This paper presents mission planning aspects of the future TanDEM-X mission scheduled for launch in 2010. In 2007 the TerraSAR-X satellite was successfully launched. Its payload consists of an earth observing Synthetic Aperture Radar, which supplies high resolution radar images. The primary goal of the TerraSAR-X mission is to supply the commercial and scientific users with radar image data on request. The TanDEM-X satellite is a nearly identical copy of the TerraSAR-X satellite. The two satellites will be orbiting the Earth in a close formation with distances from 250m to 500m. The radar instruments on both satellites may be used synchronously in a bi-static mode: One or both satellites actively transmit radar pulses; the echo is received by both satellites. This configuration gives a stereoscopic view such that information comprising all three dimensions can be retrieved from the data. The TanDEM-X mission goal is to generate a digital elevation model covering the whole Earth’s surface. In addition, the radar instruments on both the TerraSAR-X and the TanDEM-X satellites will still be operated in the TerraSAR-X mono-static mode and therefore both satellites may support the TerraSARX mission goals. The combined TerraSAR-X / TanDEM-X mission planning system will handle the two satellites as well as two completely different missions with their different mission goals. As a consequence, the new combined TerraSAR-X / TanDEM-X mission planning system not only has to support two satellites with their mutual constraints, but also will handle two different missions at the same time: the long-term mapping approach of the TanDEM-X mission and the short-term on-demand approach of the TerraSAR-X mission. There are two critical issues regarding the operational safety of the formation flight: the close distance of the two satellites implies a significant collision risk in case of anomalies. Next, illuminating the other satellite with radar pulses can cause severe damage to the illuminated satellite.