AIAA 2001-0095 Contamination Effects Due to Space Environmental Interactions Philip T

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AIAA 2001-0095 Contamination Effects due to Space Environmental Interactions Philip T. Chen NASA Goddard Space Fligh_ Center Greenbelt, MD 20771

39 _ Aerospace Sciences Meeting & Exhibit 8-11 January 2001 Reno, Nevada

I I I I I II For permission to copy or to republish, contact the American Institute of Aeronautics and Astronautics, 1801 Alexander Bell Drive, Suite 500, Reston, VA, 20191-4344. Contamination Effects due to Space Environmental Interactions

Philip T. Chcn NASA Goddard Space Flight Center Mail Code 545 Greenbelt, Maryland 20771 philip, [email protected], nasa.gov

ABSTK_CT spacecraft is accelerated by the vacuum and solar heating conditions of the space environment. Molecular and particulate contaminants are Particulate and molecular contaminants present in the commonly generated from the orbital spacecraft space environment tend to move around the spacecraft operations that are under the influence of the space due to the localized spacecraft environment. The environment. Once generated, these contaminants may presence of the contaminants will affect spacecraft's attach to the surfaces of the spacecraft or may remain in original design performance. Returned hardware from the vicinity of the spacecraft. In the event these Space Shuttles, Solar Max Repair Mission (SMRM), contaminants come to rest on the surfaces of the Long Duration Exposure Facility (LDEF), Hubble Space spacecraft or situated in the line-of-sight of the Telescope (HST), and MIR have provided evidence of observation path, they will create various degrees of both short-term and long-term effects of space contamination effect which may cause undesirable environment on spacecraft. Information learned from effects for normal spacecraft operations. There will be these renamed hardware is invaluable because the space circumstances in which the spacecraft may be subjected environment is complex and is very difficult to to special space environment due to operational reproduce in a space simulation facility on the ground. conditions. Interactions between contaminants and special space environment may alter or greatly increase The space environment provides a dynamic the contamination effect due to the synergistic effect. boundary for an operational spacecraft in orbit. When a spacecraft is operating in space, it is mostly interfered This paper will address the various types of by the spacecraft's local space environment. Therefore, contamination generation on orbit, the general effects of a key consideration of the interactions between a ti,e contamination on spacecraft systems, and the spacecraft and its space environment is the localized typical impacts on the spacecraft operations due to the conditions which change tremendously according to the contamination effect. In addition, this paper will altitude and orbit. The space environment to be explain the contamination effect induced by the space considered in this paper includes orbits ranging in environment and will discuss the intensified altitude from Low Earth Orbit (LEO) to contamination effect resulting from the synergistic Geosynchronous Earth Orbit (GEO) and beyond. effect with the special space environment. SPACE ENVIRONEMNT Key Words: Contamination, Space Environment, and Synergy The operations of a spacecraft need to consider the effect of the diverse space environment and its INTRODUCTION associated parameters. According to Purvis [_1the space environment can be characterized into four The contamination generation, transport, and categories: the ionizing radiation, the natural plasmas, impact on spacecraft systems are greatly influenced by the micrometeorites and debris, and the neutral the space environment. Material outgassing from the environment, Among these environmental factors,

