Spacecraft Materials Research Articles

Effects of electron beam and atomic oxygen irradiation on hypervelocity - Impact tested / polyimide coated carbon fiber-reinforced plates

In a low-Earth orbit, space debris orbit at approximately the first cosmic velocity. When space debris strike a spacecraft, ejecta (fragments) from the spacecraft are widely scattered. The reduction in the number of ejecta (fragments caused by impact) must also be considered when selecting spacecraft materials. The authors’ group has previously examined the size of ejecta (fragments) and reduction in the number of ejecta. In general, the mechanical properties of polymer materials and CFRP plates may be damaged by space environments, such as radiation (gamma rays and electron beams (EBs)), atomic oxygen (AO), ultraviolet rays, temperature, and thermal cycling. The authors suggested an anti-AO coating/polyimide CFRP as a material resistant to space environments. The effects of EB and AO irradiation on the fracture behavior and ejecta of anti-AO coating/polyimide CFRP were examined for hypervelocity impacts. The results of static three-point flexural tests were compared with the fracture behavior and ejecta. A two-stage light-gas gun was used for the impact tests. Spherical projectiles formed of aluminum alloy 2017-T4 with a diameter of 1.6 mm were used. Photographs of the ejecta scattered in front of each polyimide CFRP plate were captured from the side using a high-speed video camera. The number and weight of the ejecta on the front side and perforation holes were examined.

Design and Verification of Thermal Control System of Communication Satellite

The multiple working modes, complex working conditions, frequent changes in external heat flux, and high power consumption of communication satellites all pose great difficulties to their thermal design. This paper mainly describes the design of a thermal control system for high-power communication satellites. Firstly, new efficient heat transfer technologies and thermal control materials for spacecraft are introduced. Secondly, the structure and internal heat source of the satellite are introduced. Thirdly, the external heat fluxes are analyzed, and the position of the heat dissipation surface and extreme conditions are confirmed. Then, a thermal control system is designed around the difficulties of thermal control. With heat pipes, the temperature uniformity of +Y deck, −Y deck, and +Z deck increased by 8 °C, 9.9 °C, and 34.2 °C, respectively. Furthermore, the maximum temperature of the power controller, secondary power supply, bidirectional frequency converter, and solid discharge decreased by 32.5 °C, 22.0 °C, 14.0 °C, and 164 °C, respectively. Finally, a thermal balance test is performed. The test results show that the temperatures of the solid-state power amplifier, on-board computer, power controller, secondary power supply, and bidirectional frequency converter meet the requirements of the thermal control indices. In addition, the temperature of thermal-sensitive components such as batteries and the storage tank also meets the requirements. The thermal design scheme is reasonable and feasible, and the thermal balance test verifies the correctness of the thermal design.

Effect of Fatigue and Precorrosion Fatigue on Fatigue Crack Propagation and Fracture Failure Behavior of 5B70 Aluminum Alloy

Aluminum alloys are ideal lightweight structural materials for spacecraft, but they are susceptible to local corrosion during service. Herein, a systematic investigation is carried out to determine the fatigue behavior of 5B70 aluminum alloy under salt spray precorrosion. The mechanism of salt spray corrosion, fatigue life, crack propagation, and failure fracture mechanism of 5B70 aluminum alloy are studied. Under the salt spray corrosion, 5B70 aluminum alloy mainly undergoes pitting corrosion via a galvanic corrosion mechanism. After precorrosion, the formation of macroscopic fatigue cracks is accelerated, which significantly reduces its fatigue life. Fracture occurs due to single crack initiation and propagation under fatigue conditions. The fatigue crack tip undergoes a mixture of intergranular and transgranular modes, but mainly transgranular mode. Multiple strain localization phenomena occur during precorrosion fatigue, and corrosion pits on the surface provide a tortuous crack propagation path; the fatigue crack tip is mainly transgranular mode.

