**U.S. Space Radioisotope Power Systems and Applications: Past, Present and Future**

Robert L. Cataldo1 and Gary L. Bennett2 *1NASA Glenn Research Center 2Metaspace Enterprises USA* 

#### **1. Introduction**

472 Radioisotopes – Applications in Physical Sciences

National Reseach Council. Aeronautics and Space Engineering Board, Space Studies Board,

Richins, W. & Lcay, J. (June 2007). Safety Analysis for a Radioisotope Stirling Generator,

Sanchez-Torres, A.; Sanmartin, J., Donoso, J., Charro, M. (2010). The radiation impedance of

Sanmartin, J.; Martinez-Sanchez, M., Ahedo, E. (19930). Bare Wire Anode for

Sturgis, B. et al. (September 2006). Methodology Assessment and Recommandations for the

http://georgenet.net/misc/rtg/Safety%20Analysis%20for%20RTG.pdf Ritz, F. & Peterson, G. (2004). Multi-Mission Radioisotope Thermoelectric Generator

*National Academic Press*, Washington, D.C. , Avalible from

*Proceedings of Space Nuclear Conference, Paper 2024,* Avalible from

http://www.nap.edu/catalog/12653.html

ISSN: 1095-323X

4658

1050-1057. ISSN: 0273-1177

DOI: 10.2172/893553.

Engineering and Physical Sciences (2009), Radioisotope Power Systems: An Imperative for Maintaining US Leadership in Space Exploration, *Tech. Rep.*, The

(MMRTG) Program Overview, pp. 2950-2957*, IEEE Aerospace Conference Proceedings,*

electrodynamic tethers in a polar Jovian orbit, *Advances in Space Reseach*, Vol. 45, pp.

Electrodynamic Tethers*, J. of Propulsion and Power*, Vol 9, pp. 353-360, ISSN: 0748-

Mars Science Laboratory Launch Safety Analysis*, Sandia Report Sand* 2006-4563.

Radioisotope power systems (RPS) have been essential to the U.S. exploration of outer space. RPS have two primary uses: electrical power and thermal power. To provide electrical power, the RPS uses the heat produced by the natural decay of a radioisotope (e.g., plutonium-238 in U.S. RPS) to drive a converter (e.g., thermoelectric elements or Stirling linear alternator). As a thermal power source the heat is conducted to whatever component on the spacecraft needs to be kept warm; this heat can be produced by a radioisotope heater unit (RHU) or by using the excess heat of a radioisotope thermoelectric generator (RTG).

As of 2010, the U.S. has launched 45 RTGs on 26 space systems. These space systems have ranged from navigational satellites to challenging outer planet missions such as Pioneers 10/11, Voyagers 1/2, Galileo, Ulysses, Cassini and the New Horizons mission to Pluto. In the fall of 2011, NASA plans to launch the Mars Science Laboratory (MSL) that will employ the new Multi-Mission Radioisotope Thermoelectric Generator (MMRTG) as the principal power source.

Hundreds of radioisotope heater units (RHUs) have been launched, providing warmth to critical components on such missions as the Apollo 11 experiments package and on the outer planet probes Pioneers 10/11, Voyagers 1/2, Galileo and Cassini.

A radioisotope (electrical) power source or system (RPS) consists of three basic elements: (1) the radioisotope heat source that provides the thermal power, (2) the converter that transforms the thermal power into electrical power and (3) the heat rejection radiator. Figure 1 illustrates the basic features of an RPS.

The idea of a radioisotope power source follows closely after the early investigations of radioactivity by researchers such as Henri Becquerel (1852-1908), Marie Curie (1867- 1935), Pierre Curie (1859-1906) and R. J. Strutt (1875-1947), the fourth Lord Rayleigh. Almost 100 years ago, in 1913, English physicist H. G. J. Moseley (1887-1915) constructed the first nuclear battery using a vacuum flask and 20 mCi of radium (Corliss and Harvey, 1964, Moseley and Harling, 1913).

After World War II, serious interest in radioisotope power systems in the U.S. was sparked by studies of space satellites such as North American Aviation's 1947 report on nuclear space power and the RAND Corporation's 1949 report on radioisotope power. (Greenfield,

U.S. Space Radioisotope Power Systems and Applications: Past, Present and Future 475

Fig. 2. The first radioisotope thermoelectric generator (RTG).

**2. Early SNAP program** 

(Corliss and Harvey, 1964).

Figure from the Jordan and Birden 1954 report via (Corliss and Harvey, 1964).The heat source consisted of a 1-cm-diameter sphere of 57 Ci (1.8 Wt) of 210Po inside a capsule of nickel-coated cold-rolled steel all inside a container of Lucite. The thermocouples were

The AEC began the Systems for Nuclear Auxiliary Power (SNAP) program in 1955 with contracts let to the Martin Company (now Teledyne) to design SNAP-1 and to the Atomics International Division of North American Aviation, Inc. to design SNAP-2. (Under the AEC nomenclature system, the odd-numbered SNAPs had radioisotope heat sources and the even-numbered SNAPs had nuclear fission reactor heat sources.) SNAP-1 was to provide 500 We using the then readily available fission product radioisotope cerium-144 (144Ce)

The Martin Company began with a 133-We RPS design using 144Ce as the radioisotope fuel and a Rankine thermal-to-electric conversion system. From this came the 500-We SNAP-1 RPS design based on 144Ce fuel and a Rankine conversion system (see Figure 3) (Corliss and Harvey, 1964). The use of a dynamic conversion system in the first RPS is a key historical fact in understanding the current focus on developing an Advanced Stirling Radioisotope

silver-soldered chromel-constantan. The "thermal battery" produced 1.8 mWe.

1947, Gendler and Kock, 1949). Radioisotopes were also considered in early studies of nuclear-powered aircraft (Corliss and Harvey, 1964).

Fig. 1. Cutaway view of a radioisotope power source (RPS) (Image credit: DOE).

In 1951, the U.S. Atomic Energy Commission (AEC) signed several contracts to study a 1 kWe space power plant using reactors or radioisotopes. Several of these studies, which were completed in 1952, recommended the use of RPS (Corliss and Harvey, 1964). In 1954, the RAND Corporation issued the summary report of the Project Feedback military satellite study in which radioisotope power was considered (Lipp and Salter, 1954).

Paralleling these studies, in 1954, K. C. Jordan and J. H. Birden of the AEC's Mound Laboratory conceived and built the first RTG using chromel-constantan thermocouples and a polonium-210 (210Po or Po-210) radioisotope heat source (see Figure 2). While the power produced (1.8 mWe) was low by today's standards, this first RTG showed the feasibility of RPS. A second "thermal battery" was built with more Po-210, producing 9.4 mWe. Jordan and Birden concluded that the Po-210 "thermal battery" would have about ten times the energy of ordinary dry cells of the same mass (Jordan and Birden, 1954).

Fig. 2. The first radioisotope thermoelectric generator (RTG). Figure from the Jordan and Birden 1954 report via (Corliss and Harvey, 1964).The heat source consisted of a 1-cm-diameter sphere of 57 Ci (1.8 Wt) of 210Po inside a capsule of nickel-coated cold-rolled steel all inside a container of Lucite. The thermocouples were silver-soldered chromel-constantan. The "thermal battery" produced 1.8 mWe.
