In order to power the remote scientific instruments packages contained in the WISP, and to assist in the descent of the probe, a RTG will be used. The development of the RTG is focused on improving specific mass and efficiency over other power sources. Converter efficiency is particularly critical in order to maximize the usefulness of the Plutonium heat source given the isotope's cost. The distinguishing feature from other nuclear power designs is the modularity afforded by the use of multicouple thermoelectrics allowing scalability over a wider power range (19-340 W) [12]. The chosen RTG has a 150 W output, 12 year nominal lifetime, and weighs approximately 48 kg. The dimensions of the RTG are as follows: length: 9.32 cm, width: 9.72 cm. This particular RTG was modeled after the GPHS RTGs used for the Apollo missions and for the Galileo and Ulysses spacecraft. [11]