Learn English – Using all with or without a definite article

Generally the article is not used with acronyms (initials that can be pronounced as a word), whereas it is with initialisms (initials where the letters themselves are pronounced). I would actually use the article with 'ESA' in the examples you gave, and so 'NASA' (acronym) doesn't get an article, but 'FBI', 'ESA', and 'DDR' (initialisms) do.

That said, there are initialisms where one wouldn't use the article either; you can usually work it out by expanding it and seeing whether you would use the article with the full sentence. Is the initialism talking about a specific thing (use the article), or a notion in general (don't use the article)? eg.

• The CPU is overheating (The Central Processing Unit is overheating).
• DRM is bad for consumers (Digital Rights Management is bad for consumers).

Acronyms don't tend to ever get an article, however (at least I can't think of any) because once they become pronounced as a name, they are treated like proper nouns, which don't receive an article (James did this; Microsoft did that; France did this; NASA did that).

The article "the" indicates that you are referring to a specific instance. So you say, "What is the time?" if you are asking for a specific time. Usually that would mean the time right now, though in context it could mean the time of some event under discussion. (Like, "We're planning a meeting to discuss this. The date of the meeting will be August 12." "Okay. And what is the time?")

Without an article, you would be referring to time in general. If you asked, "What is time?", you would be likely get a philosophical discussion on the nature of time. (Like, "What is time?" "Time is what keeps everything from happenning all at once.")