Fixing the Hidden Problem Behind Stranded Space Rovers
When the Mars rover Spirit ground to a halt in 2009, buried axle-deep in a deceptively innocent-looking Martian sand trap, engineers at NASA spent months trying to free it remotely.
Despite their best efforts, the mission was eventually declared over. And while Spirit still provided valuable science data, its fate became a cautionary tale for planetary exploration: even the most advanced robotic explorers can falter when the terrain fights back.
Fast forward to today, and researchers at the University of Wisconsin–Madison believe they’ve uncovered a key reason why rovers like Spirit get stuck. Surprisingly, the culprit isn’t the rover itself, but the way it’s tested here on Earth.
The gravity of the situation
Until now, engineers have replicated lunar or Martian gravity in rover tests by using lightweight prototypes—essentially building a rover at one-sixth the mass when preparing for Moon missions. Then they roll these scaled-down bots across Earth-based test sites like deserts or sand pits to predict how they’d fare on the real terrain.
But here’s the rub: while the rover’s weight gets adjusted, the sand doesn’t. Earth’s gravity pulls on the testing surface far more than the Moon’s or Mars’ ever would, making the terrain seem firmer and more stable than it really is in low-gravity environments.
That means rovers get better traction in tests than they would in space—a serious miscalculation that can lead to multimillion-dollar machines becoming stuck where no human can reach them.
“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,” said Professor Dan Negrut, a mechanical engineering expert at UW–Madison.
Simulating extra-terrestrial terrain with Project Chrono
The revelation came during a NASA-backed project involving the Volatiles Investigating Polar Exploration Rover, or VIPER, scheduled for a lunar mission. As Negrut and his team ran simulations using Project Chrono—an open-source physics simulation engine—they noticed a troubling mismatch between physical rover tests and their virtual moon-based equivalents.
Project Chrono, co-developed with Italian researchers, allowed the team to model full-size rovers navigating “squishy” surfaces under different gravitational fields. It showed, unequivocally, that sand under Earth’s gravity behaves very differently from lunar regolith.
Through Chrono, researchers demonstrated that in low gravity, particles of soil interact more loosely, meaning traction is limited and the risk of getting bogged down increases. This wasn’t merely theoretical. The data clearly showed that simulations better predicted real extra-terrestrial conditions than scaled-down sandpit trials ever could.
Broader implications for engineering
While NASA stands to gain the most immediate benefits, the ripple effects of this research go well beyond the space sector.
Project Chrono has already been used by hundreds of organisations worldwide, spanning everything from horology to heavy machinery. It has helped model the movement of precision mechanical watches and the behaviour of Army trucks in off-road combat zones. In essence, it’s a virtual sandbox for engineers working in real-world conditions that are too tricky or expensive to replicate physically.
“It’s rewarding that our research is highly relevant in helping to solve many real-world engineering challenges,” said Negrut. “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 software’s open-source nature is part of what makes it so powerful. Anyone, anywhere, can use it—free of charge. But maintaining software at this level isn’t cheap or easy.
“It’s very unusual in academia to produce a software product at this level,” Negrut explained. “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.”
Keeping innovation open and accessible
One of the standout features of the UW–Madison team’s approach is their commitment to open science. By making Project Chrono freely available, they’re inviting collaboration, competition and continuous innovation.
“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.”
And move forward they do. The team continues to enhance Chrono’s capabilities to ensure it remains relevant as mission needs evolve and engineering questions grow more complex.
Rethinking how we explore
At the heart of this story lies a fundamental truth: sometimes, assumptions that seem too obvious to question are the ones that most need rethinking.
By challenging the decades-old method of rover testing and shining a light on a simple yet significant oversight, UW–Madison’s engineers have changed the game for planetary exploration. Their research is a reminder that innovation isn’t always about radical invention—sometimes, it’s about looking again at what we thought we knew and asking better questions.
The next generation of robotic explorers will likely roam the Moon and Mars with a much clearer sense of what lies beneath their wheels. And with Project Chrono running simulations behind the scenes, they’ll be less likely to meet the same fate as Spirit.
