Very low Earth orbit (VLEO) altitude, defined as altitudes below 350 km, offers significant advantages for high-resolution Earth observation, as well as low-cost launches and communications. However, developing long-lived VLEO satellites remains technically challenging due to the highly dense residual atmosphere, primarily composed of atomic oxygen (O, or AO) and molecular nitrogen (N2). At VLEO altitudes, dense AO collides with the ram face of satellites at a relative velocity of ~ 8 km s−1, causing significant erosion, roughening, and degradation of organic materials used for thermal control, structural components, and coatings. In addition, the highly dense atmosphere may induce drag on satellites, complicating orbit maintenance and affecting the propulsion system. This study investigates a polyimide film coated with a photocurable silsesquioxane (SQ, manufactured by Toagosei Co., Ltd.) as a potential AO-resistant material. Through the reaction with AO, the SQ coating forms a passivating silica layer on the surface, protecting the underlying polyimide film from further AO attack. To investigate the potential use of SQ-coated polyimide films as a material for the external surfaces of VLEO satellites, the films were exposed to AO in low Earth orbit (LEO) via Material Degradation Monitor 2 and Carbon Nanotube missions on the International Space Station, as well as a lab-based laser-detonation AO source. The mass losses and erosion yields of LEO-exposed films were higher than those of the lab-exposed films. The erosion of SQ-coated polyimides exhibited sensitivity to the environmental differences between the LEO and the laser-detonation AO source (e.g., the velocity distributions and compositions of O and O2, UV, and thermal cycling). Regardless of the exposure environment, cracks that penetrated the polyimide layer formed when AO exposure exceeded ~ 1 × 1021 atoms cm−2. The cracks formed were more severe under LEO exposure than in the lab. Similar cracking phenomena were observed at LEO and VLEO altitudes during the Material Degradation Monitor mission on Super-Low-Altitude Test Satellite, as indicated by a marked increase in visible diffusion above ~ 1 × 1021 atoms cm−2. Cross-sectional laser Raman spectroscopy of lab-exposed films revealed that the reaction of the coating with AO induced compressive stress, which was relieved by the formation of cracks. These cracks also affect thermo-optical and mechanical properties and may increase satellite drag through multiple-bounce scattering. To enable long-term VLEO missions, the molecular design of the coating must be improved to maintain nanoscale surface smoothness and prevent crack formation under high-fluence AO exposure.

Atomic oxygen resistance of silsesquioxane-coated polyimide films studied by LEO, VLEO, and lab exposures

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2025年10月21日

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Aki Goto,
Kazuki Yukumatsu,
Yugo Kimoto

摘要

Very low Earth orbit (VLEO) altitude, defined as altitudes below 350 km, offers significant advantages for high-resolution Earth observation, as well as low-cost launches and communications. However, developing long-lived VLEO satellites remains technically challenging due to the highly dense residual atmosphere, primarily composed of atomic oxygen (O, or AO) and molecular nitrogen (N₂). At VLEO altitudes, dense AO collides with the ram face of satellites at a relative velocity of ~ 8 km s⁻¹, causing significant erosion, roughening, and degradation of organic materials used for thermal control, structural components, and coatings. In addition, the highly dense atmosphere may induce drag on satellites, complicating orbit maintenance and affecting the propulsion system. This study investigates a polyimide film coated with a photocurable silsesquioxane (SQ, manufactured by Toagosei Co., Ltd.) as a potential AO-resistant material. Through the reaction with AO, the SQ coating forms a passivating silica layer on the surface, protecting the underlying polyimide film from further AO attack. To investigate the potential use of SQ-coated polyimide films as a material for the external surfaces of VLEO satellites, the films were exposed to AO in low Earth orbit (LEO) via Material Degradation Monitor 2 and Carbon Nanotube missions on the International Space Station, as well as a lab-based laser-detonation AO source. The mass losses and erosion yields of LEO-exposed films were higher than those of the lab-exposed films. The erosion of SQ-coated polyimides exhibited sensitivity to the environmental differences between the LEO and the laser-detonation AO source (e.g., the velocity distributions and compositions of O and O₂, UV, and thermal cycling). Regardless of the exposure environment, cracks that penetrated the polyimide layer formed when AO exposure exceeded ~ 1 × 10²¹ atoms cm⁻². The cracks formed were more severe under LEO exposure than in the lab. Similar cracking phenomena were observed at LEO and VLEO altitudes during the Material Degradation Monitor mission on Super-Low-Altitude Test Satellite, as indicated by a marked increase in visible diffusion above ~ 1 × 10²¹ atoms cm⁻². Cross-sectional laser Raman spectroscopy of lab-exposed films revealed that the reaction of the coating with AO induced compressive stress, which was relieved by the formation of cracks. These cracks also affect thermo-optical and mechanical properties and may increase satellite drag through multiple-bounce scattering. To enable long-term VLEO missions, the molecular design of the coating must be improved to maintain nanoscale surface smoothness and prevent crack formation under high-fluence AO exposure.