In any system that poses the potential danger that an RTG does, there must be cautious use. However, in three decades of use by the United States space program, over twenty-three missions, there have been absolutely no accidents to date involving RTG failure. The RTG is a good source of power. Without it, long duration manned missions will be much more difficult to plan and conduct. In fact, these manned missions would be impossible without RTGs or nuclear reactors because there is no other power source that has the power potential or extended lifetime of a nuclear system. The simple fact is that the engineering design that has gone into developing and implementing RTGs over the last thirty years has successfully found ways to protect the crew and peripheral systems (electronics, telecommunications, etc.) from radiation and heat hazards. The space-borne radioisotope thermoelectric generator is designed to withstand a beating and survive, all without releasing harmful, excessive radiation into the earth (or space) environment. The RTG is a marvel of power engineering and one that will surely propel mankind further in the cosmos.

Odysseus I and II will both require RTGs for power. This is a foregone conclusion. The RTGs that will be used will be, in most ways, exactly like those already in existence. The only thing we will change is the choice of radioisotope in use in the reaction tube (Sr 90 vice Pu 238). If we can discover a way to improve the thermoelectrics, namely the thermocouples, we will incorporate these changes too. This would probably increase energy conversion efficiency by five to ten percent over the current levels. At this time, we are looking more intensely at this problem. It seems that we can change the dopant material or amount of dopant used in the semiconductor to improve the thermoelectrics. We may also consider changing the semiconductor used from silicon-germanium to another material.

We will also attempt to design the means by which a significant amount of the excess thermal energy is supplied to the ship in order to provide heat for the crew and any other instruments or systems that require additional heat. By directly using the RTG waste heat rather than converting thermal energy to electrical energy and then back to thermal energy for heating, we can improve efficiency by a fair amount. This design will involve the combination of the waste heat radiator and a delivery system to successfully transfer the appropriate heat levels into the spacecraft. On the planet surface, as part of the ground mission, The RTG can provide direct heat to the crew in their "space bubble." This will free up other electric power for use in communications and experiments since we are harnessing the waste heat to provide any necessary thermal energy.

As the name of the apparatus implies, the RTG consists of radioisotopes undergoing a decay process. The decay of the radioisotopes produce large amounts of thermal energy. Using thermocouples, the device converts the thermal energy to electric energy via the thermoelectric effect. The electrical energy created can then be transferred to the spacecraft subsystems requiring power. Radioisotope thermoelectric generators operate much like nuclear reactors. A central core of radioisotope material is surrounded by a thermocouple array in a parallel series. The hot junction contacts a canister of radioisotope while the cold junction attaches to the external generator wall to provide a heat sink (radiator) to space. The radioisotope fuel is compressed into seventy-two ceramic-like cells. Each heat source consists of eighteen separate modules, each encasing four of the fuel pellets. Typically, Plutonium 238 is the radioisotope fuel of choice for the decay process. It is one of the best choices of radioisotope although Strontium 90, Cesium 144, and several other radioisotopes also provide good material properties. Still, Plutonium 238 has the more ideal qualities in terms of half-life, power production per gram, and cost for most space missions (Strontium 90 also shows great promise for usage in long-duration missions like the manned Mars mission.) The radioisotope fuel is found in the form of plutonium dioxide solid fuel. As the radioisotope decays, heat is released. The heat is then converted to electricity by a thermoelectric converter. An electromotive force is produced from the electron diffusion across the cold and hot junctions. This occurs because the junctions are at different temperatures (approximately 700 K difference) and are made of different materials. The materials are joined together to form a circuit. These junctions, consisting of different metal wires, are thermocouples. At present, the semiconductor thermoelectric elements allow for a ten to eleven percent conversion efficiency from thermal to electrical energy. RTG efficiency is limited by the conversion capability of the thermoelectric elements in the system. The RTG is also limited by the internal thermal conductivity of the radioisotope. The waste heat can be radiated throughout the ship to provide heat for the crew or other systems or it can be radiated into space using a large heat radiator surface on the skin of the ship or attached to the power plant itself if it is mounted remote of the main spacecraft.

The radioisotope thermoelectric generator is a superior power source for spacecraft design. The RTG has no moving parts, making it a highly reliable power source. This helps to eliminate problems relating to component mechanical wear and deterioration. RTGs do not require bulky dynamic machinery to produce energy. RTGs are independent of solar distances. Unlike solar arrays, they do not require solar input to function properly. RTGs are primarily limited by the half-life of the radioisotope used and the thermal-to-electric energy conversion efficiency. RTGs are also small in size for the relative amount of energy produced. The RTG used on Galileo produced 285 W of power (+/- 10%) for a weight of fifty-six kilograms. For a larger system, the power to weight ratio will improve somewhat. Of this weight, only eleven kilograms of the weight was in the form of the plutonium dioxide fuel which was pressed into seventy two solid ceramic-like cylindrical pellets. The rest of the weight was comprised of the shielding and containment devices for the RTG. We believe that by redesigning the thermocouples somewhat, we can increase the thermal-to-electric power conversion efficiency to near twenty percent. This large increase in efficiency would provide electric power levels that are sufficient for the Odysseus I and II missions. We have yet to look into the thermocouple redesign, but feel it is our best chance at improving energy conversion efficiency.

