Technically, I do believe this anime series qualifies as "hentai", i.e. erotic anime, but it is made in so beautiful, tasteful and inoffensive a manner that it is thoroughly enjoyable.
Good erotic sci-fi is preposterously rare, but this is actually the real thing. The animation is clear and good-looking, the characters are intriguing and the space adventures are interesting and... alien. Being Japanese, this show tends to have the weirdness that so often accompanies anime, from a Western perspective. But this is one show that needs to be given a chance. It is a work of rare beauty.