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I American [nstitute of" Aeronautzcs and Astronautic local environment related to gravity, B & E Fields, Plasma is defined as an electrically neutral neutral atmosphere, and plasma are strongly coupled ionized gas. Plasma are encountered in the ionosphere, with the spacecraft systems. A detailed examination plasrrtasphcre, rnagnetospheric storms, anti plasmas can be performed by analyzing the interactions between induced by the spacecraft. At LEO, the solar UV the system and its local environment. A list of ionizes the major atmospheric constituents, oxygen and environmental factors and their effects on space nitrogen atoms, to produce plasma. Interactions systems in near-Earth space was provided by Purvis in between the spacecraft and the plasma include arcing, Table 1. current collection, and spacecraft charging. The charging due to different potentials on spacecraft Table 1. The Space Environmental Effects surfaces in a plasma environment is a concern when building a spacecraft. The Space Environmental Effects • Sunlight & Earthshine A spacecraft can be impacted and damaged by the micrometeorites and man-made orbital debris Heating, Thermal Cycting, Material Damage, Sensor Noise, Drag, Torque, Photoemission environment. These impact can cause serious structure • Gravity damage and catastrophic failure of the spacecraft. Although the hazard from the natural debris Acceleration, Torque • B&EFields environment has been considered as acceptable, the Torque, Drag, Surface Changes, Potentials dangers posed by man-made orbital debris is a real concern. The impact by orbital debris environment can • Neutral Atmosphere also produce a dense localized plasma or enhance Drag, Torque, Material Degradation, Vacuum, spacecraft surface erosion. Contamination, HV Breakdown • Plasma The neutral environment consists of natural Charging, Arcing, Parasitic Currents, System species and contamination generated by the spacecraft Potentials, Sputtering, Enhanced Contamination, and released in the immediate environment. Typical ES & EM Waves (Noise), Plasma Waves & attributes associated with the neutral environment Turbulence, Change of EM Refractive Index include atomic oxygen (AO), glow, sputtering, and • Fast Charged Particles atmosplaeric drag. AO at LEO degrades spacecraft Radiation Damage, Internal Charging, Single surfaces by a chemical reaction with materials that Event Upsets, Arcing, Noise changes the surface properties. For a LEO spacecraft, • Meteoroid & Debris the neutral atoms cause significant surface heating and Mechanical Damage, Enhanced Chemical & orbital decay. To compensate the orbital decay, orbital Plasma Interactions, Local Plasma Production, adjustments are necessary to maintain a spacecraft in its Induced Arcing proper orbit. • System Generated System Dependent: Neutrals, Plasma, Fields, CONTAMINATION SOURCES Forces & Torque, Particles, Radiation Spacecraft contamination is defined as a foreign substance on or near a spacecraft surface that The ionizing radiation includes the presence in adversely affects the performance of space systems. space of ultraviolet (UV), radiation belts, cosmic, solar, The contaminant, either molecular or particulate, is and galactic radiation. Near the Van Allen belt, the generated by the spacecraft under the influence of the relatively strong Earth magnetic field traps the space environment. As shown in Table 2, the majority energetic protons. High-energy particles degrade of the contamination sources are self-generated by the detector performance by the accumulative material spacecraft systems. The returned SMP_M hardware microstructural damage. Although any spacecraft may revealed spacecraft generated molecular contaminants be susceptible to surface charging, the highest on fine Sun sensor, louver frames, and instrument occurrence seems to be with GEO spacecraft. In venting areas. Study oft.he MIR solar arrays as part of general, radiation may cause single events upset (SEU) the International Space Station (ISS) Risk Mitigation and degrade solar arrays, dielectric materials, electronic Program discovered significant thruster burn residue [-'1. components, and sensors. It can also deposit charge In addition, particles and fibers were identified from the inside dielectric materials and on surfaces internal to returned MIR solar arrays. Through examination it was the spacecraft with a result in dielectric breakdown and concluded that MIR solar arrays are sources of particle EM[ electrical upsets. and glass fiber generation and shedding I31

2 American [nstitute of Aeronautics and .\ _tronatmc Fable2. ContaminationSourccs the ttsc of a highly volatile lubricant may present a contamtnatlon threat that is detrimental to sensitive ContaminaticmSources spacecraft sur face:;. Molecular • Materialoutgassing{_,ater,hydrocarbons, Spacecraft plume products are another silicones) contamination contributors. Rocket engines are used for launch vehicles, apoapsis and periapsis motors, spacecraft • SpacecraftandMulti-LayerInsulation (MLI) attitude control, and station keeping. The release of large venting amounts of expelled propellants into the surroundings • Fluid leak, dump, and lubricant loss alters the space environment and interferes with the • Plume products spacecraft operations. Products of combustion can also • Extravehicular activiw deposit on surfaces in the vicinity of the thrusters by Particulate direct impingement. • Surface particles • Plume products Extravehicular activity (EVA) and operations of • Ice formation spacecraft mechanisms generate both molecular and • Mechanism generated particles particulate contamination. The space suits and Space • Micrometeorites & debris Shuttle storage compartments are potential • Extravehicular activity contamination sources. During an EVA, the ambient and the surface particles are disturbed with the possibility of being redistributed onto other critical Long-term outgassing originated from surfaces. spacecraft materials poses a great concern. Spacecraft outgassing materials are released when the CONTAMINATION IMPACTS ON environmental pressure is reduced and the temperature SPACECRAFT SYSTEMS is raised. Low molecular weight constituents, light reaction products, and absorbed gases such as water, Contamination may be originated from many hydrocarbons, and silicones are the most significant sources during spacecraft operations. After being outgassing constituents. Typical outgassing releasing generated, contaminants may attach to spacecraft mechanisms consist of desorption of physically or surfaces or remain in the vicinity of the spacecraft. chemL.ally adsorbed gases from the surface, Deposited or line-of-sight contaminant influences a evaporation or sublimation of the material itself, or normal spacecraft operation. This contamination effect evaporation and decomposition of gases in the bulk is further altered or enhanced by the space environment. material. Among all space environmental attributes discussed before, vacuum, temperature (solar heating), AO, and Spacecraft venting is another major radiation create obvious contamination problems. Each contamination source. Many spacecraft and instruments single environment contributes separately in producing require venting of gases from thermal blankets and a contamination related effect. However, a synergistic spacecraft interior to balance the internal pressure. effect may be produced with the interaction of multiple Additmnally, venting may be u_ed to dispose of waste space environment conditions. Relatively speaking a gases, to produce low-pressure drag makeup, or simply synergism may aggravate the contamination effect more to provide low-level propulsion for attitude control. than a single space environment. A summary of the The vented gases modify the local environment and contamination related impacts, with the interactions disturb spacecraft activities. When condensed on with space environment, on spacecraft systems is surfaces, the vented gases degrade the thermo-optical shown in Table 3. properties of the surfaces. SPACE ENVIRONMENT ENHANCED Leakage of operational fluids and gases from CONTAMINATION EFFECTS manned modules and the Space Shuttle produces molecular contamination. These fluids and gases leak The space environment provides unique from storage tanks, supply lines, or pressurized cabins conditions for orbiting spacecraft. The vacuum of during a normal vehicle operation. Inhabited space space magnifies many environmental parameters systems generally have overboard dumping of disposals (speed, temperature, voltage, etc.) more than the Earth's such as gases, liquids, and particles. Besides dumping atmosphere. Without the interference of the waste products from the crew cabin, the Space Shuttle atmosphere, the temperatures produced in vacuum by frequently discharges the fluid into space. In addition, sunlight arc extreme. Therefore, a spacecraft wtll be

