Only now noticed there was a follow-up here.
Sci-fi is a wide genre. Could be all kinds of stuff. Weird, out-there space opery alike Brian K. Vaughan and Fiona Staples'
Saga, or more down-to-earth SF like
Astronauts in Trouble by Larry Young and Charles Adlard. (Both personal favorites of mine.) Matt Fraction's Casanova (with Gabriel Bà and Fabio Moon) is right out there with Saga, minus the space. Brian Clevinger's Atomic Robo goes for good old-fashioned science adventure fun while Warren Ellis'
Ministry of Space tells an alternative histroy where England has been the driving force in space exploration. If you want to look Internationally, Christin and Mézière's Valerian is a good start - storytelling has its flaws, but the visuals are stunning. There's tons of Japanese stuff, too, but you're on your own there. (I can tell you, though, that the
Cowboy Bebop mangas sucked, compared to the anime.)
Those are just from the top of my head. Go to a comic store and ask, or browse webcomic aggregators like
The Belfry or
The Webcomic List by genre.