Degradation of far ultraviolet reflectance of aluminum films exposed to atomic oxygen. In-orbit coating application

• Published in: Optics communications Vol. 124; no. 3; pp. 208 - 215
• Main Authors: Larruquert, Juan Ignacio, Méndez, JoséAntonio, Aznárez, JoséAntonio
• Format: Journal Article
• Language: English
• Published: Amsterdam Elsevier B.V 01-03-1996; Elsevier Science
• Subjects: Artificial earth satellites; Astronomy; Earth, ocean, space; Edge and boundary effects; reflection and refraction; Exact sciences and technology; Fundamental areas of phenomenology (including applications); Fundamental astronomy and astrophysics. Instrumentation, techniques, and astronomical observations; Optics; Physics; Wave optics; Thin films; Ultraviolet radiation; Artificial satellite; Aluminium; Oxidation; Experimental study; Mirrors; Coatings; Optical reflection
• ISSN: 0030-4018; 1873-0310
• DOI: 10.1016/0030-4018(95)00679-6

The drop in reflectance at the interval 82.6–174.4 nm of ultra high vacuum prepared aluminum coatings when exposed to controlled doses of atomic oxygen with average energy 0.17 eV has been measured by the first time. We show that atomic oxygen produces a much stronger effect on the far ultraviolet (FUV) reflectance of aluminum that molecular oxygen: for the same relative drop in reflectance of 40% at 82.6 nm, the required exposure to molecular oxygen is about 5000 times larger than to atomic oxygen. These measurements allowed us to calculate a higher limit of the useful lifetime of an aluminum mirror placed in a low earth orbit (LEO, 200 to 700 km altitude; oxygen atoms with energy of 5 eV). This limit was found to be as short as a few hours. To avoid the oxidation by atomic oxygen a high altitude orbit should be used. A more practical alternative may be found by placing the aluminum mirror behind a wakeshield device, a well known proposal, in a LEO. In this case, the flux of oxygen atoms impinging on the mirror would be strongly reduced and consequently the lifetime of the mirror greatly increased. We have calculated that the energy of oxygen atoms impinging on the shielded mirror would range from 0.07 to 1.8 eV. As the energy of oxygen atoms in our experiment is within that interval of energies, a plausible estimate of the lifetime expected for an in-orbit aluminum coating behind a wakeshield can be derived from our experimental measurements. For a wakeshield operating as a free flyer at a 300 km altitude orbit an aluminum mirror would suffer a negligible drop in its FUV reflectance after a time as long as 20 years, which is a stimulating prospective.