3 .American Institute of' Acronaut_cs and A,;mmauttc Fable3 q'heContaminationImpactsOnSpace comanfination effects, material outgassmg, .+\O erosion, Systems thermal surface degradation, and particle deposition are fiequcntly studied. Detailed space environment ContaminationImpacts enhanced contamination effects are discussed below. "lqlermal Control Atmospheric Drag • Contamination buildup • Radiation effect Operational spacecraft encounter a significant - " • Degradation of thermal control system number of atmospheric particles during each orbit. Optics Neutral gas molecules are atmospheric particles that • Degradation of optical system influence a spacecraft by transferring energy and • Reduced transmittance through lenses, mirrors, momentum to a spacecraft as a drag force. windows, and detectors Atmospheric drag in LEO is a key parameter in • Increased light scattering along the optical path predicting spacecraft fuel requirements. The Power atmospheric density and its long-term fluctuations • Reduced solar cell efficiency dominate the reboot requirements for a spacecraft. To • Obscured solar array surfaces and reduced solar compensate the drag, additional fuel is consumed for cell efficiency orbital adjustments, which inevitably produces more Propulsion contaminants around the spacecraft. • Restricted propulsion operations Structure Mechanisms capable of heating the Earth's • Surface degradation atmosphere produce ambient density changes. The • Particle penetration process of heating induced drag may be resulted from Sensors geomagnetic storms and changes in solar extreme UV • Performance of Sun, Earth, and star sensors emission. An excellent atmospheric drag example can • Scatters ambient light onto sensor optics be found on the Tropical Rainfall Measuring Mission (TRMM) that operates in 2002's 23rd solar maximum leading to false response cycle. A solar maximum with roughly an 11-year Operations period is the time of largest average sunspot number. • Disturbance to spacecraft As the sun activity level increases, the atmosphere is • Increased molecular column density can affect heated and the gas is expanded. This in turn affects the instrument operation plasma density at the LEO environment and increases • Electrical discharge or arcing in high voltage atmospheric drag on a spacecraft. As a result of equipment due to high outgassing operating during a solar maximum, TRMM will • Noise on slip rings and electrical contacts consume more propellants; thus more contaminants will • Failure of precision mechanisms due to be generated. Generally, it is a valid assumption that particulate most LEO spacecraft encounter elevated contamination • Electrical discharge or arcing in high voltage levels generated by the additional fuel consumed to equipment due to contamination compensate for atmospheric drag.

Outgassing exposed to an extreme solar heating and cooling conditions in space. Consequently, spacecraft materials According to the phase equilibrium, a reduced and components will demonstrate more damage pressure of space ,,,,'ill promote polymeric materials to because of the impact of the extreme conditions. outgas. With a low ambient pressure, molecules are Additionally severe sunlight effects include surface easily released from spacecraft surfaces. Coupling with discoloration, material damage, and coating failure. the solar radiation and long-term exposure, this Similar to the sunlight, the presence of other space outgassing effect is even more significant. Contrary to environment conditions creates contamination this inverse pressure effect, outgassing rates of problems. Evidence of contamination problems polymeric materials increase as temperature goes up. associated with space environment exposure has been Thus, materials tend to outgas more in GEO than LEO obser_'ed since the beginning of the space era. based on both pressure and temperature factors. Besides Knowledge base has grown from the examination of space environment, a spacecraft's mission life also many returned flight hardware, real-time in-flight determines the degree of a spacecraft's outgassing. surface surveys, observed anomalies via telemetry, Normally a spacecraft may take several weeks before its ground tests, and predictions. Among these high outgassing rate reduces to a moderate degree.

