From helio- + -naut.
helionaut (plural helionauts)
- (science fiction, rare) Someone who travels to the Sun; an explorer of the Sun.
2008 January 10, Jared, “Re: SUNSHINE (no spoilers)”, in rec.arts.movies.current-films[1] (Usenet):IIRC from the director's commentary the name was chosen to add a bit of humility to the mission, lest the helionauts get too cocky about their cutting edge technology.