Mars: The Obstacle That Has Confounded NASA Rovers for Years Finally Uncovered

NASA robotic missions open a new window to us on neighboring planets. However, despite decades of experience, Rovers have always had an annoying tendency to stop in furniture terrains. The most famous case remains that of Spirit, a Martian rover launched in 2003, which bogged down in 2009 in a sandy area. Despite numerous efforts to free it from a distance, the machine has never moved and NASA officially ended its mission in 2011, after more than six years of activity on the red planet.

It is not an isolated incident. Spirit, just like his opportunity twin or more recently Perseverance, regularly faced hostile land that has put the traction of his wheels to the test. It is not only the unforeseen events that cause these blockages. They also reveal a deeper gap. Our still imperfect understanding of mobility in extraterrestrial environment. And until recently, no one had identified this flaw.

A breakthrough in understanding the mobility of space rivers

To prepare the missions, engineers test the Rovers in sandy land areas. They reproduce the low severity of the stars targeted by lightening the prototypes, supposed to simulate the behavior of a real machine on Mars or the Moon. But this method, widely adopted for decades, has a major bias.

This bias has just been highlighted by Dan Negrut and his team from the University of Wisconsin-Madison. Thanks to precise simulations based on physics, these researchers have shown that tests on earth neglect an essential factor: the behavior of the sand itself under a different gravity. Indeed, under terrestrial gravity, the sand is more compact and carrier. Conversely, on the Moon or Mars, it becomes more fluid, more friable. This phenomenon considerably modifies the ability of a rover to move.

Their demonstration is based on Project Chrono, an open-source simulation engine capable of modeling complex mechanical interactions between the wheels of a rover and the soil grains. By comparing the results of lunar and Martian simulations to those of tests in land conditions, the researchers noted major differences. Extraterrestrial sand, often described as “soft”, reacts differently to mechanical constraints. This explains why the Rovers, designed and validated on earth, sometimes end up trapped in land that would have seemed practicable in our deserts.

The results of this study were published in the Journal of Field Robotics, highlighting a test error that could have been avoided much earlier.

Simulate so as not to undergo

This new lighting deeply changes the way in which space agencies must design and test their robots. NASA, which funded part of the work of the Madison team as part of its Viper program, now intends to integrate these simulations into future design phases. It is no longer just a question of lightening the models to simulate gravity. It is necessary to fully rethink the conditions of the Test soil, taking into account the effect of gravity on the materials themselves.

Hundreds of organizations, civil and military, already use Project Chrono software – made available to the public for free – to model complex systems. Whether it's precision watches, all-terrain vehicles or lunar rovers, this technology makes it possible to predict mechanical behavior in extreme environments. It is a precious tool to avoid expensive failures like that of Spirit, now frozen for eternity on the plains of Gusev.

In this context, the study of the mobility of space rivers becomes a lever of innovation far beyond the interplanetary missions. Understanding how a wheel interacts with reduced gravity soil is also learning to master mechanics in the most unpredictable conditions. And it is perhaps this new rigor that will open, tomorrow, the way to a more autonomous, more reliable exploration … and less trashy.