**6. Conclusion**

454 Radioisotopes – Applications in Physical Sciences

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

Fig. 31. ASRG Engineering Unit.

small RPS unit in the future.

**5. Other potential applications** 

Radioisotope power systems will continue to play an important role in NASA's exploration efforts. These systems also have the potential for use in a variety of new applications, which would benefit from the technology's versatility in a broad range of space and planetary environments. In the near-term, the MMRTG will expand the capability for conducting science on the surface of Mars. The ASRG will enable even higher performance missions. These units will also enable more ambitious exploration of other planetary surfaces and provide a reliable means of powering spacecraft in deep space. Current activities would also allow the potential development of new systems that could expand application of RPS to smaller science missions. The key to successful implementation of RPS is to maintain close ties with potential users and the science community at large. With these advancements, radioisotope power systems and technology will offer tremendous benefits for future exploration endeavors.

### **7. References**


**21** 

*Spain* 

**Radioisotope Power Systems** 

*Universidad Politécnica de Madrid, Escuela Técnica Superior de Ingenieros Aeronáutcos,* 

At the beginning of the Space Age, both propulsion and power generation in the spacecraft has been the main issue for consideration. Considerable research has been carried out on technologies by several Space Agencies to reach outer planets and generate electric power for the systems and subsystems in the spacecraft (SC). Various types of power source such as solar photovoltaic, Radioisotope power systems (RPS) have been used by Space Agencies. New technology such as reactor based, electric solar sail and electrodynamic bare tethers might be used in the future for both propulsion and power generation. Mainly, both NASA and Russian Agency worked separately using nuclear technology to obtain more efficiency

Radioisotope Power Systems (RPS), is a nuclear-powered system to generate electric power to feed communication and scientific systems on a spacecraft. Radioisotope Thermoelectric Generators (RTGs), a type of Radioisotope Power System, were used in the past as electric power supplies for some navigational and meteorological missions, and most outer-planet missions. Radioisotope power systems use the natural decay of radionuclides produced by a nuclear reactor. The expensive, man-made Plutonium-238 (238Pu) is the appropriate source of energy used in RPS fueling; its long half-life (~87 years) guarantees long time missions. The limited avability of Plutonium-238 is inadequate to support scheduled NASA mission beyond 2018. After the Cold War, throughout the Non-Proliferation of Nuclear Weapons Treaty, the production and processing of these resources have been severally reduced. There is a high-priority recommendation to reestablish production to solve the severe 238Pu

The isotope initially selected for terrestrial and space power applications was Cerium-144 because it is one of the most useful fission products available from nuclear reactor (Furlog, 1999; Lange, 2008). Its short half-life (about 290 days) made Cerium-144 compatible with a possible short-time mission. However, the high radiation associated with a powerful beta/gamma emission produces several problems with the payload interaction and safety in the case of reentry orbit. The development of RTGs was assigned to The Atomic Energy Commission in 1955. The first system developed for space situation was the System for Nuclear Auxiliary Power (SNAP). The Cerium-144 fueled SNAP-1 power system was never used in space. The first flight with a RTG was SNAP-3 in 1961 delivering 11.6 kW over a 280 days period, using as fueling Polonium-210 (Po-210) isotope. Po-210 is an alpha emitter with

**1. Introduction** 

in their systems for deep space exploration.

demand problem (National Reseach Council, 2009).

**for Space Applications** 

Antonio Sanchez-Torres

*Departamento de Física Aplicada,* 

