In many practical applications, that can be found in control engineering, the functionality and safety of the overall control process rely on a proper function of the respective operating system. This fact makes the software one of the most safety critical elements of such practical applications, especially when the control process is placed in an inhospitable environment not directly accessible to man. One of such environments is Space. In this paper a problem of designing a safety critical real time operating system for a small space satellite called CubeSat is discussed. It is shown how to design such an operating system and how to increase its reliability and to protect it against single upset events.