Non-anime sci-fi manga
These are recommended titles with no anime adaptions.
Cannon God Exaxxion 1-4 (outstanding)
Domu: A Childs Dream 1c
Immortal Rain 1-6 (Tokyopop has caught up .. big cliffhangers)
Ray 1-3 (will be an anime soon)
sci-fi manga with limited anime adaptions
I have included these, becaause their anime adaptions were
either:
a) very short (small OVAs)
b) bad
c) substantially different from the manga
Alien 9 (this is very trippy stuff)
Appleseed (outstanding - possibly THE sci-fi manga)
Battle Angel Alita (outstanding)
Nausicaa (scope and breadth of the manga is 4 times the anime)
While sci-fi manga is not necessarily rare, it does tend to get
adapted very readily into anime. Theres also plenty of sci-fi
manga where the sci-fi aspect is a prop to a different kind of
story (enabling super-powers, mecha, or just a backdrop).
Theres other stuff out there, but I haven't read it (Blame! comes
to mind).