Finding a Dust Mitigation Strategy for Lunar Surface Operations

Introduction: Design of an effective dust mitigation system to support NASA’s new initiative to return to the surface of the Moon must be a high priority [1,2]. A number of strategies have been proposed based on brushing, magnetic susceptibility or magnetic attraction, variable EMF or other surfaces, and particle guns for charging or repelling dust [2]. Brushing was demonstrated to be effective only on a short term basis during the Apollo missions [1]. Abrasivity of particles damaged surfaces, sometimes compromised seals. High surface area/volume of the particles meant sticking (electrostatic and mechanical) and little removal of finer particles leading ultimately to mechanical joint failure. Magnetic Susceptibility/Magnetic Attraction strategies have been proposed to remove lunar fines based on the observations that metallic iron abundance and magnetic susceptibility increase with decreasing grain size in mare soils [3,4]. Tests of magnetic separation mechanisms have shown clumping and lower separation efficiency for the problematic finer grain sizes [5]. High gradient magnetic separation technique proposed for filtering fine dust from shelters [5] are not necessarily suitable for cleaning mechanical surfaces in the field. A question remains as to how much variation in magnetic susceptibility is present among all lunar fines, particularly those in uncontaminated highland soils. Electrostatic Approach to Dust Control: Fields, charged particles, and dust particle interactions on the Moon are complex, their interactions dependent on environmental conditions and highly variable particle properties including size, shape, composition, magnetic and electrical properties. Lunar fines shows low electrical conductivity and dielectric loss [5], and thus tend to remain electrostatically charged [6]. Greater illumination and temperature increase surface potential. Less mafic particles tend to have lower loss tangents and greater conductivity and are thus more apt to become electrostatically charged more quickly [6]. Electrostatic charging of the Moon occurs via interaction of solar UV light with the surface causing photoemission of electrons and interaction of the local plasma environment [7]. Charged dust grains are repelled from like-charged surface or attracted to oppositely charged surface. Surface charging on the dayside is driven by photoelectron currents, resulting in electron depletion and positive charging of the surface; on the nightside, plasma electron currents result in electron accumulation and negative charging of the surface [8]. The properties and behaviors of individual particles in the presence of plasmas or photoemissive surfaces have been studied in the laboratory [e.g., 6,9-13] and theoretically modeled (e.g., 7,14,15]. Although foundational work on dust behavior in fields has been done, empirical analysis of collective dust behavior on surfaces within the context of surrounding nonconducting, dusty plasma/regolith environment is just beginning. Starting with theoretical models as well as dust behavior already observed on the lunar surface [16-19] as input for an empirical simulation, Calle and coworkers [20] recently demonstrated that dust particles can be transported by electrostatic fields whether charged or not by applying alternative waveforms of voltage to a surface with patterned grids of electrodes. Goddard investigators have proposed an instrument to monitor the dust and plasma environment on the Moon (LEED-Lunar Emissions and Electron Detector) to support longer-term efforts like the development of SPARCLE, Space Plasma Alleviation of Regolith Concentrations in the Lunar Environment [2,15,21]. Our original concept (SPARCLE) [3] involved using electron or ion guns (Figure 1 electron gun), acting as a plasma dust sweepers, an approach similar to one used to control spacecraft potential in a highly charged environment [22]. Because Calle and coworkers [20] have already achieved success in moving dust by systematic scanning of an electrostatic mesh surface with variable EMF (Figure 2), we now propose using particle beams to control the flow and remove/collect dust across such surfaces (Figure 3). How will the nature of the spacecraft surface covered with insulating dust affect electrostatic discharge? If the spacecraft surface is conducting the discharge would be relatively rapid. As long as the discharge rate occurs as the RC time