On the problem of fluid dynamics foundation for applications in the space physics

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.