Power limitation represents a major issue within space applications aimed to human settlements on solar system planets. Among these planets, Mars is considered the most attractive because of its nearness to the Earth and the probable presence of minerals which can be used by the settlers to live off the land. In this frame, small size nuclear power plants can be an interesting solution to overcome the energy supply problem. This paper presents a preliminary feasibility study of a 100 kWe self-pressurized water space reactor, with the aim to design a system characterized by compactness, intrinsic safety and simplicity of the main reactor control components. To this end an innovative reactivity control system, based on the control of the primary coolant mass flow rate, was adopted. The introduction of this system in the reactor design required a comprehensive core neutronics analysis in order to properly quantify the effect of the coolant on the reactor behaviour also as a function of the fuel burn-up. Here only the main results of this analysis, concerning neutron flux profiles and multiplication factors, are discussed. Moreover preliminary results on long term reactivity control are presented, showing the possibility to operate the reactor for as long as 7 years with no need of human intervention.