The influence of the gas surface interaction on spacecraft orbital motion
Characterisation of the interaction between the neutral atmosphere in Low Earth Orbit and spacecraft surfaces is required for a broad range of applications to system and mission definition. For the prediction of spacecraft trajectories in this Free Molecular regime, the nature of the interaction must be accurately modelled. Accurate definition of the interaction is also required for the design of the attitude control system for vehicles such as Space Station which will generate large aerodynamic moments. The methods available to determine this interaction indirectly from observation of the motion of a satellite are reviewed. The orbital analysis technique is chosen for this study. Two methods of modelling complex spacecraft configurations are developed. The first, a Monte Carlo Test Particle approach, is able to account for all the phenomena characteristic of Free Molecular Flow. The second, adopting a panel method approach, accounts only for the possible shielding of surfaces from the flow. Discrepancies between the two modelling programs are identified but for the limited flow range relevant to the ANS-1 mission, good agreement is found and the computationally more efficient panel method program adopted. A new set of momentum accommodation coefficients are introduced to characterise the interaction. These are resolved in an aerodynamic frame. The new coefficients are found to be more robust than classical surface-resolved models and to have greater sensitivity to the nature of the interaction. The theory required to represent the relationship between the changes in the spacecraft trajectory and the gas-surface interaction is then developed. The sensitivity of the derived momentum accommodation to the parameters used in the modelling process is then determined. The most sensitive parameters are found to be the accuracy of the orbital elements semi-major axis and inclination, the rotation rate of the atmosphere, the temperature of the spacecraft surface and the complexity of the spacecraft configuration model. This theory is then applied to the orbital data derived for ANS-1 (1970-70A) by ESOC. The results suggest that the nature of the interaction is close to the classical diffuse case but the accuracy of the results are limited by uncertainties in the measured changes in orbital inclination.