"There is quite a bit of interest within NASA to pursue the tumbleweed rover design, but one of the questions regarding the concept is how it might perform on the rocky surface of Mars," said Andre Mazzoleni, mechanical and aerospace engineer and co-investigator of the research. "We set out to address that question."
In 2010, Discovery News spoke with Kim Kulman, senior research scientist of the Planetary Science Institute who has been developing the design for some time. Although the tumbleweed rover wouldn't have the stunning array of instrumentation that a wheeled rover can carry, what the tumbleweed lacks in sophistication, it certainly makes up for in scope.
"(Rovers) Spirit and Opportunity have been nothing short of spectacular. However, they have very limited mobility, which is often dictated by the terrain," Kuhlman told Discovery News.
"A fleet of Tumbleweeds could cover a much greater area using the wind for propulsion," she added. "Some of them may get stuck and become stationary platforms ... but the majority would perform a ‘random-walk' survey of an area orders of magnitude greater than that of a rover."
Unsurprisingly, Mazzoleni and lead author Alexander Hartl found that the tumbleweed would need to be big (to catch as much Martian wind as possible) and lightweight (to optimize the rover's performance).
As much of the Martian surface is covered in rock fields - on average, there is one significantly-sized rock per square meter that could be an obstruction hazard - the team needed to find the optimal size of a ball that can be efficient at catching the breeze, light enough to actually move and, fundamentally, big enough to roll over the rocks without getting stuck. It turns out that this optimal size is six meters (20 ft) in diameter.
"We found that, in general, the larger the diameter, and the lower the overall weight, the better the rover performs," Mazzoleni in the NCSU press release.
Another significant finding was that any wind-blown tumbleweed design would be in for a bumpy ride during their mission. Martian gravity is only one-third that of Earth's and the planet's atmosphere is only one-hundredth the pressure that we experience at sea-level - which means it takes a small bounce to get these things airborne. In fact, they will likely do more bouncing than actual rolling, according to the simulations.
"Computer simulations are crucial for designing Mars rovers because the only place where you find Martian conditions is on Mars," adds Mazzoleni. "Earth-based testing alone cannot establish whether a particular design will work on Mars."
Needless to say, understanding the bounciness of the tumbleweed will be an important parameter to work into any future design.
Currently, several designs are being considered. Everything from an inflatable design to a rigid "boxkite" structure are being studied. Controlling the speed and direction of the rover isn't a key parameter, but it may be desirable to include a counterweight inside the final design so future tumbleweed rover drivers can add some bias to their general direction. But to see these things operate, some big advances in miniaturization would need to be made first.
"The real constraining factor as to how many and which instruments are deployed is the amount of power that can be incorporated," said Kuhlman. "Batteries add mass, which slows down the Tumbleweed."
So might we be seeing swarms of robotic tumbleweeds spreading far and wide over the Red Planet's landscape in the near-future? Perhaps not, but it may turn out to be a low-cost means of seeing multiple wind-driven sentries traveling huge distances under the power of the Martian breeze alone.
This research is published in the March/April issue of the Journal of Spacecraft and Rockets.
Image: A pair of "boxkite" tumbleweeds explore Mars in this artist's impression. Credit: NASA LaRC/Case Western University/NASA Planetary Data System