Well, it
was predicted that the fireball would hit the ground, but as it turned out the shockwave from the explosion prevented this, and I think if they had gone for the full 100mt it would have also vaporised the Tu-95 which dropped it. But yes, part of the decision to knock it down to 50mt was to reduce fallout, which was a smart move really, would have soured the achievement a bit if it had made most of the Soviet Union radioactive.
Not even the Proton could get the Tsar-Bomba into LEO, it has a payload of 50,000lb, the Tsar Bomba was 60,000lb. I imagine that advances in science since then though could probably bring the size of a 100mt device down somewhat, so a Proton could probably take it, but as was the case with the Tsar Bomba it's a very inefficient device, most of the energy from the explosion went into space, it's more efficient to use a couple of low megaton warheads and bracket the target.
Well, yes and no, fallout is still fallout, it's tiny bits of radioactive debris, in this case tiny bits of cobalt-60, that get carried up into the atmosphere by the explosion and then fall to earth downwind of the target. The height into the atmosphere that the cobalt-60 is blown by the explosion the further it will be able to travel. In the Baker test, which was a 23 kt device, the mushroom cloud went up to 10,000ft, obviously with a megaton device you'd need to multiply that, plus there are the base surges to take into account which would likely be the things that spread the most radiation, they can get up to around 1000ft and will head downwind from the explosion. It would probably do in a city, but you'd need to detonate it pretty much at the shoreline for maximum effect, the further out to sea it is, the less effect it will have.
It's a doomsday weapon, like all nuclear weapons, but not the most efficient of them, and if Russia starts messing with cobalt bombs, then the US will no doubt resume its cobalt bomb production, and is this the kind of nuclear arms race that Russia really wants to have?
