Carlos I. Calle

Self-Cleaning Transparent Dust Shields for Protecting Solar Panels and Other Devices

The development of transparent flexible dust shields using both single- and three-phase electrodynamic shields is reported here for possible application on Mars and Earth to minimize obscuration of solar panels from the deposition of dust. The electrodynamic screens (EDS) are made of transparent plastic sheets, such as polyethylene terephthalate (PET) for its UV radiation resistance, and a set of parallel conducting electrodes made of transparent indium tin oxide (ITO) embedded under a thin transparent film. The basic principle of EDS operation, a simplified mathematical model of particle trajectories, the experimental setup used for testing the screens, and their dust removal efficiencies (DRE) are described. Results of our measurements on dust removal efficiency of EDS as a function of the particle size and electrostatic charge distributions of Mars simulant dust are reported. The results show that the EDS technology has a strong potential for protecting solar panels against dust hazards with DRE higher than 80% for dust. The power requirements will be approximately 10 watts per square meter of the panels when cleaning is needed.

Experimental Evaluation and Analysis of Electrodynamic Screen as Dust Mitigation Technology for Future Mars Missions

The electrodynamic screen (EDS) is considered to be one of the feasible dust mitigation technologies for future Mars missions. In this paper, the performance of EDS for surface cleaning was characterized with respect to the following operational parameters: 1) the efficiency of screens under both continuous and intermittent operations with different rates of dust deposition; 2) electrical power requirements for the screen operation with respect to dust removal efficiency (DRE), frequency, and excitation frequency; and 3) the optical transmission efficiency of the transparent EDSs and the corresponding power loss, when these screens were placed on solar panels. The average DRE of EDS during continuous dust loading was over 95%, whereas it was 90% when the screen was activated intermittently. Power consumption by EDS, as well as the size and weight of the power supply, is one of the critical factors for its applicability for dust removal from solar panels during future Mars mission. The power consumption by EDS was measured under several dust loadings and using different frequencies and electrical field intensities for the safe operation of power supplies without Paschen breakdown. Experiments were conducted under simulated Martian atmosphere (5.0 mb CO2 atmosphere) using a screen with an active surface area of 59 cm2. The average power consumption of screen varied between 1.02 and 2.87 mW. The optical transmission efficiency for a transparent EDS (PET substrate with indium tin oxide electrodes) was measured for a PET screen with ITO electrodes. It was found that placing the transparent EDS on a typical space-type solar panel resulted in a significant obscuration. The power output of the solar panel decreased by 15%.

Electrodynamic removal of contaminant particles and its applications

An electrodynamic screen was designed, built, and tested for the removal of particles from its surface. The technology has a large number of applications ranging from space exploration to biotechnology. An AC signal was applied to the shield and it was studied the effect of its amplitude, shape, and frequency to the shields particle removal properties. The best working conditions were found to be: the amplitude ranging from 7 to 10 kV, and the frequency ranging from 5 to 15 Hz. The shield was proved to remove also the initially uncharged particles and to separate the particles based on their charge polarities.

Controlled particle removal from surfaces by electrodynamic methods for terrestrial, lunar, and Martian environmental conditions

An Electrodynamic Dust Shield to remove already deposited micron-size particles from surfaces and to prevent the accumulation of such particles on surfaces has been developed. In addition to terrestrial application, our NASA laboratory is adapting this technology for the dusty and harsh environments of the Moon and Mars. The Apollo missions to the moon showed that lunar dust can hamper astronaut surface activities due to its ability to cling to most surfaces. NASAs Mars exploration landers and rovers have also shown that the problem is equally hard if not harder on Mars. In this paper, we show that an appropriate design can prevent the electrostatic breakdown at the low Martian atmospheric pressures. We are also able to show that uncharged dust can be lifted and removed from surfaces under simulated Martian environmental conditions. This technology has many potential benefits for removing dust from visors, viewports and many other surfaces as well as from solar arrays. We have also been able to develop a version of the electrodynamic dust shield working under hard vacuum conditions. This version should work well on the moon. We present data on the design and optimization of both types of dust shields as well substantial data on the clearing factors for transparent dust shields designed to protect solar panels for Martian exploration.

