I'm not entirely sure if this is the right section for this... if there's somewhere else more appropriate, could a Mod kick it over?
Anyway. I'm writing some science fiction, and in one scene there is a battle in Earth orbit--about 4,000 km altitude. Nuclear missiles are used to blow up a ship, and what I'm trying to determine is if this would be visible from the surface of the planet.
I know from other sources that a nuclear weapon detonated in the atmosphere has the big, brightly-glowing long-lasting fireball because the air is absorbing the x-rays emitted and re-emitting them as visible light over a longer period of time. There's obviously no air to do this in space, so most of the energy of the weapon would instead be released as a single flash of x-rays.
The upper atmosphere of the Earth is opaque to x-rays. Would this make the nuclear detonations effectively invisible from the surface, or would the x-rays be converted into visible light after being absorbed and re-emitted by the air?
Finally, if the nukes would be visible, how bright would they be from that distance if they were, say, 100 kilotons in yield, 500 kt, 1 megaton and 10 Mt. Oh, and it's nighttime on the surface below.