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I have a universe where humanity has produced various civilizations on multiple planets (and no planet has achieved a single world government); and very few of these countries possess the know-how of nuclear weapons- but space travel is possible, although it can be expensive and difficult.
My question is this: Could basically all nations have the technology to be space-faring, yet not independently produce nuclear weapons? Can these two technologies be separate?
With the comment regarding theory vs. engineering, the answer becomes a clear yes.
Building a nuclear weapon is a serious engineering challenge even if you have the nuclear physics down pat. Gun type weapons are slightly easier than implosion type weapons, but both have the problem of preventing a fizzle.
The society might even run nuclear power plants. They would know that a nuclear weapon is possible, and that some of their power plant waste could be abused, but they never start the necessary R&D project to turn the theory into practice.
Yes, this would be possible. It would even be likely if Uranium (and Plutonium) were absent or exceedingly rare on those planets.
Thorium works for reactors but cannot create a nuclear chain reaction by itself, so it would not work for nuclear bombs. The concept of a destructive chain reaction might be only esoteric theory or might be well-known but judged impossible to realize, as it would require mining an entire planet's supply of Uranium and then enriching it.
About note 2 from the question: Thorium as found on Earth only contains trace amounts of the unstable isotopes that might allow a chain reaction. Somehow finding a very young/new supply of it might allow its use as a bomb.
Rockets don't need anything nuclear. Chemistry will suffice. For finding your way into orbit and back, electronics are a great help (and crucially, provide a great economic motivation). The peaceful but profitable uses of space travel are communications, weather forecasting, and natural resources management. Less peaceful but non-nuclear, spying and surveillance.
Weapons came first here on Earth, because WW2 was the catalyst for the development of both rockets/ICBMs (German V2) and nuclear weaponry. Further, unresolved tensions between the USA and the USSR (to put it mildly) drove the further development of space technologies via ICBMs.
In another branch of the multiverse, Lenin was shot by Russian royalists and Hitler died in the trenches of WW1. That world today would probably be dominated by the British empire and the USA. Would nuclear-tipped ICBMs have been developed in advance of communications satellites? I rather think that WW1 would have provided a longer-lasting deterrent to the military and that trans-Atlantic competition would be by trade, not armed conflict.
Elsewhere in the universe, there are doubtless stars and planets which condensed out of a supernova remnant 8 Gyears ago rather than 4 Gyears ago. Which means that there will be something like 1/16 as much Uranium-235 available to their inhabitants, which will probably hold up the development of nuclear fission technology by a considerable time compared to electronics. (Half-life of U235 is about 1 Gyear). Further if the planet doesn't have Earth's history of a huge impact event to stir it up, a much greater fraction of the heavy elements like Uranium will be buried in the planet's core. (We don't know enough to know whether a planet with less heavy elements at its surface is a barrier to the evolution of life, or not).
Hmmm. This is asking for a lot, considering that humanity was on its path to discovering nuclear energy ever since the 1800's. Think Marie Curie, and various other scientists who experimented with x-rays, etc. It took until the 1940's for the technology to develop enough and for governments to be motivated to fund the research in order for the bomb to come along, but it would have happened sooner or later, even if WW2 had not occurred.
Some ideas:
Alternate Power Source
I think it MIGHT be possible to skip nuclear energy if you give people some other POWERFUL energy source to exploit. For example the discovery of an alien artifact, or some strange material that we first find in the remains of a meteor that crashes on Earth (we use that material to get into space and harvest more, but at great expense).
Nuclear Power Without The Bomb
WW2 was averted somehow, and the scientists that stumbled onto nuclear energy did so in the context of a civilian scientific research setting, not military. That way they may give world nuclear power (by far our most powerful energy generating method), but avoid weaponizing it for a few decades. In the mean time, mankind enthusiastically researches new power sources and finds something even more powerful than nuclear energy - at that point you make some new tech up.
The thing to note is that neither of these scenarios actually excludes the creation of nuclear technology, or the atom bomb. Quite simply, a lot of scientists were having similar ideas about radioactivity, and one of them would have figured it out sooner or later, no matter what. Which leads to what is probably your best bet:
Aversion To Nuclear Power
Marie Curie or another scientist who first meddled with radioactivity is more successful than they should have been. One of their experiments accidentally irradiates a major European city and kills hundreds of thousands of people. The governments of the world impose a ban on all things nuclear, and severely punish any who explore that line of research. As a result, alternate power sources are explored: electrical motors, solar power panels, etc.
Good luck, hope that helped a bit.
Most of the answers refer to nuclear FISSION power and weapons. A couple mentioned fusion. This is usually thought of as either huge, plasma-based magnetic-confinement "Tokomak" style fusion reactors or the almost as big, beam-implosion reactors. Or as also mentioned, the boondoggled cold fusion.
As was suggested, if the characters lived on worlds with little access to naturally fissionable elements, there would likely be a much lower theoretical understanding of radioactivity much less the weaponizing of said knowledge, but their sun could be a potent source of inspiration for fusion research, and as was noted, a semi-portable fusion weapon is much harder to create, if not impossible, with current technology without a fission trigger device.
