The first sci-fi story is 1800 years old!
Ironically, the tale is titled “True Story.” It is a travel tale by the Greek-speaking Syrian author Lucian of Samosata, and is the earliest known fiction about travelling to out space, alien life-forms, and interplanetary warfare.
The work was meant by Lucian to be a satire against those who quoted mythical events as the truth. In True Story, Lucian and a band of adventurous heroes discover new planets, new life forms, and new struggles.
The story ends rather abruptly with Lucian promising that the rest of their adventures will be documented in a following book (spoiler alert: he lied). Its modern equivalents include the works of Douglas Adams. For more info; may the source be with you.