That's 5x larger than the Krakatoa explosion; huge but not alone big enough to exterminate humanity.

The most dangerous nukes are actually the smallest, since politicians may convince themselves that just a few small nukes is permissible, resulting in a rapid escalation that sees every major city showered with megaton h-bombs, destroying civilization and sending humanity into a poisoned dark age.

If it seems implausible that politicians might use tactical nukes, ask what those nukes were created for. The US made many thousands of them to be used against a conventional Soviet ground invasion of West Germany; lobbing small nukes at tank columns. Escalation from a conventional war to a nuclear war was a common assumption in the military planning of the late cold war. But sides anticipated it and were prepared to perform that escalation themselves.

We already have hundreds (thousands?) of Mt devices. A handful of Gt devices is not going to change anything as regards humanity's threat to itself.

In fact I believe that anything over the low tens of Mt is considered impractical for military use.

Yes, and for the most cynical of reasons: the fireball and dirt-digging go with r^3, but the people killed and infrastructure destroyed go with r^2, and the purpose of a nuclear weapon isn't to light a fireball and dig dirt.

Agreed, a big device doesn't really alter the balance of power, which is why both the Americans and Soviets abandoned the path of bigger and bigger devices by 1962, leaving lots of engineering possibilities on the table to instead focus on miniaturization.

Yeah I don't think anyone, even the craziest of dictators, want to take over a country and then have to deal with a giant smoldering radioactive hole in the ground the size of rhode island.

Tsar bomba was 100 Mt design, about 50 Mt test. Not much of a difference, you're going to be toast anyway (multiple "smaller" warheads are more economical and harder to intercept anyways).

Good point, especially when referenced to he low chance of the strategy working.

We should consider ignoring asteroid-deflection itself as a possibility. The scale of an asteroid impact, deliberately instigated by deflection as an act of war, is far larger than what humanity's capable of with nuclear weapons alone.

Surely anyone on earth who has the technical ability to deflect an asteroid precisely to hit one part of the planet, would also have the technical ability to just bomb that part of the planet directly - with much lower risk of accidentally hitting the wrong part?

The scale of asteroid impacts is much larger than nuclear war. An actor could take out an entire continent at once, or end human civilization globally.

It's not easy, but it's something like a 3-4 order of magnitude multiplier if you figure out how to do it.

Yeah right, MAD is a deterrent not a first strike weapon. Besides that, asteroids spend most of their time extremely far from Earth. If you launched some mega project to get out there and redirect one that wasn't already headed for us it would take you years of preparation and then years of execution time. Your enemy could easily see all of this activity and decide to just start a conventional war in the mean time to take you out and still have enough time to deflect your war asteroid. It's a dumb scifi plot device not a practical weapon.