Christopher Panceri of Encounter 2001 explains the mission

The first detailed radio message to the stars is being transmitted on Monday. It asks any aliens who receive it to get in touch.

It is part of a commercial project called Encounter 2001 based in Houston, US. The company offered the public the chance to tag their own messages on the end for $15.

Chan Tysor of Encounter 2001 says: "This is a statement, sending something of yourself away from the Earth to travel in space forever."

The message will be sent from the Evpatoria in the Ukraine

The stars the messages are aimed at are 51 to 71 light years away. Radio waves travel at the speed of light, meaning that it will be at least 102 years before any reply is received. That does not include any thinking time for any alien trying to decipher the message.

But scientists involved in listening for intelligent signals from outer space are sceptical. Specialists in Seti (Search for Extraterrestrial Intelligence) say sending a message out into space is almost certainly a fruitless exercise.

Dr Frank Stootman, of Seti Australia, says that it is not a message to aliens, but to us. He adds that a reply is very unlikely and certainly not within our lifetime.

The message

The main message is based on science, logic and mathematics.

The 1974 Arecibo message

Dr Yvan Dutil, a scientist working for the Canadian Government, helped to design it. He points out that the only other signal deliberately sent into space, beamed out in 1974, was aimed at stars unlikely to have planets.

Because the new signal is aimed at a handful of stars like our Sun, in a region of the sky called the Summer Triangle, he says that "for practical purposes, this will be our first detailed interstellar transmission."

It is being transmitted on Monday by the Evpatoria radio telescope in the Ukraine.

The message consists of a series of pages repeated three times over a period of three hours. The signal is 100,000 times stronger than a TV broadcast.

Don't listen, talk

The message sent in 1974 was transmitted from the Arecibo radio telescope. It was a brief three-minute message towards the distant M13 stellar cluster.

It consisted of 1,679 pulses. When arranged into a matrix, they became an image showing atoms, molecules, our Solar System and a representation of a human.

But the new cosmic message is much longer - 400,000 bits.

Starting with basic symbols, it uses logic to describe numbers and geometry. It then goes on to introduce concepts such as atoms, planets and even DNA.

"If any aliens ever intercept this message, they will have mastered science. Therefore, much of the first part of the message, the part that deals with numbers and atoms, will be familiar to them", says Dr Dutil.

"They can then go on and deduce a few things about humans such as where we live, how big we are and how many there are of us."

Universal peace

A cosmic chemistry lesson

As well as the encrypted message, there will be a series of greetings written by the general public.

According to Chan Tysor, the greetings include peoples' hopes for a more peaceful future for mankind and other races in space. One person says that we have made a mess of our planet and asks aliens to put off a visit for another 100,000 years.

Mr Tysor says the signal is a kind of monument. "It is a kind of immortality knowing that something you wrote is beaming its way out of the solar system into the galaxy."

Listening in

In a separate project, Seti experts who listen for unsolicited messages have been pleased by the response to their release of free SETI@home software.

This screensaver program allows computers to analyse radio signals for possible signs of ET. Just a week after its launch, nearly 300,000 computers have contributed 1100 years of computer time to the search for extraterrestrial life.