4 American Institute of Aeronautics and .\_tron,,uttc Sometimesaspacecraftmaye_enbeseveralyearsinorbit to absorb water in a humid cn_ironmcnt, cat:sing be(oretheoutgassingtocease.Theretbrelbraspacecraft, expansion. Once m orbit composites release water initialonorbitcheckoutperiodprovidesagood causing the structure to contract. [he released water opportunityforrapidoutgassingtooccurbeforeon-board can reach contamination sensitive surfaces inducing instrumentsbegintooperate. additional damages. To minimize water in composites, humidity control during the manufacturing on the Foraspacecraftwithsingleinstrument,the ground and during on orbit operations is essential. molecularoutgassingisprimarilya self-contamination problem. However, cross-contamination becomes an Due to its high vapor pressure, surface water issue when multiple instruments of different may be departed from surfaces in vacuum. However, contamination sensitivities are closely located on the trapped water t'orms ice on sensitive surfaces and same spacecraft. To avoid cross-contamination, diligent reduces its mobility. Cryodeposits of ice on detectors is effort has been done to provide an optimal spacecraR a common water problem for many IR instruments layout tbr all systems and instruments. operating at cryogenic temperatures. The ice contaminant changes the reflectance and causes a Molecular contamination degrades the transmittance loss problem. Without a proper venting performance of systems by altering their surface for, water contaminant, a repetitive ice formation effect properties. For thermal control system, molecular is highly possible when operating condition cycles. The contamination increases solar absorptance (or) and Filter Wedge Spectrometer (FWS) experiment on changes emittance (_) of Multi-Layer Insulation (MLI) Nimbus IV spacecraft failed shortly after its launch and radiator surfaces. Changes of these surface April 8, 1970. It was determined that the failure was properties require a larger beginning-of-life (BOL) due to the accretion of water. design margin for the thermal control surfaces. When outgassing contaminant is deposited on optical elements, Water cloud around the spacecraft is an issue it reduces the transmittance through lenses, mirrors, too. Once in space, water released from surfaces may windows, and detectors. The contaminant also stay with the spacecraft for several orbits before its degrades optical functions by increasing the light dissipation. For a Space Shuttle mission, most of the scattering along the optical path. contamination in the bay is water with pressure measured as high as 10.4 tom By comparison, the Material outgassing is the most common ambient pressure is approximately 10 .7 ton'. Water spacecraft contamination problem. Results from many cloud around the spacecraft or high water pressure in a ground tests have demonstrated its various effect on bay obscures on orbit observations. thermal and optical properties. Due to their wide application on spacecraft components, hydrocarbon and Molecular Transport silicone contaminants have been identified on almost all short and long duration missions. Water will outgas like In space molecular contamination can move most hydrocarbons and pose similar contamination from one surface to another by direct flux or threat to spacecraft systems. Due to its physical backscattering. The surrounding vacuum and characteristics and the repetitive problems on infrared temperature conditions strongly influence these (IR) instruments, water contamination is worthwhile to transport mechanisms. Molecular transport by direct be discussed separately. flux is a line-of-sight mechanism with a Lambertian distribution for molecules originated from the Water contamination source, To avoid direct line-of-sight impingement, sensitive surfaces on a spacecraft are Unlike more strongly bonded hydrocarbons designed to point away from the contamination sources. and silicones, water is weakly adsorbed on surfaces and Under certain circumstances, protective devices may be releases readily under reduced pressure. Most installed for shielding purpose. Due to its direct spacecraft structure and ML[ surfaces contain water. impact, undoubtedly molecular transport by direct flux Even with its high vapor pressure, water may still pose can become a problem. On a returned SMR,M a potential problem because of its abundance on hardware, a single impact has penetrated three layers of spacecraft surfaces. Due to its weight/strength MLI. Chemical analysis indicated that contaminants advantage, composites are increasingly utilized in were consistent _ith urine residue from the Space spacecraft. Composites are continuous polymeric Shuttle waste management system. In addition, materials containing a fibrous or particulate filler. molecular contamination can reach non line-of-sight Therefore, composites outgas similarly to polymers in locations by multiple reflections resuhing from repeated the space environment. Furthermore, composites tend deposition and reem_ssion. For a closed system such as

5 American Institute of Aeronautics and .\stronatut_c the interior o fan instrument, molecular transport by low altitude the deBye lengths are recast, red in multiple reflections may become more important than millimeters so that the ionized molecules have less bv direct flux. opportunity to be reattracted to the spacecraft by the resultant electrostatic field around the spacecraft. At Backscattering or return flux is a mechanism high altitudes the ionosphere is much less dense with that molecules emit from and return back to the the deBye lengths in meters. Therefore, contaminants spacecraft surfaces by self or ambient scattering. Self have a lower probability of escaping the electrostatic scattering occurs when molecules departed from the attraction before being ionized. spacecraft collide with each other and return to the spacecraft. On the other hand, ambient scattering refers This space envirormlent induced to the collisions between spacecraft released molecules backscattering molecular transport effect tal is and ambient molecules. Molecular transport by sometimes called Electrostatic Return (ESD). The backscattering depends heavily on the surface released charging environment and its associated ESD effect in molecules and the molecular density around the GEO can be more severe than in LEO because the spacecraft. Therefore, backscattering is especially plasma is very energetic. The Spacecraft Charging at important when the spacecraft operates in a high High Altitudes (SCATHA), a GEO spacecraft, had ambient density environment that increases the reported that nearly 3 1% of the deposited probability of molecular collisions. In addition, the contamination was associated with the spacecraft backscattering effect is enhanced as a Ram effect when charging. the outgassing occurs in the leading-edge of the spacecraft, i.e. in the velocity vector. Field-of-View Obscuration

