The results of an experimental and theoretical investigation of the flow upstream of a thin foil model aligned with a low density, hypersonic stream are presented. Measurements of number density distributions ahead of the foil for two different foil thicknesses are compared in order to assess the relative importance of leading edge bluntness and aerodynamic surface contributions to the upstream disturbance. Numerical computation of a first collision model for the nearly free molecular flow field confirms the proposed scaling of the flow field. The upstream disturbance can be scaled on &lgr;bf, a mean free path for body emitted (wall temperature) molecules traveling into a cloud of free stream molecules.