Electrostatic charge mitigation by graphene-based thin films for optical solar reflectors in spacecraft

In the field of spacecraft engineering, in particular for geostationary and geosynchronous applications, a significant need exists for a thermal control film to be electrically conductive, to effectively manage the dissipation of accumulated charges. In this study, transparent-electrically conductive reduced graphene oxide is strategically applied to heated rigid (quartz glass) and flexible (Fluorinated Ethylene propylene) optical solar reflector substrates. This innovation harnesses the unique properties of graphene to seamlessly integrate electrical conductivity into space-appropriate substrates with minimum deviation in their optical transparency and thermal properties. The tape peel-off test, conducted in accordance with space materials standards (ASTM D903) using 3 M Scotch tape, reveals the better adhesion of the developed coating. A noteworthy aspect of the present work involves visualizing charge dissipation using Field Emission Scanning Electron Microscope (FESEM) electron bombardment through obtained images and decreased surface potential measured by Kelvin Probe Force microscopy (KPFM). Furthermore, a basic OSR model is fabricated by depositing a reflective back with Aluminium and graphene thin film protecting from the top to investigate the alteration in reflection by varying incident angles. This research contributes to the advancement of spacecraft materials and coatings aimed at improving spacecraft durability and functionality.

Robust-flexible and highly-thermostable polyimide-silica aerogels with adjustable fiber skeleton structure for efficient aerospace thermal protection

The rapid growth of the aerospace industry presents significant challenges in developing lightweight and flexible thermal protective materials for spacecraft and related equipment. One longstanding challenge has been the preparation of high-thermostable and highly flexible aerogels, which limits their practical application in flexible thermal protection. In this work, through a rational 'chain-sphere' dual-combination structure design, polyimide-silica aerogels with integrated properties of the adjustable aerogel-fiber skeleton structure, robust flexibility and high thermostable have been prepared. By incorporating 2,2-dimethylbenzidine (DMBZ), we achieve a more solid aerogel network. The aerogel membrane retains its original shape without creases or cracks even after undergoing 500 folding and unfolding tests. Additionally, the silicon nanoparticles surrounding the aerogel exhibit superior performance at high temperatures. The unique structure and components of the composite enable it to possess lightweight (0.075–0.11 g/cm−3), low thermal shrinkage (300℃∼6 %), excellent thermostability (95 % weight retention at 474℃∼557℃), and promising thermal insulating performance below 600℃. The integrated performances of the aerogel make it suitable for high-temperature thermal protection applications that require both efficient thermal insulation and robust flexibility.

The Spectral Characterization of Novel Spacecraft Materials in the Low Earth Orbit Environment

The Spectral Characterization of Novel Spacecraft Materials in the Low Earth Orbit Environment

Effect of depth-absorbed dose distribution on the flexural modulus of carbon fiber–reinforced plastics in a radiation environment in radio-astronomical satellite orbit

Carbon fiber–reinforced polymers (CFRP) are used as structural materials for spacecraft due to their excellent physical properties. However, exposure in space to environmental factors, such as radiation, ultraviolet radiation, and thermal cycling, would alter these properties. Therefore, in order to ensure the safety and reliability of the spacecraft and the feasibility of the mission, it is necessary to evaluate the material’s space-environment resistance.The space environment consists of various types of energy and radiation, resulting in a non-uniform depth-absorbed dose distribution. This means that the absorbed dose is significantly higher at the surface of the material and lower with depth. However, reproducing this distribution in a ground-based test facility that typically uses a single energy irradiation is challenging. To overcome this challenge in ground-based testing, it is recommended that multiple radiation types and energies be irradiated to simulate the depth-absorbed dose distribution in a real space environment. However, there is a lack of understanding about the agreement between the depth-absorbed dose distribution obtained in real space environments and in ground-simulated tests. Additionally, the degree to which each depth region of the absorbed dose distribution affects material degradation is unknown. Hence, a faithful reproduction of the depth-absorbed dose distribution in a real space environment requires not only massive test facilities with significant financial investment but also a significant amount of time spent on irradiating those materials containing radiation that have practically no effect on material degradation.This study focused on analyzing the changes in the mechanical properties of CFRP due to radiation degradation using composite beam theory and mathematical models while considering the non-uniform depth-absorbed dose distribution in real space environments. The results of this study provide insight into which depth region of the depth-absorbed dose distribution is dominant for radiation-induced changes in mechanical properties when the CFRP plate thickness varies.

