Kinetic Theory of Drag on Objects in Nearly Free Molecular Flow
  Using an analogy between the density expansion of the transport coefficients of moderately dense gases and the inverse-Knudsen-number expansion of the drag on objects in nearly free molecular flows, we formulate the collision integrals that determine the first correction term to the free-molecular drag limit. We then show how the procedure can be applied to calculate the drag coefficients of an oriented disc and a sphere as a function of the speed ratio. 
J. V. Sengers, Y.-Y. Lin Wang, B. Kagmar-Parsi, J. R. Dorfman
cond-mat.stat-mech