Drag and energy accommodation coefficients during sunspot maximum

Conditions appropriate to gas-surface interactions on satellite surfaces in orbit have not been successfully duplicated in the laboratory.
However, measurements by pressure gauges and mass spectrometers in orbit have revealed enough of the basic physical chemistry
that realistic theoretical models of the gas-surface interaction can now be used to calculate physical drag coefficients. The dependence of
these drag coefficients on conditions in space can be inferred by comparing the physical drag coefficient of a satellite with a drag coefficient
fitted to its observed orbital decay. This study takes advantage of recent data on spheres and attitude stabilized satellites to compare
physical drag coefficients with the histories of the orbital decay of several satellites during the recent sunspot maximum. The orbital
decay 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 were included, 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, using the Jacchia-Bowman 2006 (JB2006) model to describe the atmospheric density. After adjusting for density model bias, a comparison
of the fitted drag coefficient with the physical drag coefficient has yielded values for the energy accommodation coefficient as well
as for the physical drag coefficient as a function of altitude during solar maximum conditions. The results are consistent with the altitude
and solar cycle variation of atomic oxygen, which is known to be adsorbed on satellite surfaces, affecting both the energy accommodation
and angular distribution of the reemitted molecules.

The research was carried out by C. Pardini and L. Anselmo (Space Flight Dynamics Laboratory, ISTI/CNR, Pisa, Italy), and by K. Moe and M.M. Moe (Space Environment Technologies, Irvine, California, USA).