I wrote a post earlier but it had a factual error.
In that post, I was saying that the suffix -naut conveyed the meaning of Sea or Ocean. (Think I made the same statement on my own journal at some point.)
In fact, -naut more precisely conveys the meaning of ‘sailor’ or ‘navigator’. It’s actually the same root from which the words ‘navigate’, ‘navigator’, or ‘navigation’ are derived.
It tends to connote a sense of exploration, pioneering, and adventure, because in our human histories, those who learned to sail and to navigate the seas were the frontier-runners; risking peril for the sake of knowledge, wealth, freedom, or even adventure.
It’s not in 1000 different terms, but we do see it around sometimes:
Astronaut, as you already mentioned, ‘a navigator (or sailor) of the stars’.
Aeronaut: a sailor of the sky and wind. We don’t see that one much BUT we do see the famous NASA: (National Aeronautics and Space Administration).
Jason and the Argonauts (the sailors of the ship named ‘Argo’)
Oneironauts: those who sail and travel the seas of Dreams
And then to these would be added @Joa23 's
Psychonaut: One who sails and navigates the seas of Psyche: of mind, of soul, of spirit.
Many, many adventures, freedoms, treasures, and insights to be found in that Sea!