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Barcelona, Spain.

**20** 

*USA* 

**Radioisotope Power: A Key Technology for** 

George R. Schmidt1, Thomas J. Sutliff1 and Leonard A. Dudzinski2

Radioisotope Power Systems (RPS) generate electrical power by converting heat released from the nuclear decay of radioactive isotopes into electricity. Because all the units that have flown in space have employed thermoelectrics, a static process for heat-to-electrical energy conversion that employs no moving parts, the term, Radioisotope Thermoelectric Generator (RTG), has been more popularly associated with these devices. However, the advent of new generators based on dynamic energy conversion and alternative static conversion processes favors use of "RPS" as a more accurate term for this power technology. RPS were first used in space by the U.S. in 1961. Since that time, the U.S. has flown 41 RTGs, as a power source for 26 space systems on 25 missions. These applications have included Earthorbital weather and communication satellites, scientific stations on the Moon, robotic explorer spacecraft on Mars, and highly sophisticated deep space interplanetary missions to Jupiter, Saturn and beyond. The New Horizons mission to Pluto, which was launched in January 2006, represents the most recent use of an RTG. The former U.S.S.R. also employed RTGs on several of its early space missions. In addition to electrical power generation, the U.S. and former U.S.S.R. have used radioisotopes extensively for heating components and instrumentation. RPS have consistently demonstrated unique capabilities over other types of space power systems. A comparison between RPS and other forms of space power is shown in Fig. 1, which maps the most suitable power technologies for different ranges of power level and mission duration. In general, RPS are best suited for applications involving long-duration

use beyond several months and power levels up to one to 10 kilowatts.

It is important to recognize that solar power competes very well within this power level range, and offers much higher specific powers (power per unit system mass) for applications up to several Astronomical Units (AU) from the Sun. However, RPS offer the unique advantage of being able to operate continuously, regardless of distance and orientation with respect to the Sun. The flight history of RTGs has demonstrated that these systems are long-lived, rugged, compact, highly reliable, and relatively insensitive to radiation and other environmental effects. Thus, RTGs and the more capable RPS options of the future are ideally suited for missions at distances and extreme conditions where solarbased power generation becomes impractical. These include travel beyond the asteroid belt, operation within the radiation-intensive environments around Jupiter and close to the Sun,

**1. Introduction** 

**Deep Space Exploration** 

*1NASA Glenn Research Center,* 

*2NASA Headquarters* 
