A small thermoelectric generator is being developed for general use in space, and in particular for any of several proposed Mars atmospheric probes and surface landers that may be launched in the 2003 to 2006 time period. The design is based on using as the generator heat source an existing 1 watt radioisotope heater unit, which has already been used to provide heating alone on numerous spacecraft, including the 1997 Pathfinder/Sojourner Mars lander. The thermoelectric generating module will be bismuth-telluride alloy. The module will combine new manufacturing technology with a basic design that has a two-decade heritage of performance and lifetime data. Power output will be approximately 40 milliwatts. Important technical issues that need to be addressed in the detailed design are the mechanical integrity of the overall power supply in consideration of the impact of landing on Mars and the subsequent performance of the thermal insulation around the heat source, which is critical to delivering the output power. The power supply is intended to meet a 20-year operational lifetime. The paper describes the design status to date, and it presents the analytical approach, the testing program plan and a manufacturing schedule that is needed to meet the launch dates being considered.