The term "visionary" is tossed around a lot. It's become overused, and is pretty much ruined for those people who truly deserve it.

Like the late Arthur C. Clarke.

Clarke is often considered one of the "trinity" of great science-fiction writers -- authors who were not only superb storytellers, but had a solid grounding and thorough knowledge of science itself. And now all three have passed on -- Isaac Asimov, Robert Heinlein, and now Arthur C. Clarke.

Clarke's vision of the future was an astonishing thing. During World War II, he not only conceived of the notion of geostationary satellites (satellites that move in perfect synchronicity with the Earth's rotation so they appear to "hover" over the same spot), he caclulated the precise altitude they would need to achieve and speculated that they would make ideal communications relays. In fact, he calculated that it would only take three such satellites, situated 120 degrees apart from each other, to cover the entire Earth.

I don't think even Clarke could have imagined just how many communications satellites there would someday be, and how incredibly important they would become to our everyday lives. Cable TV, long-distance and international telephone calls, internet connections, all sorts of data is sent back and forth every instant.

Also sharing that incredibly valuable hunk of nothing 22,300 miles up are the 24 satellites that constitute the Global Positioning System. Since they don't move relative to the Earth, all you need to know is the direction and distance of three satellites, and it's just plain old geometry to know your precise position within about ten feet. We take it for granted, but it is directly traceable to Clarke.

Correction: as others have noted, the GPS satellites actually occupy a much lower orbit. But their function is still clearly precedented by Clarke's work on communication satellites -- work that has been cited in court as definitive enough to prevent anyone from patenting the idea.)

His most well-known work has to be the short story "The Sentinel," which was expanded into the novel and film "2001: A Space Odyssey." Brilliantly directed by Stanley Kubrick, the film is almost 40 years old -- and it is remarkable how un-dated it is. Indeed, I can only think of two elements in the story that Clarke got wrong (and not just premature): the "Bell System" logo on the phones and the "Pan Am" logo on the space plane.

One of Clarke's other visions -- although not one he came up with -- is the space elevator. It's a deceptively simple concept, as he outlined in his novel, "The Fountains Of Paradise:" stick a platform in geosynchronous orbit, then start paying out cables -- one "down" towards the Earth, one in the opposite direction. When the cable reaches the Earth, you have a ready-made pole running from Earth's surface right up into space. No more need for rockets, just build an "elevator car" that can run up and down the cable. The main technological hurdle is finding a material strong enough to work.

Clarke was an astonishing man, and his legacy will long be remembered. We were truly fortunate to have shared this small world with him.

It's a pity that we didn't reach out and conquer new worlds before Sir Arthur passed on. He did all he could -- and far more than most could ever dream of doing -- to see that come to pass, but we let him down.