[SteveS]

These are settings I know at least something about, that I'd call semi-hard science fiction:

Transhuman Space - probably the least divergence from known basic science, but the assumptions about the direction of technology may not appeal to everyone. It may take a lot of work to adapt GURPS rules to d20.

Traveller 2300 - a completely different future history from the Third Imperium future history, with stutterwarp instead of jump. Other than stutterwarp, it's fairly hard science. The main connection with Traveller when it came out was that both were from GDW. Its current version shares a lot of rules with Mongoose Traveller; equipment may need some adaptation to d20.

Traveller 20 - the books assume Third Imperium, about a century earlier than the classic setting, but you can use any Traveller era without much trouble. My impression was that the core book was not a strong candidate for the proofreading hall of fame, but they were usable.

Do you have a clear idea of what sort of semi-hard science fiction you and your players would like? That covers things like Total Recall (realistic space technology, magic brain science), Transhuman (realistic hard science, improbable application of technology), Aliens (hand-wave star travel, science fantasy monster hunting), assorted space wars, assorted planetside wars, traditional Traveller style adventures such as rule-bending merchants, Scouts that still have the exploration itch, mercenaries, plausibly deniable Imperial agents, etc.

Also, do you and your players have a strong preference for d20, or just have that in mind as the right balance between abstract and crunchy? If the latter, you might try Savage Worlds rules, but all of the Traveller adaptations I've found for it have been pretty rough around the edges.