I'm not an expert but I am a science freak, so please take this with a grain of salt.
1. Fallout—The bomb could kick up contaminated debris from the asteroid that could be spread over any amount of the Earth, it would depend on how far away the bomb went off and how near the asteroid’s path came. The debris would be spreading out in a cone shape and more would fall the longer Earth was within the cone.
2. EMP—There wouldn’t be much purpose in using an EMP type bomb on Nightfall unless it had a higher tonnage yield. IIRC there’s a difference if the bomb is within the atmosphere or not. Also, there is an inverse square relationship between distance from the explosion and the amount of energy received. That is, twice the distance is 1/(2*2) = 1/4th the energy, three times is 1/9th and so on. It doesn’t take a huge distance to make a huge difference in the effect. In the dark you can read using a candle close up but not 10 feet away—same effect.
3. Nukes do disrupt radio transmissions and the ionosphere.
Look up “Operation Dominic”, HANE (High Altitude Nuclear Explosion) and “Starfish Prime” about the H-bomb set off 250 miles up in 1962. There are really good descriptions of what these things can do out there. I’m amazed we survived.
4. From space I don’t think blindness would be a problem unless you were looking at it through a telescope or binoculars due to the inverse square thing. The Hiroshima people were much, much closer than anything you would set off for Nightfall.
Actually, control of the missile gets easier once you’re out of the atmosphere since there’s nothing to interfere. Out past the air, gravity is the only thing to worry about and it’s relatively easy to calculate. You could still have a malfunction if you needed one for the story. Hey, we lost a Mars probe because someone didn’t convert pounds to kilograms!
Hope this helps,
