Istituto di Scienza e Tecnologie dell'Informazione     
Pardini C., Anselmo L., Moe K., Moe M. M. Drag and energy accommodation coefficients during sunspot maximum. In: 37th COSPAR Scientific Assembly (Montréal, Canada, 13-20 July 2008).
A hundred years of laboratory measurements have shown that gas-surface interactions depend not only on the chemistry and energy of the incident particles but also on the degree of surface contamination. The conditions appropriate to gas-surface interaction in space have not been successfully duplicated in the laboratory. Consequently, knowledge of satellite drag coefficients has been dependent upon opportunities to compare theoretical models with observations of satellite decay. From such studies it is now known that the great majority of molecules which strike satellite surfaces are reemitted in a diffuse angular distribution with an energy loss given by the energy accommodation coefficient. Although a few measurements of the energy accommodation coefficient were made in the past, none was made near sunspot maximum. In the present study, we take advantage of the increasing data base to compare theoretical determinations of satellite drag coefficients with the history of satellite orbital decay during sunspot maximum. An example is the SNOE satellite which was in a circular orbit with an initial perigee altitude of 515 km during dates from October 1999 to December 2002. SNOE had a cylinder-like shape with a hexagonal cross section. It was attitude stabilized so that it maintained a constant aspect relative to the incident velocity vector, a feature which facilitated the computation of its drag coefficient as a function of the energy accommodation coefficient. The satellite drag coefficient was obtained by fitting, in a least squares sense, the semi-major axis decay inferred from the historical two-line elements acquired by the US Space Surveillance Network. All the principal orbital perturbations, namely geopotential harmonics up to the 16th degree and order, third body attraction of the Moon and the Sun, direct solar radiation pressure (with eclipses), and aerodynamic drag were included, using the Jacchia Bowman 2006 (JB2006) model to describe the atmospheric density. The average drag coefficient (fitted to JB2006), calculated over 30-day intervals from October 1999 to December 2002, was found to be 2.16. When the measurement was adjusted upward by 12 % to correct for the bias in the density model, the resulting physical drag coefficient became 2.42. Comparison of this result with the calculated dependence of the drag coefficient of SNOE on the energy accommodation coefficient has revealed agreement for an energy accommodation coefficient of 0.97. This high an energy accommodation coefficient implies that a considerable portion of the satellite surface had oxygen atoms adsorbed on it. This is physically reasonable because during sunspot maximum the increased solar UV radiation dissociates more molecular oxygen and the UV heating causes the atmosphere to expand to higher altitudes. The analysis was carried out for several satellites, having perigee altitudes between 300 and 800 km, during the last solar cycle maximum. This sample included a subset of Surrey satellites, for which the main characteristics (i.e. size, shape, mass, attitude, etc.) were known, and some spherical, or nearly spherical, objects.
Subject Drag coefficient
Energy accommodation coefficient
Thermospheric density models
J.2 Physical Sciences and Engineering
70F15 Celestial mechanics

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