Stone", developed by astronomers Yvan Dutil and Stephane Dumas from Defence Research and Development Canada in Valcartier, Quebec. Like Freudenthal's and DeVito's approaches, the images start with arithmetic and build from there. They also include graphs to illustrate physics, as well as sketches of the solar system, Earth's topography and the man and woman from the Pioneer plaque.

As well as these complex signals, Zaitsev has transmitted a more lightweight message. In "A message from Earth", sent in 2008, contributions from users of social networking site Bebo were directed at a single planetary system around the star Gliese 581. If anyone there is listening, they can expect to receive it in 2029 - followed by 26,000 unsolicited text messages collected by Cosmos magazine and transmitted last year. At 50 to 60 kilobytes, there is more information in the Cosmic Calls than in the Arecibo message, giving alien cryptologists and linguists something to work with. To give them a helping hand, a lot of what has been sent is redundant.

"Redundancy really helps," says Shostak, as it allows a recipient to make a guess about the meaning and then check it, like in a crossword. He suspects that all the polite efforts to be understood might be unnecessary. "A lot of people wonder what we should send. Music, mathematics or pictures? My first thought is it probably doesn't matter," he says.

Instead, Shostak suggests that we just gabble. "My conclusion is that you would just send them the Google servers. That's an enormous amount of information, much of it redundant and pictographic. Much of it is pornographic too, but I expect they could handle that." (Although it raises questions like, can Earth handle a trillion orders for Viagra?)

According to Shostak's calculations, sending so much stuff becomes practical if we assume that ET is very good at listening and has the technology to pick up a faint, rapidly changing signal against the background noise of the galaxy. If they have truly vast radio dishes, then we can send them encyclopedias and even libraries in a sensible amount of time.

Shostak also points out that an interstellar message is quite likely to be a one-way deal: it may not even reach its destination until long after our civilisation has ended. So as this is probably the only chance of telling an alien world about humanity, we might as well say as much as we can.

Of course, it would be much better if we could actually chat, as this would allow us to gradually teach each other our languages and histories. But interstellar distances make that nigh on impossible unless we can somehow send a representative.

Sociologist William Bainbridge of George Mason University in Fairfax, Virginia, is trying to develop avatars, human personalities encoded in software. If he

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