The largest source of error in the deduction of atmospheric densities from orbital data stems from uncertainty in our knowledge of the interaction of atmospheric molecules with satellite surfaces. A method of determining two of the important parameters, the accommodation and drag coefficients, is developed in detail. As an example, the method is applied to observational data from the paddle wheel satellite Ariel 2. Because of the importance of spherical satellites the drag coefficient for a sphere under the conditions of Ariel 2 (a moderately eccentric orbit with perigee near 300 km) is also obtained. The importance of well-designed orbital experiments to improve our knowledge of surface-particle interactions at a variety of altitudes is stressed.