UV Reflectance of Spacecraft Materials and Analog Soils: Implications for Bioburden Reductions on the Undersides of Mars Rovers

AbstractThe Mars Sample Return mission architecture will utilize three spacecraft to collect, cache, recover, launch, and return to Earth a diversity of regolith and rock samples. However, no comprehensive Mars Microbial Survival (MMS) model currently exists. As an initial effort in building an MMS model, we examined the UV reflectance of 15 spacecraft materials and seven Mars analog soils within the context of the Perseverance mission. Data were used to predict the times required to achieve one lethal dose (syn., Sterility Assurance Level [SAL]; def. as a bioburden reduction of −12 logs). Results suggest that a single SAL dosage of UVC was achieved on exposed surfaces on the upper deck of Perseverance within a few hours to a few sols post‐landing at Jezero Crater. The overall average for UVC reflectance from spacecraft materials was approximately 10%. The overall UVC reflectance from Mars analog soils was measured at 1.3%. The Adaptive Caching Assembly (ACA) on Perseverance is located on the forward edge of the underbelly of the spacecraft. Modeling of the accumulated UVC dosage for the ACA yielded a prediction of reaching one SAL for downward facing surfaces at 93 sols that receive “single‐bounce” UVC photons from the local terrain. The SAL increases to 930 sols if an additional “bounce” of the solar UV irradiation is required to reach a partially protected site in the ACA hardware. The current study is the first to report the UVC reflectance from a diversity of spacecraft materials.

Overview of aerospace materials and their applications

Aerospace materials play a crucial role in modern aerospace technology. This paper is mainly a comprehensive overview of aerospace materials, including their classification, properties and applications. Aerospace materials are used in a wide range of applications, including aircraft and spacecraft, propulsion systems, and electronics. The paper also summarizes the importance and diversity of aerospace materials in various applications. The classification of aerospace materials includes metal materials, composite materials, ceramic materials and nanomaterials. Each material has unique properties that make it suitable for different applications. Aerospace materials are used in a wide range of applications, including aircraft and spacecraft, propulsion systems, and electronic devices. Materials for aircraft and spacecraft are particularly demanding because they are subjected to extreme temperatures, pressures and vibrations. Materials for propulsion systems and electronics require properties such as high strength, high electrical conductivity and high corrosion resistance. This paper also summarizes the importance and diversity of aerospace materials in various applications. Through the reading of this article, readers will understand the key role of aerospace materials in modern aerospace technology.

Spacecraft Materials’ Reflectivity and Surface Morphology: Aging Caused by Proton Irradiation

The radiation environment in Low Earth Orbit (LEO) is dominated by protons captured by Earth’s magnetic field in the Inner Van-Allen belt. Defunct satellites and other space debris objects can be resident in this environment for several decades and even centuries. So far, there is little knowledge about the impact of long-duration proton exposure to the surface morphology and reflectivity in LEO environment. We report on a laboratory test campaign exposing typical spacecraft materials with protons of 100 keV and 2.5 keV kinetic energy and a fluence corresponding to an in-orbit duration of 100 years and 120 years, respectively, in an 800 km sun-synchronous orbit. Although we find microscopic changes in surface morphology, reflectivity changes of all tested materials were smaller than 15%. This result brings positive news for on-going efforts to use optical methods, e.g. lightcurve measurements or active polarimetry, for characterizing space objects, since it suggests that data can - to a good approximation - be analyzed without accounting for proton induced aging effects that might affect the materials’ optical properties over time.

Information technologies of simulation of beam structures using multispectral spacecraft materials

