(Using the term “fireball” for the sake of title shorthand, it should properly be called a “debris cloud.”)
Starfish Prime, nuclear test, 1962. 1.4 megaton nuclear warhead, detonated at an altitude of 250 miles.
Video here.
My question, which I’ve surprisingly not been able to find online: how big is that distinctive blue “debris cloud” as can be seen in that video clip? Data on that element of the test is hard to find or parse through easily online, and high altitude or space-based nuclear explosions behave differently enough from atmospheric ones to require special figuring.
Bonus question: what would the probable effects be on an object* about located about half a radius from the center of that debris cloud, as seen in that video?
*Let’s say, oh…what the hell, the Exxon Valdez. I don’t know what it’s doing in orbit, either.
If AI is to be believed, 3.3 miles in diameter, from here:
As for the effects on the Valdez, probably almost none to the structure, but if Joseph Hazelwoo was still aboard and alive (somehow) he would most likely get a fatal dose of Neutrons.
The AI was getting spoofed by references to other tests in documents referencing Starfish, from my own searches, I’m afraid—I forget which one, now, but an atmospheric test—by comparison, Hardtack Teak, another near space (or barely in space) test, 3.88 megatons at 47 miles altitude, created a fireball that was 11 miles wide in 0.3 seconds (!), expanding to 18 miles in 3.5 seconds.
My old buddy Nukemap is saying that a 3.88 megaton surface burst would produce a fireball 2.96 miles in diameter.
Trying to guess fireball size from test footage itself is also complicated by a lot of US footage being slowed down (thus, harder to estimate how long the fireball is actually forming/expanding), and in this case, being shot without any nearby frames of reference, and (in the video above) probably zoomed in. Though I did find a lovely restored film of the test online with a camera and a shot that probably gives a closer view of what it would have looked like to an observer on the ground.
If the focal length of the lens was known, it would be straightforward to figure out the fireball dimensions. The distance is known - 250 miles.