Evaluation of atomic oxygen fluences by analyzing transmitted light images of polyimide obtained at SLATS/MDM

Abstract

Expectations for high-resolution observations by very low Earth orbit (VLEO, below 300 km) satellites are growing. However, the density of atomic oxygen (AO) is much higher in VLEO than in LEO (300 km or higher), so severe degradation of materials is a concern due to high-flux AO collisions. To clarify the degradation, we developed and deployed the material degradation monitor (MDM) mission onboard the super-low altitude test satellite (SLATS). In the MDM, several materials were exposed to the environment at 160–560 km from 23 December 2017 to 1 October 2019 and observed optically by a CCD camera. In general, AO fluence is evaluated by measuring the volume or mass loss of a polyimide, but this was difficult to do because the samples could not be returned to the ground in this mission. This study evaluated AO fluences using the CCD images of a polyimide, Vespel. The sample had nine non-penetrating holes with different bottom-plate thicknesses and was monitored by the CCD while being illuminated by LED light from the back. The CCD images reveal the intensity of the light transmitted through the holes in the bottom plate. The erosion depths of the polyimide, corresponding to the AO fluences, can be evaluated from the pixel values at the hole positions on the images. Here, the correlation between the pixel values and thickness was investigated for pristine and AO-exposed Vespel samples using the MDM engineering models. The total green pixel values followed the Beer-Lambert law for the bottom-plate thicknesses, regardless of AO exposure. Applying the correlation to the MDM flight images, we found that the AO fluence increased significantly after the end of March 2019, finally reaching (6.5–9.8) × 1021 atoms/cm2. This optical imaging method would be useful for evaluating the AO effects on the other MDM samples and high AO fluences in future VLEO missions.