**8. Conclusion**

In space application, Radioisotope Power Systems takes some advantages over solar panels. In several space operations there are long periods of darkness, and RPS will be the best actual technology. For outer planet missions, RTGs are more useful than solar panels to generate electric power for feeding communication systems and scientific instruments on the spacecraft. Additionally, there are new space technologies that use natural resources with/without radioisotope power systems. Future mission such as Europa Jupiter System Mission (EJSM), which is a joined NASA/ESA mission, will intend to study Jovian system, focusing two particular Jovian moons. NASA-led will use one type of RPS on Jupiter Europa Orbiter (JEO) to reach Europa, whereas ESA will consider solar arrays for Ganymede exploration. NASA's Juno mission will use solar panels for Jovian system exploration, in spite of the low solar light reaching Jupiter. The JEO spacecraft is designed to meet the planetary protection requirements. The flight system will use five multi-mission RTGs (MMRTG) to generate 540 W of electrical power at the end of the mission. The high radiation environment (>50 the dose supported of Juno mission) makes the RPS more useful than solar array, because of the low solar wind reaching Jupiter. Waste heat from the MMRTGs would be used for thermal control in order to reduce electrical power.

Safety analysis of RPS requires a combination of deterministic and probabilistic steps to accurately predict the probability of system failure. The system failure is defined as rupture of one or more of the internal containment capsules surrounding the radioisotope fuel. To reduce the accident probability, we would have to identify among credible accidents, and analyze typical accident scenarios and consequences for overall flight phases of the spacecraft. The Launch Accident Scenario Evaluation Program (LASEP) computer program analyze the overall response of the GPHS-RTG in the various on-pad and near-pad launch accidents.

Actual high-magnitude earthquakes events occurred in Japan in 2011, has severally damaged the Fukushima reactor. This marks the difficult to change the public opinion about nuclear energy. Besides, the low disposal of Plutonium-238 is a serious drawback. The reestablishment of this man-made radioisotope production will be more difficult with these events. For using less plutonium than required, RPS efficiency must improve. Using lowconductivity materials and high thermoelectric rating, *Z*, RPS efficiency would improve. A high-efficiency Stirling-type system would give an apparent mass/power benefit, as well as using less plutonium for a similar power output. If we want to continue using RPS with Plutonium-238 as fuelling, we have to develop more high-efficiency systems, avoiding vibrations on the attitude on the spacecraft, as itself occurs with dynamic-conversion system. The current RPS power conversion efficiency is not too high. It is also required lower cost power systems.

Tethers might be used as alternative to solve the severe power generation problem. An electrodynamic tether, which is a very long wire capable to generate the suggested power, might radiate waves to satisfy communication requirements itself (Sanchez-Torres et al., 2010). The large electromotive force produced by the tether moving in some plasma ambient near the planet generate induced current and then electric power (Sanmartin et al., 1993). Tethers might be very useful for generating electric power both in Low Earth Orbit (high plasma density and moderate magnetic field) and in Jovian conditions (low plasma density and high magnetic field).
