Bruce A. Banks and Katelyn T. Seroka

Alphaport, Inc., Cleveland, Ohio

Jason B. McPhate

University of California, Berkeley, California

Sharon K. Miller

Glenn Research Center, Cleveland, Ohio

Attenuation of Scattered Thermal Energy

Atomic Oxygen

NASA/TM—2011-217028

April 2011
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Bruce A. Banks and Katelyn T. Seroka

Alphaport, Inc., Cleveland, Ohio

Jason B. McPhate

University of California, Berkeley, California

Sharon K. Miller

Glenn Research Center, Cleveland, Ohio

Attenuation of Scattered Thermal Energy

Atomic Oxygen

NASA/TM—2011-217028

April 2011

National Aeronautics and

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Glenn Research Center

Cleveland, Ohio 44135

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NASA/TM—2011-217028 1 

Attenuation of Scattered Thermal Energy Atomic Oxygen 

Bruce A. Banks and Katelyn T. Seroka 

Alphaport, Inc. 

Cleveland, Ohio 44135 

Jason B. McPhate 

University of California 

Berkeley, California 94720 

Sharon K. Miller 

National Aeronautics and Space Administration 

Glenn Research Center 

Cleveland, Ohio 44135 

Abstract 

The attenuation of scattered thermal energy atomic oxygen is relevant to the potential damage that 

can occur within a spacecraft which sweeps through atomic oxygen in low Earth orbit (LEO). Although 

there can be significant oxidation and resulting degradation of polymers and some metals on the external 

surfaces of spacecraft, there are often openings on a spacecraft such as telescope apertures, vents, and 

microwave cavities that can allow atomic oxygen to enter and scatter internally to the spacecraft. Atomic 

oxygen that enters a spacecraft can thermally accommodate and scatter to ultimately react or recombine 

on surfaces. The atomic oxygen that does enter a spacecraft can be scavenged by use of high erosion yield 

polymers to reduce its reaction on critical surfaces and materials. Polyoxymethylene and polyethylene can 

be used as effective atomic oxygen scavenging polymers. 

Introduction 

Spacecraft in low Earth orbit (LEO) sweep through atomic oxygen resulting in hyperthermal 

(~4.5 eV) atomic oxygen attack on ram facing surfaces. Although there can be significant oxidation and 

resulting degradation of polymers and some metals on the external surfaces of spacecraft, there can also 

be damage to materials and sensitive surfaces within a spacecraft if the atomic oxygen is allowed to enter. 

Atomic oxygen can enter spacecraft through openings such as telescope apertures, vents, and microwave 

cavities. The atomic oxygen that does enter will either react with organic or other reactive surfaces, 

reassociate to become chemically inactive diatomic oxygen, or lose its hyperthermal energy as it scatters 

off multiple surfaces and becomes thermally accommodated atomic oxygen (Refs. 1 to 3). This thermally 

accommodated atomic oxygen is still fully capable of reacting with materials even though its reaction 

probability is greatly reduced compared to hyperthermal atomic oxygen. Sensitive optical filters, optical 

sensors that operate on photoelectric effect principals, or microwave cavities can be compromised by the 

presence of a fluence of thermal energy atomic oxygen. 

The objective of this investigation is to compare the degree to which scattered thermal energy atomic 

oxygen is attenuated by the material it scatters from. This is relevant to considerations of what materials 

may be best to have within a spacecraft to scavenge the atomic oxygen and to reduce its damage to 

sensitive materials and surfaces. 
NASA/TM—2011-217028 2 

Apparatus and Procedure 

The experiments were conducted in radio frequency plasma ashers operating on air (Ref. 4). 

