Abstract. When the space age began, some aerodynamicists expected that the surfaces of spacecraft would be cleaned by desorption in the high vacuum of space; while others, familiar with experiments on engineering surfaces, believed that satellite surfaces would be contaminated. During subsequent decades, satellite evidence has accumulated, showing that surfaces in low-Earth orbit are contaminated by adsorbed atomic oxygen and its reaction products. These contaminants cause accommodation coefficients to be high, and the angular distribution of reemitted molecules to be nearly diffuse. These surface conditions must be considered in calculating satellite drag coefficients in free-molecular flow. We describe the experimental and theoretical developments which have led to these conclusions.
Densities derived from accelerometer measurements depend on the drag coefficient assigned to the satellite. Although laboratory measurements increase our understanding of gassurface interactions, they are not adequate to determine the appropriate drag coefficient because it is not known how the surface conditions at any particular altitude relate to the heterogeneous chemisorption and physisorption revealed by measurements in the laboratory. Therefore it is necessary to rely on drag and accommodation coefficients which have been measured in orbit. We use knowledge of these coefficients from our recent review of satellite measurements, and insights gained from laboratory measurements, to construct a table showing how the accommodation coefficient of compact satellites varies wtih altitude and solar activity. We then insert the accommodation coefficients in theoretical calculations to provide recommended drag coefficients for a variety of satellite shapes in low earth orbit. By using all of the information on aerodynamic coefficients measured by previous satellites, we can minimize errors in density measurements made by future satellites. Soon a new aeronomic satellite, STEP-1, will be flown. If its large flat plates sometimes are oriented at several different angles to the airstream, our knowledge regarding the dependence of drag coefficients on altitude and angle of incidence can be improved. The Upper Mesosphere and Lower Thermosphere: A Review of Experiment and Theory Geophysical Monograph 87 Copyright 1995 by the American Geophysical Union 349 accuracy of the assumed value of Cd. Its numerical value depends on the reference area, which often is chosen to be the cross sectional area of the satellite normal to the airstream. With that choice, the value of Cd which is customarily used for convex satellites of compact shapes is 2.2 [Cook, 1965, 1966]. By "compact shape" we mean that the ratio of the satellite's maximum to minimum diameter is less than 1.5, and the satellite does not have large external structures like solar paddles. A Cd in the neighborhood of 3.0 to 3.5 is used for long cylindrical satellites that fly like an arrow, depending on the length-to-diameter ratio of the satellite and the air temperature [Sentman, 1961; DeVries, 1972; Moe, et al., 1993].
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