  Renewed interest in Very Low Earth Orbits (VLEO) - i.e. altitudes below 450
km - has led to an increased demand for accurate environment characterisation
and aerodynamic force prediction. While the former requires knowledge of the
mechanisms that drive density variations in the thermosphere, the latter also
depends on the interactions between the gas-particles in the residual
atmosphere and the surfaces exposed to the flow. The determination of the
aerodynamic coefficients is hindered by the numerous uncertainties that
characterise the physical processes occurring at the exposed surfaces. Several
models have been produced over the last 60 years with the intent of combining
accuracy with relatively simple implementations. In this paper the most popular
models have been selected and reviewed using as discriminating factors
relevance with regards to orbital aerodynamics applications and theoretical
agreement with gas-beam experimental data. More sophisticated models were
neglected, since their increased accuracy is generally accompanied by a
substantial increase in computation times which is likely to be unsuitable for
most space engineering applications. For the sake of clarity, a distinction was
introduced between physical and scattering kernel theory based gas-surface
interaction models. The physical model category comprises the Hard Cube model,
the Soft Cube model and the Washboard model, while the scattering kernel family
consists of the Maxwell model, the Nocilla-Hurlbut-Sherman model and the
Cercignani-Lampis-Lord model. Limits and assets of each model have been
discussed with regards to the context of this paper. Wherever possible,
comments have been provided to help the reader to identify possible future
challenges for gas-surface interaction science with regards to orbital
aerodynamic applications.