Kapton H polyimide witness samples were used to measure the flux of atomic oxygen inside and outside 

of scattering chambers. Drawings and photos of the three types of scattering chambers used include: an 

open-ended chamber (Fig. 1), a cylindrical Faraday cage chamber (Fig. 2), and two rectangular Faraday 

cage chambers (Fig. 3). In the first two scattering chamber configurations, the walls were lined with 

various test materials to assess the impact of the material’s interaction with atomic oxygen. The atomic 

oxygen Kapton H effective flux within the scattering chambers were measured by means of mass loss of 

the Kapton H witness sample at the bottom of the chambers. Kapton H effective flux in a thermal energy 

plasma asher is the hyperthermal flux in LEO that produces the same rate of erosion of Kapton H. A 

comparison of flux inside-to-outside scattering chambers was made by placing 2.54 cm diameter by 

0.0127 cm thick Kapton H witness samples inside and outside the chambers and comparing weight loss 

per area. Outside the scattering chambers 1.905 cm diameter Kapton H witness samples were used to 

measure the ratio of inside-to-outside effective flux. The open-ended chamber typically used 0.05 mm 

thick sample materials to line the cylinders that were 5.65 cm long by 2.61 cm inside diameter. The 

Faraday cage chamber used the same set of materials but was 1.56 cm long by 2.61 cm inside diameter. 

The rectangular Faraday cage was used to assess the impact of various size inner lid openings on the 

mass loss of a Kapton H sample inside the cage. 

The open-ended chamber (Fig. 1) allowed a greater flux of atomic oxygen, ions, electrons and UV 

radiation to enter the scattering chamber than the Faraday cage chamber (Fig. 2). The Faraday cage 

chamber prevented line of sight entrance of atomic oxygen, ions or electrons. To accomplish this, the 

Faraday cage utilized three aluminum half-circles as shown in Figure 2. 
NASA/TM—2011-217028 3 

The rectangular Faraday cages were used to provide information on the attenuation of atomic oxygen 

due to varying the size of the opening into the cage as well as the fractional area of Kapton within the 

Faraday cage. The outer lid of the cage in Figure 3(a) and (b) measured approximately 15.2 by 11.4 by 

2.1 cm high. This lid is shown upside down on the right side of Figure 3(b) and fit over the inner chamber 

which is shown on the left side of Figure 3(b). The inner chamber measured approximately 11.7 by 7.5 by 

2.5 cm high and had four wires attached to it to hold the lid 0.7 cm above the chamber. A representative 

opening into the Faraday cage shown in Figure 3(a) and (b) can be seen as the small hole through the 

aluminum foil on the left side of the chamber. This opening was varied in size and in some cases two 

openings were used. The Faraday cage shown in Figure 3(c) and (d) was a simple hinged overlapping box 

that allowed covering the inner surface with varying amounts of Kapton to assess the attenuation of 

atomic oxygen due to fractional coverage of Kapton. The lids of both Faraday cages overlapped the lower 

chambers shown in Figure 3 thus preventing line of sight of plasma into the cages. Samples of 2.54 cm 

diameter Kapton H were placed on top of the lid and inside the chamber to assess the inside-to-outside 

mass loss ratio. 
NASA/TM—2011-217028 4 

 
NASA/TM—2011-217028 5 

The Kapton H witness samples were vacuum dehydrated for at least 48 hr prior to all weight loss 

measurements in accordance with ASTM-E 2089-00, “Standard Practices for Ground Laboratory Atomic 

Oxygen Interaction Evaluation of Materials for Space Applications,”(Ref. 4). The mass of the Kapton H 

witness samples was measured by averaging three mass measurements made within 5 min after removal 

from vacuum desiccation using a 10 μg sensitivity Sartorius balance. 

To assess the effects of various materials on the attenuation of the thermal energy atomic oxygen flux 

in the scattering chambers, linings of various metals, metal oxides, polymers, and graphite were used on 

the vertical sides of the chambers. The ratio of Kapton H effective flux, inside-to-outside, was measured 

by comparing the Kapton mass loss and correcting for area differences (in the case of open-ended and 

cylindrical Faraday cages). As can be seen by the Figures 1 to 3, the geometries of the scattering 

chambers were sufficiently different that direct comparison of atomic oxygen attenuation for any specific 

material cannot be made between the types of chambers. However, relative comparison of the differences 

from material to material can be made in terms of which sidewall causes the greatest attenuation in atomic 

oxygen for each configuration. The sidewall materials chosen for evaluation were: aluminum, copper, 

300 series stainless steel, gold, silicon dioxide coated Kapton H, flexible graphite made from pure, natural 

graphite flake (GRAFOIL), polyethylene, fluorinated propylene ethylene (FEP) Teflon, 

polyoxymethylene (POM), and chlorotrifluoroethylene (CTFE). 