Another potentially useful version of fusion research is called "inertial confinement fusion". It was originated in the 1960's by television pioneer Philo T. Farnworth using a electrostatically-driven vacuum tube-like device he called a fusor.
Fusion can be easily (commercially even) achieved but a net power gain has not yet been proven doable. However, the late, great Dr. Robert W. Bussard (of "Bussard ram-jet fame", AEC member, etc.) was convinced it held much better prospects than Tokomaks (after something like 20 years on them) and was involved in a company called Energy Matter Conversion Corporation (i.e. EMC2). They claimed to have finally licked the engineering problems a few years ago... just before their U.S. DoD funding ran out AND their coils burned out. Since then the surviving researchers have been mostly publicly silent, although there are some links on their website to published papers.
Back in December 1986, Analog Magazine published an article by Tom Ligon, who worked with Bussard and it is now online at a website dedicated to the fusor topic fusor.net. The article also suggested a potential fuel, proton-boron 11 or p-B11, as ideal for a variety of reasons, including suitability for a spaceship drive and power plant. Hope that helps a bit and good luck with your writing!
You can't have thorium for your deep space work because it's not that hard to convert it into a fissionable isotope and thus make a bomb. You'll need a world virtually completely lacking in elements heavier than Bismuth.
Powering spacecraft that operate in the outer parts of the star system is going to be very problematic in such an environment—the only normal RTG isotope you would have would be Sr90 I see no good way to produce this. Sr89 is quite hot, you'll need two neutron captures within a fairly short period of time to convert stable Sr88 into Sr90. Trying to create it from Rubidium won't work because the only stable isotope is Rb85—neutron capture & decay yields Sr86. Likewise, I can't find anything heavier to bombard something with that will end up with enough neutrons. Likewise, decay from Yttrium isotopes yields something without enough neutrons.
Note that producing the heavy elements is likewise basically impossible. Neutron capture is stopped at Astatine unless you have a huge neutron flux (a supernova.) Bombardment with heavier atoms can bridge the gap but the results are so neutron-poor they promptly decay.
If you want a world with no big booms then I'm afraid you'll have to wait for fusion power to head beyond where solar power can do the job.
It actually depends on the level of space flight you assume for your civilizations.
Newtonian physics allow you to achieve space flight without the knowledge of special theory of relativity. The last is an integral part of understanding matter to energy conversion needed to create a nuclear weapon.
However - even if they totally do no research subatomic structure - a spacefaring civilization will sooner or later notice the effects of special and general theory of relativity when they use sattelites for applications like GPS.
If you want to generate enough power to go to outer space, you will however sooner or later need power generation through radioactive decay, fission or fusion at some point because it just becomes uneconomical to do that via conventional means.
If you go a step further, and say they do not develop nuclear weapons - even if they only develop nuclear power generation (fission/fusion) - and they would be in a position to alter the course of astronomical objects, they could still also achieve the same destructive power as nuclear weapons.
Chemical energy is sufficient to get into space, and has even been used to get to the Earth's Moon and back, so we have some pretty convincing proof that it is indeed possible. Wernher von Braun wrote a pretty detailed plan for a mission to Mars based on the technology available in the late 1940's/early 1950's, which resembled a fleet action more than anything else (there were about 7 manned ships just to get there and back, and electrical "hotel" power was through the use of solar concentrators to boil mercury to run turbines). The assembled fleet of manned ships, landers and cargo craft looked like this:
So spaceflight, and even large scale manned spaceflight is certainly possible using chemical energy and 1950 era technology. Without nuclear power, you are sharply limited by the energy density of chemical fuels, and the diminishing energy of the Sun as you go farther into the Solar System (an expedition to Saturn could take decades using chemical rockets), but you if there was some economic, social or military invectives to carry out a space program,, then you can go into space.
Given the physics are similar to our world. The rule for creating from matter is Einsteins famous formula: E=MC2.
Knowing this you would have the first step in building a nuclear weapon. It is also the same formula you use for cold fusion, nuclear power, oxygen and so on.
"Could basically all nations have the technology to be space-faring, yet not independently produce nuclear weapons?" : Yes, it could be possible.
Can these two technologies be separate? : It is fairly much the same principles, can you make a gasoline motor without knowing how to make a gasoline bomb. It is producing energy, the difference is just, how much energy, can it be contained, and are you making too much of it?
Space Travel Without Understanding How to Create Atomic Weapons? : There is other alternative to atomic weapons, you can make an Ethanol engine without knowing about gasoline. And you can make space travel without knowing about atomic weapons, you just need to have another power source just as good.
An example that this is possible — Nuclear Fusion
It's quite possible that a civilisation could understand and develop nuclear fusion power capabilities without ever developing nuclear fission capabilities. The two processes don't have much to do with each other. If it could be put under control (something that scientists are working towards right now), fusion could easily provide enough energy to travel interplanetary distances.
You can make fusion work without having any uranium or plutonium, all that's needed in principle is hydrogen, so you might guess that your world doesn't have uranium at all.
Making a hydrogen bomb is not possible without first making the fission bomb (the fission bomb sets off the fusion part of things), so this avoids misusing the technology to create nuclear weapons.
