Arthur C. Clark; Ray Bradbury; Isaac Asimov; Robert Heinlein's "juveniles", such as "Starship Troopers" and "Tunnel in the Sky" (much of his other stuff is loaded with adult situations and themes), H.G. Wells...

I read a lot of other incredibly well-written and thought-provoking science fiction at that age that I am dying to recommend, (such as Ursula le Guin's "The Dispossesed", "The Left Hand of Darkness" and "The Lathe of Heaven", for starters) but I'm pretty sure that my parents would not have considered them appropriate for an 11 year old if they had read them.