From Atomic Rockets
I would like to clarify that this post does not refer to radioisotope thermoelectric generators (RTG) that have been used in space since the ’60s. These are still used and will be used in future missions. What I am talking about here is a primary propulsion system that uses nuclear fission or fusion to generate thrust.
When Will the First Atomic Rocket Lift Off Earth?
Most likely never. The main reason for this is quite simple; even the smallest propulsion engine will likely qualify as a potential weapon of mass destruction. Even a few kilograms of radioactive material could cause a huge loss of life if it were to burn up on reentry and that would be the best case scenario of an accident involving such a craft. It would be so easy to weaponize such an engine that there would be massive international opposition to its development. Those that think that countries like the U.S.A. and China would not be effected by such opposition how simple huge boycotts could cripple the economies of any country and how universal the opposition would be.
Will We Ever Have Atomic Rockets?
Yes, atomic propulsion in space will be an important part of our expansion into space. The catch here is that the radioactive element used will not likely come from the Earth’s surface and will likely never approach it closer than Geosynchronous orbit. The main reason for this is that even intentional nuclear weapon will not be that much more destructive than more conventional weapons in space. A large mass traveling as a significant speed has the potential energy and destructive power of even our largest nuclear weapons and the radiation produced is not that much greater than is ‘natural’ in the local environment. As the Moon is made up of the same material as the Earth’s crust there will likely be enough heavy metal isotope there to use in the engine. Mining it will also not have the issue of contaminating the local environment that exists on Earth.
Atomic propulsion will be used but it most likely will be uncommon. There will be cheaper and cleaner ways to get from point A to B but there will be situation where the high fuel density and power output provided by an ‘atomic rocket’ will be the right tool for the job.
Above our Cradle Earth 
It amazes me that people still think in the rather limited scope of what is basically a 1950’s sci fi magazine. Yes, there is enormous energy density per pound of fissionable material, however, in orbital, or even interplanetary uses, such density is overkill. Just look how little it took to get two men into lunar orbit versus the expenditure required to get two men into Earth orbit. (Apollo LEM ascent stage versus the Gemini module on a Titan II ICBM.) Now remove the .16G lunar gravity field. You can move objects that are in trans-lunar orbit (Earth to or from Luna) with really, little more force than a can of RAID, or the canned air you clean your keyboard with. After all, as you noted, speeds are high in space, which makes any solid object a potential weapon of mass destruction, so moving things around, needs to be done gently, slowly.
For moving further out than lunar orbit, solar sails become a viable alternative, as do many other lower energy fuels than nuclear. To go beyond the solar system, the leap in speed needed precludes nuclear as even THIS fuel won’t be up to the task. Frankly, we don’t know WHAT would fill that need.
SO, will there be atomic powered space ships? Maybe, but they’d be in the same category as the jet powered motorcycles and cars, or the curiosities like the car/boat.
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I think they will eventually be more than curiosities but they may be fairly rare. I think they will remain unknown until we become a truly interplanetary race. Once travel between planets becomes regular the ability to keep a fairly high thrust for the entire trip could cut transit time by as much as an order of magnitude. The ability to change transit times from months to weeks will likely be essential to human successfully colonizing the rest of the solar system.
I agree even with the best power sources we know today it is unlikely we will be able to send anything but a few probes to other stars. I also agree that I do not know what power source will be able to take us to the stars. I strongly suspect that the limits of our of knowledge today will not stop humans in the future reaching that goal. Humanity has always been good at reaching new heights. To borrow from Churchill, once we have exhausted all other options.
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Unfortunately, your first paragraph contains a variety of unjustifiable statements. Atmospheric, ground-level, & underwater detonations of atomic & hydrogen bombs, from the 1940s into the 1960s, dispersed literally tonnes of highly-active fission products, along with more tonnes of fast-neutron irradiation products, into the atmosphere & hydrosphere. If you read the reports of the United Nations Special Commission on the Effects of Atomic Radiation, the effect on human health of even such huge releases has been remarkably small. Likewise, despite all the hysterical pronouncements, deaths attributable to radiation from the Chernobyl event of 1986, which is about the most severe thing that could happen to any type of reactor, are to be numbered in the dozens only.
When we consider that the fuels used in atomic rocket engines of the type built & tested by the United States & the Soviet Union are, naturally, designed to stand up to high temperatures, thermal shock, & the erosion of ultrahot gas ; when we remember that a similar fuel was used in the plutonium-238 radioisotope thermoelectric generator carried by Apollo 13, which survived an unplanned reentry & is sitting placidly at the bottom of the Pacific Ocean ; when we understand that the buildup of highly-active fission products in a reactor is a function of its power level and the time it has been running, amounting to one gramme per megawatt-day, and that the total firing time of a nuclear-thermal rocket is generally measured in minutes ; when we observe that the uncontrolled reentry of a power reactor (not built for robustness) from a Soviet reconnaissance satellite, known as Cosmos-954, had in the event minimal radiological consequences ; and when we take account of the results of the tests performed as part of Project Rover, including the deliberate explosion of a rocket-reactor under the name of Kiwi-TNT, which proved to be about as benign as an uncontained nuclear reactor accident could possible be ; we are justified in supposing that the Devil is not so black as he is painted.
But that is the trouble with atomic energy generally. People have been taught to fear, and to pay no attention to facts. Hence all the clamour for “improved safety” when it is already the safest source of industrial-scale energy available, & all the carrying-on about Fukushima, when the earthquake & tsunami killed twenty thousand people & ruined entire cities, while the nuclear reactor problems have required only the temporary evacuation of limited areas, & neither have nor likely ever will (according, again, to the World Health Organization & UNSCEAR) result in deaths from radiation exposure. People have died as a result of unjustifiable “protective” measures (such as the workman who recently fell off a tank storing decontaminated water), & from the increased air pollution from the substitution of fossil fuels, but apparently they don’t count!
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I stand by my statement that an atomic rocket will be viewed by a majority of the world as a potential WMD. You might as well ask Exon to admit climate change is real. A majority of government will never admit atomic rocket are safe. A large part of their citizens will always view them as a threat to their safety and the government will see them as a potential change in the balance of power.
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publius:
A little devil’s advocate, using current political concerns as a backdrop. How long do you think it would take Iran to make a rocket capable of carrying one ton into orbit? How long would it ALSO take Iran to build a one ton dirty bomb? This alone would be enough to have the major governments of the world writing treaties to keep nuclear power of any significant size out of space. (See “Treaty on Principles Governing the Activities of States in the Exploration and Use of Outer Space, including the Moon and Other Celestial Bodies” signed on 27 January 1967)
It is less about whether nuclear power PROPERLY USED is safe in space, but more about the fact that not everyone who can reach space is going to “play nice.”
As for the safety of nuclear power on the ground, your data has some flaws as well. Yes, it is entirely probably that deaths attributable to Chernobyl are about a dozen, SO FAR. Remember, however, that new deaths attributed to the 1945 Hiroshima and Nagasaki blasts were recorded well into the 1990’s due to the long term cancer effects of radiation. We don’t yet know how many Chernobyl has killed, and may not know in our lifetime. The same holds for Fukushima.
“(U)njustifiable “protective” measures”? When nuclear waste material is hazardous for years after it has stopped producing power, and we currently have no realistic safe disposal plan for it, how can you call what LITTLE safety measures in place “unjustifiable”? Unless you can show a reliably SAFE means of disposing of hundreds of thousands of tons of radioactive waste water, plus hundreds of tons of reactor core waste material, calling what means are being used “unjustifiable protective measures” is reckless and irrational.
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