Although humanity is getting better at sending robotic probes out into the Solar System to explore the places no human can tread, we’re still very much on a learning curve.
The first extraterrestrial robotic rover was launched from Earth in 1970. It’s only now, more than half a century later, that scientists have figured out why these marvels of ingenuity and engineering keep getting stuck in the soils of alien worlds.
“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,” explains mechanical engineer Dan Negrut of the University of Wisconsin-Madison.
“Our findings underscore the value of using physics-based simulation to analyze rover mobility on granular soil.”
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Making a rover that will operate in an alien environment is more complicated than making one that will work on Earth. We’ve lost more than one Mars mission to giant dust storms that leave drifts of sand on solar panels, preventing the machinery from being able to generate power, for instance.
Gravity is another one. The Solar System bodies on which we have deployed robotic rovers have lower gravity than Earth, and this has an effect on how things move around. Engineers, when designing rovers, have therefore taken into account the effects the target gravitational environment will have.
Nevertheless, rovers still manage to get stuck pretty often, requiring control teams to conduct a series of maneuvers to try and free the poor robot. It’s usually fine, if annoying, although in one notable case it was not: NASA’s Mars rover Spirit got stuck in soft soil in 2009, and there it remains to this day.
Using computer simulations running on a physics-based engine called Project Chrono, Negrut and his colleagues set out to get to the bottom of this recurring problem. Comparing their results with real-world tests on sandy surfaces revealed a discrepancy that pointed right to it.
Previous tests of rover designs in Moon- and Mars-simulated dirt omitted one very, very important detail: sand, also, behaves differently under different gravitational conditions.
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The dust that coats the Moon and Mars is fluffier and squishier than dust on Earth, shifting more easily, and hindering traction – making it far easier for their wheels to get stuck. Think of a vehicle on Earth that has driven into slippery mud, or very loose desert sand.
This eureka moment could be the missing piece of the puzzle that could keep future space exploration rovers out of a dusty jam.
“It’s rewarding that our research is highly relevant in helping to solve many real-world engineering challenges,” Negrut says. “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 research has been published in the Journal of Field Robotics.