The work is devoted to solving the scientific and practical task of modeling beam structures based on the materials of multispectral space images based on aerospace and contact measurements. The effectiveness of the use of space survey materials for the study of soil cover largely depends on the time of the survey. The linear forms of erosion (waterholes, ravines) are deciphered on space photographs, which are displayed only on images with a spatial resolution of 1-2 meters in the form of narrow, clearly delineated contours that have a jagged shape.The developed clustering algorithm makes it possible to more clearly identify beam structures on satellite images in combination with data from digital terrain models (DRM). A number of experiments were conducted on some set of remote sensing data of Boryspil and c. Panchevo, Novoukrainsky District, Kirovohrad Region. The experiments consisted in determining the spectral and other features of the beam structures (for example, the shape, soil types, etc.), which are located in different places of the territory that was investigated.On images with a spatial resolution of more than 10 meters, ravines are usually not displayed, but networks of beams with elongated wavy tree-like shapes are clearly visible. In the course of the study, data from multispectral imaging from the Sentinel-2 satellite (MSI scanner) and DEM (Digital Elevation Model, DEM) data obtained through the SRTM 3 archive were used.The research results showed that automating the process of highlighting beam structures on multispectral images with a spatial resolution of 30 m is very difficult. First of all, this can be explained by the fact that the beam consists of vegetation that is also present outside the beam. In addition, the number of channels of the ETM+ scanner and its spatial ability are not enough for a clearer separation of the "beam" class.

A Data-Driven Variance Reduction Technique for Efficiently Modelling Astronaut Radiation Doses in Spacecraft in High-Energy Isotropic Radiation Fields.

The ionizing radiation exposure to crew on current and future space missions can significantly increase their health risks for cancers, degenerative diseases, and other acute and late effects. A common approach for estimating risk to crew is by completing stochastic (e.g., Monte Carlo) or deterministic particle transport simulations. Within the simulated environment, a small fraction of the particle histories tracked will interact with the astronaut or detector, particularly for larger spacecraft such as the International Space Station, Tiangong Space Station or Lunar Gateway. These simulations can be computationally intensive as they require a very large number of particle histories to achieve a low statistical uncertainty. Variance reduction techniques are applied to simulations to reduce the computational time of the simulation while maintaining the same (or less) statistical uncertainty. The variance reduction technique developed herein involves applying a directional source bias to an isotropic radiation field, such as galactic cosmic rays, to reduce the quantity of particles that have a low probability of interacting with the astronaut or detector. A custom application has been developed utilizing the Geant4 Toolkit that computes the trajectories and energies of particles in three dimensions in the International Space Station using the Monte Carlo method. The results demonstrate the impact of our variance reduction technique on effective dose equivalence depending on: primary and secondary particle type (proton, neutron, photon, heavy ion, etc.), geometric volumes and spacecraft materials. Our variance reduction technique can be tuned by the user to optimize the simulation time depending on their objectives and enables rapid testing of different shield configurations and materials. This variance reduction technique is implemented easily using several input parameters for boundary conditions. Recommended values are presented for rapid implementation in simulations.

A new numerical model of the HF antenna of the Mars Reconnaissance Orbiter’s (MRO) Shallow Radar (SHARAD) and first results from a test at a high-angle spacecraft roll

This communication deals with a newly developed electromagnetic model of the Mars Reconnaissance Orbiter Shallow Radar (SHARAD) HF antenna and its interaction with the spacecraft. The model has been developed considering the effect of spacecraft materials and components and the orientation of appendages such as the solar arrays and high-gain antenna. The model appears very useful in designing new operational scenarios of the SHARAD sub-surface sounder, confirming the ability to increase up to 10 dB the signal-to-noise ratio in radar sounding performances by altering the geometry of the observations. We show results from a first test wherein the spacecraft roll angle was set to 120°while obtaining an observation over a ground track previously observed at 0°roll angle, and the expected performance improvement was achieved.

Analysis of Spacecraft Materials Discrimination Using Color Indices for Remote Sensing for Space Situational Awareness

The increasing number of space missions have resulted in an augmented density of artificial objects positioned in orbital domains near Earth. Knowing the material composition of the resident space object can be of value in object identification and risk assessment using remote sensing techniques. To perform this task, it is increasingly imperative to optically characterize spacecraft materials to identify unique material-specific spectroscopic markers. In this work, a variety of materials frequently utilized by the aerospace industry in spacecraft design and construction were analyzed using reflectance spectroscopy. The collected data provide a spectral characterization baseline for modern-day and historical spacecraft materials. The color index was computed for standard astronomical and suggested theoretical filter passbands and compared to each other in their capability to discriminate materials belonging to different classes. The color index was calculated from reflectance spectra of common spacecraft materials in their pristine, as-received conditions that fell under different family groupings. Different color-index combinations were studied using color-color diagrams. Visual and quantitative analysis of the color-color diagrams were used to evaluate the possibility of discriminating materials from one another by means of optical measurements. Results of the analysis show that polyimide and photovoltaic materials are most easy to discriminate from all other materials via color indices using different filter passbands evaluated in this study.

