system
Given the following post, respond as if you're commenting on Reddit:
human
Starfish Prime was the largest nuclear test conducted in outer space, by the US in 1962. What was its purpose and what did we learn from it?

Edit: Thank you all for taking the time to produce such interesting answers!
gpt
Starfish Prime was part of a larger series of high-altitude tests called Operation Fishbowl (a subset of Operation Dominic). As the researcher Chuck Hansen puts it pithily in his _Swords of Armageddon_ (v2):

> The purpose of the FISHBOWL program was to satisfy JCS requirements for weapons effects data about nuclear fireball transparency, growth and rise rates; intensity and duration of atmospheric ionization; missile RV structural response to thermal radiation; radiation flux measurements; electromagnetic pulse (EMP) effects and range; nuclear, thermal, and x-radiation output and effects; and radio and radar "blackout" effects (which would bear directly on antiballistic missile targeting and control). Knowledge of these effects was required to evaluate ICBM "kill" mechanisms and vulnerabilities; ABM effectiveness; communications and control; and the value of ICBM penetration aids.

At the time, both the US and USSR were deploying anti-ballistic missile systems that would try to intercept incoming missiles at high altitudes with nuclear warheads, and used radio waves for communication and coordination of their forces. So understanding what would happen when a weapon went off very high above the atmosphere was important for this, especially since many of the effects of a nuclear weapon are somewhat different in versus outside of the atmosphere. And if you imagine lots of these things going off in the upper atmosphere, you get a picture of how "messy" it would be to try and detect incoming missiles and planes, and communicate outside of your home country, in the event of all-out war.

To highlight two of the most important of the above:

* The "blackout" effects pertain to the fact that a high-altitude nuclear weapon will interfere with radar and radio. That means that there is a period after a weapon has detonated at that height that the radars on the ground can no longer see any incoming weapons. Understanding this is crucial if you are really trying to field a nuclear-armed ABM system, because every "hit" makes it harder for you to see any further, incoming missiles, and makes it very easy to defeat (just send a lot). 

* The electromagnetic pulse (EMP) was somewhat understood prior to these tests but Starfish Prime in particular highlighted its effects. Because it ionized the upper atmosphere, it produced a massive EMP effect over a very large area. This was of interest for a lot of reasons relating to both defense and attack strategies — if you are able to interfere with electronics on a large scale, that can be useful; if you have electronics you don't want interfered with in that way, you have to design them to be able to resist it.

Starfish was an "effects" test — the goal was to see "what happened" not to learn about whether it would work or not. This is different than, say, Frigate Bird, which was a "systems" test (does the whole system work?) or the other tests in the Dominic series that tried out new warhead ideas ("design" tests).