"The stars look the same from night to night. Nebulae and galaxies are dully immutable, maintaining t..." — Seth Shostak

While human space travel is daunting, machines - with their indefinitely long lifetimes - could travel the galaxy. It might make little difference to them that bridging the distance from one star to the next could take hundreds of thousands of years or more.

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.

Sending greeting cards to aliens is hardly a new idea. In 2005, Craigslist solicited messages for broadcast to space by a transmitter in Florida, and in 2008, NASA beamed a Beatles song to the North Star (Polaris), on the assumption that any putative Polarians would appreciate the Fab Four's 1960s-genre compositions.

Ever since the Second World War, television signals (as well as FM radio and radar) have served as Homo sapiens' emissaries into deep space. High-frequency, high-power broadcasts have filled an Earth-centered bubble more than 60 light-years in radius with signals.