These days, if you talk about Sir Arthur Clarke, you need to know that since he got the knighthood in 2001, he is referred to in polite circles as the "space visionary and author". That will probably arouse a little cynicism in some people, in whose innermost thoughts may be, carefully concealed, this heretic notion: Arthur Clarke is as nutty as a fruitcake.

He is half a step away from being dragged off to a loony bin and locked up forever. It's a thought with which I confess to being familiar, and I suspect half the rest of the people who know who he is are, too. Did you hear what the man said in June? He was the key speaker in the Wernher von Braun Memorial Lecture series held in Washington at the Smithsonian's National Air and Space Museum. He was speaking to the audience by telephone from his home in Sri Lanka.

He said he'd been poring over images on his home computer taken by the now-orbiting Mars Global Surveyor (MGS). There are signs of vegetation evident in the photos, he said. "I'm quite serious when I say, have a really good look at these new Mars images. Something is actually moving and changing with the seasons that suggests, at least, vegetation," he said. Maybe it was trees, he thought.

Trees? Trees? Where are the little men in white coats, you wonder. Why are they taking so long? He struck it lucky, you think, with the film, '2001: A Space Odyssey' - but that's probably only because its director, Stanley Kubrick was there to keep his feet on the ground. Then you reflect that this is the man who said, in the 60s, when the word technology was nothing more than just another dry, vague little twiglet of the English language: "Any sufficiently advanced technology is indistinguishable from magic."

That's a very smart thought. It may be that people who belong in loony bins are capable of having such thoughts occasionally, but when you look at Sir Arthur Clarke's record, you realise that he must have thoughts like that every day, probably a dozen of them before breakfast, most days.

He is one of the most celebrated science fiction authors of our time. He is the author of more than 60 books with more than 50 million copies in print, winner of all the field's highest honours. He invented satellite communication with satellites in geostationary orbit in 1945. That got him quite a lot of attention and a number of honours, such as the 1982 Marconi International Fellowship; a gold medal of the Franklin Institute, the Vikram Sarabhai Professorship of the Physical Research Laboratory in Ahmedabad in India; the Lindbergh Award and a Fellowship of King's College in London.

He is a past Chairman of the British Interplanetary Society, a member of the International Academy of Astronautics, the Royal Astronomical Society and a host of other scientific organisations. There is no question. Sir Arthur Clarke is the genuine article.

The talk at the National Air and Space Museum, when Sir Arthur gave his lecture, was about space travel, and about the possibility of life in space. There were a number of other space authorities there, including Eugene Cernan, one of the Apollo 17 astronauts who walked on the moon. He might rather have put his finger on something. He suggested that there is no longer any real difference between science fiction and science fact. That's what it is about Sir Arthur Clarke - one keeps thinking of him as a science fiction writer, full of ideas about terrible green furry men with ray guns. So when he talks about trees on Mars, one thinks, 'there he goes again with his damned green men!' But actually, these days, terrible green furry men may actually be in the picture - sort of.

We've given up on our image of Life in Space as equating to humanoids, and we've accepted the possibility of what seem to me to be far stranger ideas - that is, life forms shaped by the habitat of the planet on which they live. The Astronomer Royal, Sir Martin Rees says: "They could be balloon-like creatures floating in dense atmospheres; they could be the size of insects, on a big planet where gravity pulled strongly. Or they may be freely-floating in space. They could even, as some science fiction reminds us, be super-intelligent computers, created by a race of alien beings that had already died out."

Balloon-like creatures floating in dense atmospheres? I'd say that makes green furry men look pretty sick, even with the ray guns.

It's bad news for Hollywood, among other things. Who's going to play the balloon-like creature? I remember reading somewhere that when Zero Mostel was once caught in a restaurant in a foreign country, unable to speak the language, he acted the part of every single ingredient in a salad for the bemused waiter, and got precisely what he wanted. But poor old Zero Mostel died some years ago, and they don't come like him any more.

Scientists seem more and more certain these days that we are going to find life out there somewhere. They point out that our Sun is just one star among billions. And in the vastness of space far beyond our own Solar System we can rule out nothing. Astronomers have discovered, just within the last five years, that many stars have their own retinue of planets. There are millions of other Solar Systems. And there would surely, among this vast number, be many planets resembling our Earth.

So if there are intelligent aliens, why haven't they visited us already? Some people, of course, claim that aliens have indeed visited us. But as Sir Martin Rees says: "The evidence for UFOs is no better than that for ghosts, and I'm personally quite unconvinced. Some astronomers cite this as evidence that aliens are rare. They note that some stars are billions of years older than our Sun, and point out that, if life were common, its emergence should have had a 'head start' on planets around these ancient stars.

"But the fact that we haven't been visited doesn't, in my view, imply that aliens don't exist - the question remains open. It would be far harder to traverse the mind-boggling distances of interstellar space than to send a radio signal.

"That's perhaps how aliens would reveal themselves first. The nearest stars are so far away that signals would take many years in transit. For this reason alone it makes sense to 'listen' rather than transmit - if a signal were detected, there would be time to send a measured response, but no scope for quick repartee! (Aliens equipped with large radio antennae could in any case pick up the combined output of all our TV transmitters - if they could decode them, it's hard to think what they might conclude about 'intelligent' life on Earth!)."

Good point. Life off-world. It's one of the more exciting puzzles left un-puzzled out. And I know what answer I'm hoping for. My own personal working assumption is that Sir Arthur Clarke is right - there are trees on Mars. I'd be chuffed if there were little space squirrels hopping around in them.

gshortoibl.bm