The EHD (electro-hydrodynamic) thruster seems suitable for a device of mobility at planetary exploration because of its propellant-less nature. In this paper, the fundamental characteristics of the EHD thruster are investigated from a viewpoint of the applicability to the planetary exploration. The numerical analysis of the drift-diffusion equations shows that it works irrespective to the polarity of the electrodes because the polarity of the produced ions is also changed. This fact implies that it will work in different atmospheric composition with different polarity of the produced ions. It is also numerically found that the thrust tends to increase with the decrease in the ambient pressure. For simplicity of discussion, a simple analytical model to describe the energy conversion efficiency is derived, and it indicates that higher efficiency is expected in higher Knudsen number regime. Both the above results suggest that the EHD thruster is promising especially in planetary atmosphere with low density.