Constructing a quantitative theoretical model for the prediction and explan
ation of experimental data is an important goal in various branches of scie
ntific knowledge. However, such a model is useful if it has a reliable and
physically correct theoretical basis. This paper is devoted to the foundati
on of gas dynamic equations to be used for constructing theoretical models
of physical phenomena in the space physics. Several examples of pioneering
models are presented to show an important role of the cosmic gas dynamics i
n problems of the space physics. Kinetic foundation of the fluid dynamic eq
uations is discussed. Basic criteria to derive the hydrodynamic equations f
or a fully ionized plasma on the basis of the Boltzmann equation with the i
ntegral of collisions in the Landau form (Braginsky, 1963) are given. Hydro
dynamic equations for a collisionless plasma are discussed. In conclusion a
model description of two-component flows where one of the components canno
t be described in the framework of the hydrodynamic approximation is sugges
ted.