But before this plan can advance beyond the concept phase, more funding is needed to develop the miniaturized instrumentation that would need to be carried aboard the Tumbleweeds. If researchers can get the funds, the physical size of scientific experiments could be shrunk, making them easier for the spherical probes to tote.
The Tumbleweeds are intended to track atmospheric conditions, geographical location, communicate with orbiters (to relay data back to Earth), and even probe the chemistry of Martian soil, so the smaller the better. They could even generate their own power by harnessing the kinetic energy their motion generates.
There's also the tantalizing possibility that a fleet of Tumbleweeds - each with different instruments on board - could "swarm" and act as one unit to carry out a sophisticated array of measurements.
"The instrumentation is constantly being miniaturized and some components could actually come off the shelf. The real constraining factor as to how many and which instruments are deployed is the amount of power that can be incorporated. Batteries add mass, which slows down the Tumbleweed. One can certainly envision a fleet of Tumbleweeds that have various configurations of instruments and the capability to swarm if one Tumbleweed finds something of great interest. This would, of course, require a means of controlling the direction of movement of the Tumbleweeds. This technology is not very mature because funding has not been available." - Kim Kuhlman.
They Come in All Shapes and Sizes
If the project is given the go-ahead, NASA will need to decide what configuration of Tumbleweed would be most efficient and/or practical.
For example, the inflatable Tumbleweed concept is a tried and tested vehicle, having undergone extensive field tests in Greenland and Antarctica in 2003 and 2004. The inflatable Tumbleweed traversed hundreds of miles while continually relaying atmospheric measurements and location data.
It is also hoped the inflatable design could be commanded to deflate at locations of interest, causing it to "sit down" and stop rolling. As the underside will be in greater contact with the ground, perhaps analysis tools can be lowered into the cavity beneath, sampling any gases vented from the ground. (Watch the video to see the inflatable Tumbleweed in action.)
Another prominent Tumbleweed design is a ridged "box kite" type. Although this technique is less mature than the inflatable design, the sail-like paddles have a better drag coefficient. This means they will use the available Mars winds more efficiently, perhaps traveling further and faster.
To aid control over the Tumbleweed, an offset weight housed in the center of the sphere could be commanded to alter position, shifting the Tumbleweed's center of mass and essentially steering it that way.
These concepts, along with others - namely the Dandelion, Eggbeater, and Tumble-cup configurations - have been tested and developed by a collaboration of research institutions including NASA Langley Research Center, North Carolina State University, Texas Tech University, the Biorobotics Laboratory at Case Western Reserve University and Planetary Science Institute.
Ready to Roll?
Although we probably won't see a fleet of Tumbleweeds bouncing across the Martian surface any time soon, it is certainly a novel approach to planetary exploration. However, Kuhlman will have her work cut out to convince the world that this paradigm shift in robotic exploration is a viable one.
"I've actually had a very influential scientist in astrobiology call the idea "loopy" to my face," she added.
But critics be warned, the Tumbleweed scientists have been working on this project for a decade and Kuhlman is lead author of a chapter titled "Tumbleweed: A New Paradigm for Surveying the Surface of Mars for In-situ Resources" of the book Mars: Prospective Energy and Material Resources.