Thruster operations also contribute to an Molecular contaminants released from the increase in density in the backflow region. To prevent spacecraft remain within the orbit of the spacecraft. the thruster plume contamination, spacecraft surfaces Material outgasasing, venting, leakage, and plume are designed to avoid direct plume impingement or products are major contamination contributors. protected by a plume shield if necessary. Because of Depending on conditions, these contaminants may the high velocity of the plume, the effluent tends to be remain in a spacecraft's orbit for extended period. confined to a narrow region around the thruster exit. While travelling with the spacecraft, molecular Regardless of relatively small quantity, the possibility contaminants form a cloud around the spacecraft and of scattering into the be.ckflow region by the thruster greatly increase the local ambient density. A plume is still a concern for sensitive surfaces. For contamination cloud above the instrument surface example, a monitoring package on NOAA-7 spacecraft increases number column density (NCD) along any has shown that effluent from an apogee kick motor intended line-of-sight and affects any instrument's field- (AKM) flowed into the area of sensitive spacecraft of-view observation. This will drastically reduce the components and could have degraded their measured signal and diminish the effectiveness of the performance. instrument. An example of the operation of a star tracker interfered by particle clouds was visually observed on Electrostatic Return Skylab. The persistence of a long-term cloud was also indicated on NOAA-7 spacecraft. Molecular contamination is often considered as transport by direct flux or backscattering only. The AO Erosion reattraction of molecular contaminants onto spacecraft under the influence of spacecraft charging is an unusual AO is abundant in LEO environment at phenomenon. The transport by reattraction requires altitude from 200 to 650 km. Energetic AO reacts with both spacecraft surfaces and molecules to be charged. spacecraft surface materials by either erosion or fixing. Spacecraft charging is a variation of electrostatic Residual atmosphere of AO erodes hydrocarbons on potential on the spacecraft surface. Spacecraft surfaces spacecraft surfaces and develop a characteristic surface may be ionized by solar UV to create electrostatic fields morphology. A surface morphology such as AO within a few deBye lengths of the spacecraft. undercutting was obse_'ed in samples on early Space Shuttle missions. If ot, tgassed become ionized in the region close to the spacecraft, then the ionized contaminants The space environment is particularly harsh on can be pulled by spacecraft surface charging the solar arrays and MLI due to their large size, the distributions. Reattraction of the ionized contaminants constant exposure, and the nature of the materials. By depends on the plasmas shielding of the spacecraft. At attacking the polymers used in construction, AO causes

6 American Instttute of Aeronautic,; and Astroraautic defctenous effects on solar arrays and MLI. Kapton, a of 350 km for 3.5 ycarr;, The AO flucncc for TRMM in polyimide film, used for construction is one of the the velocity vector has been calculated to be 8.9 x I0": polymers highly susceptible to chain scission by AO AO atoms, era-' This lluence is about I0 times higher _,_th an AO erosion rate, or reaction efficiency, of than the AO impingement on LDEF's leading-edge. In 30 x 10:_ cm_;atom. this environment, unprotected thermal blanket surfaces of Kapton will virtually disintegrate. In order to endure In addition to the solar arrays and MLI, AO under this severe AO environment, a protective environment in LEO attacks the composite materials. composite coating was developed for the TRMM Carbon/epoxy and Kaptorv'epoxy samples flown on spacecraft. Space Shuttle missions showed extensive erosion of both the matrix and filler. Graphite and Kapton fibers AO Fixing originally covered by epoxy were eroded into a corduroy pattern. The fibers themselves appeared as The AO reacts with spacecraft surface porous ridges with little apparent residual strength or materials in _'o distinct ways. Depending on the stiffness. Since composites will be used extensively in reaction efficiency and the AO fluence, hydrocarbon the future, the long-term effects of AO must be materials can be eroded by AO to a varying degrees. examined in order to prevent surface degradation and Colatrarily silicone materials used by a spacecraft are structural failure. The most feasible method of permanently fixed by the oxidation with AO. AO protection available is coating the exposed composite fixing produces a protective coating on the spacecraft surface with resistant materials. Selected metal foils, surfaces. like aluminum for example, are an obvious choice due to their impermeable oxide coatings. Room temperature vulcanizing (RTV) silicone adhesives and sealants are designed for a wide range of AO may cause dimensional changes in either sealing, bonding, and protection of spacecraft the surface or the bulk of some materials. A significant applications. Methyl silicones, methyl phenyl silicones, oxidation with silver with the formation of an oxide can and fluorinated silicones are most frequently used. In expand and does not remain adherent with the space these silicone materials outgas in response to unreacted silver. The reaction with silicones to form vacuum and temperature conditions similar to silicone oxides causes contraction and cracking. These hydrocarbon materials. AO attacks outgassed silicones AO effects were observed on SMRM returned that transport to spacecraft surfaces. hardware. The reaction between the silicones and AO AO erosion of a material is proportional to its generates silicone oxide (SiOx). The strong surface exposure to a total AO fluence. AO fluence is def'med adhesion of SiOx results from strong covalent as a product of AO density and exposure time. bondings. Heaters, commonly applied for on orbit Therefore AO erosion is notable for a spacecraft with a decontamination, can not easily remove these fixed long contact with AO environment or a spacecraft silicones. As a result, AO fixed contaminants have exposed to a high AO density. When studying AO detrimental effects on both optics and the thermal erosion, ISS is a primary example with a 15-year design control system by totally altering their optical and lifetime in LEO orbit. The combination equates to a thermal properties. In many spacecraft applications, the total fluence of 5.5 x 10'2 AO atoms/era z for ISS. usage of silicone materials has been restricted.