The RTG itself has multiple layers of protective material to shield the other spacecraft components from the high temperature levels and the radiation output. An aeroshell heat shield contains the carbon bonded carbon fiber sleeves and disks. These components have graphite impact shells which contain the fuel pellets and the rest of the internal RTG instruments. Since the fuel is stored in independent modular units, each having its own heat shield and impact shell, the chance for fuel release is minimized. This is because the design limits the impact distribution on each vessel. The multiple layers of protective materials , namely iridium capsules and high strength graphite blocks, help in the prevention of fuel leaks. These two materials are both corrosion resistant and strong. They also are compatible with the fuel pellet material. The graphite outer coverings protect against structural, thermal, and post-impact situations. The iridium cladding helps contain the fuel pellets in the event of a crash or other destructive end. Finally, the graphite and iridium are very heat resistant, making them ideal choices to protect in a high-heat environment.

The RTG does, however, have some complications associated with its use. The RTG cant be turned off once it is activated. If a temperature differential exists, the thermoelectric elements will continue to produce energy. The radioisotope continues to decay. Therefore, the RTG is stored in a shorted condition so that the temperature differential is kept at a minimum to keep energy production at a minimum. It is very difficult to design the thermal pathway such that a minimum temperature drop occurs from the isotope near the hot junction to the cold junction outer casing. This is a primary design goal, along with minimizing the heat leakage between the points that bypass the thermoelectrics. This is further limited by the material limits on the hot side (source) and the radiator size on the cold side (sink).

Another problem with the RTG occurs only over very long periods of lifetime usage. There will be a noticeable loss in energy output due to degradation of the thermoelectric elements. This is caused by dopant migration at higher temperatures. The semiconductor materials are often laced with small amounts of dopant material like the Boron or Phosphorous that are added to Silicon-Germanium semiconductors. This is done because it produces an excess or deficiency of electrons. This makes the semiconductor a more efficient power converter than conventional metals. Insulation breakdown due to temperature and radiation effects is another performance reducing condition in the RTG. Also, as the radioisotope decays over an extended period of time, the rate of heat release decreases somewhat. Therefore, when designing a power system requiring RTGs, the designer should plan for the end of mission power outputs of the system rather than the optimal energy output at the beginning of radioisotope decay.

Radioisotope thermoelectric generators are also very dangerous to handle. The high temperature and radiation effects make installation, repair and replacement an arduous task. If a great deal of work needs to be done on the RTG, the crew needs to be larger in order to reduce exposure to the ionizing radiation. To limit radiation exposure, it is possible to mount the RTG on a boom that extends away from the spacecraft main body or to shield it with a cladding material. Since the RTGs Plutonium 238 or other primary radioisotopes generally emit short-range alpha particles, the power source is often mounted on a five meter boom that extends away from the spacecrafts main body and electronics suites. The alpha particles arent very dangerous and only travel a few inches in air. When working on the RTG, personnel can be protected from these particles if wearing a protective suit. The alpha radiation only becomes a problem when it is deposited inside of the human body. One of the reasons the plutonium dioxide is in a solid form is to prevent the easy inhalation of small particles should the radioisotope be destroyed or dropped. The neutron and gamma radiation from an RTG is also quite small, and poses no significant health hazard if the external dose is limited as much as possible.

When being implemented, the radioisotope thermoelectric generators are designed to withstand both impact and other forms of damage and destruction. The ceramic-form plutonium fuel is heat resistant, thus making it more difficult to be vaporized in case of fire or reentry environmental exposure. The fuel is also very insoluble. It has a low chemical reactivity and breaks in large pieces, not small parts that can be inhaled or ingested. Unlike in nuclear accidents, RTGs cannot explode because no fusion or fission processes are occurring. Hence, the acute radiation sickness associated with nuclear explosions wont be witnessed in an RTG accident..

As mentioned previously, the mechanical containment vessels protect the RTG from impact damage and excessive heat. To protect the crew and the electronic instruments that can be damaged, the RTG can be mounted on a boom that extends away from the main body of the spacecraft. The separation distance does a lot to limit exposure to alpha, gamma, and neutron radiation. Shielding or cladding can be used to further protect the spacecraft hardware and personnel. The shield can either contain the entire RTG internally, or a shadow shield can be used. A shadow shield is lighter in weight and smaller in size. It only blocks radiation exposure over a small area of importance. This keeps the radiation from penetrating vital areas of the spacecraft, while saving precious space and weight. If a shadow shield is used though, it must be large enough to properly shield the crew and the sensitive electronic instruments like the telecommunications and solar arrays.