Thermal energy isotropic atomic oxygen behaves as a reactive gas where its partial pressure in a 

chamber determines the erosion rate of the polymer inside the chamber. The difference between the 

erosion rate inside the chamber and outside the chamber is influenced by the entry aperture area and the 

atomic oxygen partial pressure losses due to reaction with the wall and recombination within the chamber. 

Results and Discussion 

The flux in the asher was ~51015 atoms/(cm2 sec) and the durations of exposures were typically 

~24 to 68 hr. Thus, the fluence exposed to the outside Kapton H samples was at least ~3.41020 atoms/(cm2

). 

The ratio of effective flux outside-to-inside for the open-ended scattering chamber is shown in 

Figure 4. 

As can be seen, in general, the chamber lining materials that do not react with atomic oxygen tend to 

cause less attenuation of the atomic oxygen thus allowing a higher ratio of mass loss inside the scattering 

to outside the chambers. Copper reacts with atomic oxygen to form a nonprotecting brown Cu2O copper 

oxide (Ref. 5) that was found to spall from smooth copper surfaces. Thus, the copper lining of the open 

scattering chamber tended to reduce the amount of atomic oxygen mass loss. To prevent copper oxide 

from spalling, the copper surfaces were roughened by cross scratching the surface with rough sandpaper. 

A comparison can be made between the above experimental results for SiO2 coated Kapton and a 

Monte Carlo computational model prediction (Ref. 3) with SiO2 walls. The Monte Carlo model is for a 

similar geometry but has sweeping or fixed arrival (at 45 from normal) of hyperthermal LEO ram atomic 

oxygen rather than the isotropic thermal energy plasma of an asher environment. Slight differences exist 

in the geometry in that the Monte Carlo model assumes a square cross section well and the thermal energy 

plasma experiment uses a cylindrical scattering chamber. Figure 5 compares the ratio of Kapton H 

effective flux at the bottom of both scattering chambers to that outside the chambers.

As can be seen in Figure 5, the sweeping hyperthermal atomic oxygen in the Monte Carlo model 

predicts a higher erosion rate of Kapton H at the bottom of the scattering chamber than for the fixed 45

arrival. This may be due to the fact that for a portion of the sweep angles, hyperthermal atomic oxygen 

can directly erode the Kapton H at the bottom of the chamber. For the fixed 45 arrival only scattered, 

partial, or fully thermally accommodated atomic oxygen can react at the bottom. Because the reaction 

probability is much lower for the thermal energy than hyperthermal atomic oxygen, the erosion rate of the 

Kapton H at the bottom will also be lower. The experimental results for thermal energy isotropic plasmas 

indicate even more attenuation than for both Monte Carlo predictions involving hyperthermal atoms but 

reasonably close to that of the 45 LEO atoms. 
NASA/TM—2011-217028 6 

 

 
NASA/TM—2011-217028 7 

The results of the Faraday cage scattering chamber are shown in Figure 6. 

Although the geometries are quite different between the open and the Faraday scattering chambers, a 

comparison of ranking between materials indicates only minor differences in atomic oxygen attenuation. 

Again as in the open scattering chamber case, the chamber lining materials that do not react with atomic 

oxygen tend to prevent atomic oxygen attenuation. This leads to higher mass loss of the Kapton inside 

relative to outside the chambers. However, the best polymers, of those tested, to have in the Faraday cage 

to attenuate atomic oxygen are POM and polyethylene. Graphite from GRAFOIL is thought to be too 

friable to be safely used inside spacecraft. 

It is also interesting to compare the amount of attenuation in atomic oxygen flux inside the Faraday 

cage as a function of the erosion yield measured in an asher environment. This was done assuming that 

the erosion yield for Kapton H is 310–24 cm3

/atom (Refs. 6 and 7). Figure 7 is a plot of ratio of Kapton H 

effective flux inside a Faraday cage to outside the Faraday cage for various cage-lining materials as a 

function of the effective atomic oxygen erosion yield in an asher environment. 

