A novel concept of satellite design, named PETSAT, is proposed in this paper. In this concept, a satellite is made of several Functional Panels such as the CPU panel, Battery panel, Communication panel, Attitude control panel, or Thruster panel, each of which has a special dedicated function. By connecting these panels with a reliable connection mechanism in a plug-in fashion, the total integrated system has a full satellite’s function. Various combinations of functional panels, (for example, two CPU panels + one communication panels + three attitude control panels + two battery panels, etc.) provide flexibility to deal with various mission requirements, even though the basic panels are the same for various missions. The concept, technical issues and conceptual study results of PETSAT will be discussed. 1. Concept of PETSAT A novel concept of satellite design, named PETSAT, is proposed in this paper. In this concept, a satellite is made of several Functional Panels each of which has a special dedicated function. By connecting these panels by reliable connection mechanism in plug-in fashion, the total integrated system has a full satellite’s function. Various combinations of different kinds of functional panels, sometimes with multiple instances of identical panels, provide flexibility to deal with various mission requirements, even though the basic panels are the same for various missions. These panels are stowed during launch into a small volume (left figure in Fig. 1), and are extended on orbit (right figure in Fig. 1), potentially realizing a satellite with a large antenna, large solar cell area or large boom. PETSAT intends to change the satellite development cycle in the following way: 1. Each functional panel can be produced in mass quantities so that the reliability can be improved, and the produced panels can be stocked. 2. When a certain mission is given, the satellite bus suitable to the mission requirements can be configured only by connecting the appropriate panels in appropriate quantities in plug-in fashion without much effort needed in ground test of the total system. This semi-customizable satellite production process is expected to dramatically reduce required manufacture and test time, as well as workload, resulting in a drastic reduction of the individual satellite cost and development time. Mass production of the panels is the key to reducing cost and improving reliability, but in the conventional satellite concept, mass production is difficult even at subsystem level due to the wide variety of mission requirements. PETSAT tries to make this possible by modularizing the various basic subsystem functions into standard panels, and by dealing with the variety of mission requirements with the quantity of the utilized panels of different functions. The concept of PETSAT was initially proposed at the Satellite Design Contest in 2002, where the concept was the winner of the Best Design Award. Five year research project of PETSAT just started in