Modifying Bucket Wheels for an Asteroid Mining Context

Space mining is the future, being essential to meet the demands of supplying critical rare materials—abundant on the Moon, Asteroids, and other celestial bodies—required to produce electrical and electronic components for daily life in the coming decades. Asteroids are of particular interest due to their high resource highness and energy accessibility. Both of these properties of asteroids are owed in large part to their low gravity, which poses a challenge to mining them. Asteroid mining ventures must contend with an environment with little gravity bounding machinery to the surface and little cohesion to push against. For low cohesion environments, research shows that the bucket wheel design is the best candidate among other excavators for mining the regolith and can be well suited to the lunar mining context. Adopting the bucket wheel to function in milligravity will provide a way to build off preexisting space excavation technologies to solve a novel problem. Using bucket wheels to mine asteroids and being conscientious of reaction force generation provides scalability in a realm where many leading designs involve the capture of the entire asteroid, limiting viable targets to below the mid-sized (<1 km) range. The majority of well-characterized asteroids like Bennu (~500 m) are within this range. Thus, there is a need for alternative technologies to mine the mid-size asteroid like Bennu. In this paper, we consider the deployment of spacecraft with modified bucket wheel systems used for regolith manipulation on the asteroid surface. The modified bucket wheel system must also contend with uneven terrain and electrostatically charged, coarse grains while minimizing environmental impact. Bucket wheels will be iteratively modified using digital modeling and rapidly prototyped with 3D printing, with the intended final design incorporating a rotating array of extensible claws that will execute a biting motion to scoop regolith. Testing the bucket wheel necessitates the development of a simulant that is mechanically analogous to asteroid surface conditions. Candidate simulants must be able to mimic the low shear strength present on asteroids without the use of buoyant suspension, as that would produce inaccurate force data. The particle size accuracy and affordability of the simulants are other leading criteria for selection. The test bed for each asteroid-mining bucket wheel will read the horizontal cutting force, vertical resistive force, excavation rate, electric power consumption, and the filling efficiency of the mechanism. The resolved force on the bucket wheel will be low enough such that the bucket wheel can remain attached to the surface of a representative asteroid such as Bennu only through the gravitational force. To kickstart the design of the modified asteroid-mining bucket wheel, engineering factors that have the greatest impact on cutting force were identified to be cutting velocity and the volume of the buckets. By examining a simplified bucket wheel design of constrained size bucket shape, hypothetical asteroid-mining excavators were compared to the most developed Lunar excavators to determine the bounds on their bucket volume and cutting velocity in asteroid environments of varying gravity, density, and resource richness. These bounds represent the intersection of an upper limit—the force curve—which determines whether the bucket wheel remains gravitationally attached to the asteroid surface while mining, and a lower limit—the productivity curve—which determines whether the bucket wheel can excavate rare metals faster than a Lunar excavator. Density was found to be the most impactful asteroid parameter on both cutting velocity and bucket volume. Increased gravity had no discernible impact on bucket volume but had a beneficial effect on maximum allowable velocity. Resource richness had little impact on constraining bucket wheel design. High-mass excavators appear to be favored in this model. To keep high-mass excavators economically feasible, continuous excavation must be prioritized. Using a screw feeder to offload mined regolith to a central storage facility is a proposed method of achieving this. Screw feeders also speed up the centripetal draining of regolith into a bucket drum, making them a necessity for fast-rotating small bucket wheels to operate continuously. Extensible claws theoretically should allow for more buckets to be added to one wheel without increasing reaction forces, thereby improving excavation rates. The outsized impact of high cutting velocity and the volume tradeoffs incurred by doing so discourage the usage of orbiting spacecraft to mine asteroids with bucket wheels, as they would be too fast.