As can be seen, in general, those materials that have a high erosion yield in the thermal energy plasma 

environment tend to cause less erosion of the Kapton H witness samples within the Faraday cages. This is 

probably due to a reduction in the atomic oxygen density in the Faraday cage caused by greater reaction 

with the sidewall materials. 

It is interesting to see the effect of the atomic oxygen entry area into the Faraday cage upon the 

erosion rate within the cage. The number density of atomic oxygen within a Faraday cage is dependent on 

flux impinging upon the cage, the area for the atomic oxygen entrance into the cage, and the losses within 

the cage. The losses include reaction and recombination with the wall materials and Kapton H sample at 

the base of the cage. Thus, the greater the area and erosion yield of materials that react with atomic 

oxygen, the more scavenging of atomic oxygen will occur. This will reduce the mass loss of Kapton H 

within the chamber relative to outside the chamber. If there is relatively little reaction with the walls of 

the chamber then the flux inside the chamber will be close to that outside the chamber. Only when the 

aperture into the chamber is sufficiently small will the losses within the chamber become significant 

enough to start showing reductions in polymer erosion relative to outside the chamber. Figure 8 compares 

the ratio of mass loss inside-to-outside Faraday cages as a function of the fractional area of the entrance 

aperture into the inside chamber plotted on a semi-log format. 
NASA/TM—2011-217028 8 

The data appears to have a simple power law dependence upon aperture area ratio. The ratio of mass 

loss of Kapton H inside-to-outside the Faraday cage is related to the ratio of aperture area to maximum 

inner lid area raised to the 0.268 power (shown as the curve fit line in Fig. 8). This results in a correlation 

coefficient of 0.95. The reason the ratio of mass loss inside-to-outside is not one for an inner lid fractional 

area of one is because there is also attenuation of the flux as the atomic oxygen travels between the outer 

lid and the outside of the inner lid. As can be seen from Figure 8, an aperture of only 0.1 percent of the 

inner lid area still produces a flux within the chamber that is ~10 percent of that outside the Faraday cage. 

This implies that a small aperture on a spacecraft, which is oriented in the ram direction, has the potential 

to react a significant fraction of the ram fluence within the spacecraft. Even though the atomic oxygen 
NASA/TM—2011-217028 9 

becomes thermally accommodated resulting in low reaction probabilities (Ref. 3), it can bounce around 

until it reacts with an organic or reactive surface. This can be overcome by using sacrificial polymers 

within the spacecraft to scavenge the atomic oxygen that scatters within it. 

The resulting attenuation of atomic oxygen flux within a Faraday cage, due the scavenging effect of 

varying amounts of Kapton H within the cage, is shown in Figure 9 for the Faraday cage shown in 

Figure 3(c) and (d). This figure shows the dependence of the ratio of inside flux to outside flux upon the 

fraction of surface area inside the Faraday cage that is covered with Kapton H. The profile reasonably 

matches (a correlation coefficient of 0.9) and exponential dependence upon fractional coverage with 

Kapton. The results indicate that to reduce the flux by almost a factor of 5 within the cage, the Kapton 

fractional area coverage needs to be at least 90 percent. Thus, for effective scavenging of trapped atomic 

oxygen it is important to use a significant area coverage of polymers that have high reaction probabilities. 

Summary 

A ground laboratory investigation was conducted to assess the erosion rate of atomic oxygen that 

enters cavities in an effort to simulate telescope, vents, or microwave openings on spacecraft in low Earth 

orbit. To accomplish this, tests were conducted in RF plasma ashers to measure the ratio of mass loss of 

Kapton H polyimide placed inside and outside of three types of scattering chambers: an open-ended 

chamber which allowed a flux of atomic oxygen, ions, electrons and UV radiation to enter, a cylindrical 

Faraday cage chamber in which atomic oxygen could enter but very little ion, electron, or UV radiation, 

and a larger rectangular Faraday cage chamber was used to assess the effect of the atomic oxygen entry 

area into the Faraday cage upon the erosion rate within the cage. 

