We study the fundamental physics that dictate the behavior of grains on planetary surfaces and the subsequent dynamics of the grains. We are especially interested in the influence of non-gravitational forces on grain dynamics, specifically cohesion and electrostatic forces. We seek to understand the morphological evolution of planetary surfaces (specifically asteroids and the Moon) as well as improve the design of spacecraft to explore these destinations.

Magnetorheological Robotic Gripper

The Magnetorheological Robotic Gripper (MR Gripper) is a robotic end-effector concept that has initially been designed for use by free-flying spacecraft inside the International Space Station. The main benefits of the MR Gripper are that it has no moving parts and it requires very little a priori knowledge of the object to be gripped. The MR Gripper consists of an elastomeric membrane filled with magnetorheological (MR) fluid and attached to an electromagnet. When the electromagnetic is activated, the MR fluid becomes rigid, gripping whatever object is entrained in the gripper. We are doing both experimental characterization (with Co-I Dr. Norm Wereley at UMD) and computational modeling of this new robotic end-effector. The computational modeling is based on the LIGGGHTS open-source Soft-Sphere Discrete Element Method code, with custom add-ons to include magnetic forces and the deformable membrane. This work is funded by the NASA Early Stage Innovations Program.

Electrostatic Dust Lofting

Electrostatic dust motion has been hypothesized to occur on the Moon and asteroids. Airless bodies (i.e. those bodies with negligible or minimal atmospheres) interact directly with the solar wind plasma. Under certain conditions, the electrostatic force on dust grains may be stronger than the gravitational and cohesive forces holding grains against the surface, leading to dust lofting. We seek to answer questions associated with understanding the feasibility and implications of dust lofting: What size dust can be lofted? What plasma environment is required for lofting to occur? We have analytically and experimentally demonstrated the significance of cohesion in evaluating the feasibility of electrostatic lofting of small grains.

Electrostatic Dust Levitation

If dust grains are launched off the surface of a small body within a narrow range of initial velocities, the grains can levitate due to the opposition of the gravitational and electrostatic forces. We study the dynamics of lofting dust grains to determine what range of initial velocities lead to levitation, the locations where levitating grains may be observed, and the locations where grains are redeposited. Through our studies of levitation in 1D plasma sheaths and gravity models, we have shown that grains up to 3 microns radius are capable of stable levitation above the asteroid Itokawa and that the size of levitating grain decreases as the mass of the central body increases. Additionally, we have shown that the range of launching velocities that leads to levitation is more dependent on grain size than central body mass.

Neutral Beam Asteroid Control Device

The Neutral Beam for Asteroid Control (NBAC) is a globally neutral plasma thruster whose applications range from touchless de-orbit of large space junk to deflecting asteroids on an Earth impact trajectory. The Neutral Beam Technology Demonstrator (NBTD) is a scaled down version of NBAC aiming to achieve the theoretical high global neutralization of ion through a gas diffusion cell. The NBTD consists of two main components: the ion source and the gas diffusion neutralizer. The ion source provides a steady 1keV argon ion beam which is directed into the gas diffusion neutralizer. Gas is azimuthally pumped into the neutralizer and evenly distributed through a diffusion ring into the main chamber. Through recombination and ionization reactions the ions interact with this cold gas, changing their charge state. This beam can then be directed at a piece of space debris or an asteroid to redirect it without issues in electro-potential differences between the spacecraft and the object. In the case of asteroids, this eliminates the problem of dust being caught in a quasi-neutral plasma and traveling back to the spacecraft. The NBTD was successfully tested at NASA MSFC on the VAHPER thrust stand. This research is funded through the NASA Space Technology Research Fellowship (NSTRF) NNX15AP68H.

Triboelectric Charging of Regolith

Triboelectric charging is the phenomena of charge exchange between two surfaces when they collide or rub together: for example, it is the mechanism of charge exchange that occurs when you rub a balloon on your hair. On Earth, tribocharging of dirt is a negligible effect because the humidity in the air quickly neutralizes any charges. However, on airless bodies (e.g. the Moon, or asteroids), there is essentially no atmosphere. Thus, when regolith (dust) grains collide and rub (for instance, during an avalanche or during rover operations), the grains get charged. The amount of charge on a grain will depend on its size and its chemical composition. Our goal is to develop a predictive model of the tribocharging of regolith grains. We are currently developing the experimental apparatus to test our model. This research is funded through the NASA Space Technology Research Fellowship (NSTRF).

Granular Flows

We are interested in understanding the physics of granular flows in low gravity environments, specifically avalanching on low-gravity bodies like asteroids and the Moon. In order to study granular flows in this type of environment, we are developing a Contact Dynamics (CD) simulation. CD simulations are similar to Discrete Element Method simulations where there is a constant timestep and the interactions of grains are controlled through coefficients of friction and restitution. We intend to use this type of simulation to also aid in the design of future spacecraft sample collection, anchoring and mobility devices.