Nuclear data for space exploration

Understanding the harmful effects of galactic cosmic rays (GCRs) on space exploration requires a substantial amount of nuclear data. Specifically, the interaction of energetic GCR charged particles with spacecraft materials generates secondary radiations that, through energy deposition, can harm astronauts and electronic systems. By identifying the gaps in our knowledge of the relevant nuclear data—such as interaction cross sections—and identifying ways to fill those gaps—with measurements, compilations, evaluations, dissemination, reaction modeling, sensitivity studies, and uncertainty quantification—the safety and viability of space exploration can be improved. This work surveys the state of the art in this interdisciplinary field and identifies promising collaborative research topics that have significant potential to advance our understanding of the effects of the space radiation environment on space exploration.

Relevancy of Pulsed Electroacoustic Measurements for Investigating Spacecraft Charging

The magnitude and spatial distribution of charge embedded in dielectric materials and the evolution of the charge distributions with time are paramount for the understanding and mitigation of spacecraft charging. Spacecraft materials are charged primarily by incident fluxes of low-energy electrons, with electron fluxes in the 10–50 keV range often responsible for the most deleterious arcing effects. While the pulsed electroacoustic (PEA) method can provide sensitive nondestructive measurements of the internal charge distribution in insulating materials, it has often been limited for spacecraft charging applications by typical spatial resolutions of <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">$\le $</tex-math> </inline-formula> 10 <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">$\mu $</tex-math> </inline-formula> m, with a 10- <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">$\mu $</tex-math> </inline-formula> m range of electrons in common spacecraft materials (e.g., polyetheretherketone (PEEK), polytetrafluoroethylene (PTFE), low-density polyethylene (LDPE), or SiO <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">$_{2})$</tex-math> </inline-formula> at incident energies from <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">$\sim$</tex-math> </inline-formula> 20 to <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">$\sim$</tex-math> </inline-formula> 40 keV. A series of PEA tests over a range of incident electron energies were devised to investigate the relevance of the PEA method for typical spacecraft charging applications. Thin-film samples of vacuum-baked PEEK were irradiated with 10–80-keV monoenergetic electron beams. PEA measurements of deposited charge profiles determined the peak positions and magnitude of deposited charge. These were used to establish the minimum incident energies for which PEA measurements provided meaningful results and thus to characterize the merits of PEA measurements over energy ranges of relevancy to spacecraft charging issues.

Adding Radiation-Induced Conductivity Test Capability to the JPL Dynamitron

The Europa Clipper space environments test campaign has made extensive use of the Dynamitron electron accelerator located at the Jet Propulsion Laboratory. One of the most common uses of this facility has been gathering data on the internal electrostatic discharge (IESD) properties of the various elements of the spacecraft. These testing needs often required some reconfiguration of the accelerator system. One of the most recent upgrades was adding the ability to measure both dark conductivity and radiation-induced conductivity (RIC) for spacecraft materials. Adding this new one-of-a-kind capability required incorporating a low-energy electron flood gun to the existing higher energy electron beam from the Dynamitron accelerator. The upgraded system was also designed to obtain data on multiple samples per test run allowing for the acquisition of a large quantity of data in a relatively short time period. Dark and radiation-induced conductivities measured using the new system are provided demonstrating examples of results that can be obtained.

