A team of researchers from the University of Wisconsin-Madison reveals that global space agencies, including NASA and Chinese space administration, have fundamentally misunderstood the physics of other celestial bodies during the design of their Rovers. This major methodological error would explain why so many landing missions still fail today. The study, published in the Journal of Field Robotics, questions the decades of practices established in space exploration.
The enigma of repeated failures
Despite considerable technological progress, robotic exploration of the solar system remains strewn with pitfalls. The failed landings on the moon are multiplying, while on Mars, the NASA Rover Spirit experienced a premature end after having banged in soft sand, its wheels having pierced a more fragile than expected superficial crust.
These incidents do not fall under bad luck or isolated technical failures. They reveal a fundamental gap in our approach to spatial exploration: a systematic misunderstanding of the way in which the reduced gravity affects not only our machines, but also the surfaces on which they evolve.
A faulty test approach for decades
Traditionally, space agencies test their Rovers in supposedly analogous terrestrial environments: deserts, landscaped land as the famous “Mars Yard” of the Jet Propulsion Laboratory. To compensate for the reduced gravity of March (around 38% of that of the earth), engineers alleviate their test prototypes.
The example of the Rover Curiosity perfectly illustrates this method: its terrestrial prototype was relieved of 567 kilograms, going from 907 to 340 kilograms to reproduce its Martian weight. This approach seemed logical and was applied for years by all space agencies.
Bryan Martin, manager of flight software at JPL, summed up this philosophy: ” We are testing a lot to determine what to avoid. What we have gone through here allows rovers to plan their journeys on Mars. »
The error hidden in the equation
The Dan Negrut team has identified the fatal defect of this logic: by modifying only the mass of the rover, the engineers forget that the earthly gravity continues to act on the test soil with its normal strength. Result: the terrestrial terrain retains a resistance and cohesion greater than what they really be on Mars or the Moon.
« The idea is simple retrospectively ”explains Negrut. “” We must consider not only the gravitational attraction on the rover, but also the effect of gravity on the sand to better anticipate lunar performance. »
On Mars, each particle of dust and sand undergoes a gravitational attraction three times less than on earth. This difference radically modifies the mechanical properties of the soil: its elasticity limit, its resistance to shear, its ability to endure the weight of a rover.
Photograph taken by the Rover Spirit of his landing on the surface of March on January 18, 2004 (Spirit Sol 16). Credits: NASA/JPL
Revealing simulations
Working on the Viper mission (today canceled), the team used Project Chrono, an open source physics simulator. The results revealed significant differences between the performance provided during land tests and those simulated in realistic lunar conditions.
These simulations also highlighted that individual wheel tests provide more reliable data than those on complete Rovers, and that the mass reduction of prototypes could even be superfluous in certain cases.
« The nominal vehicle has produced the same sliding curves as a function of the slope on the moon and on earth“, Note the study, suggesting that maintaining the original geometry of the rover could be more relevant than modifying its mass.
A necessary methodological revolution
This discovery imposes a complete overhaul of the test protocols. Future missions will have to rely massively on terramedicanic models integrating the physics of reduced gravity environments, rather than approximate terrestrial analogies.
The stake exceeds the simple improvement of performance: each mission failure represents hundreds of million euros lost and years of destitute research. More fundamentally, these errors slow down our scientific understanding of the solar system.
Towards a new era of exploration
« There are certain types of applications relevant to NASA and the planetary exploration that our simulator can solve, unlike the tools of large technological companies“, Ugruts enthusiastically.
This revelation arrives at a crucial moment, while spatial exploration intensifies with Artemis programs, Chinese Martian missions and the exploration projects of the moons of Jupiter and Saturn. Correct this fundamental error could considerably improve the success rates of future missions and accelerate our expansion in the solar system.
The lesson is clear: to explore other worlds requires to completely rethink our relationship to physics, even the most basic.
