This paper presents an “Inquiry by Design” approach to the problem of architectural design for the crew habitat of an interplanetary vehicle. This habitat must meet a range of difficult requirements to protect the crew's health and safety during the approximately 6 to 12 month voyage each way. It must provide a habitable environment that affords the crew privacy, group activities, recreation, exercise, communications, training facilities, and health care. It must incorporate countermeasures against prolonged exposure, to zero gravity and shielding against radiation from solar flares and galactic cosmic rays.The design research involves the investigation of a prototype interplanetary habitat that incorporates substantial radiation shielding for the crew quarters and a human powered, short-arm centrifuge for zero gravity countermeasures. It includes private crew quarters, life support system, stowage and equipment volumes. This inquiry by design exercise explores the habitat components subsystem by subsystem to find the challenges that emerge for integrating them. This exercise in designing an optimized interplanetary habitat reveals some key design and engineering issues for NASA to consider in developing a Mars vehicle.