Seth Shostak

Lamentably, alien audiences may be frustrated by the switch to digital television. That's because the transmitter power for DTV is fairly evenly spread across the spectrum. The spikiness is gone, and from afar, the attention-grabbing squeals of analog television's carriers have been replaced by DTV's smooth, low hiss.

There will be an end point to how good TV pictures can get. The boob tube has hugely benefited from the rapid advance of digital electronics. Consequently, the strategy for hardware has changed. In the old days, sets had to be as simple as Elmer Fudd to keep them inexpensive. All the technical 'smarts' were at the transmitter end.

As far as I can tell, the only unambiguous consequence of the claimed invasion of Earth by beings from another star system has been a nonstop torrent of TV specials. So if you're one of the many who believe the aliens are here, you really do have to admit this: They're the best houseguests ever.

Any beings advanced enough to traverse interstellar distances are at least a thousand years beyond our technical level. Spending gobs of time examining our missiles is equivalent to sending the Air Force back to the Middle Ages and insisting they examine the chain mail factories.

Of course there is still unexplored terrestrial territory, but most of it is waterlogged. Submersed secret places, such as the Challenger Deep, which today lure hi-tech adventurers like Richard Branson and James Cameron, will undoubtedly provide welcome fodder for 'National Geographic.'

It seems obvious that if a species has the brainpower for speech, along with the sort of appendages that can manipulate a pair of pliers, it will eventually blunder into science, technology, and radio.

In days gone by, scientists would speak solemnly about our solar system's 'habitable zone' - a theoretical region extending from Venus to Mars, but perhaps not encompassing either, where a planet would be the right temperature to have liquid water on its surface.

The planets and moons of our solar system are blatantly visible because they reflect sunlight. Without the nearby Sun, these planets would be cryptic and dark on the sky.

Everything you see is filtered through your visual system (imperfect) and your brain (also imperfect, despite what your mom told you). Witness testimony is the worst kind of evidence in science.

It's hardly a secret that I'm skeptical of declarations that the aliens are out and about on our planet. Still, I try to answer every one of these mails and phone calls because, after all, it's not a violation of physics to travel from one star system to another.

A century ago, scientists believed there was only one obvious stomping ground for alien biology in our solar system: Mars. Because it was reminiscent of Earth, Mars was assumed to be chock-a-block with animate beings, and its putative inhabitants got a lot of column inches and screen time.

Estimates are that at least 70 per cent of all stars are accompanied by planets, and since the latter can occur in systems rather than as individuals (think of our own solar system), the number of planets in the Milky Way galaxy is of order one trillion.