The 69-month LDEF mission resulted in a Surface Damage by Environment long space exposure of material surfaces. During the mission, the leading-edge materials were exposed to Spacecraft surface damage by the space approximately 8.8 x 10-'j AO atoms/era z, a level at environment can be found by examining almost all of which erosion of over 10 mils was expected for many the hardware exposed in space. Most space exposed polymers Isl. The trailing-edge exposure was only Kapton was drastically different from the original about 10"_AO atoms/cm', making AO effects Kapton. The space exposed Kapton was dull, rather insignificant compared to solar UV and charged than shiny or glossy. Surface damages by space particles. A 1 rail of erosion observed on LDEF was environment are discovered on returned HST, Spartan, reported due to the combined effects of vacuum and SMRM hardware. Returned hardware from the ultraviolet (VUV) and AO environment [61. LDEF showed brownish discoloration near uncontrolled venting areas. In general the discoloration on thermal The AO environment was tbund to be even control and other spacecraft surfaces are the most more severe for the TRMM, which requires an altitude common surthce damage by space environment.

7 American Institute of Aeronauncs anti -\stronautlc Surl:ace discoloration caused primarily by UV causing various reactions. The reaction, eithcr by radiation, increases solar absorptance of thermal control crosstmking or chain scission, will degrade the surfaces. Under some severe space environments, polymers. The UV radiation causes the same chemical Kapton on MLI surface can him almost black after changes as its higher energy counterparts. Instead of several years in orbit. SI3G white paints are affected ionizing the atoms through electron removal, the most severely by UV radiation and charged particles electrons are excited into higher energy state by UV. (protons and electrons). Its solar absorptance degraded The unstable, excited atoms undergo subsequent from 0.20 to 0.70 in justa few years in geosynchrous reactions to form free radicals. Further reaction with orbit. High absorptance black paints generally do not the presence of free radicals can cause polymerization degrade. A slight reduction in solar absorptance of to occur. black paints over time has been observed as a result of UV bleaching. The thermal control surfaces is prone to photopolymerization due to its exposure to space Solar cells converts sunlight into electricity. The environment. The thermal control surfaces are affected solar cell's glass cover provides environmental and in orbit by the charged particles, UV radiation, high radiation protection for the solar array. Once the vacuum, and the contaminant films that deposit on molecular contamination is deposited on the cover glass, alr'post all spacecraft surfaces. By forming a darkened it is darkened by UV radiation. The darkened molecular film on radiator surfaces, phtopolymerization will film reduces power output by reducing transmittance in increase the solar absorptance significantly and affect the power generating wavelength range and by increasing the radiator's performance. Other photopolyrnerization solar cell temperatures, A spacecraft in GEO for a 7- example is the degradation of aluminized or silvered year mission has been reported with its solar cell Teflon films caused by both charged-particle damage and performance degraded by as much as 25%. To contamination deposition. minimize UV damage, a UV filter coating may be applied to the underside of the cell to deflect the UV radiation. The direct UV radiation causes photopolymerization. However, indirect UV radiation Besides the thermal control and the power may also have a similar effect on contaminants. From a systems, the UV radiation causes damage on optics. returned HST hardware, 500 ,_ of polymerized The UV energized particles discolor optical thin film contamination was found on the pickoffmirror. The and the bulk transmission mater/Ms and thus modify the reflectance of the pickoffmirror was decreased from broadband transmittance. The UV radiation also 0.7 to 0.05 at 1216 ,_ wavelength as measured by an interacts with some optical materials to cause reduction EUV scatterometer. In orbit this plckoffmJrror was not in reflectance and transmittance. In 1975, two exposed to direct UV radiation, but rather to an indirect instruments aboard the Orbiting Solar Observatory radiation source. After a material study, albedo was (OSO) failed due to the complete loss of sensitivity in concluded to be the reason for such a the 1216 A Lyman-Alpha wavelength caused by the photopolymerization. The Earth albedo, a fraction of LrV degradation of optics from electronics box solar flux incident on the Earth surface, is reflected to outgassing. the pickoff mirror surface and reacts with contaminants. The molecular contamination can have a devastating Photopolymerization. effect on flae reflectance of UV optics, since most molecular contaminants are very absorptive in the UV Radiation damage caused by the high energy range. particles or UV has damaging effects on spacecraft polymeric materials. The UV and the charged panicles Micrometeorites and Debris Impact can act independently or simultaneously, causing the contaminants to form a polymerized film. The degree Orbital debris in the LEO presents a potential of polymerization depends on the UV and the charged hazard to both the spacecraft and its payloads. The orbital particles exposure, the type of the condensed debris consists of either natural meteorites or man-made contaminants, and the conditions of the surfaces. debris from such sources as propulsion stage explosions in space or spent stages. The debris impacts on high- Through an ionization process, the high- pressure fluid tanks and thermal control fluid lines can energy gamma rays, X-rays, electrons, and the protons penetrate and cause a leaking contamination source. can break molecular bonds to from free radicals. A free Collisions with large space debris can be characterized as radical is a chemical radical or molecular fragment highly energetic events, which will likely generate containing an unpaired electron. It is longer lived than millions of panicles posing even greater hazards to ions and can migrate throughout a polymer structure operational spacecrat't. Besides affecting operations,