The Use of Tribocharging in the Electrostatic Beneficiation of Lunar Simulant

The use of tribocharging as a potential method to provide sufficient charge to several different lunar simulants for electrostatic beneficiation was investigated. The objective was to determine whether specific minerals of interest (e.g., ilmenite) that are present in lunar regolith could be enriched in concentration by beneficiation that would therefore allow for more efficient extraction for in situ resource utilization use. The production of oxygen, water, and other resources on the Moon from raw materials is vital for future missions to the Moon. Successful separation of ilmenite was achieved for a prepared simulant (KSC-1), which is a mixture of pure commercially supplied pyroxene, olivine, feldspar, and ilmenite, in a 4 : 4 : 1 : 1 ratio, showing proof of concept when tribocharged against three different charging materials, namely, Al, Cu, and PTFE. Separation by chemical composition was also observed for existing lunar simulants JSC-1 and JSC-1A; however, the interpretation of the separation was difficult due to the complex mineralogy of the simulants compared to the simple prepared mixture.

Use of non-thermal atmospheric plasmas to reduce the viability of Bacillus subtilis on spacecraft surfaces

Atmospheric pressure glow-discharge (APGD) plasmas have been proposed for sterilizing spacecraft surfaces prior to launch. The advantages of APGD plasmas for the sterilization of spacecraft surfaces include low temperatures at treatment sites, rapid inactivation kinetics of exposed microbial cells, physical degradation and removal of microbial cells, physical removal of organic biosignature molecules, and short exposure times for the materials. However, few studies have tested APGD plasmas on spacecraft materials for their effectiveness in both sterilizing surfaces and removal of microbial cells or spores. A helium (He)+oxygen (O2) APGD plasma was used to expose six spacecraft materials (aluminum 6061, polytetrafluoroethylene (PTFE), polycarbonate, Saf-T-Vu, Rastex, and Herculite 20) doped with spores of the common spacecraft contaminant, Bacillus subtilis, for periods of time up to 6 min. Results indicated that greater than six orders of magnitude reductions in viability were observed for B. subtilis spores in as short of time as 40 s exposure to the APGD plasmas. Spacecraft materials were not affected by exposures to the APGD plasmas. However, Saf-T-Vu was the only material in which spores of B. subtilis adhered more aggressively to plasma-treated coupons when compared to non-plasma treated coupons ; all other materials exhibited no significant differences between plasma and non-plasma treated coupons. In addition, spores of B. subtilis were physically degraded by exposures to the plasmas beginning at the terminal ends of spores, which appeared to be ruptured after only 30 s. After 300 s, most bacteria were removed from aluminium coupons, and only subtle residues of bacterial secretions or biofilms remained. Results support the conclusion that APGD plasmas can be used as a prelaunch cleaning and sterilization treatment on spacecraft materials provided that the biocidal and cleaning times are shorter than those required to alter surface properties of materials.

Test Method for In Situ Electrostatic Characterization of Lunar Dust

The unmanageable effects of the dust on the surface of the moon made lunar traverses for the Apollo astronauts nearly impossible after three days exposure. These effects, governed by electrostatic properties, are responsible for their behavior and, to this date, are largely unclassified. Although many of the electrostatic characteristics of lunar soil have been measured, there are other electrostatic properties yet to be determined, such as chargeability (or charge-to-mass ratio), charge decay characteristics, and triboelectric properties. The purpose of this paper is to present a future in situ instrument capable of simultaneously measuring four important electrostatic properties: dielectric permittivity, volume resistivity, charge decay, and the chargeability of triboelectrically charged soil. This paper serves to illustrate the testing methods necessary to classify the electrostatic properties of lunar dust using in situ instrumentation and the required techniques therein. A review of electrostatic classification of lunar simulant materials is provided as is its relevance to the success of future human lunar missions.

