A milliwatt power source (MPS) has been developed to satisfy the requirements of several potential solar system exploration missions. The MPS is a small power source consisting of three major components: A space qualified heat source (RHU), a thermopile (thermoelectric converter or TEC) and a container to direct the RHU heat to the TEC. Thermopiles from Hi-Z Technology, Inc. of San Diego and the Institute of Thermoelectricity of Chernivtsi Ukraine suitable for the MPS were tested and shown to perform as cxpcctcd, producing 40 mW of power with a temperature difference of about 170°C. Such thermopiles were successfully life tested for up to a year. A MPS container designed and built by Swales Aerospace was tested with both a TEC simulator and actual TEC, The Swales unit, tested under dynamic vacuum, provided less temperature difference than anticipated, such that the TEC produced 20 mW of power, with heat input equivalent to a RHU.