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.

Orbital Debris Detection

Small (sub-cm) orbital debris is currently undetectable using optical methods. However, even small orbital debris can be hazardous to spacecraft, causing pits in Space Shuttle windows and holes in electronics. We are developing a new method for detecting currently undetectable orbital debris. Instead of measuring the optical signature of the debris, instead we propose to measure the plasma wave that is generated by the debris as it travels through the upper ionosphere. It may be possible to map small orbital debris using a constellation of cubesats that can "listen" for plasma waves. This project is supported by NASA's NIAC program.

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.

Cohesion Measurements via Electrostatic Dust Lofting

Because gravity on the surface of an asteroid is very small, other forces (e.g., electrostatics and cohesion) can dominate the behavior of the surface dust. In order to design spacecraft to explore the surface of asteroids (for science, mining or to prevent an Earth impact), it is necessary to be able to predict how the surface of an asteroid will respond to disturbances. Thus, we must understand the magnitude of the cohesive force in the regolith. Currently, there is a several order of magnitude uncertainty in our estimates of the strength of cohesion in regolith. We are designing a technology to measure cohesion on the surface of an asteroid by attracting dust to a surface using electrostatic forces. When an attractor plate is biased to a non-floating electric potential, regolith will be attracted to the plate. If the surface gravity is known, then by observing the grain size that detaches from the surface and moves towards the plate, we can calculate the cohesive force on the dust.

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).

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.

Dust Motion on Bennu

The OSIRIS-REx mission is a NASA mission to collect a regolith sample from the asteroid Bennu (find out more about OSIRIS-REx here). Because gravity is very small on the surface of the asteroid, non-gravitational forces become significant in driving the behavior of grains on the surface. We are looking for evidence of electrostatic dust motion on Bennu.

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.