8 American Institute of Aeronautics and .-\stronaut_c particles generated by the impact create an obstructive a surface and cause an increase in the level of stray particle cloud in the field-of view of observation. light scattering. Light scattered into the regions of image plane can seriously affect the performance of The micrometeorites and debris impact on some detector systems. High velocity particulate, operational spacecraft is apparent and substantial. A which impact surfaces directly, can diminish both their NASA Shuttle window with substrate fracturing was throughput and their ability to image. Indirect impacts discovered m 1983 to have been caused by a of high velocity micrometeorites or particles may also subrrulhmeter particle. From returned SMRM hardware create a problem by causing small pieces of spacecraft in 1984, a survey of approximately one-half square material to break loose from the surface. These meter of ML[ has revealed over 1,500 impacts. A particles may then either adhere to a critical surface or number of particles completely penetrated all of the drift through the sensor's field-of-view. MLI layers. Particulate impacts were also observed on the LDEF returned hardware surfaces. LDEF used a Particles in space brings m a new problem. A specifically designed altanmi.trn plate to measure the wide released particle can take on an orbit with a trajectory spectrum of impacts. During the recent Midcourse returning to the vicinity of the spacecraft. The particles Space Experiment (MSX) mission, two discrete particle can also become charged and be drawn back to a releases caused by micrometeorites or debris impacts charged spacecraft by the ESD effect. In addition, were observed [71. Orbital debris is a growing concern, particle clouds, similar to molecular clouds, in the field- which may necessitate changes in overall system of-view of observation can greatly impair the decisions involving larger safety factors, protective performance of sensors. surfaces, and different orientations for the solar arrays. svn_.r._n___s_ Particulate Contamination Sources A synergy is a simultaneous action of separate There are two major particulate contamination forces to achieve an effect of which each is individually sources for an operational spacecraft. One is particles incapable. The synergistic contamination effect created accumulated on spacecraft surfaces during ground by the space environment is twofold. A synergy can be operations. The other is particles induced by the space produced by an individual space environment element environment when in orbit. Prior to launch, surface influencing each step of the contamination generation, particles are controlled to meet acceptable levels for a transport, deposition, and degradation. As a result, a normal spacecraft. Per MIL-STD-1246, the cleanliness specific contamination effect is induced by a single requirements could _ange from levels 100-400 for space environment element. On the other hand, a sensitive surfaces and levels 300-1,000 for less synergy can also be generated by multiple space sensitive surfaces. These deposited particles may be environment elements act together to create an redistributed by extemal forces imposed on them during enhanced contamination effect. orbital operations. In addition, solar heating, thermal cycling, AO erosion, and radiation damage can generate One obvious example of synergy is material new particles. Regardless of the particle sources, outgassing in space. Outgassing is strongly affected by particles released in orbit may interfere with operations the spacecraft's ambient conditions. Therefore, the of space systems. combination of vacuum, temperatures, and the duration of space environment exposure determines actual Particle build-up during ground operations is outgassing rates. very common. For the Geostationary Operational Environmental Satellite (GOES), particles were The discoloration of the thermal control accumulated on the radiant cooler surfaces prior to surfaces is the result of the combined effect of vacuum, launch. The particle accretion will scatter the light and UV, charged particles, and contamination. A darkened allow solar radiation to reach the instrument cooler. surface with an increase in solar absorptance affects the Ultimately this contamination effect will degrade the performance of a thermal control system. Other performance of the cooler tSl. A consequential cleaning combined effect of radiation and thermal cycling was performed before the launch using CO,. cleaning induces thermal stresses on the structure and the MLI technique in 1995. Despite all controls and cleanings, surfaces. After long-tem_ space environment exposure, some particles still cling on the launched spacecraft. the damages on these surfaces further generate particulate contamination. In general a spacecraft Particle contamination degrades the continues to generate particles on orbit due to the performance of space systems by a number of effects of material degradation by UV, radiation, AO. mechanisms. Low velocity particles can accumulate on thermal cycling, and the impact from micrometeorites