Electrostatic precipitation of dust in the Martian atmosphere: Implications for the utilization of resources during future manned exploration missions

Future human missions to Mars will require the utilization of local resources for oxygen, fuel, and water. The In Situ Resource Utilization (ISRU) project is an active research endeavor at NASA to develop technologies that can enable cost effective ways to live off the land. The extraction of oxygen from the Martian atmosphere, composed primarily of carbon dioxide, is one of the most important goals of the Mars ISRU project. The main obstacle is the relatively large amount of dust present in the Martian atmosphere. This dust must be efficiently removed from atmospheric gas intakes for ISRU processing chambers. A common technique to achieve this removal on earth is by electrostatic precipitation, where large electrostatic fields are established in a localized region to charge, precipitate and collect dust particles. This technique is difficult to adapt to the Martian environment, with an atmospheric pressure of about one-hundredth of the terrestrial atmosphere. At these low pressures, the corona discharges required to implant an electrostatic charge to the particles to be collected is extremely difficult to sustain and the corona easily transitions to a glow/streamer discharge, which is unsuitable for particle charging. In this paper, we report on our successful efforts to establish a stable corona under Martian simulated conditions. We also present results on dust collecting efficiencies with an electrostatic precipitator prototype that could be effectively used on a future mission to the red planet.

Reduced gravity flight demonstration of the Dust Shield technology for optical systems

The Electrodynamic Dust Shield (EDS), an active dust mitigation technology for lunar exploration systems, has been under development in our laboratory at the Kennedy Space Center for the last three years. The EDS uses electrostatic and dielectrophoretic forces to remove dust from opaque, transparent, rigid, and flexible surfaces. The EDS consists of an array of electrodes on a substrate that are coated with a material possessing a high dielectric constant. The EDS has been tested with JSC-1A lunar dust simulant at high vacuum pressures of the order of 10−6 kPa. In this paper, we report on our demonstration of the EDS at high vacuum (10−6 to 10−7 kPa) and under lunar gravity (g/6) during a Reduced Gravity Flight. Over one hundred and twenty experiments were performed to test the removal of different dust particle sizes using several EDS configurations and coatings. Particle sizes ranged from under 10 micrometers to 450 micrometers, separated in four different size fractions.

Development of an electrostatic precipitator to remove Martian atmospheric dust from ISRU gas intakes during planetary exploration missions

Manned exploration missions to Mars will need dependable in situ resource utilization (ISRU) for oxygen production. The Martian atmosphere is composed of 95.3% CO2, other gases, and 0.13% O2 at ~ 9 mbar (1% of the Earths pressure). However, it also contains 2-10- μm dust uploaded by dust devils and high winds. Oxygen extraction requires removal of the dust with little pressure drop (Δp). An electrostatic precipitator (ESP) has lower Δp than a filter, but the low pressure causes an electrical breakdown at electric fields ( ~ 1 kV/cm) ~ 30× lower than on Earth, making implementation challenging. Molecular mean free paths (λ = 4 μm) and ion mobility values (b = 0.008 m2/V·s) are ~ 100× larger than at Earths pressure (λ = 44 nm) and (b = 8.4 ×10-5). The large λ lowers Stokes drag, particularly for smaller particles. Pauthenier field charging dominates for particles with and diffusion charging for d<;2 μm. The low E proportionally decreases both Pauthenier particle charging and the F = qE collection force. This greatly reduces the particle migration velocities (w), e.g., for d = 10 μm, w = 0.01 m/s compared with 0.4 m/s on Earth. However, for small particles (d = 1 μm), this is compensated by diffusion charging and reduced drag ( w = 0.04 m/s on Mars, 0.05 m/s on Earth). The Martian atmosphere was simulated with 95% CO2/5% humid air at 9 mbar. Paschen curves were measured, and I- V curves ( I ~ 5 - 300 μA for V ~ 1.3 - 2.3 kV) were obtained for 5-10-cm-diameter wire/rod-cylinder ESPs. Only positive polarity yielded stable uniform corona. Charging of 0.5-1.3-cm-diameter spheres agreed with the Pauthenier theory. A Martian dust simulant collection efficiency test is in progress.