Solar Absorptivity Degradation of Spacecraft Materials Due to UV and Charged Particles in the Gateway Environment

Gateway will be an outpost in cislunar space designed to survive a 15-year mission to support human Moon landings and future deep space exploration. The Gateway Passive Thermal Control System (PTCS) Group has identified charged particle and UV degradation of potential Gateway materials as a knowledge gap for the Near Rectilinear Halo Orbit (NRHO) and transit environment, especially the slow spiral transit planned for the first two modules. Solar absorptivity degradation is important to quantify because the end-of-life absorptivity impacts thermal performance of the system. To address this, ground testing has been completed at NASA Goddard Space Flight Center in which potential Gateway materials, including radiators, multi-layer insulation (MLI), and structural materials are subjected to the expected transit plus 15-year on-orbit charged particle fluence (9.86x1015 protons/cm2 at 2.5 keV and 3.1x1016 electrons/cm2 at 10 keV) and 5093 Equivalent Solar Hours (ESH). Reflectivity of a single sample of each material was measured in atmosphere and in vacuum inside the chamber after exposure to incremental levels of ESH and charged particle fluence. Increase in absorptivity of most samples were seen throughout the test including large differences in absorptivity of materials of the same category. Future planned testing will validate these results and include additional materials of interest.

High temperature atomic oxygen testing for ESA’s Envision mission to Venus

EnVision will be ESA’s next Venus orbiter, providing a holistic view of the planet from its inner core to upper atmosphere to determine how and why Venus and Earth evolved so differently. The spacecraft will be equipped with a suite of instruments including a sounder to reveal underground layering, and spectrometers to study the atmosphere and surface. The thermal fluxes in orbit around Venus are high, the solar flux at Venus distance is twice its value on Earth (around 2600 W/m2), and the reflected sunlight flux has a similar order of magnitude due to the albedo. Besides, there will also be an aerobraking phase, when a third thermal flux, the aerothermal flux, needs to be considered, with a similar order of magnitude as the two others. These high total thermal fluxes, together with the cold instruments’ requirements and high-power dissipation make the thermal environment a design driver for the mission. The chemistry of the atmospheric environment during aerobraking needs also to be taken into account. Atomic oxygen exists in Venus atmosphere at the altitude range foreseen for aerobraking, with densities of up to 1017 oxygen atoms per cubic metres at aerobraking altitudes. Though the concentrations are small, their accumulation over up to 2000 atmospheric passes lead to total atomic oxygen fluences on the exposed spacecraft surfaces which are comparable to those encountered on typical Low Earth orbiting satellites (upwards of 1021 atomscm-2). This drives the selection of spacecraft materials on the most exposed surfaces. Whilst significant data exists about atomic oxygen erosion effects on materials for Low Earth Orbit, there is little knowledge about high temperature atomic oxygen effects on materials under environmental conditions representative of the Venus atmosphere. To analyse the potential risks for the Envision mission, and to assist the designers in selecting the best materials, an atomic oxygen test campaign is being performed in the ESA ESTEC atomic oxygen ground-based simulator (LEOX facility). The facility has been adapted so that material samples can be simultaneously exposed to atomic oxygen and high temperatures up to at least 200°C. For the test campaign, the Envision instrument teams and designers of the prospective satellite systems are providing candidate materials to be exposed. This includes a variety of materials to be utilised on the outside of the satellite such as thermal control coatings, optical materials, coatings for appendages and antennas and multi-layer insulation. The paper describes the modifications to the facility and the first results of the testing which has been performed.

Evaluation of Decontamination Potential of Wet Wipes Against Microbial Contamination of Chinese Spacecraft Materials.

Abstract There are many kinds of microorganisms that inhabit the environment of manned space stations. Wet wipes are a common tool used in space stations to clean and reduce microorganisms on surfaces. Here, we compared the performance of five types of wipes used by the Chinese Space Station (CSS) on orbit before 2021 in terms of microbial decontamination. In previous studies, we found that Bacillus sp. TJ-1-1 and Staphylococcus sp. HN-5 were the most abundant microorganisms in the assembly environment of the CSS. In this study, we used these two bacteria to build different microbial load models to represent the occurrence and non-occurrence of microbial outbreaks in the on-orbit CSS. The results show that the number of microorganisms that can be removed when wiping the surface with high microbial load by wet wipes was higher than that when wiping the surface with low microbial load. For on-orbit daily cleaning and keeping the microbial population within the regulation concentration range, it is suitable to use two pure water wipes per 100 cm2. When the number of microorganisms increases to a degree where astronauts can see the colonies with their naked eyes, the best way to eliminate the problem is to wipe them thoroughly and repeatedly with at least four quaternary ammonium-based wipes every 100 cm2.