Trying to verify theories with astronomy on the other hand is impractical since we don't have a method to move stars around to see if they are what caused a phenomenon or if it's just a coincidence.
Yet. We can't do this, or blow up stars, or set up colliding black holes *yet.*
No. The main problem is that they are weapons of mass destruction that can vaporize entire cities in an instant. They are weapons that are specifically designed to kill a large number of people over a large area very quickly. THAT is the main problem with them. Let's not lose sight of why nukes are scary. The fallout merely adds the problem.
The term collateral damage when applied to nukes is kind of meaningless. The entire point of a nuke is to destroy everything in a rather large radius. There really is no such thing as collateral damage when using explosions of that size because you are unavoidably and intentionally targeting non-combatants and infrastructure when you make the decision to use one. Yes this remains true for "tactical nukes" too.
Exactly. We also, though we could, don't create firestorms in cities anymore, nor do we engage in unlimited civilian target bombing. We seem to, as a species, decided that these things are off the table, both due to adverse reactions to civilian deaths, as well as the possibility of nuclear response ("You burned my capital to the ground, we don't have the air superiority to do the same to you, but the NORKs sold us a little bomb we've sent over in a cargo container.") As most "tactical" nukes are more powerful than the original strategic WWII-era nukes, it is pretty clear that their use, no matter what they are called, would be as city burners, as that's where the modern battlefields are.
"Once both sides of a conflict start playing with nukes, even if it starts out with small, tactical, targeted nukes, the other side will too, and whichever side is losing will be tempted to scale up, "
Even if the conflict is with a non-nuclear country, or one with no long-distance delivery technology, there is a fear that a contained strike, say the US blasting an ISIS underground redoubt, would 'normalize' nuclear warfare in the future.
Not to mention that if the fallout is encountered by even one citizen of another nuclear state, let alone an embassy or crosses a border into a nuclear armed country, they may well consider that an attack and retaliate. Nuking Daesh should be safe-ish in that one regard, but even there you have Israel (still denying they have nukes), would they show restraint if, say, fallout from a Russian nuke contaminated their northern territories? How would Turkey, a member of NATO, respond if their country was irradiated? If a Chinese embassy was rendered uninhabitable by fallout, what would they do? Best to leave that can of radioactive worms unopened.
50 kilometers above the surface the temperature and pressure are earth-normal. Huge dirigibles using oxygen and nitrogen would float in the denser co2 atmosphere.
Sure, but what are you going to make your drigibles out of? There's only two sources of nonvolatile raw materials on Venus, and if it's not to be found in Venus's atmosphere, you're either going to have to import from Earth (or elsewhere in the System) or go down to the surface and mine it. The first is prohibitively expensive just in terms of energy costs, and the second is undoable using current technology. The idea of a colony is to be self-sufficient enough to be able to establish an settlement and expand according to your needs. We'll need some significant technological development before that's even possible on Venus, but Mars and/or the Moon seem doable with just extending current engineering capabilities.