Many spacecraft cannot run on batteries or solar power alone. This is simply due to impracticalities. The number of batteries that would be required to power a spacecraft such as Galileo would create such a weight that the power output from the batteries would not be sufficient to lift the craft and power the craft’s systems.

Solar panels would not be practical either as, using Galileo as an example once more (when it under went a trip to Jupiter), it would’ve required 186m² of solar panels, which would weigh more than 454kg to produce the same power output as two Radioisotope Thermoelectric Generators (RTGs). Also, the shuttle carrying Galileo would not have been able to accommodate a solar array of this size and weight.

The solution therefore, is to use a Radioisotope Power System (RPS). This (in it’s most basic form) consists of two parts:

1. A source of heat;
2. A system to convert the heat to electricity.

These power sources come in two main forms, each with it’s own purpose.

Radioisotope Thermoelectric Generators

The most common form of an RPS is the Radioisotope Thermoelectric Generator (RTG). These are used to power the spacecraft, the scientific instruments on board and the engineering systems. They have been used by many craft in past space exploration missions, including Galileo, the Apollo Moon Mission series, the Viking Mars Landers and the Pioneer series.

An RTG is made up of slightly more than a heat source and a temperature transducer*. It also includes a gas-pressure venting system, connectors, a heat-rejecting system* and mounting brackets. The RTGs are usually mounted in tandem (end-to-end) on a deployable boom*, usually in groups of three. A typical RTG usually measures around 115cm in height, 45cm in diameter and weighs about 56kg.

RTGs are lightweight, cost-effective and highly reliable. This high reliability is mainly due to the fact that they have are no moving parts, therefore reducing the possibility of the mechanics failing. The level of power output from an RTG is low, making them suitable for deep space exploration, so suitable, in fact, that NASA have used RTGs for the past three decades.

Nuclear fission does not take place inside the RTG and neither does nuclear fusion. Instead the RTG houses a nuclear material that becomes physically hot as it decays. It is for this reason that the material is often referred to as the ‘heat source’. The heat from the nuclear material is converted into electrical energy by using a thermoelectric converter, which employs the principles of the Seebeck effect, an explanation of which follows:

If two wires of different materials are joined together at their ends and one end is maintained at a higher temperature than the other end, a potential difference will arise and an electric current will flow between the hot and cold junctions.

Using doped semi-conductors such as silicon-germanium with impurities such as boron or phosphorus, instead of pure metal, produces an excess or deficiency of electrons, therefore making the semi-conductor a more efficient power converter. The combination of these thermionic conductors with the hot isotopes produces a temperature difference of about 700K (426.85°C).

The nuclear heat source is an isotope of an element that is radio-active (hence Radioisotope). The most commonly used isotope is Plutonium-238, but in the form of the oxide, Plutonium Dioxide, PuO₂.

Plutonium Dioxide becomes hot when it decays because of the emitted alpha particle colliding with other particles in the Plutonium Dioxide.

This material is used due to its relatively high heat-to-mass ratio*, long half-life of 87.7 years and low gamma ray emissions. The Plutonium Dioxide is separated into 2.54cm long by 3.81cm diameter ceramic fuel pellets and each pellet is contained within three layers or protective material. These layers are designed to protect the Plutonium Dioxide from fires, explosions, fragment impacts and re-entry. Finally, a 1.91cm thick packet of metal and fibreglass-like insulation, which provides additional protection, surrounds the entire assembly. A typical RTG can provide about 1.06x10⁶ Joules of energy.

The selection of a specific radioisotope for a radioisotope power generator involves several considerations, depending upon the requirements of the spacecraft. In order to minimise the craft’s payload, the isotope must generate a large amount of heat from a small amount of material. A second consideration is the isotopes half-life. If the majority of the radio nuclide decays in less than the operational lifetime of the space mission, it is not suitable.

A third point to be considered is the type of radiation released by the radio nuclide. Certain radio nuclides decay by alpha particle emission. These require no shielding because alpha particles cannot penetrate the skin. However, in the case of those materials that release gamma rays, careful shielding is required to protect the launch preparation workers from external exposure. The dose that the workers would’ve received if insufficient shielding had been used can be calculated using becquerels, grays and sieverts. To calculate the dose equivalent one of the workers would receive in sieverts, you would need to multiply the dose in grays, by the quality factor. The quality factor of an alpha particle is 20. The standard dose equivalent a person receives form working with nuclear power is 3mSv.

Although RTGs have never been responsible for any failures onboard spacecraft, they have been in use during three accidents. In all cases, the RTGs performed exactly as designed. The earlier RTG models were designed to burn up at a high altitude during an accidental re-entry. In 1964, the RTG design was changed so that they would contain their plutonium payload during any accidental re-entry. This criterion was met during the Apollo 13 mission.

In 1997, NASA planned to launch a space probe named Cassini. There was a protest from the public that it should not be launched, as there was a fear that Cassini would accidentally re-enter the Earth’s atmosphere while returning from it’s trip round Venus, and much of the plutonium fuel source would be vaporised, resulting in a collective dose to the world’s population. The idea was that the probe would arrive back at Earth, travelling at a velocity of 18.8 km s^-1, and would pass within 700 km of the Earth’s surface. A 1995 NASA Environmental Impact Statement states that in the event of an inadvertent re-entry, approximately 5 billion of the world population could receive 99 percent or more of the radiation exposure.

Radioisotope Heater Units

The other main form of a Radioisotope Power System is Radioisotope Heater Units (or RHUs).

These systems are designed to heat the electrical devices aboard spacecraft, as, besides requiring power, the scientific systems and instruments also need to be at a maintained, standard temperature in order to operate efficiently and optimally.

RHUs work on almost the same principle as RTGs. They contain an isotope, which, when it decays, becomes physically hot. Again, plutonium-238 oxide is used, in the form of a pellet. The heat generated by the decay is transferred directly to the system, without the use of moving parts or intersecting electronic components.

RHUs are usually very compact, measuring 3.2cm long, 2.6cm in diameter and weighing 40g. The fuel pellet itself weighs around 2.7g. They have a very rugged containment system to prevent the release of the radioactive fuel. Like the pellets in the RTGs, the pellets in the RHUs have multiple layers that are resistant to the heat and impact that might be encountered during an accident. An external graphite shell and a graphite insulator also protect the fuel.

In addition to these safety features, the plutonium in a ceramic form, which means that if it were to break, it would break into very large pieces, rather than shattering into tiny fragments. This means that the shards would have a lesser surface area with which to interact with the environment around them. Also, in the extremely unlikely event of a breach of the multiple fuel containment barriers, it minimises the potential for human exposure.

Transportation of Radioisotope Power Systems

The transportation system for RPSs was developed for the DoE (Department of Energy) under a contract with the Westinghouse Hanford Company. The system compromises of a conventional packing system for the transport of radioactive materials, (in this case plutonium) and an environmental ‘cocoon’ to surround and protect the fragile RPS from overheating, shock and vibrations.

The system also includes two leak-tight packages, redundant active refrigeration systems for the normal transport and handling of heat sources, provisions for accommodation of a variety of possible RPS payloads and an aerospace-quality shock and vibration isolation system.

The entire system is housed in a customized 12.2m van trailer and is monitored with an instrumentation system for on-line temperature and vibration measurement, as well as post trip data interpretation.

The Future

Advanced Radioisotope Power Systems, that make use of higher efficiency thermal conversion, are currently under study. The plan is that the quantity of radioisotope material necessary to accomplish a mission will be reduced and to provide a smaller, more efficient radioisotope power system.