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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 expected, 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. IntroductionSeveral potential solar system exploration missions are being developed that require a power source smaller than the current redioisotope thermoelectric generator (RTG) or planned advanced radioisotope power source (ARPS). Several systems have been proposed utilizing the standard radio isotope heater unit (RHU) as a heat source. The RHU was developed by DOE for the Galileo and Ulysses missions to maintain specific spacecraft components within normal operating temperatures. The RHU provides 1 W of heat and can be expected to produce about 40 mW of electrical power using a thermoelectric generator. Using a MPS instead of only a RHU will provide electrical power in addition to 96% of the RHU heat. These electrical power systems, generally called milliwatt power sources (MPS) have also been referred to as the "powerstick" concept (Chmielewski, 1994). Various MPS systems have been developed at tested in Russia for terrestrial and space applications (Pustovalov, 1999). The MPS is designed to be used in a small planetary rover, small communication transmitter, microspacecraft or an autonomous science package such as a remote weather station, that either uses continuous power at a level of a fraction of a watt or can operate intermittently by using energy from a battery that is continuously charged by a MPS. For example, the proposed Mars Network missions (e.g. PASCAL) could take advantage of MPS's for their minimeteorological stations. The MPS is designed to produce enough continuous power to operate a sequencer while trickle charging the batteries. The battery power could then be used every couple of weeks to operate the instruments and transmit

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