**5. Other potential applications**

MMRTG and ASRG should satisfy most RPS mission requirements well beyond 2010, particularly for those applications involving several hundred watts of power. However, there will likely be a demand for additional types of units in the future. One potential need identified by the space science community is for small RPS units ranging in power from ~10 milliwatts (mWe) to ~20 We. These so-called 'milliwatt' and 'multiwatt-class' power supplies could extend the capability of small, low cost missions supported through NASA's small to mid-size programs, and augment human missions involving deployment of monitoring stations and autonomous devices. They would likely utilize the GPHS or other existing heat sources. Although flight-qualified systems in this size range do not presently exist, the promise of RPS has led NASA and DOE to evaluate the possible development of a small RPS unit in the future.

Nuclear Electric Propulsion (NEP) has been studied since the early 1960's because of its potential for future high-energy space missions. Almost all NEP assessments to date have assumed fission as the nuclear energy source. Unlike solar-powered electric propulsion (SEP) systems, NEP operation is generally independent of distance and orientation with respect to the Sun. Over the last decade, several studies have pointed to Radioisotope Power Systems (RPS), instead of reactor power sources, as the best way of implementing NEP. Radioisotope-based NEP, also known as Radioisotope Electric Propulsion (REP), has been evaluated before, but has not been seriously considered for flight due to the low specific power range of traditional RPS (e.g., 3 to 5 We/kg). However, the prospects for REP have improved substantially with the advent of the ASRG and its likely improvement in specific power.

In this capacity, REP would principally be used as an interplanetary stage for long-duration deceleration and acceleration in deep space. At remote destinations, REP would perform deceleration, orbit insertion and maneuvers around outer planets and other planetary bodies. REP-based spacecraft could also provide ample power at destination for sophisticated science instruments and communications, but it would fit better within the relatively modest kilowatt-scale power requirements of the space science community.
