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In this particular case, don't, because the term NASA is more well known than its expansion. Sometimes acronyms evolve into words, such as scuba and radar. Once they become words, use them as words...
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Source: https://writers.stackexchange.com/a/47854 License name: CC BY-SA 4.0 License URL: https://creativecommons.org/licenses/by-sa/4.0/
#3: Attribution notice added
Source: https://writers.stackexchange.com/a/47854 License name: CC BY-SA 4.0 License URL: https://creativecommons.org/licenses/by-sa/4.0/
#2: Initial revision
In this particular case, don't, because the term NASA is more well known than its expansion. Sometimes acronyms evolve into words, such as scuba and radar. Once they become words, use them as words. As a matter of usage, NASA is no longer an acronym but a word (like FBI or CIA). Use it as such. As a general rule of thumb, if the acronym is better known than its expansion, just use the acronym. If you are writing fiction and you are referring to a real world acronym that your characters would all know, just use the acronym. If the reader does not know what it means, that is what Google is for. Keep the use in your text natural. If it is a made up acronym, think twice about whether you really want a made up acronym in your story. And if you decide you really do, find a way to introduce it in narrative before your characters use it in dialogue.