Abstract Satellites operating in the Very-Low-Earth-Orbit regime, that is, at altitudes where the residual atmosphere significantly impacts spacecraft dynamics, encounter notable aerodynamic forces, which are traditionally considered disturbances in spacecraft attitude control design. Instead, active control methods seek to harness these forces using the torques generated by adjustable aerodynamic surfaces as an additional means of external actuation. In this work, we examine the nonlinear structure of the control problem for a satellite featuring a reference geometry in a feathered configuration with four rotatable surfaces. We argue that challenges for the control design include uncertainty in aerodynamic and atmospheric models as well as time-varying parameters such as density and thermospheric temperature. Furthermore, our work emphasizes that low lift-to-drag ratios not only reduce control authority around the roll axis but also significantly restrict the region of validity of the linearized system, complicating actuator allocation procedures. To highlight the effects on control performance, we introduce a custom simulation tool designed for testing rotatable panel geometries under VLEO conditions and present a case study that illustrates the limitations of linear control design methods. Finally, we discuss potential nonlinear control strategies and the need for stability guarantees to further advance the field of aerodynamic attitude control in VLEO.

Abstract 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.

Abstract A new gas–surface interaction (GSI) model for atmospheric drag evaluation of very low Earth orbit is proposed. The variable-gamma (VG) and variable-gamma energy-transfer (VGET) models are proposed based on the data from thermal and hyperthermal beam scattering experiments. The VG model takes into account the incident angle dependences of the accommodation coefficient and the VGET model adds the translational energy loss of the incident atoms in the scattering event. The VG model is an empirical model applicable for real surface scattering of molecules with thermal velocities and VGET is for hyperthermal velocities. These models are expected to provide a more realistic evaluation of the shear drag at the side surfaces of a satellite compared to the Maxwell GSI model for non-reactive molecules such as N 2 in VLEO environment. A model calculation of the SLATS satellite indicated that the drag coefficient C d is evaluated 38% lower than that evaluated by the conventional Maxwell model with a constant γ = 0.9.