A free molecule aerodynamic investigation using multiple satellite analysis

Abstract The inaccuracies of numerical satellite positioning in Low Earth Orbit are predominantly due to the errors in calculating the aerodynamic forces. This is due to the unknown interactions of the rarefied atmosphere on the satellite surface, and the errors of the atmospheric density models. To improve knowledge of the gas-surface interactions, the effects of aerodynamic forces upon the orbits of satellites can be isolated, using precise orbital analysis (POA). When examining the orbit of a single satellite, the solution is restricted by the limited observational data, atmospheric model errors and the inevitable correlations between the atmospheric and aerodynamic models. To address these, three satellite orbits are examined simultaneously. The satellite orbits analysed are those of ERS1, a remote sensing satellite, and the geodetic satellite's STARLETTE and STELLA. They orbit at 800 to 1000 km where helium and atomic oxygen are dominant. The collisional energies of helium and atomic oxygen upon the spacecraft surface are 1.25 and 5 eV, respectively. This preliminary analysis suggests that the re-emission of these particles are at lower speeds and lower emitted angles than those measured in laboratory molecular beam experiments. The analysis suggests that the bulk speed of the emitted particles is about 2 km s−1, and in a direction approximately half way between the specular direction and the surface normal. This emission produces a drag coefficient on an aluminium sphere of 2.52.