The results of the investigation indicate that even small openings which allow ram atomic oxygen to 

enter a spacecraft can cause considerable reaction within the spacecraft to sensitive surfaces. This is in 

spite of the fact that the atomic oxygen thermally accommodates to have low reaction probabilities. The 

use of polymers within a spacecraft can contribute to scavenging of the atomic oxygen thus significantly 

reducing its flux. Tests were conducted on five metals or metal oxides, five polymers and graphite to 

assess their ability to reduce atomic oxygen flux within spacecraft. The results of the testing indicate that 

metals or metal oxides that do not react with atomic oxygen do little to reduce the flux of atomic oxygen 
NASA/TM—2011-217028 10 

within the spacecraft. However, copper which does not form a protective oxide and does react with 

atomic oxygen and reduces atomic oxygen flux. Polymers that have low atomic oxygen erosion yields 

tend not to scavenge atomic oxygen, whereas polymers that have high atomic oxygen erosion yields tend 

to scavenge atomic oxygen. Thus, use of atomic oxygen scavenging polymers that have high atomic 

oxygen erosion yields are recommended to minimize atomic oxygen attack within spacecraft that have 

openings to the low Earth orbital environment. 

References 

1. Banks, Bruce A., de Groh, Kim K., and Miller, Sharon K., “Low Earth Orbital Atomic Oxygen 

Interactions with Spacecraft Materials,” NASA/TM—2004-213400 and paper presented at the 

Fall 2004 Meeting of the Materials Research Society, Boston Massachusetts, November 29- 

December 3, 2004. 

2. Banks, B.A., Miller, S.K., and de Groh, K., “Low Earth Orbital Atomic Oxygen Interactions with 

Materials,” paper presented at the Second International Energy Conversion Engineering Conference 

sponsored by the American Institute of Aeronautics and Astronautics, AIAA–2004–5638, NASA/ 

TM—2004-213223, Providence, Rhode Island, August 16–19, 2004. 

3. Banks, B., Miller, S., de Groh, K., and Demko, R., “Scattered Atomic Oxygen Effects on Spacecraft 

Materials,” NASA/TM—2003-212484, June 2003 and Proceedings of the 9th International 

Symposium on Materials in a Space Environment, Noordwijk, The Netherlands, June 16–20, 2003, 

ESA SP-540, September 2003. 

4. ASTM E 2089-00, “Standard Practices for Ground Laboratory Atomic Oxygen Interaction Evaluation 

of Materials for Space Applications,” June 2000. 

5. Silverman, E., “Space Environmental Effects on Spacecraft LEO Materials Selection Guide,” NASA 

Contractor Report 4661, August 1995, section 6.2.2. 

6. de Groh, K.K., Banks, B.A., McCarthy, C.E., Rucker, R.N., Roberts, L.M., and Berger, L.A., 

“MISSE 2 PEACE Polymers Atomic Oxygen Erosion Experiment on the International Space 

Station,” Journal of High Performance Polymers, vol. 20, no. 4/5, August/October 2008. 

7. Stambler, A.H., Inoshita, K.E., Roberts, L.M., Barbagallo, C.E., de Groh, K.K., Banks, B.A., 

“Ground-Laboratory to In-Space Atomic Oxygen Correlation for the PEACE Polymers,” 

Proceedings of the 9th International Conference on Protection of Materials and Structures From 

Space Environment, Toronto, Canada May 20–23, 2008. 
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Attenuation of Scattered Thermal Energy Atomic Oxygen 

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14. ABSTRACT

The attenuation of scattered thermal energy atomic oxygen is relevant to the potential damage that can occur within a spacecraft which 

sweeps through atomic oxygen in low Earth orbit (LEO). Although there can be significant oxidation and resulting degradation of polymers 

and some metals on the external surfaces of spacecraft, there are often openings on a spacecraft such as telescope apertures, vents, and 

microwave cavities that can allow atomic oxygen to enter and scatter internally to the spacecraft. Atomic oxygen that enters a spacecraft can 

thermally accommodate and scatter to ultimately react or recombine on surfaces. The atomic oxygen that does enter a spacecraft can be 

scavenged by use of high erosion yield polymers to reduce its reaction on critical surfaces and materials. Polyoxymethylene and

polyethylene can be used as effective atomic oxygen scavenging polymers.

15. SUBJECT TERMS

Atomic oxygen; low Earth orbit (LEO); Spacecraft polymer 

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