For those of you who know who he was, he probably had as much influence on you as he did on me.  We are far from a small group- so rather than echo what so many others have said about him, here are links to a Space.com tribute, part 2,  and his Wikipedia page.

Many, many scientists, engineers, and aerospace types were inspired by his work, and as a vector toward the subject of recruiting, it should never be underestimated how powerful real inspiration can be.

His passing marks the end of an era- although its been well and truly gone since at least the early eighties, when the visual arts began to dominate popular SF.