  As an application of quantum fluid mechanics, we consider the drag force
exerted on a sphere by an ultra-dilute gas. Quantum mechanical diffraction
scattering theory enters in that regime wherein the mean free path of a
molecule in the gas is large compared with the sphere radius. The drag force is
computed in a model specified by the ``sticking fraction'' of events in which a
gaseous molecule is adsorbed by the spherical surface. Classical inelastic
scattering theory is shown to be inadequate for physically reasonable sticking
fraction values. The quantum mechanical scattering drag force is exhibited
theoretically and compared with experimental data.
