By Guest Blogger, Nathan Edge  This month Nathan Edge discusses the nature and operation of nuclear devices being used in unmanned space operations. As an aside, I can't believe I wasn't the first person to type into Google, "nuclear engine...USS Enterprise"?! Nathan provides links to sites for more information throughout the piece. Anyway, on with the article! ------------------------------------------------- Nuclear power isn’t just applicable to terrestrial electricity generation; it has also been used in space travel since 1961 – whereby it is still a tremendous source of potential for propulsion mechanisms. Many well known space vehicles such as the deep space probes Voyagers 1 & 2 – which are over 18,000,000,000 and 15,000,000,000 km from Earth respectively – rely on nuclear power. It is even being used on NASA’s Curiosity Rover, which made a spectacular touchdown on Mars last year. Voyager 2. One of the most distant man-made objects in existence.The popularised spacecraft don’t actually use conventional nuclear reactors. Radioisotope thermoelectric generators (RTGs) - which harness Plutonium-238 as fuel - are the primary propulsion source. These so-called “nuclear batteries” exploit the principle that plutonium undergoes alpha-decay to produce heat. This is subsequently converted in to electricity using thermoelectric generators. Since they have no moving parts, RTGs are very reliable; the RTG on Voyager 2 has worked continuously since 1977, and is expected to continue working until 2025. A general RTG configurationThis is not to say, however, traditional nuclear reactors have not also been used in space operations in the past … albeit with some serious modifications. For example, the Soviet RORSAT satellites used 90% enriched uranium fuel as a power source. The safety implications of using such a system in earth orbit are obvious, and any fears of utilising such a technology are not exactly unfounded. The nuclear-powered Cosmos 954 satellite, for instance, fell into Canada in 1978 after a systems failure, distributing radioactive debris over 124,000 square kilometres.  Due to events like these, it is likely that any future reactors will be confined to deep space like RTGs. However, this does not mean that they will only be used for unmanned exploration probes. NASA has its eyes on a manned Mars mission, but traditional chemical rockets would take six to seven months to reach Mars. A 1MW fission reactor powering 100-400 kW electric ion thrusters would take 3 months, thus limiting the health degradation astronauts face on long space journeys. Modern RTGs are designed to survive the possible accidents which could befall it during operation, including propellant fires during launch and land/water impacts. Reassuringly, RTGs cannot explode like nuclear weapons: the plutonium associated with weapons is Pu-239, not Pu-238. A similar safety feature which has been specified for nuclear reactor propulsion tech is that they are not activated until they are confirmed to have reached space successfully. Ultimately, this is to ensure fission fragments and other components of nuclear waste are not present if failure occurs during launch. Space: The future of waste disposal?Lastly, when using an RTG some additional thought must be applied to the disposal of the spacecraft when they come to the end of their working life. This is not just a nuclear issue: it’s important not to disturb any areas which potentially harbour extraterrestrial life. For example the RTG-powered Galileo spacecraft was sent into Jupiter’s atmosphere and destroyed to stop it crashing into a potential ocean under Europa’s crust. The disposal of RTGs and future nuclear reactors actually touch on one of the more outlandish proposals for dealing with nuclear waste: firing it out of the solar system. In short, the space and nuclear industry