It appears easy at first. The Moon and Mars are much less huge than Earth and have weaker gravity. To check how a rover will traverse their surfaces, engineers make much less huge engineering variations of rovers and check them in simulated floor circumstances right here on Earth. Because the Moon’s gravity is six instances weaker than Earth’s, for instance, engineers make variations of lunar rovers which might be one sixth as huge. The information then tells engineers what they should know.
Nonetheless, rovers hold getting caught. NASA’s Spirit rover ended its mission when it bought caught in deep sand. It could not free itself and orient its photo voltaic panels towards the Solar. With out the power to recharge its batteries, the rover ultimately succumbed to Mars’ frigid circumstances.
Researchers have discovered why, and now that they know, it appears apparent.
The weaker gravity of the Moon and Mars not solely impacts how the rovers behave, it additionally impacts how unfastened driving surfaces behave, since they’re additionally topic to weaker gravity. Right here on Earth, sandy, unfastened surfaces are topic to larger gravity and are extra strong. The reverse is true on the Moon and Mars. These unfastened surfaces aren’t as secure.
Engineers on the College of Wisconsin-Madison discovered this flaw and reported it in new analysis. It is offered in a analysis article titled “A Study Demonstrating That Using Gravitational Offset to Prepare Extraterrestrial Mobility Missions Is Misleading” within the Journal of Area Robotics. Dan Negrut, a professor of mechanical engineering at UW–Madison, led the analysis.
“Lately, there was a surge of worldwide curiosity in extraterrestrial exploration concentrating on the Moon, Mars, the moons of Mars, and numerous asteroids,” the authors write. “This contribution discusses how present state-of-the-art Earth-based testing for designing rovers and landers for these missions at the moment results in overly optimistic conclusions in regards to the habits of those gadgets upon deployment on the focused celestial our bodies.”
At difficulty is what engineers name terramechanics. Terramechanics is the research of how automobiles journey over deformable terrain like sand, gravel, snow, and dirt. With the intention to design a automobile successfully, engineers must know the way wheels or tracks reply when shifting throughout surfaces that may shift beneath them. M.G. Bekker, an engineer and professor initially from Poland, initiated the sector of terramechanics, and labored on armoured automobile designs in Canada throughout WW2.
One of many core ideas of terramechanics is knowing how totally different surfaces reply underneath stress. How a lot load can they bear? What are their shear strengths? How do their properties have an effect on a automobile’s efficiency?
“Looking back, the thought is straightforward: We have to contemplate not solely the gravitational pull on the rover but additionally the impact of gravity on the sand to get a greater image of how the rover will carry out on the moon,” Negrut mentioned in a press release. “Our findings underscore the worth of utilizing physics-based simulation to investigate rover mobility on granular soil.”
A rover on sand within the Simulation-Primarily based Engineering Lab. The researchers found discrepancies between the Earth-based testing and the Chrono physics simulation. Picture Credit score: Joel Hallberg
Constructing an under-massed engineering model of a rover can mimic the gravity felt by the rover. However it could possibly’t inform you something about how the bottom itself behaves. That is the place a physics simulation referred to as Venture Chrono is available in.
NASA was creating a mission referred to as VIPER earlier than it was cancelled in 2024 resulting from price overruns. VIPER stands for Volatiles Investigating Polar Exploration Rover, and its mission was to map water ice in shadowed craters on the Moon’s south pole area. Negrut and his colleagues developed Venture Chrono to mannequin advanced techniques like rovers interacting with totally different surfaces. “We developed such a physics-based simulator and utilized it to gauge the mobility of early prototypes of the Volatiles Investigating Polar Exploration Rover,” the authors write of their analysis article.
When the workforce was simulating VIPER’s efficiency, they found discrepancies between their simulation outcomes and the Earth-based testing outcomes. They labored with Chrono to find out what was occurring. “If one lowered the rover mass for Earth testing, a lightweight rover can be positioned on granular materials acted upon by the Earth’s gravitational pull, resulting in overoptimistic efficiency for the nominal rover when deployed on a celestial physique of decrease gravity,” the researchers clarify of their article.
This schematic illustrates the workflow within the group’s simulations and testing. The experimental check outcomes are used to validate the simulator, which is subsequently used to foretell the VIPER rover’s efficiency on the Moon. Picture Credit score: Hu et al. 2025. Journal of Area Robotics.
Chrono is free to make use of and publicly obtainable worldwide, and a whole bunch of organizations are using it to mannequin advanced mechanical techniques. These embody the US military’s wheeled automobiles and tanks, in addition to issues like precision watches. “The simulator, which is open-source and publicly obtainable, additionally helps research for in situ useful resource utilization actions, for instance, digging, bulldozing, and berming, in low-gravity environments,” the paper states.
This schematic exhibits the cosimulation framework used within the analysis. “The rover was modeled as a multibody system whose dynamics was solved utilizing a multicore CPU,” the researchers clarify. “The deformable terrain was modeled as a continuum whose dynamics was solved utilizing GPU computing. The 2 modules communicated passing drive and torque info from the terrain to the rover; and place, velocity, orientation, and angular velocity coming from every wheel and going to the terrain CRM solver.” Picture Credit score: Hu et al. 2025. Journal of Area Robotics.
“It’s rewarding that our analysis is extremely related in serving to to resolve many real-world engineering challenges,” Negrut mentioned. “I’m pleased with what we’ve completed. It’s very troublesome as a college lab to place out industrial-strength software program that’s utilized by NASA.”
“All our concepts are within the public area and the competitors can undertake them rapidly, which is drives us to maintain shifting ahead,” Negrut mentioned. “We’ve got been lucky over the past decade to obtain help from the Nationwide Science Basis, U.S. Military Analysis Workplace and NASA. This funding has actually made a distinction, since we don’t cost anybody for the usage of our software program.”
“Though this research drew on VIPER testing outcomes and was meant to evaluate that rover’s trafficability attributes, the teachings discovered are related within the strategy of designing any wheeled or tracked rovers aimed to function on the Moon, Mars, or different celestial our bodies in low gravity,” the researchers write of their article’s conclusion. “The conclusion of this research is that full-vehicle checks on Earth augmented with simulations and GSLs (granular scaling legal guidelines) insights present a framework that may anchor the design strategy of a automobile working in non-Earth gravitational fields.”
“Finally, the outcomes obtained on this effort make a case for relying closely on physics-based terramechanics fashions when designing rovers and landers for extraterrestrial exploration,” they conclude.
