**5. Modification of material optical signatures after exposure to a space plasma environment**

In general, it has been found that the space environment fundamentally changes spacecraft materials [15, 44–46]. The nature of particles primarily responsible for the damage is dependent on the orbit; in GEO, high-energy electrons are the primary damaging species in terms of energy deposition into the bulk of the material [5]. This energy deposition breaks chemical bonds within the material, resulting in the creation of new electronic transitions in the material. These new electronic transitions manifest themselves in altered optical and physical properties, such as color change and electrical conductivity.

Over the last decade, astronomical reflectance spectroscopy has been proposed to characterize the material properties of artificial space objects, such as satellites, rocket bodies, and human-made debris [47, 48]. In this application, reflected light is collected from a remote target illuminated by a continuous source, the Sun. The reflected light produces a spectrum whose shape and absorption features are indicative of a specific material composition. Since each material has a unique spectral fingerprint, it may be unambiguously identified if its spectral features are well differentiated.

**6. Conclusion**

spacecraft materials.

particle environment [5].

**Acknowledgements**

**Conflict of interest**

**Author details**

Daniel P. Engelhart<sup>1</sup>

and Ryan C. Hoffmann<sup>2</sup>

Albuquerque, NM, USA

No conflict of interest is declared.

\*, Elena A. Plis<sup>1</sup>

1 Assurance Technology Corporation, Carlisle, MA, USA

\*Address all correspondence to: afrl.rvborgmailbox@us.af.mil

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

Space Plasma Interactions with Spacecraft Materials http://dx.doi.org/10.5772/intechopen.78306 241

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

We would like to acknowledge support from the Air Force Office of Scientific Research, Remote Sensing and Imaging Physics Portfolio (Dr. Stacy Williams). Grant 17RVCOR414.

, Dale Ferguson<sup>2</sup>

2 Air Force Research Laboratory, Space Vehicles Directorate, USA, Kirtland AFB,

, W. Robert Johnston<sup>2</sup>

, Russell Cooper<sup>2</sup>

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

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

**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.