9 ,,kmcrtcan Institute of Aeronautlcs and Astronautic ordebris.SubstantialmaterialdegradationonHST both particulate and molecular contamination. Light thermalcontrolsurfaceswasobservedduringthein- scattering, obscuration, and throughput change caused flightsurfacesurvey.Thesedamagedsurfaceswere by contamination buildups are major reasons for the repairedduringtheHSTsen'icemissions. degradation of optics. In addition, many other spacecraft systems are affected by the space AOreactswithallpolymericmaterialscausing environment to a certain degree. variousdegreesofsurfaceerosion.Othercontributing factorssuchasdamagecausedbyradiationandorbital The materials that significantly degraded debrisincreasetheerosionrate.Additional provided the opportunity to study the space impingementeffectduetospacecraftorientationisalso environment and material interaction. One single space possible.Withthesesynergisticfactors,aholeonan environment element causes various degree of effect on oxidecoatingcanexposeanon-oxidesubstratetoAO spacecraft systems. The synergistic effect is an attacks.Thepresenttheoryisthatmaterialserodeata enhancement of multiple space environment elements, fasterratebyAOwhensimdltaneouslyexposedto which may aggravate the contamination effect. Post ultravioletandelectronorprotonradiation.One flight materials investigations performed on SMRM, exampleindicatedthatthesimultaneousexposureto LDEF, Space Shuttles, MIR, and HST are of great AOandelectronst91increasedtherateoferosionof inte4"est. Study results help spacecraft engineers in polysulfone.OneadditionalsynergyisthelowTeflon gaining the knowledge of the contamination effects (purefluorocarbon)reactivitywiththepresenceofAO. enhanced by the space environment interactions. TheC-FandC-CbondsinTeflonappearinerttoAO. However,it hasbeenfoundthatexposuretothe Most of the contaminants are self-generated by combinationofAOandUVwill causeamoresevere the spacecraft and its systems. By eliminating or TeflonreactionthanAOalone. reducing sources, problems can generally be minimized. Careful selection of materials, especially Photopolymerizationisachemicalreactionof low outgassing materials, and proper system designs are molecularcontaminantsinthepresenceofenergy critical while building a spacecraft. Specific spacecraft sources.EithertheUV,highenergyparticles,orthe requirements including mission lifetime, charging Earthalbedocreatesfreeradicalsforthereactionto restrictions, thermal conditions, optics throughput, occur.Thesynergyoftheseenergysourcesenhances instrument sensitivity, and cleanliness levels must be thephotopolymerizationprocess.Thermalcontroland considered. Long-term space environment conditions opticssystemsareimpactedmostbythe should be taken into account in light of systems' end- photopolymerizationresult.Oncepolymerizationis of-life (EOL) performance. After the deployment of formedon the surfaces, the removal of the the spacecraft, additional applicable contamination contaminants to restore systems' original performance mitigation methods L_01can be taken. Among them, de- is difficult. contamination and operational control are most effective in mitigating contamination effects involving In GEO, the charge buildup on the spacecraft spacecraft operations. attracts charged contaminants to sensitive surfaces by the ESD effect. This contamination, in turn, can alter REFERENCES the properties of the spacecraft surface by making a conducting surface less conductive. The change in 1. Purvis, C. K., "Overview of Environmental Factors", charging characteristics is a synergism of the plasma NASA/SDIO Space Environmental Effects on and the neutral environment. Materials Workshop, NASA Conference Publication 3035, NASA Langley Research Center, Hampton, VA, CONCLUSIONS June 28-July 1. 1988.

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10 American Institute of Aeronautzcs and Astronautic 4.Cauffman,D, "IonizationandAttractionof Neutral MoleculestoaChargedSpacecraft",ReportSD-TR-80- 78,TheAerospaceCorporation,1978. 5.Bourassa,R.etal.."LDEFAtomicOxygenFluence Update",MaterialsWorkshop"91,NASAConference Publication3162,November19-22,1991. 6. Hemminger,C.S.,etal.,"SpaceEnvironmental EffectsonSilveredTeflon Thermal Control Coatings," LDEF-69 Months in Space First Post-Retrieval Symposium, NASA Conference Publication 3134, 1992.

7. Galica, G., et al., "Long-term Observations of the Particle Environment Surrounding the MSX Spacecraft", Proceedings of 2000 SPIE International Symposium, Optical Systems Contamination and Degradation, San Diego, CA, August 2-3, 2000.

8. Chen, P., et al., "Contamination Effects on the Geostationary Operational Environmental Satellite Instrument Thermal Control System", Journal of Thermophysics and Heat Transfer, page 467-471, volume 11, Number 3, July-September 1997.

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I0. Chen, P., "In-flight Space Environments Related Operational Contamination Concerns and Mitigation Methods", 1997 AIAA Defense & Space Programs Conference & Exhibit, Huntsville, AL, September 23- 25 1997.

I1 American Institute of Aeronautics and Astronautic