Drag coefficient is a major source of uncertainty in calculating the aerodynamic forces on satellites in low Earth orbit. Closed-form solutions are available for simple geometries under the assumption of free molecular flow; however, most satellites have complex geometries, and a more sophisticated method of calculating the drag coefficient is needed. This work builds toward modeling physical drag coefficients using the direct simulation Monte Carlo method capable of accurately modeling flow shadowing and concave geometries. The direct simulation threedimensional visual program and the direct simulation Monte Carlo analysis code are used to compare the effects of two separate gas-surface interaction models: diffuse reflection with incomplete accommodation and quasi-specular Cercignani-Lampis-Lord models. Results show that the two gas-surface interaction models compare well at altitudes below ∼500 km during solar maximum conditions and below ∼400 km during solar minimum conditions. The difference in drag coefficient of a sphere at ∼800 kmcalculated using the two gas-surface interaction models is ∼6% during solar maximum and increases to ∼10% during solar minimum. The difference in drag coefficient of the GRACE satellite computed using the two gas-surface interaction models at ∼500 km differs by ∼15% during solar minimum conditions and by ∼2-3% during solar maximum conditions.