstanding of gravity, the research team was initially working on NASA’s Viper rover in preparation for a possible future lunar mission when they discovered the potential discrepancy. Specifically, the team found that when modeling the traction and other components of a rover’s motion on alien soil using the UW-Madison open-source physics simulation engine, Project Chrono. The team said the robust simulator can accurately model complex mechanical systems “like full-size rovers operating on ‘squishy’ sand or soil surfaces,” to determine how they will operate in real-world conditions.
When comparing the computer model of the rover’s movement in the simulated lunar soil to actual laboratory tests using weight-scaled models, the numbers didn’t match. Further analysis of the conflicting results revealed that previous efforts, which accounted for the effect of gravity on the rover by scaling, had not considered the effects of lower gravity on Martian and Lunar soils simulated with Earth sand.
“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,” explained Dan Negrut, a professor of mechanical engineering at UW–Madison.
According to the team’s published study, the stronger gravitational pull on Earth increases the rigidity of the sand. making it less likely to shift under a rover’s wheels. Conversely, the surface material on Mars and the Moon is likely fluffier and less compact. The team concluded that this misunderstanding of gravity, particularly its effects on alien soil, is likely what caused simulated rover extraction plans that worked in the lab to fail when sent to Spirit, which never escaped its final resting place.
The team noted that their successful resolution of a previous misunderstanding of gravity is a perfect example of the value of collaborative tools like Chrono, which have been built and expanded by hundreds of researchers worldwide. Negrut said the team’s findings underscore the value of using physics-based simulation when analyzing rover mobility in alien environments.
The team also highlighted current applications for Chrono that included simulating the inner workings of precision mechanical watches and U.S. Army trucks and tanks operating in off-road conditions. Negrut said it is very unusual for academia to produce software at this level that can “solve problems that no other tool can solve,” including simulation tools made by the largest tech companies.
“It’s rewarding that our research is highly relevant in helping to solve many real-world engineering challenges,” the researcher explained. “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.”
After solving the mystery of why rovers on other solar system bodies get stuck and Earth-based extraction plans often fail due to a misunderstanding of gravity, the team will continue to improve and update Chrono, which is available to researchers everywhere.
“All our ideas are in the public domain, and the competition can adopt them quickly, which drives us to keep moving forward,” Negrut said. “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.”
The study “A Study Demonstrating That Using Gravitational Offset to Prepare Extraterrestrial Mobility Missions Is Misleading” was published in the Journal of Field Robotics.
Christopher Plain is a Science Fiction and Fantasy novelist and Head Science Writer at The Debrief. Follow and connect with him on X, learn about his books at plainfiction.com, or email him directly at christopher@thedebrief.org.