Learn English – Articles, prepositions, and conjunctions in acronyms and initialisms

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 BBC is not as rigid about these matters as some other bodies. For example, it was recently written that the BBC had standardised on "CE" rather than "AD" for denoting years -- but in fact the BBC simply doesn't have a set of rigid standards for such things.

The Guardian Style Guide, however, states:

Use all capitals if an abbreviation is pronounced as the individual letters: BBC, VAT, etc; if it is an acronym (pronounced as a word) spell out with initial capital, eg Nasa, Nato, unless it can be considered to have entered the language as an everyday word, such as awol, laser and, more recently, asbo, pin number and sim card. Note that pdf and plc are lowercase.

There is no universal rule; the Guardian's style guide is just one set of rules you can choose. The important thing is to be consistent.