**6. Conclusion**

each material has a unique spectral fingerprint, it may be unambiguously identified if its

One example of applied optical characterization of materials in space is the assessment of high area-to-mass ratio (HAMR) objects, consisting primarily of spacecraft MLI. Several layers of MLI have comprised of aluminum sputtered polyimide. It has been shown that after electron irradiation the hemispheric reflectance spectrum, the sum of all diffuse and specular reflectance of a surface under diffuse illumination, of this aluminized PI changes considerably. This change in reflectance is due in part to the change in absorbance of the polymer portion of the material. Since the PI is backed by a thin reflective aluminum layer, the light that would otherwise pass through the PI is instead reflected back into the spectrometer. Pristine PI is fairly transparent between 500 and 700 nm the absorption edge at 500 nm is what gives PI its characteristic amber color. The damaged material's new electronic structure results in resonant absorption of lower energy photons, which results in a shift of the absorption band edge to around 730 nm. This phenomenon manifests itself as a darkening of the material in the visible spectrum [15, 49].

Because the change in optical absorption is greater at some wavelengths than others, the ratio of one spectral band to another, known as a color–color plot, can be used to characterize the extent of space plasma exposure experienced by spacecraft material. **Figure 9** shows the evolution of the color–color plots of two different aluminum sputtered PI layers of MLI after various electron exposures. The color–color plots were generated using the passbands associated with the r' and i' filters defined by the Sloane Digital Sky Survey [50–53]. For a more

Conceivably, the magnitude of change of these color–color indices is great enough to enable identification and/or characterization of space debris clouds. Knowledge of the origin of these

**Figure 9.** r'-i' plots for space- and space-craft facing Kapton. The magnitude of change for all materials is comparable.

However, the absolute magnitude of the dose differs due to the material composition.

detailed description of the experiment, refer to reference [13].

debris clouds can be used to facilitate space debris remediation [54, 55].

spectral features are well differentiated.

240 Plasma Science and Technology - Basic Fundamentals and Modern Applications

As the population of man-made objects in space grows ever more rapidly, understanding the interaction of space plasma with commonly used materials becomes only ever more important. Clearly, knowledge of the different facets of this interaction can be used to the advantage of aerospace scientists either as a diagnostic tool or as guidelines that can result in more efficient and robust spacecraft design. Further, detailed knowledge of how space plasma interacts with materials in Earth's orbit will guide the development of next-generation spacecraft materials.

While the information presented in this chapter is not exhaustive, it serves to illustrate the broad variety of chemical and physical changes that can occur when a given material interacts with space-like plasma. The examples presented here were chosen due to the ubiquity of polyimide and solar arrays on existing spacecraft. The simulated GEO environment (high-energy electrons) used to study these interactions is a first-order approximation for the rather more complicated GEO environment. Although electrons are the primary damaging species in GEO in terms of energy deposition, a more representative space weather simulation would include protons and UV photons in order to study synergistic damage processes that occur during the interaction of materials with a more complicated charged particle environment [5].
