Exploring the surface of Mars can be a perilous endeavor for a robot. Consider the plight of the wheeled Mars rover Spirit: The six-wheeled, 400-pound robot has been stuck in the Martian sand­ since January 2010—when it was downgraded from rover to science station—and the dust collecting on its solar panels may prevent it from even being able to carry out this task. As we look back on the successes and setbacks of the six-wheeled Spirit, researchers are testing out a variety of other rover ideas that could last longer, collect more data and go places on Mars that no bot has ever been.

One such concept is the Tumbleweed Rover—a simple, wind-powered, wheel-less rover that would roll over the Martian surface just like, well, a tumbleweed. The Tumbleweed has inspired scientists for more than 10 years, but to date there is no consensus regarding how exactly the rover should be designed. Now researchers at North Carolina State University have developed a computer model they say will allow engineers to test different vehicle designs and predict how they will perform on the Martian surface. The research, funded by NASA and the North Carolina Space Grant Consortium, is described in a paper published in the Journal of Spacecraft and Rockets on June 1.

The researchers created a two-part model. The first part simulates the physical parameters of the vehicle itself—the radius, weight, elasticity (how much, for example, the rover deforms when it strikes a rock) and rotational inertia. The second part re-creates specific features of the Martian surface, like rock fields and craters, based on video imaging data collected by NASA's Viking and Mars Pathfinder missions. The surface of Mars is characterized by drastic changes in landscape, and it is crucial that engineers understand the challenges each terrain type poses.

The biggest challenge in the project, according to NC State aerospace engineer and paper co-author Alexandre Hartl, was developing a detailed wind model—an extremely important element for designing the rover, given that Tumbleweed is wind-powered. "The surface wind speeds on Mars are relatively uncharted, with limited data, so you have to make some compromises on how you model the wind flow through a rock field or crater," he says. "We know what the magnitude of the wind speed is, but we don't know how it varies and its duration." The little data that is available comes from the Viking and Mars Pathfinder missions.

This model is not the first of its kind, Hartl says, but to date it is the most detailed. "We consider a lot more dynamic phenomena," Hartl says. "We're not the first one, but I think we've used it in the most sophisticated sense." Of course, getting a Tumbleweed Rover to the surface of Mars is still, at best, years away. But NASA's Jet Propulsion Laboratory and Langley Research Center are both currently developing prototypes, and the North Carolina State model will now bolster those efforts.

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