Mobility in the Small Body Environment: Close Proximity Landing and Surface Dynamics

Historically, missions to small bodies have been fly-by trajectories, but recent missions have increasingly attempted to interact with the target body for touch-and-go, impact, or landing operations. This motivates the development of technological capabilities needed for small body in situ exploration, sample acquisition, and crewed operations. Specifically my research examines electrostatic dust lofting on asteroids in the Solar System and the role it plays in the natural evolution of the body as well as design of mission operations near the surface. Electrostatic dust lofting may be a common occurrence on small bodies, whereby the upward electrostatic force on a grain is able to overcome the surface gravity and cohesion binding it to the surface. This electrostatic lofting may serve to redistribute and transport dust across the surface of these bodies to produce features such as those found on Eros, or even to rid bodies of small particles completely. Classical models, which distribute charge evenly across a dust grain, have historically predicted electric field strengths which are insufficient to loft dust. However, recent studies have developed grain-scale charging models which account for the buildup of charge in the microcavities of regolith and assume unequal distribution of charge on a dust grain. These models predict electric field strengths orders of magnitude larger than classical models, which may explain how electrostatic dust lofting occurs on small bodies. My work extends the most recent experimental results and grain-level simulations to a more global, comprehensive small body environment model to better understand the complex interactions affecting dust behavior there. In doing so, my research aims to develop models and tools to characterize and utilize the small-body environment for close proximity landing and surface operations with a specific focus on charged dust behavior. Progress towards achieving this goal included 1) developing a small body environment model 2) determining realistic dust grain lofting requirements given new grain-scale charge models, 3) simulating and analyzing dust behavior in the small body environment given various surface conditions, and (4) making predictions on particle populations at specific bodies such as Ryugu, Bennu, Itokawa, and Eros. The first half of my research involved development of the small body environment model, which models the relevant forces acting on a dust grain in the vicinity of the small body surface. The small body environment model combines several independent models into a single, comprehensive simulation that is able to provide higher fidelity analysis of the complex motions of dust particles near the surface than any of the separate models alone. Individual models include a constant density polyhedron gravity model which takes into account the unique, complex shape of the primary body, a solar radiation model which accounts for eclipsing behind the small body, a regolith cohesive strength model, and a dynamic electric field model which accounts for the different charging environments near the surface as the body rotates through its local day. The second half of my research focused on simulating and analyzing dust behavior in the small body environment given various surface conditions, examining dust particle initial condition requirements for electrostatic lofting, and making predictions on particle populations and behavior at small bodies. A series of simulations were conducted, giving insights into charged dust dynamics at various asteroids in the Solar System. The first simulation studied the effect of grain size and location on particle behavior by launching particles with different initial velocities from the surface of an asteroid. The results give insight into cleaner working regions on the surface, as well as which populations of particles are expected to persist in the dusty working environment. The second simulation studied the relationship between grain charge and size by lofting particles with different initial charges. The results give insight into charge requirements for natural dust lofting—namely that the required charge is orders of magnitude larger than that observed experimentally. This implies that the classical electrostatic models are not sufficient for dust lofting to occur and another mechanism must be at work. The third simulation used the supercharging model to study this theory, particularly how cohesion limits and initial charge requirements change as a function of particle size and primary body spin period. The results give insight into how the regolith properties and spin rate of the small body affect different grain populations on the surface and provide predictions on the maximum cohesion a particle can realistically overcome. The fourth simulation studied coordination of the small body rotation rate with the dust grain supercharging rate to find relationships between lofting time of day, particle size, and general grain behavior. The results give insight into characteristic grain curves, which describe the different regimes of behavior grains undergo throughout their trajectories depending on the lofted time of day. This simulation also provides preliminary predictions on grain size populations expected at spherical Bennu-like and Ryugu-like asteroids. The fifth study examined the rates at which charge accumulates on dust grains when accounting for photoemission, the solar wind and conductivity of the grain. Grain-scale charging was found to occur very quickly and was not limited by the half-rotation rate of the primary body. From this, we developed a method of determining the charge and ejection velocity of lofted grains at maximum charge conditions. A final sixth study simulated grain lofting using this new method of initial condition generation at Ryugu, Itokawa, and Eros. The results give insight into how the solar incidence angle at the surface of a small body affects lofted dust behavior and make predictions about lofting requirements and the mobility of various dust grain populations on these bodies. Overall my research furthers our understanding of charged dust grain dynamics on small bodies by 1) providing a novel and comprehensive small body environment model, 2) developing a new method of determining lofting requirements for electrostatically charged grains on small bodies, and 3) examining dust grain lofting events and behaviors on small bodies to make predictions about past and current dust grain populations. These tools, insights and predictions of dust particle behavior can be used by NASA scientists to better understand the natural evolution of small bodies in the Solar System and equally by NASA mission designers to develop safer, more efficient exploration activities and operations on these bodies.