The DLR (German Aerospace Center) plans to launch the microsatellite BIRD (Bi Spectral Infrared Detection) in 1999 as part of a Earth remote sensing mission with hot spot detection as matter of priority. This project represents the begin of a line of small satellite missions with ambitious scientific and technological objectives by application of new technology and respecting the limitations of microsatellites.The spacecraft bus design is based on the proposed orbit and the payload requirements. The scientific payload is a novel multi-spectral sensor system, consisting of two cooled infrared sensor arrays and the Wide Angle Opto-electronic Stereo Scanner (WAOSS). A serious constraint of the satellite design is the required compatibility to a piggyback launch. The concept of the satellite bus fits to the requirements with the satellite dimensions of about 550x610x620 mm3 and a total mass of approx. 85kg.The presentation describes the approach for the thermal design of the satellite bus with focus on the mission profile and the requirements of the payload.